The Diet-Heart Hypothesis
The diet-heart hypothesis is the belief that eating foods high in saturated fat contributed to heart disease was first proposed in the 1950s by a scientist named Ancel Keys who believed that by replacing saturated fat from meat, butter and eggs with newly-created industrial polyunsaturated vegetable oil (such as soybean oil) that heart disease and the deaths allegedly associated with it would be reduced by lowering blood cholesterol levels.
In 1952, Keys suggested that Americans should reduce their fat consumption by 1/3 – while at the same time acknowledged that he had no idea whether he was right;
“Direct evidence on the effect of the diet on human arteriosclerosis is very little and likely to remain so for some time” .
In 1953, Ancel Keys published the results of his “Six Countries Study“ , where he said that he had demonstrated that there was an association between dietary fat as a percentage of daily calories and death from degenerative heart disease.
Four years later, in 1957, Yerushalamy published a paper with data from 22 countries , which showed a much weaker relationship between dietary fat and death by coronary heart disease than was suggested by Keys’s Six Countries Study data.
Nevertheless, in 1970, Keys went on to publish his Seven Countries Study in which maintained there was an associative relationship between increased dietary saturated fat and Coronary Heart Disease -basically ignoring the data presented in Yerushalamy’s 1957 study and failing to study countries where Yerushalamy found no relationship, such as France. In a paper published in 1989 based on food consumption patterns in the 1960s in the seven countries , Keys said that the average consumption of animal foods (with the exception of fish) was positively associated with 25 year CHD mortality (death) rates and the average intake of saturated fat was strongly related to 10 and 25 year CHD mortality rates. Keys knew of Yerushalamy’s data from 1957 and ignored it.
Keys methodology has been widely criticized for selecting data only from the 7 countries that best fit his hypothesis.
The Sugar Industry Funding of Research Vilifying Fat
In August of 1967, just as Ancel Keys had published his Seven Country Study, Stare, Hegsted and McGandy – the 3 Harvard researchers paid by the sugar industry published their review in the New England Journal of Medicine, titled “Dietary fats, carbohydrates and atherosclerotic vascular disease” which vindicated sugar as a contributor of heart disease and laid the blame on dietary fat and in particular, saturated fat and dietary cholesterol (see previous article on that topic).
Stare, Hegsted and McGandy concluded that there was “only one avenue” by which diet contributed to the development and progression of “hardening of the arteries” (atherosclerosis) and resulting heart disease and that was due to how much dietary cholesterol people ate and its effect on blood lipids;
“Since diets low in fat and high in sugar are rarely taken, we conclude that the practical significance of differences in dietary carbohydrate is minimal in comparison to those related to dietary fat and cholesterol…the major evidence today suggests only one avenue by which diet may affect the development and progression of atherosclerosis. This is by influencing the levels of serum lipids [fats], especially serum cholesterol.” 
These researchers concluded that there was major evidence available at the time which suggested that there was only ONE avenue for diet to contribute to hardening of the arteries and the development of heart disease – yet a year later in 1968 the report of the Diet-Heart Review Panel of the National Heart Institute made the recommendation that a major study be conducted to determine whether changes in dietary fat intake prevented heart disease because such a study had not yet been done ;
“the committee strongly recommended to the National Heart Institute that a major definitive study of the effect of diet on the primary prevention of myocardial infarction be planned and put into operation as soon as possible. ”
This is an important point; prior to a major study having ever been conducted to determine whether changes in dietary cholesterol impacts heart disease, 3 Harvard researchers paid by the sugar industry concluded that there was “only one avenue” by which diet contributed to the development and progression of atherosclerosis (i.e. “hardening of the arteries”) and heart disease and that was due to how much dietary cholesterol people ate and its effect on blood lipids.
Researcher Paid by the Sugar Industry Helps Develop the 1977 US Dietary Guidelines
Only ten years after the sugar industry paid Stare, Hegsted and McGandy to write their reviews, the same Dr. Hegsted was directly involved with developing and editing the 1977 US Dietary Guidelines  which recommended an increase in dietary carbohydrate and a decrease in saturated fat and cholesterol in the diet.
Historic changes in the Dietary Recommendation in Canada have largely been based on changes to the Dietary Recommendations in the US, and as a result both stemmed from a belief that eating saturated fat increases total cholesterol and therefore increases the risk of heart disease.
