Part II. Basic nutrition Chapter 8. Body composition, the functions of food, metabolism and energy Dietary constituents and the functions of food Body composition Metabolism and energy Chapter 9. Macronutrients: Carbohydrates, fats and proteins Carbohydrates Fats Proteins Chapter 10. Minerals Calcium Iron Iodine Fluorine Zinc Other trace elements Chapter 11. Vitamins Vitamin A (Retinol) Thiamine (Vitamin B₁) Riboflavin (Vitamin B₂) Niacin (nicotinic acid, nicotinamide, vitamin PP) Vitamin B₁₂ (Cyanocobalamin) Folic acid or folates Vitamin C (ascorbic acid) Vitamin D Other vitamins

Part II. Basic nutrition

Chapter 8. Body composition, the *functions of food, metabolism and energy***

The phrase "we are what we eat" is frequently used to signify that the composition of our bodies is dependent in large measure on what we have consumed. The many chemical elements in the human body occur mainly in the form of water, protein, fats, mineral salts and carbohydrates, in the percentages shown in Table 8. Each human body is built up from food containing these five constituents, and vitamins as well. Food serves mainly for growth, energy and body repair, maintenance and protection. Food also provides enjoyment and stimulation, since eating and drinking are among the pleasures of life everywhere. Truly food nourishes both the body and soul. Even if technology could produce a perfect diet in terms of content, such a diet could still lack, for example, the aroma and flavour of a curry, or the stimulating taste of hot coffee. What controls appetite or the feeling of hunger is not fully understood. The hypothalamus in the brain has a role, as do other central nervous system sites. Other probable factors include blood sugar levels, body hormones, body fat, many diseases, emotions and of course food type and availability, personal likes and dislikes and the social setting where food is to be consumed.

Dietary constituents and the functions of food

A simple classification of dietary constituents is given in Table 9. Human beings eat food, and not individual nutrients. Most foods, including staples such as rice, maize and wheat, provide mainly carbohydrate for energy but also significant quantities of protein, a little fat or oil and useful micronutrients. Thus cereal grains provide some of the constituents needed for energy, growth and body repair and maintenance. Breastmilk provides all the macro- and micronutrients necessary to satisfy the total needs of a young infant up to six months of age including those for energy, growth and body repair and maintenance. Cows' milk has the balance of nutrients for all the requirements of a calf. Water Water can be considered the most important dietary constituent. A normal man or woman can live without food for 20 to 40 days, but without water humans die in four to seven days. Over 60 percent of human body weight is made up of water, of which approximately 61 percent is intracellular and the rest extracellular. Water intake, except under exceptional circumstances (e.g. intravenous feeding), comes from the food and fluids consumed. The amount consumed varies widely in individuals and may be influenced by climate, culture and other factors. Often as much as 1 litre is consumed in solid food, and 1 to 3 litres as fluids drunk. Water is also formed in the body as a result of oxidation of macronutrients, but the water thus obtained usually constitutes less than 10 percent of total water. Water is excreted mainly by the kidneys as urine. The kidneys regulate the output of urine and maintain a balance; if smaller amounts of fluid are consumed, the kidneys excrete less water, and the urine is more concentrated. While most water is eliminated by the kidneys, in hot climates as much or more can be lost from the skin (through perspiration) and the lungs. Much smaller quantities are lost from the gut in the faeces (except in the presence of diarrhoea, when losses may be high). TABLE 8 Chemical composition of a human body weighing 65 kg

Component	Percentage of body weight
Water	61.6
Protein	17
Fats	13.8
Minerals	6.1
Carbohydrate	1.5

TABLE 9 Simple classification of dietary constituents

Constituent	Use
Water	To provide body fluid and to help regulate body temperature
Carbohydrates	As fuel for energy for body heat and work
Fats	As fuel for energy and essential fatty acids
Proteins	For growth and repair
Minerals	For developing body tissues and for metabolic processes and protecfion
Vitamins	For metabolic processes and protection
Indigestible and unabsorbable particles, including fibre	To form a vehicle for other nutrients, add bulk to the diet, provide a habitat for bacterial flora and assist proper elimination of refuse

Metabolism of sodium and potassium, which are known as electrolytes, is linked with body water. The sodium is mainly in the extracellular water and the potassium in the intracellular water. Most diets contain adequate amounts of both these minerals. In fluid loss caused, for example, by diarrhoea or haemorrhage, the balance of electrolytes in the blood may become disturbed. Water intake and electrolyte balance are particularly important in sick infants. In healthy infants, breastmilk alone from a healthy mother provides adequate quantities of fluids and electrolytes without additional water for the first six months of life even in hot climates. Infants with diarrhoea and disease, however, may require additional fluids. While food intake is largely regulated by appetite and food availability, fluid intake is influenced by the sensation termed thirst. Thirst may arise for various reasons. In dehydration it may be caused by drying of the mouth but also by signals from the same satiety centre in the hypothalamus that controls hunger sensations. Dehydration, an important feature of diarrhoea, is discussed in Chapter 37. The phenomenon of water accumulation in the body is manifested in the condition known as oedema, when disease causes an excess of extracellular fluid. Two important deficiency diseases in which generalized oedema is a feature are kwashiorkor (see Chapter 12) and wet beriberi (see Chapter 16). The excess fluid may result from electrolyte disturbances and accumulation of water in the extracellular compartment. A person can have oedema and still be dehydrated from diarrhoea; this condition is a form of heart failure. Water can also collect in the peritoneal cavity, in the condition known as ascites, which may be caused by liver disease.

Body composition

The human body is sometimes said to be divided into three compartments, accounting for the following shares of the total body weight of a well-nourished healthy adult male: · body cell mass, 55 percent; · extracellular supporting tissue, 30 percent; · body fat, 15 percent. The body cell mass is made up of cellular components such as muscle, body organs (viscera, liver, brain, etc.) and blood. It comprises the parts of the body that are involved in body metabolism, body functioning, body work and so on. The extracellular supporting tissue consists of two parts: the extracellular fluid (for example, the blood plasma supporting the blood cells) and the skeleton and other supporting structures. Body fat is nearly all present beneath the skin (subcutaneous fat) and around body organs such as the intestine and heart. It serves in part as an energy reserve. Small quantities are present in the walls of body cells or in nerves. Physiologists and those interested in metabolism have developed various ways to estimate body composition, including the amount of fluids in the body and body density. A common determination is to estimate lean body mass (LBM) or the fat-free mass of the body. These measures vary from the very simple to the very difficult. The simpler ones are of course less precise. Anthropometry using weight, height, skinfold thickness and body circumferences is relatively easy and very cheap to undertake, and does provide some estimate of LBM and body composition. In contrast, methods using, for example, bioelectrical impedance, computerized axial tomography (CAT scans) and nuclear magnetic resonance require expensive apparatus and highly trained staff. The fluid in the cells (intracellular fluid) has mainly potassium ions, and the extracellular fluid is mainly a solution of sodium chloride. Both also have other ions. Total body water can be estimated using different methods including dilution techniques to measure, for example, plasma volume. Body fat is estimated using different methods. Because a large portion of adipose tissue is present beneath the skin, it can be estimated by using a skinfold calliper to measure skinfold thickness in different sites (see Chapter 33). Another method is to weigh the person both in air and under water using a special apparatus and tank. This method really provides an estimate of body density. The various methods of determining body composition are described in detail in textbooks of physiology or nutrition (see Bibliography). Body composition is much influenced by nutrition. The two extremes are the wasting of nutritional marasmus (see Chapter 12) and starvation (see Chapter 24) and the overweight of obesity (see Chapter 23). Body composition differs between the genders and, perhaps only slightly, among races. African Americans have been shown to have heavier skeletons than whites of the same body build in the United States. In females pregnancy and lactation influence body composition. The body composition of children is influenced by their age and growth. Disturbances of growth resulting from nutritional deficiencies influence body composition, including the eventual size of the body and of body organs.

Metabolism and energy

The general term for all the chemical processes carried out by the cells of the body is "metabolism". Chief among these processes is the oxidation (combustion, or burning) of food which produces energy. This process is analogous to a car engine burning petrol to produce the energy that makes it run. In most forms of combustion, be it in the car or in the human, heat is produced as well as energy. Classical physics taught that energy can be neither created nor destroyed. Although this law of nature is not completely correct (as the conversion of matter to energy in a nuclear reactor shows), it is still true in most instances. All three macronutrients in food - carbohydrate, protein and fat provide energy. Energy for the body comes mainly from food, and in the absence of food it can be produced only by the breakdown of body tissues. All forms of energy can be converted into heat energy. It is possible to measure the heat produced by burning a litre of petrol, for example. Food energy can also be and is expressed as heat energy. The unit of measurement used has been the large calorie (Cal) or kilocalorie (kcal) (which is 1 000 times the small calorie used in physics), but this measure is increasingly being replaced by the joule (J) or kilojoule (kJ). The kilocalorie is defined as the heat necessary to raise the temperature of 1 litre of water from 14.5° to 15.5°C. Whereas the kilocalorie is a unit of heat, the joule is truly a unit of energy. The joule is defined as the amount of energy used when 1 kg is moved 1 m by 1 newton (N) of force. In nutrition the kilojoule (1000 J) is used. The equivalent of 1 kcal is 4.184 kJ. These are units of measurement in the same way that litres and pints are measures of quantity, and metres and feet are measures of length. In many scientific journals the joule is being introduced in place of the kilocalorie (see conversion tables, Annex 5), but the general public and most health workers still prefer to express food energy in kilocalories rather than joules. Kilocalories are therefore used in this book. The human body requires energy for all bodily functions, including work, the maintenance of body temperature and the continuous action of the heart and lungs In children energy is essential for growth Energy is also needed for breakdown, repair and building of tissues. These arc metabolic processes. The rate at which these functions are carried out while the body is at rest is the Basal metabolic rate (BMR). Basal metabolic rate BMR for an individual person is usually defined as the amount of energy [expressed in kilocalories or megajoules (MJ, per day] expended when the person is al complete rest, both physical (i.e. lying down) and psychological. It can also be expressed as kilocalories per hour or per kilogram of weight. BMR provides the energy required by the body for maintenance of body temperature; for the work of body organs such as the beating heart and the muscles working for normal, at rest, breathing; and for the functioning of other organs such as the liver, kidneys and brain. BMR varies from individual to individual. Important general factors influencing BMR are the person's weight, gender, age and state of health. BMR is also influenced by the person's body composition, for example the amounts of muscle and adipose tissue and therefore the amounts of protein and fat in the body. In broad terms, bigger people with more muscle and larger body organs have higher BMR than smaller people. Elderly people tend to have lower BMR than they had when they were young, and females tend to have lower BMR than males even on a per kilogram body weight basis. There are exceptions, however, to all these generalizations. BMR is important as a component of energy requirements. Table 10 shows BMR of adult men and women according to height and weight, both per kilogram body weight and as total energy per day. The table shows, for example, that in females aged 30 to 60 years BMR ranges from 1 190 to 1 420 kcal per day. This is the amount of energy required by a woman at complete rest for 24 hours. Of course many adult females in developing countries are smaller than 1.4 m in height and 41 kg in weight; their BMR might then be a little lower than 1 190 kcal per day. TABLE 10 Basal metabolic rate in adult men and women in relation to height and median acceptable weight for height

