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Lipid hypothesis: Difference between revisions

Revision as of 22:09, 19 December 2018

Development

The German pathologist Rudolf Virchow described lipid (medical term for fat-soluble molecules) accumulation in arterial walls.^[6] In 1913, a study by Nikolai Anitschkow showed that rabbits fed on cholesterol developed lesions in their arteries similar to atherosclerosis, suggesting a role for cholesterol in atherogenesis.^[7][8] By 1951, it was accepted that, although the causes of atheroma were still unknown, fat deposition was a major feature of the disease process. "The so-called fatty flecks or streaks of arteries are the early lesions of atherosclerosis and... may develop into the more advanced lesions of the disease.^[9]

Keys and the Seven Countries Study

With the emergence of cardiovascular disease as a major cause of death in the Western world in the middle of the 20th century, the lipid hypothesis received greater attention. In the 1940s, a University of Minnesota researcher, Ancel Keys, postulated that the apparent epidemic of heart attacks in middle-aged American men was related to their mode of life and possibly modifiable physical characteristics. He first explored this idea in a group of Minnesota business and professional men that he recruited into a prospective study in 1947, the first of many cohort studies eventually mounted internationally. The men were followed through 1981 and the first major report appeared in 1963.^[10] After fifteen years follow-up, the study confirmed the results of larger studies that reported earlier on the predictive value for heart attack of several risk factors: blood pressure, blood cholesterol level, and cigarette smoking. Meanwhile, in the mid-1950s, with improved methods and design, Keys recruited collaborating researchers in seven countries to mount the first cross-cultural comparison of heart attack risk in populations of men engaged in traditional occupations in cultures contrasting in diet, especially in the proportion of fat calories of different composition, the Seven Countries Study still under observation today. Even before the study had begun, there had been criticism of its methods. Yerushalmy and Hilleboe pointed out that Keys had selected for the study the countries that would give him the results he wanted, while leaving out data from sixteen countries that would not. They also pointed out that Keys was studying a "tenuous association" rather than any possible proof of causation.^[11]

The Seven Countries Study was formally started in fall 1958 in Yugoslavia. In total, 12,763 males, 40–59 years of age, were enrolled in seven countries, in four regions of the world (United States, Northern Europe, Southern Europe, Japan). One cohort is in the United States, two cohorts in Finland, one in the Netherlands, three in Italy, five in Yugoslavia (two in Croatia, and three in Serbia), two in Greece, and two in Japan. The entry examinations were performed between 1958 and 1964 with an average participation rate of 90%, lowest in the USA, with 75% and highest in one of the Japanese cohorts, with 100%.^[12] Keys' book Eat Well and Stay Well^[13] popularized the supplementary idea that reducing the amount of saturated fat in the diet would reduce cholesterol levels and the risks of serious diseases due to atheroma.^[14] Keys was followed during the rest of the 20th century by an accumulation of work that repeatedly demonstrated associations between cholesterol levels (and other modifiable risk factors including smoking and exercise) and risks of heart disease. These led to the acceptance of the lipid hypothesis as orthodoxy by much of the medical community;^[3]

By the end of the 1980s, there were widespread academic statements that the lipid hypothesis was proven beyond reasonable doubt,^[15][16][17] or, as one article stated, "universally recognized as a law."^{[18][19][20][21][22]}

A 2017 consensus statement from the European Atherosclerosis Society concluded that "consistent evidence from numerous and multiple different types of clinical and genetic studies unequivocally establishes that LDL causes ASCVD."^[23]

Controversy

There was ongoing contention about the results and meaning of intervention studies undertaken before the introduction of statins.^{[24][25][26][27]} A meta-analysis of cholesterol-lowering trials found that trials that were supportive of the lipid hypothesis were cited almost six times as often as those that were not, and although there was a similar number of trials unsupportive of the hypothesis, none of them were cited after 1970; some of the supportive reviews also exclude and ignore certain trials which were less favorable to the hypothesis. This meta-analysis, including the less-cited trials, found that mortality was not decreased by lowering cholesterol, and that the lowering of cholesterol was unlikely to prevent coronary heart disease.^[28][29][30]

In particular, Keys' supplementary hypothesis that reducing saturated fat in the diet will reduce cardiovascular disease has been described as a "fallacy".^[4][5] A meta-analysis in 2014 finds that "current evidence does not clearly support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats".^[31] This meta-analysis was called "seriously misleading" by Walter Willet, chair of the Department of Nutrition at Harvard, who explained that the paper contained major errors and omissions, most notably that the foods used to replace saturated fats were other unhealthy foods like refined sugars and starches.^[32]