The problem is this belief is just that, a belief.
There have been many studies that have disproved this including a randomized, controlled dietary intervention trial from 2008 which compared a low calorie, low in fat with a low carbohydrate, high fat diet of the same number of calories. This study found that overall heart health is significantly improved when carbohydrate is restricted, rather than fat [7,8].
Not all LDL cholesterol is “bad” cholesterol.
Small, dense LDL (“Pattern B”) causes more “hardening of the arteries” than the large, fluffy LDL particles (“Pattern A”).
It has been reported that when dietary fat is replaced by carbohydrate, the percentage of the small, dense LDL particles (the ones that cause hardening of the arteries) is increased, increasing risk for heart disease. Furthermore, the low carb diet increased HDL (so-called “good” cholesterol), which are protective against heart disease and HDL and small, dense LDL were made worse on the low fat diet. Quite opposite to the “Diet-Heart Hypothesis, this study demonstrated improvements in the risk of heart disease for those eating a low carbohydrate, high fat diet compared to those eating a low fat, low calorie diet – which is not all that surprising given that it had been reported previously that a diet high in saturated fat actually lowers small, dense LDL (the type of LDL that causes hardening of the arteries) and raises the large fluffy LDL; actually improving risk factors for heart disease .
There are also other randomized controlled trials from 2004-2008 which demonstrate that a low carb diet improves blood cholesterol test results more than a low fat diet [10,11,12,13,14] – yet despite this, the belief that eating saturated fat increases blood cholesterol, persists.
Both the American and Canadian governments are in the process of revising their Dietary Guidelines and what is clear is that what is needed is an external, independent scientific review of the current evidence-base for the enduring false belief that dietary fat, especially saturated fat contributes to heart disease.
What are the findings of current scientific literature?
Eight recent meta-analysis and systemic reviews which reviewed evidence from randomized control trials (RCT) that had been conducted between 2009-2017 did not find an association between saturated fat intake and the risk of heart disease [16-21].
Furthermore, recently published results of the largest and most global epidemiological study published in December 2017 in The Lancet  found that those who ate the largest amount of saturated fats had significantly reduced rates of mortality and that low consumption (6-7% of calories) of saturated fat was associated with increased risk of stroke.
Here is a synopsis of the findings of the eight meta-analysis and systemic reviews;
“Intake of saturated fatty acids was not significantly associated with coronary heart disease mortality” and “saturated fatty acid intake was not significantly associated coronary heart disease events”
Skeaff CM, PhD, Professor, Dept. of Human Nutrition, the University of Otago, Miller J. Dietary Fat and Coronary Heart Disease: Summary of Evidence From Prospective Cohort and Randomised Controlled
“There were no clear effects of dietary fat changes on total mortality or cardiovascular mortality”.
Hooper L, Summerbell CD, Thompson R, Reduced or modified dietary fat for preventing cardiovascular disease, 2012 Cochrane Database Syst Rev. 2012 May 16;(5)
“Current evidence does not clearly support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats.”
Chowdhury R, Warnakula S, Kunutsor S et al, Association of Dietary, Circulating, and Supplement Fatty Acids with Coronary Risk: A Systematic Review and Meta-analysis, Ann Intern Med. 2014 Mar 18;160(6):398-406
“The present systematic review provides no moderate quality evidence for the beneficial effects of reduced/modified fat diets in the secondary prevention of coronary heart disease. Recommending higher intakes of polyunsaturated fatty acids in replacement of saturated fatty acids was not associated with risk reduction.”
Schwingshackl L, Hoffmann G Dietary fatty acids in the secondary prevention of coronary heart disease: a systematic review, meta-analysis and meta-regression BMJ Open 2014;4
“The study found no statistically significant effects of reducing saturated fat on the following outcomes: all-cause mortality, cardiovascular mortality, fatal MIs (myocardial infarctions), non-fatal MIs, stroke, coronary heart disease mortality, coronary heart disease events.”
Note: The one significant finding was an effect for saturated fats on cardiovascular events however this finding lost significance when subjected to a sensitivity analysis (Table 8, page 137).