Height	Weight^a	18-30 years		30-60 years		Over 60
(m)	(kg)	kcal(kJ)^b/kg/day	kcal(kJ)/day	kcal(kJ)/kg/day	kcal(kJ)/day	kcal(kJ)/kg/day	kcal(kJ)/day
Men
1.5	49.5	29.0 (121)	1 440 (6.03)	29.4 (123)	1450 (6.07)	23.3 (98)	1150 (4.81)
1.6	56.5	27.4 (115)	1 540 (6.44)	27.2 (114)	1530 (6.40)	22.2 (93)	1250 (5.23)
1.7	63.5	26.0 (109)	1 650 (6.90)	25.4 (106)	1620 (6.78)	21.2 (89)	1 350 (5.65)
1.8	71.5	24.8 (104)	1 770 (7.41)	23.9 (99)	1710 (7.15)	20.3 (85)	1450 (6.07)
1.9	79.5	23.9 (100)	1 890 (7.91)	22.7 (95)	1 800 (7.53)	19.6 (82)	1560 (6.53)
2.0	88.0	23.0 (96)	2030 (8.49)	21.6 (90)	1 900 (7.95)	19.0 (80)	1 670 (6.99)
Women
1.4	41	26.7 (112)	1100 (4.60)	28.8 (120)	1190 (4.98)	25.0 (105)	1 030 (4.31)
1.5	47	25.2 (105)	1 190 (4.98)	26.3 (110)	1 240 (5.19)	23.1 (97)	1090 (4.56)
1.6	54	23.9 (100)	1290 (5.40)	24.1 (101)	1 300 (5.44)	21.6 (90)	1160 (4.85)
1.7	61	22.9 (96)	1 390 (5.82)	22.4 (94)	1 360 (5.69)	20.3 (85)	1 230 (5.15)
1.8	68	22.0 (92)	1500 (6.28)	20.9 (87)	1 420 (5.94)	19.3 (81)	1 310 (5.48)

Source: WHO, 1985. ^a Median acceptable weight for height; body mass index (BMI = wt/ht²) = 22 in men, 21 in women (see Chapter 23). ^b Kilojoules are given in parentheses. Energy requirements The mean daily energy requirements of adult men and women doing work classified as light, moderate and heavy are given in Table 11, expressed as multiples of BMR. The table shows, for example, that a woman doing heavy work requires energy equal to 1.32 times her BMR. If the woman is aged 25 years, is 1.4 m tall and weighs 41 kg, according to Table 10 her BMR would be 1100 kcal per day. Thus her daily requirements are: 1 100 kcal x 1.32 = 2 002 kcal. It is often useful to estimate energy needs for various activities that a person may do for particular lengths of time. The energy expenditure is usually calculated by multiplying an activity factor or metabolic constant, which varies according to the activity, by the individual's BMR. Table 12 gives the activity factors for calculating gross energy expenditure of various activities for adult males and females. The average human burns energy at his or her BMR only when at complete rest. All ordinary movements require additional energy, and physical work, of course, requires more still. For a healthy male with BMR of 1 kcal/min, an average day may involve the energy expenditure shown in Table 13. TABLE 11 Average daily energy requirements of adults by category of occupational work expressed as a multiple of BMR

Classification of work	Men	Women
Light	1.55	1.56
Moderate	1.78	1.64
Heavy	2.10	1.82

Source: WHO, 1985. TABLE 12 Activity factors for calculating gross energy expenditure (multiply by BMR)

Activity	Adult males	Adult females
Sleeping	1.0	1.0
Lying	1.2	1.2
Sitting quietly	1.2	1.2
Standing quietly	1.5	1.5
Walking slowly	2.8	2.8
Walking at normal pace	3.2	3.3
Walking fast uphill	7.5	6.6
Cooking	1.8	1.8
Office work (moving around)	1.6	1.7
Driving lorry	1.4	1.4
Labouring	5.2	4.4
Cutting sugar cane	6.5	-
Pulling loaded cart	5.9	-
Playing soccer	6.6	6.3
Fetching water from well	-	4.1
Pounding grain	-	4.6

Source: WHO, 1985. Note: These values apply only as approximate mean values for the time actually spent on the activity. They do not allow for rests. In heavy work individuals usually take frequent pauses or rests. If the person in this example did instead of eight hours of light work - five hours of herding and three hours of heavy work, hoeing hard ground at 8 kcal/min, then his output of energy would be as shown in Table 14. If the individual undertaking the activities in the first example gets exactly 2 640 kcal in his food, his weight will be steady, and he will be functioning normally. However, if he then undertakes the activities in the second example and eats no extra food, his weight will gradually drop, because he will have to burn up his fuel reserve, which forms part of his own body. He would fairly soon, however, begin to limit his activities in order to stop this process. He would therefore probably work much less hard at hoeing, so that instead of burning 8 kcal/min he might use only, say, 3.2 kcal/min; he would also tend to be tired at the end of the day, and might well increase his period of complete rest (at 1 kcal/min) by reducing the period of minor activities. He would therefore have reduced his energy requirements to 2 646 kcal, as shown in Table 15. This is just an example. In most in stances, when people increase their output of energy, including work, they feel more hungry and increase their consumption of their staple food, be it rice, millet, maize wheat, cassava or anything else. The energy requirements of a human being are affected by several factors. The important ones are: · Body size. A small person needs less energy than a large person. · Basal metabolic rate. BMR varies and can be affected by factors such as disease of the thyroid gland. · Activity. The more physical work or recreation performed, the more energy is required. · Pregnancy. A woman requires extra energy to develop the foetus and to carry its additional weight. · Lactation. The lactating mother needs additional energy to produce energy-containing milk for the suckling baby. The relatively long duration of breastfeeding among most Asians and Africans results in a large proportion of women requiring extra energy. · Age. Infants and children need more energy, for growth and activity, than adults. In older persons, the need for energy is sometimes reduced because there is a decline in activity and because their BMR is usually lower. · Climate. In warm climates, i.e. in most of the tropics and subtropics, less energy is necessary to keep the body at its normal temperature than in cold climates. TABLE 13 Energy expenditure of an average day for a healthy male

Activity	Time	Energy expenditure	Calculation	Total energy expenditure
	*(hours)*	*(kcal/min)*		*(kcal)*
Sleep	8	1 (=BMR)	88 x 60 x 1	480
Light work: herding animals	8	2.5	8 x 60 x 2.5	1 200
Other: sitting and minor activities	8	2	8 x 60 x 2	960
Total				2 640

TABLE 14 Energy expenditure when the person in Table 13 performs three hours of hard work

Activity	Time	Energy expenditure	Total energy expenditure
	*(hours)*	*(kcal/min)*	*(kcal)*
Sleep	8	1	480
Light work: herding	5	2.5	750
Hard work: hoeing	3	8	1440
Other: sitting and minor activities	8	2	960
Total			3 630

TABLE 15 Energy expenditure when the person in Table 14 adjusts his work to a less adequate diet

Activity	Time	Energy expenditure	Total energy expenditure
	*(hours)*	*(kcal/min)*	*(kcal)*
Sleep	10	1	600
Light work	5	2.5	750
Less hard work: hoeing	3	3.2	576
Other: sitting and minor activities	6	2	720
Total			2 646

*Chapter 9. Macronutrients: Carbohydrates, fats and proteins***

Carbohydrates

The main source of energy for most Asians, Africans and Latin Americans is carbohydrates in the food they eat. Carbohydrates constitute by far the greatest portion of their diet, as much as 80 percent in some cases. In contrast, carbohydrates make up only 45 to 50 percent of the diet of many people in industrialized countries. Carbohydrates are compounds containing carbon, hydrogen and oxygen in the proportions 6:12:6. They are burned during metabolism to produce energy, liberating carbon dioxide (CO2) and water (H2O). The carbohydrates in the human diet are mainly in the form of starches and various sugars. Carbohydrates can be divided into three groups: · monosaccharides, e.g. glucose, fructose, galactose; · disaccharides, e.g. sucrose (table sugar), lactose, maltose; · polysaccharides, e.g. starch, glycogen (animal starch), cellulose. Monosaccharides The simplest carbohydrates are the monosaccharides, or simple sugars. These sugars can pass through the wall of the alimentary tract without being changed by the digestive enzymes. The three most common are glucose, fructose and galactose. Glucose, sometimes also called dextrose, is present in fruit, sweet potatoes, onions and other plant substances. It is the substance into which many other carbohydrates, such as the disaccharides and starches, are converted by the digestive enzymes. Glucose is oxidized to produce energy, heat and carbon dioxide, which is exhaled in breathing. Because glucose is the sugar in blood, it is most often used as an energy-producing substance for persons fed intravenously. Glucose dissolved in sterile water, usually in concentrations of 5 or 10 percent, is frequently used for this purpose. Fructose is present in honey and some fruit juices. Galactose is a monosaccharide that is formed, along with glucose, when the milk sugar lactose is broken down by the digestive enzymes. Disaccharides The disaccharides, composed of simple sugars, need to be converted by the body into monosaccharides before they can be absorbed from the alimentary tract. Examples of disaccharides are sucrose, lactose and maltose. Sucrose is the scientific name for table sugar (the kind that is used, for example, to sweeten tea). It is most commonly produced from sugar cane but is also produced from beets. Sucrose is also present in carrots and pineapple. Lactose is the disaccharide present in human and animal milk. It is much less sweet than sucrose. Maltose is found in germinating seeds. Polysaccharides The polysaccharides are chemically the most complicated carbohydrates. They tend to be insoluble in water, and only some can be used by human beings to produce energy. Examples of polysaccharides are starch, glycogen and cellulose. Starch is an important source of energy for humans. It occurs in cereal grains as well as in root foods such as potatoes and cassava. Starch is liberated during cooking when the starch granules rupture because of heating. Glycogen is made in the human body and is sometimes known as animal starch. It is formed from monosaccharides produced by the digestion of dietary starch. Starch from rice or cassava is broken down in the intestines to form monosaccharide molecules, which pass into the bloodstream. Those surplus monosaccharides that are not used to produce energy (and carbon dioxide and water) are fused together to form a new polysaccharide, glycogen. Glycogen is usually present in muscle and in the liver, but not in large amounts. Any of the digestible carbohydrates when consumed in excess of body needs are converted by the body into fat which is laid down as adipose tissue beneath the skin and at other sites in the body. Cellulose, hemicellulose, lignin, pectin and gums are sometimes called unavailable carbohydrates because humans cannot digest them. Cellulose and hemicellulose are plant polymers that are the main components of cell walls. They are fibrous substances. Cellulose, which is a polymer of glucose, is one of the fibres of green plants. Hemicellulose is a polymer of other sugars, usually hexose and pentose. Lignin is the main component of wood. Pectins are present in plant tissue and sap and are colloidal polysaccharides. Gums are also viscous carbohydrates extracted from plants. Pectins and gums are both used by the food industry. The human alimentary tract cannot break down these carbohydrates or utilize them to produce energy. Some animals, such as cattle, have microorganisms in their intestines that break down cellulose and make it available as an energy-producing food. In humans, any of the unavailable carbohydrates present in food pass through the intestinal tract. They form much of the bulk and roughage evacuated in human faeces, and are often termed "dietary fibre". There is increasing interest in fibre in diets, because high-fibre diets are now considered healthful. A clear advantage of a high-fibre diet is a lower incidence of constipation than among people who consume a low-fibre diet. The bulk in high-fibre diets may contribute a feeling of fullness or satiety which may lead to less consumption of energy, and this may help reduce the likelihood of obesity. A high-fibre diet results in more rapid transit of food through the intestinal tract and is thus believed to assist normal and healthy intestinal and bowel functioning. Dietary fibre has also been found to bind bile in the intestines. It is now recognized that the high fibre content of most traditional diets may be an important factor in the prevention of certain diseases which appear to be much more prevalent in people consuming the low-fibre diets common in industrialized countries. Because it facilitates the rapid passage of materials through the intestine, fibre may be a factor in the control of diverticulitis, appendicitis, haemorrhoids and also possibly arteriosclerosis, which leads to coronary heart disease and some cancers. Frequent consumption of any sticky fermentable carbohydrates, either starch or sugar, can contribute to dental caries, particularly when coupled with poor oral hygiene. Adequate intake of fluoride and/or a topical application is the best protection against caries (see Chapter 21).