In 2016, Uffe Ravnskov, director of The International Network of Cholesterol Skeptics (THINCS) and colleagues published a systematic review to examine how low-density lipoprotein cholesterol is associated with mortality in older adults. They concluded that "High LDL-C is inversely associated with mortality in most people over 60 years" and since "elderly people with high LDL-C live as long or longer than those with low LDL-C, our analysis provides reason to question the validity of the cholesterol hypothesis."^[33] The review has been criticized for cherry picking data and confirmation bias. Nine of the authors are members of THINCS. The National Health Service noted that "the review searched only a single literature database, excluded studies only available in non-English language, and excluded studies where the title and abstract did not appear to contain information on the link between LDL and mortality in older adults."^[34] Several medical experts responded to the review claiming it had a number of "serious weaknesses" and was contradicted by robust experimental data from clinical trials. Professor Jeremy Pearson Associate Medical Director of the British Heart Foundation commented that "there is nothing in the current paper to support the authors’ suggestions that the studies they reviewed cast doubt on the idea that LDL cholesterol is a major cause of heart disease or that guidelines on LDL reduction in the elderly need re-evaluating."^[35]

In 2017, Aseem Malhotra, Robert H. Lustig and Maryanne Demasi, authored an article in The Pharmaceutical Journal which disputed the link between blood cholesterol levels and occurrence of heart disease.^[36] The article was criticized by the medical community. Cardiologist Tim Chico commented that "high cholesterol has been proven beyond all doubt to contribute to coronary artery disease and heart attack ... to say the cholesterol hypothesis is dead is simply incorrect."^[37]

Other biological lipid hypotheses

Lipid hypothesis of osteoporosis

The "lipid hypothesis of osteoporosis" postulates that lipids involved in causing heart disease also contribute to causing osteoporosis. Osteoporosis is characterized by a decrease of bone marrow cells, or osteoblasts, and an increase of fat cells, or adipocytes. The formation of osteoblasts from pre-osteoblasts is reduced by oxidized lipids and in mice fed with a high fat diet. Observations from this model suggest that LDL oxidation products can cause osteoporosis through changing the developmental fate of bone cells leading to a reduced number of osteoblasts and increased numbers of fat cells.^[38]

Lipid hypothesis of cold tolerance

Inplants and microbes, changes in the lipid composition of cell membranes have been linked to cold tolerance.^[39] The enhanced resistance to cold treatment appears to be caused by an increased amount of fatty acid desaturases produced under cold stress transforming saturated into unsaturated fatty acids in the membrane. This effect can be reproduced artificially in genetically engineered plants.^[40] The changes in membrane lipid composition lead to a higher membrane fluidity, thus keeping the membrane from "freezing" at low temperatures. This "lipid hypothesis of cold tolerance" is less well supported in animals. In fruit flies, cold acclimation does not coincide with a reduced amount of saturated fatty acids,^[41] and recent genetic studies on a nematode indicate that the mechanisms involved in cold adaptation in animals may be different from those in plants and microbes.^[42]

See also

• AIM-HIGH trial
• Atherosclerosis
• Cholesterol
• Hypercholesterolemia
• The International Network of Cholesterol Skeptics
• Saturated fat and cardiovascular disease

Notes and references

External links

• A 5-part history of the lipid hypothesis and the "cholesterol controversy" by Daniel Steinberg, published in the Journal of Lipid Research in 2004–2006. These are condensed from his The Cholesterol Wars. Academic Press, 2007. ISBN 0-12-373979-9.
• Steinberg D (September 2004). "Thematic review series: the pathogenesis of atherosclerosis. An interpretive history of the cholesterol controversy: part I". J. Lipid Res. 45 (9): 1583–93. doi:10.1194/jlr.R400003-JLR200. PMID 15102877.{{cite journal}}: CS1 maint: unflagged free DOI (link).
• Steinberg D (February 2005). "Thematic review series: the pathogenesis of atherosclerosis. An interpretive history of the cholesterol controversy: part II: the early evidence linking hypercholesterolemia to coronary disease in humans". J. Lipid Res. 46 (2): 179–90. doi:10.1194/jlr.R400012-JLR200. PMID 15547293.{{cite journal}}: CS1 maint: unflagged free DOI (link).
• Steinberg D (October 2005). "Thematic review series: the pathogenesis of atherosclerosis: an interpretive history of the cholesterol controversy, part III: mechanistically defining the role of hyperlipidemia". J. Lipid Res. 46 (10): 2037–51. doi:10.1194/jlr.R500010-JLR200. PMID 15995167.{{cite journal}}: CS1 maint: unflagged free DOI (link).
• Steinberg D (January 2006). "The pathogenesis of atherosclerosis. An interpretive history of the cholesterol controversy, part IV: the 1984 coronary primary prevention trial ends it--almost". J. Lipid Res. 47 (1): 1–14. doi:10.1194/jlr.R500014-JLR200. PMID 16227628.{{cite journal}}: CS1 maint: unflagged free DOI (link).
• Steinberg D (July 2006). "Thematic review series: the pathogenesis of atherosclerosis. An interpretive history of the cholesterol controversy, part V: the discovery of the statins and the end of the controversy". J. Lipid Res. 47 (7): 1339–51. doi:10.1194/jlr.R600009-JLR200. PMID 16585781.{{cite journal}}: CS1 maint: unflagged free DOI (link).