Hooper L, Martin N, Abdelhamid A et al, Reduction in saturated fat intake for cardiovascular disease, Cochrane Database Syst Rev. 2015 Jun 10;(6)
“Epidemiological evidence to date found no significant difference in CHD mortality and total fat or saturated fat intake and thus does not support the present dietary fat guidelines. The evidence per se lacks generalizability for population-wide guidelines.”
Harcombe Z, Baker JS, Davies B, Evidence from prospective cohort studies does not support current dietary fat guidelines: a systematic review and meta-analysis, Br J Sports Med. 2017 Dec;51(24):1743-1749
“Available evidence from randomized controlled trials (1968-1973) provides no indication of benefit on coronary heart disease or all-cause mortality from replacing saturated fat with linoleic acid rich vegetable oils (such as corn oil, sunflower oil, safflower oil, cottonseed oil, or soybean oil).”
Ramsden CE, Zamora D, Majchrzak-Hong S, et al, Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73), BMJ 2016; 353
“Available evidence from adequately controlled randomised controlled trials suggest replacing saturated fatty acids with mostly n-6 PUFA is unlikely to reduce coronary heart disease events, coronary heart disease mortality or total mortality. These findings have implications for current dietary recommendations.”
Hamley S, The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomised controlled trials, Nutrition Journal 2017 16:30
Only one recent meta analysis conducted by the American Heart Association (by the authors of the Diet-Heart Policy for Americans, mentioned above) found a relationship between saturated fat intake and coronary heart disease, yet failed to examine cardiovascular mortality (death) or total mortality .
NOTE: In 1961, the American Heart Association was the author of the original policy paper recommending to limit saturated fats to protect against heart disease and therefore has a significant interest in defending its longtime institutional position.
With the exception of the American Heart Association review, the conclusion of 9 different meta-analysis and review papers of randomized control trials conducted by independent teams of scientists worldwide do not support the belief that dietary intake of saturated fat causes heart disease.
The PURE (Prospective Urban Rural Epidemiology) was the largest-ever epidemiological study and was published in The Lancet in December 2017 . It recorded dietary intake in 135,000 people in 18 countries over an average of 7 1/2 years, including high-, medium- and low-income nations. It found;
“High carbohydrate intake was associated with higher risk of total mortality, whereas total fat and individual types of fat were related to lower total mortality. Total fat and types of fat were not associated with cardiovascular disease, myocardial infarction, or cardiovascular disease mortality, whereas saturated fat had an inverse association with stroke. Global dietary guidelines should be reconsidered in light of these findings.”
Dehghan M, Mente A, Zhang X et al, The PURE Study – Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. Lancet. 2017 Nov 4;390(10107):2050-2062
Those critical of the study say that it has methodological problems, including problems related to the authors dividing consumption of macronutrients (protein, fat and carbohydrate) into 4 groups (quintiles). Some say that this is reason the data showed an inverse relationship between saturated fat and cardiovascular disease . Criticisms also include that one cannot compare data between countries of substantially different level of income because “low fat consumption is very uncommon in high income countries” and that ‘the ability to afford certain foods may change the dietary pattern (e.g. high-carbohydrate and low-fat diets may be associated with poverty) .
Both the American and Canadian governments are currently in the process of revising their Dietary Guidelines and I feel that what is needed now is an external, independent scientific review of the current evidence-base for the belief that saturated fat contributes to heart disease.
Have questions about which types of fats are best to include in your diet and which are best to limit? Please send me a note using the “Contact Me” tab above and I will reply shortly.
- Keys, A. Atherosclerosis: a problem in newer public health. J. Mt. Sinai Hosp. N. Y.20, 118–139 (1953).
- Yerushalmy J, Hilleboe HE. Fat in the diet and mortality from heart disease. A methodologic note. NY State J Med 1957;57:2343–54
- Kromhout D, Keys A, Aravanis C, Buzina R et al, Food consumption patterns in the 1960s in seven countries. Am J Clin Nutr. 1989 May; 49(5):889-94.
- McGandy, RB, Hegsted DM, Stare,FJ. Dietary fats, carbohydrates and atherosclerotic vascular disease. New England Journal of Medicine. 1967 Aug 03; 277(5):242–47
- The National Diet-Heart Study Final Report.” Circulation, 1968; 37(3 suppl): I1-I26. Report of the Diet-Heart Review Panel of the National Heart Institute. Mass Field Trials and the Diet-Heart Question: Their Significance, Timeliness, Feasibility and Applicability. Dallas, Tex: American Heart Association; 1969, AHA Monograph no. 28.