Fats

In many developing countries dietary fats make up a smaller part of total energy intake (often only 8 or 10 percent) than carbohydrates. In most industrialized countries the proportion of fat intake is much higher. In the United States, for example, an average of 36 percent of total energy is derived from fat. Fats, like carbohydrates, contain carbon, hydrogen and oxygen. They are insoluble in water but soluble in such chemical solvents as ether, chloroform and benzene. The term "fat" is used here to include all fats and oils that are edible and occur in human diets, ranging from those that are solid at cool room temperatures, such as butter, to those that are liquid at similar temperatures, such as groundnut or cottonseed oils. (In some terminologies the word "oil" is used to refer to those materials that are liquid at room temperature, while those that are solid are called fats.) Fats (also referred to as lipids) in the body are divided into two groups: storage fat and structural fat. Storage fat provides a reserve storehouse of fuel for the body, while the structural fats are part of the essential structure of the cells, occurring in cell membranes, mitochondria and intracellular organelles. Cholesterol is a lipid present in all cell membranes. It has an important role in fat transport and is the precursor from which bile salts and adrenal and sex hormones are made. Dietary fats consist mainly of triglycerides, which can be split into glycerol and chains of carbon, hydrogen and oxygen called fatty acids. This action, the digestion or breakdown of fats, is achieved in the human intestine by enzymes known as lipases, which are present primarily in the pancreatic and intestinal secretions. Bile salts from the liver emulsify the fatty acids to make them more soluble in water and hence more easily absorbed. The many fatty acids in human diets are divided into two main groups: saturated and unsaturated. The latter group includes both polyunsaturated and mono-unsaturated fatty acids. Saturated fatty acids have the maximum number of hydrogen atoms that their chemical structure will permit. All fats and oils eaten by humans are mixtures of saturated and unsaturated fatty acids. Broadly speaking, fats from land animals (i.e. meat fat, butter and ghee) contain more saturated fatty acids than do those of vegetable origin. Fats from plant products and to some extent those from fish have more unsaturated fatty acids, particularly polyunsaturated fatty acids (PUFAs). There are exceptions, however. For example, coconut oil has a large amount of saturated fatty acids. These groupings of fats have important health implications because excess intake of saturated fats is one of the risk factors associated with arteriosclerosis and coronary heart disease (see Chapter 23). In contrast, PUFAs are believed to be protective. PUFAs also include two unsaturated fatty acids, linoleic acid and linolenic acid, which have been termed "essential fatty acids" (EFAs) as they are necessary for good health. EFAs are important in the synthesis of many cell structures and several biologically important compounds. Recent studies have also shown the benefits of other longer-chain fatty acids in the growth and development of young children, and arachidonic acid and docosa-hexaenoic acid (DHA) should conditionally be considered essential during early development. Experiments with animals and studies in humans have shown definite skin and growth changes and abnormal vascular and neural function in the absence of these fatty acids, and there is no doubt that they are essential for the nutrition of individual cells and tissues of the body. Fat is desirable to make the diet more palatable. It also yields about 9 kcal/g, which is more than twice the energy yielded by carbohydrates and proteins (about 4 kcal/g); fat can therefore reduce the bulk of the diet. A person doing very heavy work, especially in a cold climate, may require as many as 4 000 kcal a day. In such a case it is highly desirable that a good proportion of the energy should come from fat; otherwise the diet would be very bulky. Bulky diets can be a particularly serious problem in young children as well. A reasonable increase in the fat or oil content of the diets of young children raises the energy density of predominantly bulky carbohydrate diets and is highly desirable. Fat also functions as a vehicle that assists the absorption of fat-soluble vitamins (see Chapter 11). Thus fats, and even specific types of fat, are essential to health. However, practically all diets provide the small amount required. Fat deposited in the human body serves as a reserve fuel. It is an economic way of storing energy, because, as mentioned above, fat yields about twice as much energy, weight for weight, as does carbohydrate or protein. Fat is present beneath the skin as an insulation against cold, and it forms a supporting tissue for many organs such as the heart and intestines. All fat in the body is not necessarily derived from fat that has been eaten. However, excess calories from the carbohydrate and protein in, for example, maize, cassava, rice or wheat can be converted into fat in the human body.

Proteins

Like carbohydrates and fats, proteins contain carbon, hydrogen and oxygen, but they also contain nitrogen and often sulphur. They are particularly important as nitrogenous substances, and are necessary for growth and repair of the body. Proteins are the main structural constituents of the cells and tissues of the body, and they make up the greater portion of the substance of the muscles and organs (apart from water). The proteins in different body tissues are not all exactly the same. The proteins in liver, in blood and in specific hormones, for example, are all different. Proteins are necessary · for growth and development of the body; · for body maintenance and the repair and replacement of worn out or damaged tissues; · to produce metabolic and digestive enzymes; · as an essential constituent of certain hormones, such as thyroxine and insulin. Although proteins can yield energy, their main importance is rather as an essential constituent of all cells. All cells may need replacement from time to time, and their replacement requires protein. Any protein eaten in excess of the amount needed for growth, cell and fluid replacement and various other metabolic functions is used to provide energy, which the body obtains by changing the protein into carbohydrate. If the carbohydrate and fat in the diet do not provide adequate energy, then protein is used to provide energy; as a result less protein is available for growth, cell replacement and other metabolic needs. This point is especially important for children, who need extra protein for growth. If they get too little food for their energy requirements, then the protein will be diverted for daily energy needs and will not be used for growth. Amino acids All proteins consist of large molecules which are made of amino acids. The amino acids in any protein are linked together in chains, called peptide linkages. The various proteins are made of different amino acids linked together in different chains. Because there are many different amino acids, there are many different possible configurations, so there are many different proteins. During digestion proteins break down to form amino acids much as complex carbohydrates such as starches break down into simple monosaccharides and fats break down into fatty acids. In the stomach and intestines various proteolytic enzymes hydrolyse the protein, releasing amino acids and peptides. Plants are able to synthesize amino acids from simple inorganic chemical substances. Animals do not have this ability; they derive all the amino acids necessary for building their protein from consumption of plants or animals. As the animals eaten by humans initially derived their protein from plants, all amino acids in human diets have originated from this source. Animals have differing abilities to convert one amino acid into another. In the human this ability is limited. Conversion occurs mainly in the liver. If the ability to convert one amino acid into another were unlimited, then the question of the protein content of diets and the prevention of protein deficiency would be simple. It would be enough merely to supply sufficient protein, irrespective of the quality or amino acid content of the protein supplied. Of the large number of amino acids, 20 are common in plants and animals. Of these, eight have been found to be essential for the adult human and have thus been termed "essential amino acids" or "indispensable amino acids", namely: phenyl-alanine, tryptophan, methionine, lysine, leucine, isoleucine, valine and threonine. A ninth amino acid, histidine, is required for growth and is essential for infants and children; it may also be necessary for tissue repair. Other amino acids include glycine, alanine, serine, cystine, tyrosine, aspartic acid, glutamic acid, proline, hydroxyproline, citrulline and arginine. Each protein in a food is composed of a particular mixture of amino acids which might or might not contain all eight of the essential ones. Protein quality and quantity To assess the protein value of any food it is useful to know how much total protein it contains, which amino acids it has and how many essential amino acids are present and in what proportion. Much is now known about the individual proteins present in various foods, their amino acid content and therefore their quality and quantity. Some have a better mixture of amino acids than others, and these are said to have a higher biological value. The proteins albumin in egg and casein in milk, for example, contain all the essential amino acids in good proportions and are nutritionally superior to such proteins as zein in maize, which contains little tryptophan or lysine, and the protein in wheat, which contains only small quantities of lysine. It is not true, however, to say that the proteins in maize and wheat are not valuable. Although they contain less of certain amino acids, they do contain some amount of all the essential amino acids as well as many of the other important ones. The relative deficiency of maize and wheat proteins can be overcome by providing other foodstuffs containing more of the limited amino acids. It is therefore possible for two foods with low-value protein to complement each other to form a good protein mixture when eaten together. Humans, especially children on diets deficient in animal protein, require a variety of foods of vegetable origin, not just one staple food. In many diets, pulses or legumes such as groundnuts, beans and cowpeas, though short of sulphur-containing amino acids, supplement the cereal proteins, which are often short of lysine. A mixture of foods of vegetable origin, especially if taken at the same meal, can serve as a substitute for animal protein. FAO has produced tables showing the content of essential amino acids in different foodstuffs, from which it can be seen which foods best complement each other. It is also necessary, of course, to ascertain the total quantity of protein and amino acids in any food. The quality of the protein depends largely on its amino acid composition and its digestibility. If a protein is deficient in one or more essential amino acids, its quality is lower. The most deficient of the essential amino acids in a protein is called the "limiting amino acid". The limiting amino acid determines the efficiency of utilization of the protein present in a food or combination of foods. Human beings usually eat food in meals which contain many proteins; they seldom consume just one protein. Therefore nutritionists are interested in the protein quality of a person's diet or meals, rather than just one food. If one essential amino acid is in short supply in the diet, it limits the use of the other amino acids for building protein. Readers who wish to become familiar with the methods used for determining protein quality are advised to consult comprehensive textbooks on nutrition, which describe them in detail (see Bibliography). One method uses experiments on growth and nitrogen retention in young rats. Another involves determination of the amino acid or chemical score, usually by examining the efficiency of utilization of proteins in the foods consumed by comparing their amino acid composition with that of protein known to be of high quality, such as that in whole eggs. The chemical score may thus be defined as the efficiency of utilization of food protein in comparison with whole egg protein. Net protein utilization (NPU) is a measure of the amount or percentage of protein retained in relation to that consumed. As an example, Table 16 gives the chemical score and NPU of the protein in five foods. It is not usual or easy to obtain NPU values in people, and in most studies rats are used. Table 16 suggests that there is a good correlation between the values in rats and in children, and that chemical score provides a reasonable estimate of protein quality. For the professional involved in nutritional activities to help people - be it a dietitian in a health facility, an agricultural extension worker or a nutrition educator what is important is that the protein value differs among foods and that mixing foods improves the protein quality of the meal or the diet. Table 17 gives the protein content and the limiting amino acid score of some commonly eaten plant-based foods. Because Iysine is most commonly the limiting amino acid in many foods of plant origin, the Iysine score is also given. Protein digestion and absorption Proteins consumed in the diet undergo a series of chemical changes in the gastrointestinal tract. The physiology of protein digestion is complicated; pepsin and rennin from the stomach, trypsin from the pancreas and erepsin from the intestines hydrolyse proteins into their component amino acids. Most of the amino acids are absorbed into the bloodstream from the small intestine and thus travel to the liver and from there all over the body. Any surplus amino acids are stripped of the amino (NH2) group, which goes to form urea in the urine, leaving the rest of the molecule to be transformed into glucose. There is now some evidence that a little intact protein is taken up into certain cells lining the intestines. Some of this protein in the infant may have a role in the passive immunity conveyed from the mother to her newborn child. A little of the protein and amino acids released in the intestines is not absorbed. The unabsorbed amino acids, plus cells shed from the intestinal villi and acted upon by bacteria, together with gut organisms, contribute to the nitrogen found in faeces. TABLE 16 Chemical score and net protein utilization in selected foods

Food	Chemical score	NPU determined in children	NPU determined in rats
Eggs (whole)	100	87	94
Milk (human)	100	94	87
Rice	67	63	59
Maize	49	36	52
Wheat	53	49	48

Source: Adapted from FAO/WHO, 1973. TABLE 17 Protein content, limiting amino acid score and Iysine score of selected plant foods

Food	Protein content (%)	Limiting amino add score	Lysine score
Cereals
Maize	9.4	49 (Lys)	49
Rice (white)	7.1	62 (Lys)	62
Wheat flour	10.3	38 (Lys)	38
Millet	11.0	33 (Lys)	33
Legumes
Kidney beans	23.6	100	118
Cowpea	23.5	100	117
Groundnut	25.8	62 (Lys)	62
Vegetables
Tomato	0.9	56 (Leu)	64
Squash	1.2	70 (Thr)	95
Pepper,sweet	0.9	77 (Lys Leu)	77
Cassava	1.3	44 (Leu)	56
Potato	2.1	91 (Leu)	105

Source: Adapted from Young and Pellett. 1994. Much of the protein in the human body is present in muscle. There is no true storage of protein in the body as there is with fat and to a small extent glycogen. However, there is now little doubt that a well-nourished individual has sufficient protein accumulated to be able to last several days without replenishment and to remain still in good health. Protein requirements Children need more protein than adults because they need to grow. Infants in the first few months of life require about 2.5 g of protein per kilogram of body weight. This requirement drops to about 1.5 g/kg at nine to 12 months of age. Unless energy intakes are adequate, however, the protein will not all be used for growth. A pregnant woman needs an additional supply of protein to build up the foetus inside her. Similarly, a lactating woman needs extra protein, because the milk she secretes contains protein. In some societies it is common for women to breastfeed their babies for as long as two years. Thus some women need extra protein for two years and nine months for every infant they bear. Protein requirements and recommended allowances have been the subject of much research, debate and disagreement over the past 50 years. FAO and the World Health Organization (WHO) periodically assemble experts to review current knowledge and to provide guidelines. The most recent guidelines were the outcome of an Expert Consultation held jointly by FAO, WHO and United Nations University (UNU) in Rome in 1981 (WHO, 1985). The safe level of intake for a one-year-old child was put at 1.5 g per kilogram of body weight. The amount then falls to 1 g/kg at age six years. The United States recommended dietary allowance (RDA) is a little higher, namely 1.75 g/kg at age one year and 1.2 g/kg at age six years. In adults the FAO/WHO/UNU safe intake of protein is 0.8 g/kg for females and 0.85 g/kg for males. The safe levels of intake of protein by age and gender, including those for pregnant and lactating women, are given in Annex 1. Values are provided both for a diet high in fibre, comprising mainly cereals, roots and legumes with little food of animal origin, and for a mixed balanced diet with less fibre and plenty of complete protein. As an example, a non-pregnant adult woman weighing 55 kg requires 49 g of protein per day for the first diet and 41 g per day for the second. Fibre reduces protein utilization. Inadequate protein intake jeopardizes growth and repair in the body. Protein deficiency is especially dangerous for children because they are growing and also because the risk of infection is greater during childhood than at almost any other time of life. In children inadequate energy intake also has an impact on protein. As stated above, in the absence of adequate energy some protein needs to be diverted and therefore will not be used for growth. In many developing countries (though not all), the intake of protein is relatively low and of predominantly vegetable origin. The paucity of foods of animal origin in the diet is not always a matter of choice. For example, many low-income Africans and Latin Americans like animal products but find them less freely available, more difficult to produce and store and more expensive than most vegetable products. Diets low in meat, fish and dairy products are very common in countries where most people are poor. Infections lead to an increased loss of nitrogen from the body, which has to be replaced by proteins in the diet. Therefore children and others who have frequent infections will have greater protein needs than healthy persons. This fact must constantly be borne in mind, for in developing countries many children suffer an almost continual series of infectious diseases; they may frequently get diarrhoea, and they may harbour intestinal parasites.

Chapter 10. Minerals

Minerals have a number of functions in the body. Sodium, potassium and chlorine are present as salts in body fluids, where they have a physiological role in maintaining osmotic pressure. Minerals form part of the constitution of many tissues. For example, calcium and phosphorus in bones combine to give rigidity to the whole body. Minerals are present in body acids and alkalis; for example, chlorine occurs in hydrochloric acid in the stomach. They are also essential constituents of certain hormones, e.g. iodine in the thyroxine produced by the thyroid gland. The principal minerals in the human body are calcium, phosphorus, potassium, sodium, chlorine, sulphur, copper, magnesium, manganese, iron, iodine, fluorine, zinc, cobalt and selenium. Phosphorus is so widely available in plants that a shortage of this element is unlikely in any diet. Potassium, sodium and chlorine are easily absorbed and are physiologically more important than phosphorus. Sulphur is consumed by humans mainly in the form of sulphur-containing amino acids; thus sulphur deficiency, when it occurs, is linked with protein deficiency. Copper, manganese and magnesium deficiencies are not believed to be common. The minerals that are of most importance in human nutrition are thus calcium, iron, iodine, fluorine and zinc, and only these are discussed in some detail here. Some mineral elements are required in very tiny amounts in human diets but are still vital for metabolic purposes; these are termed "essential trace elements". The table giving the nutrient content of selected foods in Annex 3 shows the relative content of some important minerals in different foods.

Calcium

The body of an average-sized adult contains about 1 250 g of calcium. Over 99 percent of the calcium is in the bones and teeth, where it is combined with phosphorus as calcium phosphate, a hard substance that gives the body rigidity. However, although hard and rigid, the skeleton of the body is not the unchanging structure it appears to be. In fact, the bones are a cellular matrix, and the calcium is continuously taken up by the bones and given back to the body. The bones, therefore, serve as a reserve supply of this mineral. Calcium is present in the serum of the blood in small but important quantities, usually about 10 mg per 100 ml of serum. There are also about 10 g of calcium in the extracellular fluids and soft tissues of the adult body. Properties and functions In humans and other mammals, calcium and phosphorus together have an important role as major components of the skeleton. They are also important, however, in metabolic functions such as muscular function, nervous stimuli, enzymatic and hormonal activities and transport of oxygen. These functions are described in detail in textbooks of physiology and nutrition. The skeleton of a living person is physiologically different from the dry skeleton in a grave or museum. The bones are living tissues, consisting mainly of a mineralized protein collagen substance. In the living body there is continuous turnover of calcium. Bone is laid down and resorbed all the time, in people of all ages. Bone cells called osteoclasts take up or resorb bone, while others, termed osteoblasts, lay down or form new bone. The bone cells in the mineralized collagen are called osteocytes. Up to full growth or maturity (which has usually taken place by age 18 to 22 years), new bone is formed as the skeleton enlarges to its adult size. In young adults, despite bone remodelling, the skeleton generally maintains its size. However, as persons get older there is some loss of bone mass. A complex physiological system maintains proper calcium and phosphorus levels. The control involves hormones from the parathyroid gland, calcitonin and the active form of vitamin D (1,25-dihydroxy-cholecalciferol). Small but highly important amounts of calcium are present in extracellular fluids, particularly blood plasma, as well as in various body cells. In serum most of the calcium is in two forms, ionized and protein bound. Laboratories usually measure only total plasma calcium; the normal range is 8.5 to 10.5 mg/dl (2.1 to 2.6 mmol/litre). A drop in the level of calcium to below 2.1 mmol/litre is termed hypocalcaemia and can lead to various symptoms. Tetany (not to be confused with tetanus resulting from the tetanus bacillus), characterized by spasms and sometimes fits, results from low levels of ionized calcium in the blood. Dietary sources All the calcium in the body, except that inherited from the mother, comes from food and water consumed. It is especially necessary to have adequate quantities of calcium during growth, for it is at this stage that the bones develop. The foetus in the mother's uterus has most of its nutritional requirements satisfied, for in terms of nutrition the unborn child is almost parasitic. If the mother's diet is poor in calcium, she draws extra supplies of this mineral from her bones. An entirely breastfed infant will obtain adequate calcium from breastmilk as long as the volume of milk is sufficient. Contrary to popular belief, the calcium content of human milk varies rather little; 100 ml of breastmilk, even from an undernourished mother on a diet very low in calcium, provides approximately 30 mg of calcium (Table 18). A lactating mother secreting 1 litre will thus lose 300 mg of calcium per day. Cows' milk is a very rich source of calcium, richer than human milk. Whereas a litre of human milk contains 300 mg of calcium, a litre of cows' milk contains 1200 ma. The difference arises because a cow has to provide for her calf, which grows much more rapidly than a human infant and needs extra calcium for the hardening of its fast-growing skeleton. Similarly, the milk of most other domestic animals has a higher calcium content than human milk. This does not mean, however, that a child would be better off drinking cows' milk rather than human milk. Cows' milk yields more calcium than a child needs. A child (or even a baby) who drinks large quantities of cows' milk excretes any excess calcium, so it is of no benefit; it does not increase the child's growth rate beyond what is optimal. Milk products such as cheese and yoghurt are also rich sources of calcium. Small saltwater and freshwater fish such as sardines and sprats supply good quantities of calcium since they are usually eaten whole, bones and all. Small dried fish known as dagaa in the United Republic of Tanzania, kapenta in Zambia and chela in India add useful calcium to the diet. Vegetables and pulses provide some calcium. Although cereals and roots are relatively poor sources of calcium, they often supply the major portion of the mineral in tropical diets by virtue of the quantities consumed. TABLE 18 Calcium content of various milks commonly used in developing countries

Source of milk	Calcium content (mg/100 ml)
Human	32
Cow	119
Camel	1 20
Goat	1 34
Water buffalo	169
Sheep	1 93

The calcium content of drinking-water varies from place to place. Hard water usually contains high levels of calcium. Absorption and utilization The absorption of calcium is variable and generally rather low. It is related to the absorption of phosphorus and the other important mineral constituents of the bones. Vitamin D is essential for the proper absorption of calcium. Thus a person seriously deficient in vitamin D absorbs too little calcium, even if the intake of calcium is more than adequate, and could have a negative calcium balance. Phytates, phosphates and oxalates in food reduce calcium absorption. Persons customarily consuming diets low in calcium appear to have better absorption of calcium than those on high-calcium diets. Unabsorbed calcium is excreted in the faeces. Excess calcium is excreted in the urine and in sweat. Requirements It is not easy to state categorically the human requirements for calcium, because there are several factors influencing absorption and considerable variations in calcium losses among individuals. Needs for calcium are increased during pregnancy and lactation, and children require more calcium because of growth. Those on high-protein diets require more calcium in the diet. The following are recommended levels of daily calcium intake: · adults, 400 to 500 mg; · children, 400 to 700 mg; · pregnant and lactating women, 800 to 1 000 mg. Deficiency states Disease or malformation caused primarily by dietary deficiency of calcium is rare. There is little convincing evidence to show that the many diets of adults in developing countries supplying perhaps only 250 to 300 mg of calcium daily are harmful to health. It is assumed that adults achieve some sort of balance when intakes of calcium are low. Females who go through a series of pregnancies and long lactations may lose calcium and be at risk of osteomalacia. However, vitamin D deficiency, not calcium deficiency, is more often implicated in this condition. In children the development of rickets results from vitamin D deficiency, not from dietary lack of calcium, in spite of increased calcium requirements in childhood. Calcium balance in childhood is generally positive, and calcium deficiency has not been shown to have an adverse influence on growth. Osteoporosis is a common disease of ageing, especially in women (see Chapter 23). The skeleton becomes demineralized, which leads to fragility of bones and commonly to fractures of the hip, vertebrae and other bones, particularly in older women. High calcium intake is often recommended but has not been proved effective in prevention or treatment. Exercise appears to reduce the loss of calcium from bones; this may explain, in part, why osteoporosis is less prevalent in many developing countries, where women work hard and are very active. There is now clear evidence that providing the female hormone oestrogen to women after menopause reduces bone loss and osteoporosis.