- Introduction to the Dietary Goals for the United States – by Dr D.M. Hegsted. Professor of Nutrition, Harvard School of Public Health, Boston, MASS., page 17 of 130, https://naldc.nal.usda.gov/naldc/download.xhtml?id=1759572&content=PDF
- Volek JS, Fernandez ML, Feinman RD, et al. Dietary carbohydrate restriction induces a unique metabolic state positively affecting atherogenic dyslipidemia, fatty acid partitioning, and metabolic syndrome. Prog Lipid Res 2008;47:307–18
- Forsythe CE, Phinney SD, Fernandez ML, et al. Comparison of low fat and low carbohydrate diets on circulating fatty acid composition and markers of inflammation. Lipids 2008;43:65–77
- Tribble DL, Holl LG, Wood PD, et al. Variations in oxidative susceptibility among six low density lipoprotein subfractions of differing density and particle size. Atherosclerosis 1992;93:189–99
- Foster GD, Wyatt HR, Hill JO, et al. A randomized trial of a low-carbohydrate diet for obesity. N Engl J Med 2003;348:2082–90.
- Stern L, Iqbal N, Seshadri P, et al. The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: one-year follow-up of a randomized trial. Ann Intern Med 2004;140:778–85
- Gardner C, Kiazand A, Alhassan S, et al. Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women. JAMA 2007;297:969–77
- Yancy WS Jr., Olsen MK, Guyton JR, et al. A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia: a randomized, controlled trial. Ann Intern Med 2004;140:769–77
- Shai I, Schwarzfuchs D, Henkin Y, et al. Dietary Intervention Randomized Controlled Trial (DIRECT) Group. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. N Engl J Med 2008;359:229–41
- Dreon DM, Fernstrom HA, Campos H, et al. Change in dietary saturated fat intake is correlated with change in mass of large low-density-lipoprotein particles in men. Am J Clin Nutr 1998;67:828–36
- Skeaff CM, PhD, Professor, Dept. of Human Nutrition, the University of Otago, Miller J. Dietary Fat and Coronary Heart Disease: Summary of Evidence From Prospective Cohort and Randomised Controlled Trials, Annals of Nutrition and Metabolism, 2009;55(1-3):173-201
- Hooper L, Summerbell CD, Thompson R, Reduced or modified dietary fat for preventing cardiovascular disease, 2012 Cochrane Database Syst Rev. 2012 May 16;(5)
- Chowdhury R, Warnakula S, Kunutsor S et al, Association of Dietary, Circulating, and Supplement Fatty Acids with Coronary Risk: A Systematic Review and Meta-analysis, Ann Intern Med. 2014 Mar 18;160(6):398-406
- Schwingshackl L, Hoffmann G Dietary fatty acids in the secondary prevention of coronary heart disease: a systematic review, meta-analysis and meta-regression BMJ Open 2014;4
- Hooper L, Martin N, Abdelhamid A et al, Reduction in saturated fat intake for cardiovascular disease, Cochrane Database Syst Rev. 2015 Jun 10;(6)
- Harcombe Z, Baker JS, Davies B, Evidence from prospective cohort studies does not support current dietary fat guidelines: a systematic review and meta-analysis, Br J Sports Med. 2017 Dec;51(24):1743-1749
- Ramsden CE, Zamora D, Majchrzak-Hong S, et al, Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73), BMJ 2016; 353
- Hamley S, The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomised controlled trials, Nutrition Journal 2017 16:30
- Sachs FM, Lichtenstein AH, Wu JHW et al, Dietary Fats and Cardiovascular Disease: A Presidential Advisory From the American Heart Association, Circulation. 2017 Jul 18;136(3)
- Dehghan M, Mente A, Zhang X et al, The PURE Study – Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. Lancet. 2017 Nov 4;390(10107):2050-2062
- Sigurdsson, AF, The Fate of the PURE Study – Fat and Carbohydrate Intake Revisited, Doc’s Opinion, October 16 2017, www.docsopinion.com/2017/10/16/pure-study-fats-carbohydrates/
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