Iron

Iron deficiency is a very common cause of ill health in all parts of the world, both South and North. The average iron content in a healthy adult is only about 3 to 4 g, yet this relatively small quantity is vital. Properties and functions Most of the iron in the body is present in the red blood cells, mainly as a component of haemoglobin. Much of the rest is present in myoglobin, a compound occurring mainly in muscles, and as storage iron or ferritin, mainly in the liver, spleen and bone marrow. Additional tiny quantities are found binding protein in the blood plasma and in respiratory enzymes. The main, vital function of iron is in the transfer of oxygen at various sites in the body. Haemoglobin is the pigment in the erythrocytes that carries oxygen from the lungs to the tissues. Myoglobin in skeletal and heart muscle accepts the oxygen from the haemogIobin. Iron is also present in peroxidase, catalase and the cytochromes. Iron is an element that is neither used up nor destroyed in the properly functioning body. Unlike some minerals, it is not required for excretion, and only very small amounts appear in urine and sweat. Minute quantities are lost in desquamated cells from the skin and intestine, in shed hair and nails and in the bile and other body secretions. The body is, however, efficient, economical and conservative in the use of iron. Iron released when the erythrocytes are old and broken down is taken up and used again and again for the manufacture of new erythrocytes. This economy of iron is important. In normal circumstances, only about 1 mg of iron is lost from the body daily by excretion into the intestines, in urine, in sweat or through loss of hair or surface epithelial cells. Because iron is conserved, the nutritional needs of healthy males and postmenopausal females are very small. Women of child-bearing age, however, must replace the iron lost during menstruation and childbirth and must meet the additional requirements of pregnancy and lactation. Children have relatively high needs because of their rapid growth, which involves increases not only in body size but also in blood volume. Dietary sources Iron is present in a variety of foods of both plant and animal origin. Rich food sources include meat (especially liver), fish, eggs, legumes (including a variety of beans, peas and other pulses) and green leafy vegetables. Cereal grains such as maize, rice and wheat contain moderate amounts of iron, but because these are often staple foods and eaten in large quantities, they provide most of the iron for many people in developing countries. Iron cooking pots may be a source of iron. Milk, contrary to the notion that it is the "perfect food", is a poor source of iron. Human milk contains about 2 mg of iron per litre and cows' milk only half this amount. Absorption and utilization Absorption of iron takes place mainly in the upper portion of the small intestine. Most of the iron enters the bloodstream directly and not through the lymphatic system. Evidence indicates that absorption is regulated to some extent by physiological demand. Persons who are iron deficient tend to absorb iron more efficiently and in greater quantities than do normal subjects. Several other factors affect iron absorption. For example, tannins, phosphates and phytates in food reduce iron absorption, whereas ascorbic acid increases it. Studies have indicated that egg yolk, despite its relatively high iron content, inhibits absorption of iron - not only the iron from the egg yolk itself, but also that from other foods. Healthy subjects normally absorb only 5 to 10 percent of the iron in their foods, whereas iron-deficient subjects may absorb twice that amount. Therefore, on a diet that supplies 15 mg of iron, the normal person would absorb 0.75 to 1.5 mg of iron, but the iron-deficient person would absorb as much as 3 mg. Iron absorption generally increases during growth and pregnancy, after bleeding and in other conditions in which the demand for iron is enhanced. Of greatest importance is the fact that the availability of iron from foods varies widely. Absorption of the haem iron in foods of animal origin (meat, fish and poultry) is usually very high, whereas the non-haem iron in foods such as cereals, vegetables, roots and fruits is poorly absorbed. However, people usually eat meals, not single individual foods, and a small amount of haem iron consumed with a meal where most of the iron is non-haem iron will enhance the absorption of all the iron. Thus the addition of a quite small amount of haem iron from perhaps fish or meat to a large helping of rice or maize containing non-haem iron will result in much greater absorption of iron from the cereal staple. If this meal also includes fruits or vegetables, the vitamin C in them will also enhance iron absorption. However, if tea is consumed with this meal, the tannin present in the tea will reduce the absorption of iron. Requirements The dietary requirements for iron are approximately ten times the body's physiological requirements. If a normally healthy man or post-menopausal woman requires 1 mg of iron daily because of iron losses, then the dietary requirements are about 10 mg per day. This recommendation allows a fair margin of safety, as absorption is increased with need. Menstrual loss of iron has been estimated to average a little less than 1 mg per day during an entire year. It is recommended that women of child-bearing age have a dietary intake of 18 mg per day. During pregnancy, the body requires on average about 1.5 mg of iron daily to develop the foetus and supportive tissues and to expand the maternal blood supply. Most of this additional iron is required in the second and third trimesters of pregnancy. Breastfeeding women use iron to provide the approximately 2 mg of iron per litre of breastmilk. However, during the first six to 15 months of intensive breastfeeding they may not menstruate, so they do not lose iron in menstrual blood. Newborn infants are born with very high haemoglobin levels (a high red blood cell count), termed polycythaemia, which provides an extra store of iron. This iron, together with that present in breastmilk, is usually sufficient for the first four to six months of life, after which iron from other foods becomes necessary. Premature and other low-birth-weight infants may have lower iron stores and be at greater risk than other infants. An excess intake of iron over long periods can lead to the disease siderosis or haemachromatosis. This disease is reported to occur most commonly where beer or other alcoholic beverages are brewed in iron cooking pots, particularly in South Africa. In alcoholics siderosis leading to iron deposits in the liver may be associated with cirrhosis. Average safe levels of iron intake are provided in Annex 1. Deficiency states Consideration of the iron requirements and the iron content of commonly eaten foods might suggest that iron deficiency is rare, but this is not the case. Food iron is poorly absorbed. Iron is not readily excreted into the urine or the gastro-intestinal tract; thus severe iron deficiency is usually associated with an increased need for iron resulting from conditions such as pregnancy, blood loss or expansion of the total body mass during growth. Iron deficiency is most common in young children, in women of child-bearing age and in persons with chronic blood loss. The end result of iron deficiency is anaemia. Anaemia is described in detail in Chapter 13, and its control is discussed in Chapter 39. Hookworm infections, which are extremely prevalent in many countries, result in loss of blood which may cause iron deficiency anaemia. In some parts of the tropics schistosomiasis is also common, and this disease also causes blood loss.

Iodine

The body of an average adult contains about 20 to 50 mg of iodine, much of it in the thyroid gland. Iodine is essential for the formation of thyroid hormones secreted by this gland. Properties and functions In humans iodine functions as an essential component of the hormones of the thyroid gland, an endocrine gland situated in the lower neck. Thyroid hormones, of which the most important is thyroxine (T4), are important for regulating metabolism. In children they support normal growth and development, including mental development. Iodine is absorbed from the gut as iodide, and excess is excreted in the urine. The adult thyroid gland, in a person consuming adequate iodine, traps about 60 µg of iodine per day to make normal amounts of thyroid hormones. If there is insufficient iodine, the thyroid works harder to trap more; the gland enlarges in size (a condition known as goitre), and its iodine content might become markedly reduced. Thyroid stimulating hormone (TSH) from the pituitary gland influences thyroxine secretion and iodine trapping. In severe iodine deficiency, TSH levels are raised and thyroxine levels are low. Dietary sources Iodine is widely present in rocks and soils. The quantity in different plants varies according to the soil in which they are grown. It is not meaningful to list the iodine content of foodstuffs because of the large variations in iodine content from place to place, depending on the iodine content of the soil. Iodine tends to get washed out of the soil, and throughout the ages a considerable quantity has flowed into the sea. Sea fish, seaweed and most vegetables grown near the sea are useful sources of iodine. Drinking-water provides some iodine but very seldom enough to satisfy human requirements. In many countries where goitre is prevalent the authorities have added iodine to salt, a strategy which has successfully controlled iodine deficiency disorders (IDD). Iodine has usually been added to salt in the form of potassium iodide, but another form, potassium iodate, is more stable and is better in hot, humid climates. Iodated salt is an important dietary source of iodine. Deficiency states A lack of iodine in the diet results in several health problems, one of which is goitre, or enlargement of the thyroid gland. Goitre is extremely prevalent in many countries. There are other contributing causes of goitre, but iodine deficiency is by far the most common. Iodine deficiency during pregnancy may lead to cretinism, mental retardation and other problems, which may be permanent, in the child. It is now known that endemic goitre and cretinism are not the only problems caused by iodine deficiency. The decrease in mental capacity associated with iodine deficiency is of particular concern (see Chapter 14). IDD, although previously prevalent in Europe, North America and Australia, is now seen predominantly in developing countries. The greatest prevalence tends to be in mountainous areas such as the Andes and the Himalayas and in plateau areas far from the sea. For example, an investigation carried out by the author in the Ukinga Highlands of Tanzania revealed that 75 percent of the population had some enlargement of the thyroid.

Fluorine

Fluorine is a mineral element found mainly in the teeth and skeleton. Traces of fluorine in the teeth help to protect them against decay. Fluorides consumed during childhood become a part of the dental enamel and make it more resistant to the weak organic acids formed from foods that adhere to or get stuck between the teeth. This strengthening greatly reduces the chances of decay or caries developing in the teeth. Some studies have suggested that fluoride may also help strengthen bone, particularly later in life, and may thus inhibit the development of osteoporosis. Dietary sources The main source of fIuorine for most human beings is the water they drink. If the water has a fluorine content of about one part per million (1 ppm), then it will supply adequate fluorine for the teeth. However, many water supplies contain much less than this amount. Fluorine is present in bone; consequently small fish that are consumed whole are a good source. Tea has a high fluorine content. Few other foods contain much fluorine. Deficiency If the fluoride content of drinking-water in any locality is below 0.5 ppm, dental caries will probably be much more prevalent than where the concentration is higher. The recommended level of fluoride in water is between 0.8 and 1.2 ppm. In some countries or localities where the content of fluorine in the water is less than 1 ppm, it has now become the practice to add fluoride to the water supply. This practice is strongly recommended, but it is only practicable for large piped-water supplies; in some developing countries where most people do not have piped water, it is not feasible. The addition of fluoride to toothpaste also helps reduce dental caries. Fluorine does not totally prevent dental caries, but it can reduce the incidence by 60 to 70 percent. Excess An excessively high intake of fluoride causes a condition known as dental fluorosis, in which the teeth become mottled. It is usually caused by consuming excessive fluoride in water supplies that have high fluoride levels. In some parts of Africa and Asia, natural waters contain over 4 ppm of fluoride. Very high fluorine intakes also cause bone changes with sclerosis (added bone density), calcification of muscle insertions and exostoses. A survey carried out by the author in Tanzania revealed a high incidence of fluorotic bone changes (as shown by X-ray) in older subjects who normally drank water containing over 6 ppm of fluoride. Similar findings have been well described in India. Skeletal fluorosis can cause severe pain and serious bone abnormalities.

Zinc

Zinc is an essential element in human nutrition, and its importance to human health has received much recent attention. Zinc is present in many important enzymes essential for metabolism. The body of a healthy human adult contains 2 to 3 g of zinc and requires around 15 mg of dietary zinc per day. Most of the zinc in the body is in the skeleton, but other tissues (such as the skin and hair) and some organs (particularly the prostate) have relatively high concentrations. Dietary sources Zinc is present in most foods both of vegetable and of animal origin, but the richest sources tend to be protein-rich foods such as meat, seafoods and eggs. In developing countries, however, where most people consume relatively small amounts of these foods, most zinc comes from cereal grains and legumes. Absorption and utilization As with iron, absorption of zinc from the diet is inhibited by food constituents such as phytates, oxalate and tannins. No simple tests of human zinc status are known, however. Indicators used include evidence of low dietary intake, low blood serum zinc levels and low quantities of zinc in hair specimens. Much research on this mineral has been undertaken in the last two decades, and a great deal of knowledge concerning zinc metabolism and zinc deficiency in animals and humans has been gathered. Nonetheless, there is little evidence to suggest that zinc deficiency is an important public health problem for large numbers of people in any country, industrialized or developing. However, research now under way may show that poor zinc status is responsible for poor growth, reduced appetite and other conditions; in this way zinc deficiency may contribute especially to what is now called protein-energy malnutrition (PEM). Zinc deficiency is responsible for a very rare congenital disease known as acrodermatitis enteropathica. It responds to zinc therapy. Some patients receiving all of their nutrients intravenously have developed skin lesions which also respond to zinc treatment. In the Near East, particularly in the Islamic Republic of Iran and Egypt, a condition has been described in which adolescent or near-adolescent boys are dwarfed and have poorly developed genitalia and delayed onset of puberty; this condition has been said to respond to zinc treatment. Zinc deficiency has also been reported as secondary to, or as a part of, other conditions such as PEM, various malabsorption conditions, alcoholism including cirrhosis of the liver, renal disease and metabolic disorders.

Other trace elements

Numerous minerals are present in the human body. For most of the trace elements, besides those discussed above, there is no evidence that deficiency is responsible for major public health problems anywhere. Some of these minerals are very important in metabolism or as constituents of body tissues. Many of them have been studied, and their chemistry and biochemistry have been described. Experimental deficiencies have been produced in laboratory animals, but most human diets, even poor diets, do not appear to lead to important deficiencies. These minerals therefore are not of public health importance. Other trace elements are present in the body but do not have any known essential role. Some minerals, for example lead and mercury, are of great interest to health workers because excess intake has commonly resulted in toxic manifestations. Cobalt, copper, magnesium, manganese and selenium deserve mention because of their important nutritional role, and lead and mercury because of their toxicity. These minerals are considered in detail in large comprehensive textbooks of nutrition. Cobalt Cobalt is of interest to nutritionists because it is an essential part of vitamin B₁₂ (cyanocobalamin). When isolated as a crystalline substance, the vitamin was found to contain about 4 percent cobalt. However, cobalt deficiency does not play a part in the anaemia that results from vitamin B₁₂ deficiency. Copper Copper deficiency is known to cause anaemia in cattle, but no such risk is known in adult humans. Some evidence suggests that copper deficiency leads to anaemia in premature infants, in people with severe PEM and in those maintained on parenteral nutrition. An extremely rare congenital condition known as Menkes' disease is caused by failure of copper absorption. Magnesium Magnesium is an essential mineral present mainly in the bones but also in most human tissues. Most diets contain adequate dietary magnesium, but under some circumstances, such as diarrhoea, severe PEM and other conditions, excessive body losses of magnesium occur. Such losses may lead to weakness and mental changes and occasionally to convulsions. Selenium Both deficiency and excess of selenium have been well described in livestock. In areas of China where the soil selenium, and therefore the food selenium, is low, a heart condition has been described; termed Keshan's disease, it is a serious condition affecting heart muscle. Chinese researchers believe it can be prevented by providing dietary selenium. Selenium deficiency has also been associated with certain cancers. Lead Lead is of great public health importance because it commonly causes toxicity. Human lead deficiency is not known. Lead poisoning is especially an urban problem and is most important in children. It may lead to neurological and mental problems and to anaemia. Excess lead intake may result from consumption of lead in the household (from lead-based paint or water pipes containing lead) and from intake of atmospheric lead (from motor vehicle emissions). Mercury Mercury deficiency is not known in humans. The concern is with excessively high intakes of mercury and the risks of toxicity. Fish in waters contaminated with mercury concentrate the mineral. There is a danger of toxicity in those who consume fish with high mercury content. Mercury poisoning resulting from consumption of seeds coated with a mercury-containing fungicide has been described in Asia, Latin America and the Near East. The effects include severe neurological symptoms and paralysis.

Chapter 11. Vitamins

Vitamins are organic substances present in minute amounts in foodstuffs and necessary for metabolism. They are grouped together not because they are chemically related or have similar physiological functions, but because, as their name implies, they are vital factors in the diet and because they were all discovered in connection with the diseases resulting from their deficiency. Moreover, they do not fit into the other nutrient categories (carbohydrates, fats, protein and minerals or trace metals). When vitamins were first being classified, each was named after a letter of the alphabet. Subsequently, there has been a tendency to drop the letters in favour of chemical names. The use of the chemical name is justified when the vitamin has a known chemical formula, as with the main vitamins of the B group. Nevertheless, it is advantageous to include certain vitamins under group headings, even if they are not chemically related, since they do tend to occur in the same foodstuffs. In this book only vitamin A, five of the B vitamins (thiamine, riboflavin, niacin, vitamin B₁₂ and folic acid), vitamin C and vitamin D are described in detail. Other vitamins known to be vital to health include pantothenic acid (of which a deficiency may cause the burning feet syndrome mentioned below), biotin (vitamin H), para-aminobenzoic acid, choline, vitamin E and vitamin K: (antihaemorrhagic vitamin). These vitamins are not described in detail here for one or more of the following reasons: · deficiency is not known to occur under natural conditions in humans; · deficiency is extremely rare even in grossly abnormal diets; · lack of the vitamin results in disease only if it follows some other disease process that is adequately described in textbooks of general medicine; · the role of the vitamin in human nutrition has not yet been elucidated. None of the vitamins omitted from discussion is important from the point of view of workers studying nutrition as community health problems in most developing countries. Those wishing to learn more about these vitamins are referred to textbooks of general medicine or more detailed textbooks of nutrition. A summary of the conditions associated with vitamin Deficiencies is given in Chapter 33, Table 37.

Vitamin A (Retinol)

Vitamin A was discovered in 1913 when research workers found that certain laboratory animals stopped growing when lard (made from pork fat) was the only form of fat present in their diet, whereas when butter was supplied instead of lard (with the diet remaining otherwise the same) the animals grew and thrived. Further animal experiments showed that egg yolk and cod-liver oil contained the same vital food factor, which was named vitamin A. It was later established that many vegetable products had the same nutritional properties as the vitamin A in butter; they were found to contain a yellow pigment called carotene, some of which can be converted to vitamin A in the human body. Properties Retinol is the main form of vitamin A in human diets. (Retinol is the chemical name of the alcohol derivative, and it is used as the reference standard.) In its pure crystalline form, retinol is a very pale yellow-green substance. It is soluble in fat but insoluble in water, and it is found only in animal products. Other forms of vitamin A exist, but they have somewhat different molecular configurations and less biological activity than retinol, and they are not important in human diets. Carotenes, which act as provitamins or precursors of vitamin A, are yellow substances that occur widely in plant substances. In some foodstuffs their colour may be masked by the green plant pigment chlorophyll, which often occurs in close association with carotenes. There are several different carotenes. One of these, beta-carotene, is the most important source of vitamin A in the diets of most people living in non-industrialized countries. The other carotenes, or carotenoids, have little or no nutritional importance for humans. In the past, food analyses have often failed to distinguish beta-carotene from other carotenes. Vitamin A is an important component of the visual purple of the retina of the eye, and if vitamin A is deficient, the ability to see in dim light is reduced. This condition is called night blindness. The biochemical basis for the other lesions of vitamin A deficiency has not been fully explained. The main change, in pathological terms, is a keratinizing metaplasia which is seen on various epithelial surfaces. Vitamin A appears to be necessary for the protection of surface tissue. Several studies have shown that adequate vitamin A status reduces infant and child mortality in certain populations. Vitamin A supplementation reduces case fatality rates from measles. In other illnesses such as diarrhoea and respiratory infections, however, there is not strong evidence that the prevalence or duration of morbidity is reduced by vitamin A dosing.

Calculalating vitamin A content in foods

1 IU retinol = 0.3 µg retinol = 0.3 RE

1 RE = 3.33 IU retinol

1 RE = 6 µg beta-carotene

Since pure crystalline vitamin A, which is termed retinol alcohol, is now available, the vitamin A activity in foods is now widely expressed and measured using retinol equivalents (RE) rather than the international units (IU) previously used. One IU of vitamin A is equivalent to 0.3 1 µg retinol. Humans obtain vitamin A in food either as preformed vitamin A (retinol) or as carotenes which can be converted to retinol in the body. Beta-carotene is the most important in human diets and is better converted to retinol than other carotenes. It has been determined that six molecules of beta-carotene are needed to produce one molecule of retinol; thus it takes 6 µg of carotene to make 1 µg of retinol, or 1 RE. Dietary sources Vitamin A itself is found only in animal products; the main sources are butter, eggs, milk, meat (especially liver) and some fish. However, most people in developing countries rely mainly on beta-carotene for their supply of vitamin A. Carotene is contained in many plant foods. Dark green leaves such as those of amaranth, spinach, sweet potato and cassava are much richer sources than paler leaves such as those of cabbage and lettuce. Various pigmented fruits and vegetables, such as mangoes, papayas and tomatoes, contain useful quantities. Carotene is also present in yellow varieties of sweet potatoes and in yellow vegetables such as pumpkins. Carrots are rich sources. Yellow maize is the only cereal that contains carotene. In West Africa much carotene is obtained from red palm oil, which is widely used in cooking. The cultivation of the very valuable oil palm has spread to other tropical regions. In Malaysia it is widely cultivated as a cash crop, but its products are mainly exported rather than consumed locally. Both carotene and vitamin A withstand ordinary cooking temperatures fairly well. However, a considerable amount of carotene is lost when green leaves and other foods are dried in the sun. Sun-drying is a traditional method of preserving wild leaves and vegetables often used in arid regions. Since serious disease from vitamin A deficiency is common in these areas, it is important that other methods of preservation be established. Absorption and utilization The conversion of beta-carotene into vitamin A takes place in the walls of the intestines. Even the most efficient intestine can absorb and convert only a portion of the beta-carotene in the diet; therefore 6 mg of beta-carotene in food is equivalent to about 1 mg of retinol. If no animal products are consumed and the body must rely entirely on carotene for its vitamin A, consumption of carotene must be great enough to achieve the required vitamin A level. Carotene is poorly utilized when the diet has a low fat content, and diets deficient in vitamin A are often deficient in fat. Intestinal diseases such as dysentery, coeliac disease and sprue limit the absorption of vitamin A and the conversion of carotene. Malabsorption syndromes and infections with common intestinal parasites such as roundworm, which are prevalent in the tropics, may also reduce the ability of the body to convert carotene into vitamin A. Bile salts are essential for the absorption of vitamin A and carotene, so persons with obstruction of the bile duct are likely to become deficient in vitamin A. Even in ideal circumstances, infants and young children do not convert carotene to vitamin A as readily as adults do. The liver acts as the main store of vitamin A in the human and most other vertebrates, which is why fish-iiver oils have a high content of this vitamin. Retinol is transported from the liver to other sites in the body by a specific carrier protein called retinol binding protein (RBP). Protein deficiency may influence vitamin A status by reducing the synthesis of RBP. Storage in the body The storage of vitamin A in the liver is important, for in many tropical diets foods containing vitamin A and carotene are available seasonally. If these foods are eaten in fairly large quantities when available (usually during the wet season), a store can be built up which will help tide the person over the dry season, or at least part of it. The short mango season provides an excellent opportunity for youngsters, who may happily spend their leisure hours foraging for this fruit, to replenish the vitamin A stored in the liver. Toxicity If taken in excess, vitamin A has undesirable toxic effects. The most marked toxic effect is an irregular thickening of some long bones, usually accompanied by headache, vomiting, liver enlargement, skin changes and hair loss. Cases of vitamin A toxicity from dietary sources are rare, but toxicity can be a serious problem with supplemental doses of vitamin A. A high risk of birth defects is associated with supplements given before or during pregnancy. Human requirements The intake recommended by FAO and the World Health Organization (WHO) is 750 µg of retinol per day for adults; lactating mothers need 50 percent more, and children and infants less. It should be noted that these figures are based upon mixed diets containing both vitamin A and carotene. When the diet is entirely of vegetable origin, larger amounts of carotene are suggested, because the conversion from carotene to retinol is not very efficient. Deficiency Deficiency results in pathological drying of the eye, leading to xerophthalmia and sometimes keratomalacia and blindness. Other epithelial tissues may be affected; in the skin, follicular keratosis may be the result. These conditions are described in detail in Chapter 15.

Thiamine (Vitamin B₁)

During the 1890s in Java, Indonesia, Christiaan Eijkman of the Netherlands noticed that when his chickens were fed on the same diet as that normally consumed by his beriberi patients, they developed weakness in their legs and other signs somewhat similar to those of beriberi. The diet of the beriberi patients consisted mainly of highly milled and refined rice (known as polished rice). When Eijkman changed the diet of the chickens to whole-grain rice, they began to recover. He showed that there was a substance in the outer layers and germ of the rice grain that protected the chickens from the disease. Researchers continued to work on isolating the cause of the different effects of diets of polished and whole-grain rice, but despite many attempts it was not until 1926 that vitamin B₁ was finally isolated in crystalline form. It was synthesized ten years later, and now the term thiamine is used, rather than vitamin B₁. Properties Thiamine is one of the most unstable vitamins. It has a rather loosely bound structure and decomposes readily in an alkaline medium. Thiamine is highly soluble in water. It resists temperatures of up to 100°C, but it tends to be destroyed if heated further (e.g. if fried in a hot pan or cooked under pressure). Much research has been carried out on the physiological effects and biochemical properties of thiamine. It has been shown that thiamine has a very important role in carbohydrate metabolism in humans. It is utilized in the complicated mechanism of the breakdown, or oxidation, of carbohydrate and the metabolism of pyruvic acid. The energy used by the nervous system is derived entirely from carbohydrate, and a deficiency of thiamine blocks the final utilization of carbohydrate, leading to a shortage of energy and lesions of the nervous tissues and brain. Because thiamine is involved in carbohydrate metabolism, a person whose main supply of energy comes from carbohydrates is more likely to develop signs of thiamine deficiency if his or her food intake is decreased. For this reason, thiamine requirements are sometimes expressed in relation to intake of carbohydrate. Thiamine has been synthesized in pure form and is now measured in milligrams. Dietary sources Thiamine is widely distributed in foods of both vegetable and animal origin. The richest sources are cereal grains and pulses. Green vegetables, fish, meat, fruit and milk all contain useful quantities. In seeds such as cereals, the thiamine is present mainly in the germ and in the outer coats; thus much can be lost during milling (see Chapter 32). Bran of rice, wheat and other cereals tends to be naturally rich in thiamine. Yeasts are also rich sources. Root crops are poor sources. Cassava, for example, contains only about the same low quantity as polished, highly milled rice. It is surprising that beriberi is not common among the many people in Africa, Asia and Latin America whose staple food is cassava. Because it is very soluble in water, thiamine is liable to be lost from food that is washed too thoroughly or cooked in excess water that is afterwards discarded. For people on a rice diet, it is especially important to prepare rice with just the amount of water that will be absorbed in cooking, or to use water that is left over in soups or stews, for this water will contain thiamine and other nutrients. Cereals and pulses maintain their thiamine for a year or more if they are stored well, but if they are attacked by bacteria, insects or moulds the content of thiamine gradually diminishes. Absorption and storage in the body Thiamine is easily absorbed from the intestinal tract, but little is stored in the body. Experimental evidence indicates that humans can store only enough for about six weeks. The liver, heart and brain have a higher concentration than the muscles and other organs. A person with a high intake of thiamine soon begins to excrete increased quantities in the urine. The total amount in the body is about 25 mg. Human requirements A daily intake of 1 mg of thiamine is sufficient for a moderately active man and 0.8 mg for a moderately active woman. Pregnant and lactating women may need more (see Annex 1). FAO and WHO recommend an intake of 0.4 mg per 1 000 kcal for most persons. Deficiency Deficiency of thiamine leads to the disease beriberi, which in advanced forms produces paralysis of the limbs. In alcoholics thiamine deficiency leads to a condition termed Wernicke-Korsakoff syndrome. These disorders are described in Chapter 16.

Riboflavin (Vitamin B₂)

Early work on the properties of vitamins in yeast and other foodstuffs showed that antineuritic factors were destroyed by excessive heat, but that a growth-promoting factor was not destroyed in this way. This factor, riboflavin, was later isolated from the heat-resistant portion. It was synthesized in 1935. Properties Riboflavin is a yellow crystalline substance. It is much less soluble in water and more heat resistant than thiamine. The vitamin is sensitive to sunlight, so milk, for example, if left exposed may lose considerable quantities of riboflavin. Riboflavin acts as a coenzyme involved with tissue oxidation. It is measured in milligrams. Dietary sources The richest sources of riboflavin are milk and its non-fat products. Green vegetables, meat (especially liver), fish and eggs contain useful quantities. However, the main sources in most Asian, African and Latin American diets, which do not contain much of the above products, are usually cereal grains and pulses. As with thiamine, the quantity of riboflavin present is much reduced by milling. Starchy foods such as cassava, plantains, yams and sweet potatoes are poor sources. Human requirements Approximately 1.5 mg of riboflavin per day is an ample amount for an average adult, but rather more may be desirable during pregnancy and lactation. The FAO/WHO requirement is 0.55 mg per 1 000 kcal in the diet. Deficiency In humans a deficiency of riboflavin is termed ariboflavinosis. It may be characterized by painful cracking of the lips (cheilosis) and at the corners of the mouth (angular stomatitis). The clinical manifestations are described in Chapter 22. Ariboflavinosis is common in most countries but is not life threatening.

Niacin (nicotinic acid, nicotinamide, vitamin PP)

As the history of thiamine is linked with the disease beriberi, so the history of niacin is closely linked with the disease pellagra. beriberi is associated with the East and a rice diet, and pellagra with the West and a maize diet. Pellagra was first attributed to a poor diet over 200 years ago by the Spanish physician Gaspar Casal. At first, it was believed that pellagra might be caused by a protein deficiency, because the disease could be cured by some diets rich in protein. Later it was shown that a liver extract almost devoid of protein could cure pellagra. In 1926 J. Goldberger, in the United States, demonstrated that yeast extract contained a pellagra-preventing (PP) non-protein substance. In 1937 niacinamide or nicotinamide (nicotinic acid amide) was isolated, and this was found to cure a pellagra-like disease of dogs known as black tongue. Because pellagra was found mainly in those whose staple diet was maize, it was assumed that maize was particularly poor in niacin. It has since been shown that white bread contains much less niacin than maize. However, the niacin in maize is not fully available because it is in a bound form. The discovery that the amino acid tryptophan prevents pellagra in experimental animals, just as niacin does, complicated the picture until it was shown that tryptophan is converted to niacin in the human body. This work vindicated and explained the early theories that protein could prevent pellagra. The fact that zein, the main protein in maize, is very deficient in the amino acid tryptophan further explains the relationship between maize and pellagra. It has also been shown that a high intake of leucine, as occurs with diets based on sorghum, interferes with tryptophan and niacin metabolism and may cause pellagra. Properties Niacin, a derivative of pyridine, is a white crystalline substance, soluble in water and extremely stable. It has been synthesized. The main role of niacin in the body is in tissue oxidation. The vitamin occurs in two forms, nicotinic acid and nicotinamide (niacinamide). Niacin is measured in milligrams. Dietary sources Niacin is widely distributed in foods of both animal and vegetable origin. Particularly good sources are meat (especially liver), groundnuts and cereal bran or germ. As for other B vitamins, the main source of supply tends to be the staple food. whole-grain or lightly milled cereals, although not rich in niacin, contain much more than highly milled cereal grains. Starchy roots, plantains and milk are poor sources. Beans, peas and other pulses contain amounts similar to those in most cereals. Although the niacin in maize does not seem to be fully utilizable, treatment of maize with alkalis such as lime water, which is a traditional method of processing in Mexico and elsewhere, makes the niacin much more available. Cooking, preservation and storage of food cause little loss of niacin. Human requirements An adequate quantity for any person is 20 mg per day. Niacin requirements are affected by the amount of tryptophan containing protein consumed and also by the staple diet (i.e. whether it is maize-based or not). The FAO/WHO requirement is 6.6 mg per 1000 kcal in the diet. Deficiency A deficiency of niacin leads to pellagra (see Chapter 17), the "disease of the three Ds": dermatitis, diarrhoea and dementia. Initially manifested as skin trouble, pellagra, if untreated, can continue for many years, growing steadily worse.

Vitamin B₁₂ (Cyanocobalamin)

Pernicious anaemia, so named because it invariably used to be fatal, was known for many years before its cause was determined. In 1926 it was found that patients improved if they ate raw liver. This finding led to the preparation of liver extracts, which controlled the disease when given by injection. In 1948 scientists isolated from liver a substance they called vitamin B₁₂. When given in very small quantities by injection, this substance was effective in the treatment of pernicious anaemia. Properties Vitamin B₁₂ is a red crystalline substance containing the metal cobalt. It is necessary for the production of healthy red blood cells. A small addition of vitamin B₁₂ or of foods rich in this substance to the diet of experimental animals results in increased growth. It is measured in micrograms. Dietary sources Vitamin B₁₂ is present only in foods of animal origin. It can also be synthesized by many bacteria. Herbivorous animals such as cattle get their vitamin B₁₂ from the action of bacteria on vegetable matter in their rumen. Humans apparently do not obtain vitamin B₁₂ by bacterial action in their digestive tracts. However, fermented vegetable products may provide vitamin B₁₂ in human diets. Human requirements The human daily requirement of this vitamin is quite small, probably around 3µg for adults. Diets containing smaller amounts do not seem to lead to disease. Deficiency Pernicious anaemia is not caused by a dietary deficiency of vitamin B₁₂ but by an inability of the subject to utilize the vitamin B₁₂ in the diet because of a lack of an intrinsic factor in gastric secretions. It may be that an autoimmune reaction limits absorption of vitamin B₁₂ In pernicious anaemia the red blood cells are macrocytic (larger than normal) and the bone marrow contains many abnormal cells called megaloblasts. This macrocytic or megaloblastic anaemia is accompanied by a lack of hydrochloric acid in the stomach (achlorhydria). Later, serious changes take place in the spinal cord, leading to progressive neurological symptoms. If left untreated, the patient dies. Treatment consists of injection of large doses of vitamin B₁₂ When the blood characteristics have returned to normal, the patient can usually be maintained in good health if given one injection of 250 mg of vitamin B₁₂ every two to four weeks. Vitamin B₁₂ will also cure the anaemia accompanying the disease sprue. This is a tropical condition in which the absorption of vitamin B₁₂, folic acid and other nutrients is impaired. The tapeworm Diphyllobothrium latum, acquired from eating raw or undercooked fish, lives in the intestines and has a propensity for removing vitamin B₁₂ from the food of its host. This results in the development in humans of a megaloblastic anaemia which can be cured by injection of vitamin B₁₂ and treatment to rid the patient of the tapeworm. Some medicines interfere with absorption of vitamin B₁₂. Except in the above conditions deficiency of vitamin B₁₂ is likely to occur only in those on a vegetarian diet. Deficiency causes macrocytic anaemia and may produce neurological symptoms; however, even though strict vegetarians get very little vitamin B₁₂ in their diet, it appears that macrocytic anaemia due to vitamin B₁₂ deficiency is not prevalent and is not a major public health problem.

Folic acid or folates

In 1929 Lucy Wills first described a macrocytic anaemia (an anaemia in which the red cells are abnormally large) commonly found among pregnant women in India. This condition responded to certain yeast preparations even though it did not respond to iron or any known vitamin. The substance present in the yeast extract that cured the macrocytic anaemia was at first called "Wills' factor". In 1946 a substance called folic acid, which had been isolated from spinach leaves, was found to have the same effect. Properties Folic acid is the group name (also termed folates or folacin) given to a number of yellow crystalline compounds related to pteroglutamic acid. Folic acid is involved in amino acid metabolism. The folic acid in foodstuffs is easily destroyed by cooking. It is measured in milligrams. Dietary sources The richest sources are dark green leaves, liver and kidney. Other vegetables and meats contain smaller amounts. Human requirements The recommended daily intake for adults has been set at 400 µg in the United States. Deficiency Folate deficiency is most commonly due to poor diets, but it may result from malabsorption. It can be induced by medicines such as those used in treatment of epilepsy. A deficiency leads to the development of macrocytic anaemia. Anaemia resulting from folate deficiency is the second most common type of nutritional anaemia, after iron deficiency. Folic acid deficiency during pregnancy has been found to cause neural tube defects in newborn babies. The role of folic acid in prevention of ischaemic heart disease has also recently received increased attention. The main therapeutic use of folic acid is in the treatment of nutritional macrocytic or megaloblastic anaemias of pregnancy and infancy and for the prevention of neural tube defects. A dose of 5 to 10 mg daily is recommended for an adult. Although administration of folic acid will improve the blood picture of persons with pernicious anaemia, the nervous system symptoms will neither be prevented nor improved by it. For this reason, folic acid should never be used in the treatment of pernicious anaemia, except in conjunction with vitamin B₁₂.

Vitamin C (ascorbic acid)

The discovery of vitamin C is associated with scurvy, which was first recorded by seafarers who made prolonged journeys. In 1497 Vasco da Gama described scurvy among the crew of his historical voyage from Europe around the southern tip of Africa to India; more than half the crew died of the disease. It gradually became apparent that scurvy occurred only in persons who ate no fresh food. It was not until 1747, however, that James Lind of Scotland demonstrated that scurvy could be prevented or cured by the consumption of citrus fruit. This finding led to the introduction of fresh food, especially citrus products, to the rations of seafarers. Subsequently scurvy became much less common. In the nineteenth century, however, scurvy began to occur among infants receiving the newly introduced preserved milk instead of breastmilk or fresh cows' milk. The preserved milk contained adequate carbohydrate, fat, protein and minerals, but the heat used in its processing destroyed the vitamin C, so the infants got scurvy. Later vitamin C was found to be ascorbic acid, which had already been identified. Properties Ascorbic acid is a white crystalline substance that is highly soluble in water. It tends to be easily oxidized. It is not affected by light, but it is destroyed by excessive heat, especially when in an alkaline solution. It is a powerful reducing agent and antioxidant and can therefore reduce the harmful action of free radicals. It is also important in enhancing the absorption of the non-haem iron in foods of vegetable origin. Ascorbic acid is necessary for the proper formation and maintenance of intercellular material, particularly collagen. In simple terms, it is essential for producing part of the substance that binds cells together, as cement binds bricks together. In a person suffering from ascorbic acid deficiency, the endothelial cells of the capillaries lack normal solidification. They are therefore fragile, and haemorrhages take place. Similarly, the dentine of the teeth and the osteoid tissue of the bone are improperly formed. This cell-binding property also explains the poor scar formation and slow healing of wounds manifest in persons deficient in ascorbic acid. It is a common belief, claimed also by some scientists, that very large doses of vitamin C both prevent and reduce symptoms of the common cold (coryza). This claim has not been verified. One large study did suggest a modest reduction in the severity of cold symptoms in those taking vitamin C medicinally, but the vitamin Did not prevent colds from occurring. It is not advisable to take very large doses of medicinal vitamin C for long periods of time. Dietary sources The main sources of vitamin C in most diets are fruits, vegetables and various leaves. In pastoral tribes milk is often the main source. Plantains and bananas are the only common staple foods containing fair quantities of vitamin C. Dark green leaves such as amaranth and spinach contain far more than pale leaves such as cabbage and lettuce. Root vegetables and potatoes contain small but useful quantities. Young maize provides some ascorbic acid, as do sprouted cereals and pulses. Animal products such as meat, fish, milk and eggs contain small quantities. As vitamin C is easily destroyed by heat, prolonged cooking of any food may destroy much of the vitamin C present. Ascorbic acid is measured in milligrams of the pure vitamin. Human requirements Opinions regarding human requirements differ widely. It seems clear that as much as 75 mg per day is necessary if the body is to remain fully saturated with vitamin C. However, individuals appear to remain healthy on intakes as low as 10 mg per day. A recommendation of 25 mg for an adult, 30 mg for adolescents, 35 mg during pregnancy and 45 mg during lactation seems to be a reasonable compromise. Deficiency Scurvy and the other clinical manifestations of vitamin C deficiency are described in Chapter 19. Scurvy is not now a prevalent disease. Outbreaks have occurred in famine areas and recently in several refugee camps in Africa. In its early stages vitamin C deficiency may lead to bleeding gums and slow healing of wounds.

Vitamin D

Vitamin D is associated with prevention of the disease rickets and its adult counterpart osteomalacia (softening of the bones). Rickets was for many years suspected to be a nutritional deficiency disease, and in certain parts of the world cod-liver oil was used in its treatment. However, it was not until 1919 that Sir Edward Mellanby, using puppies, demonstrated conclusively that the disease was indeed of nutritional origin and that it responded to vitamin D in cod-liver oil. Later it was proved that action of sunlight on the skin leads to the production of the vitamin D used by humans. Properties A number of compounds, all sterols closely related to cholesterol, possess antirachitic properties. It was found that certain sterols that did not have these properties became antirachitic when acted upon by ultraviolet light. The two important activated sterols are vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol). In human beings, when the skin is exposed to the ultraviolet rays of sunlight, a sterol compound is activated to form vitamin D, which is then available to the body and which has exactly the same function as vitamin D taken in the diet. Dietary vitamin D is only absorbed from the gut in the presence of bile. The function of vitamin D in the body is to allow the proper absorption of calcium. Vitamin D formed in the skin or absorbed from food acts like a hormone in influencing calcium metabolism. Rickets and osteomalacia, though diseases in which calcium is deficient in certain tissues, are caused not by calcium deficiency in the diet but by a lack of vitamin D which would allow proper utilization of the calcium in the diet. Vitamin D is often expressed in international units; 1 IU is equivalent to 0.025 µg of vitamin D3. Dietary sources Vitamin D occurs naturally only in the fat in certain animal products. Eggs, cheese, milk and butter are good sources in normal diets. Meat and fish contribute small quantities. Fish-liver oils are very rich. Cereals, vegetables and fruit contain no vitamin D. Storage in the body The body has a considerable capacity to store vitamin D in fatty tissue and in the liver. An adequate store is important in a pregnant woman, to avoid predisposition to rickets in the child. Human requirements It is not possible to define human dietary requirements, because the vitamin is obtained both by eating foods containing vitamin D and by the action of sunlight on the skin. There is no need for adults to have any vitamin D in their diets, provided they are adequately exposed to sunlight, and many children in Asia, Latin America and Africa survive in good health on a diet almost completely devoid of vitamin D. It has been shown that fish-liver oil containing 400 IU (10 µg) of vitamin D will prevent the occurrence of rickets in infants or children not exposed to sunlight. This amount seems to be a safe allowance. Deficiency Rickets and osteomalacia, two diseases resulting from a deficiency of vitamin D, are described in Chapter 18. As vitamin D is produced in humans by the action of the sun on the skin, deficiency is not common in tropical countries, although synthesis of vitamin D may possibly be reduced in darkly pigmented skin. Rickets and osteomalacia are seen sporadically but are more common in areas where tradition or religion keeps women and children indoors. Many cases have been reported from Yemen and Ethiopia. The conditions are manifested mainly by skeletal changes. Toxicity Like other fat-soluble vitamins, vitamin D taken in excess in the diet is not well excreted. Consumption of large doses, which has most commonly resulted from overdosing of children with fish-liver oil preparations, can lead to toxicity. Overdosing may lead to hypercalcaemia, diagnosed from high levels of calcium in the blood. Toxicity usually begins with loss of appetite and weight, which may be followed by mental disorientation and finally by kidney failure. Fatalities have been recorded.

Other vitamins

The two fat-soluble vitamins (A and D) and the six water-soluble vitamins (thiamine, riboflavin, niacin, vitamin B₁₂ folates and vitamin C) have been described in some detail because these are the vitamins most likely to be deficient and to be of public health importance in non-industrialized countries. Five other vitamins, although vital to human health, are not very commonly deficient in human diets and so are of less public health importance. These are vitamin B₆, biotin, pantothenic acid, vitamin E and vitamin K. Vitamin B₆ (pyridoxine) Vitamin B₆ is a water-soluble vitamin widely present in foods of both animal and vegetable origin. It is important as a coenzyme in many metabolic processes. Primary dietary deficiency is extremely rare, but vitamin B₆ deficiency became common in tuberculosis patients treated with the drug isoniazid. The patients developed neurological signs and some times also anaemia and dermatosis. Now it is common to provide 10 mg of vitamin B₆ by mouth daily to those receiving large doses of isoniazid. Vitamin B₆ is relatively expensive, however, and the routine administration of vitamin B₆ to patients receiving isoniazid increases the cost of treatment of tuberculosis. Biotin Biotin is another water-soluble vitamin of the B complex group. It is found widely in food, and deficiency in humans is extremely rare. The vitamin is very important, however, in physiological and biochemical metabolic processes. Avidin in uncooked egg white prevents absorption of biotin in animals and humans. Rats fed egg white as their only source of protein become thin and wasted and develop neuropathies and dermatitis. Biotin deficiency has been reported in a very few cases, in people consuming mainly egg white and in a few intravenously-fed patients with some special forms of malabsorption. Pantothenic acid Pantothenic acid, a water-soluble vitamin, is present in adequate amounts in almost all human diets. It has important biochemical functions in various enzyme reactions, but deficiency in humans is very rare. A neurological condition described as burning feet syndrome, reported in prisoners of war held by the Japanese between 1942 and 1945, was ascribed to a deficiency of this vitamin. Vitamin E (tocopherol) Vitamin E, a fat-soluble vitamin, is obtained by humans mainly from vegetable oils and whole-grain cereals. It has been termed the "anti-sterility vitamin" or even the "sex vitamin" because rats fed on tocopherol-deficient diets cannot reproduce: males develop abnormalities in the testicles and females abort spontaneously. Because of its relationship to fertility and to many conditions in animals, vitamin E is widely self-prescribed and is not uncommonly recommended by physicians for a variety of human ills. However, true deficiency is probably rare; it occurs mainly in association with severe malabsorption states (when fat is poorly absorbed), in genetic anaemias [including glucose-6-phosphatase dehydrogenase (G-6PD) deficiency] and occasionally in very low-weight babies. Vitamin E (like vitamin C) is an antioxidant, and because of its ability to limit oxidation and to deal with damaging free radicals it is sometimes recommended as a possible preventive for both arteriosclerosis and cancer. Its presence in oils helps prevent the oxidation of unsaturated fatty acids. Vitamin K Vitamin K has been termed the "coagulation vitamin" because of its relationship to prothrombin and blood coagulation, and because it is successfully used to treat a bleeding condition of newborn infants (haemorrhagic disease of the newborn). Humans obtain some vitamin K from food, and some is also made by bacteria in the intestines. Newborn infants have a gut free of organisms, so they do not get vitamin K from bacterial synthesis. It is now believed that intravenously-fed or starved patients receiving broad-spectrum antibiotics that kill gut bacteria may bleed because of vitamin K deficiency. In many hospitals vitamin K is given routinely to newborn infants to prevent haemorrhagic disease.