Monday, December 14, 2009

The Dirty Little Secret of the Diet-Heart Hypothesis

The diet-heart hypothesis is the idea that saturated fat, and in some versions cholesterol, raises blood cholesterol and contributes to the risk of having a heart attack. To test this hypothesis, scientists have been studying the relationship between saturated fat consumption and heart attack risk for more than half a century. What have these studies found?

The large majority of observational studies have found no connection between habitualsaturated fat consumption and heart attack risk. The scientific literature contains dozens of these studies, so let's narrow the field to prospective studies only, because they are considered the most reliable. In this study design, investigators find a group of initially healthy people, record information about them (in this case what they eat), and watch who gets sick over the years.

A Sampling of Unsupportive Studies

Here are references to ten high-impact prospective studies, spanning half a century, showing no association between saturated fat consumption and heart attack risk. Ignore the saturated-to-polyunsaturated ratios, Keys/Hegsted scores, etc. What we're concerned with is the straightforward question: do people who eat more saturated fat have more heart attacks? Many of these papers allow free access to the full text, so have a look for yourselves if you want:

A Longitudinal Study of Coronary Heart Disease. Circulation. 1963.

Diet and Heart: a Postscript. British Medical Journal. 1977. Saturated fat was unrelated to heart attack risk, but fiber was protective.

Dietary Intake and the Risk of Coronary Heart Disease in Japanese Men Living in Hawaii. American Journal of Clinical Nutrition. 1978.

Relationship of Dietary Intake to Subsequent Coronary Heart Disease Incidence: the Puerto Rico Heart Health Program. American Journal of Clinical Nutrition. 1980.

Diet, Serum Cholesterol, and Death From Coronary Heart Disease: The Western Electric Study. New England Journal of Medicine. 1981.

Diet and 20-year Mortality in Two Rural Population Groups of Middle-Aged Men in Italy. American Journal of Clinical Nutrition. 1989. Men who died of CHD ate significantly less saturated fat than men who didn't.

Diet and Incident Ischaemic Heart Disease: the Caerphilly Study. British Journal of Nutrition. 1993. They measured animal fat intake rather than saturated fat in this study.

Dietary Fat and Risk of Coronary Heart Disease in Men: Cohort Follow-up Study in the United States. British Medical Journal. 1996. This is the massive Physicians Health Study. Scroll down to table 2 and see for yourself that the association between saturated fat intake and heart attack risk disappears after adjustment for several factors including family history of heart attack, smoking and fiber intake. That's because, as in most modern studies, people who eat steak are also more likely to smoke, avoid vegetables, eat fast food, etc.

Dietary Fat Intake and the Risk of Coronary Heart Disease in Women. New England Journal of Medicine. 1997. From the massive Nurse's Health study. The abstract claims that saturated fat was associated with heart attack risk. However, the association disappeared when they adjusted for monounsaturated and polyunsaturated fat intake. Have a look at table 3.

Dietary Fat Intake and Early Mortality Patterns-- Data from the Malmo Diet and Cancer Study. Journal of Internal Medicine. 2005.

I just listed 10 prospective studies published in top peer-reviewed journals that found no association between saturated fat and heart disease risk. This is less than half of the prospective studies that have come to the same conclusion, representing by far the majority of studies to date. If saturated fat is a dominant cause of cardiovascular disease, why are its effects essentially undetectable in the best studies we can muster?

Studies that Support the Diet-Heart Hypothesis

To be complete, some studies have found an association between saturated fat consumption and heart attack risk. Here's a list of all four that I'm aware of, with comments:

Ten-year Incidence of Coronary Heart Disease in the Honolulu Heart Program: relationship to nutrient intake. American Journal of Epidemiology. 1984. "Men who developed coronary heart disease also had a higher mean intake of percentage of calories from protein, fat, saturated fatty acids, and polyunsaturated fatty acids than men who remained free of coronary heart disease." The difference in saturated fat intake between people who had heart attacks and those who didn't, although statistically significant, was very small.

Diet and 20-Year Mortality From Coronary Heart Disease: the Ireland-Boston Diet-Heart Study. New England Journal of Medicine. 1985. "Overall, these results tend to support the hypothesis that diet is related, albeit weakly, to the development of coronary heart disease."

Relationship Between Dietary Intake and Coronary Heart Disease Mortality: Lipid Research Clinics Prevalence Follow-up Study. Journal of Clinical Epidemiology. 1996. "...increasing percentages of energy intake as total fat (RR 1.04, 95% CI = 1.01 – 1.08), saturated fat (RR 1.11, CI = 1.04 – 1.18), and monounsaturated fat (RR 1.08, CI = 1.01 – 1.16) were significant risk factors for CHD mortality among 30 to 59 year olds... None of the dietary components were significantly associated with CHD mortality among those aged 60–79 years." Note that the associations were very small, also included monounsaturated fat (like in olive oil), and only applied to the age group with the lower risk of heart attack.

The Combination of High Fruit and Vegetable and Low Saturated Fat Intakes is More Protective Against Mortality in Aging Men than is Either Alone. Journal of Nutrition. 2005. Higher saturated fat intake was associated with a higher risk of heart attack; fiber was strongly protective.

The Review Papers

Over 25 high-quality studies conducted, and only 4 support the diet-heart hypothesis. In case you're concerned that I'm cherry-picking studies, here are links to review papers on the same data that have reached the same conclusion:

Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease.  American Journal of Clinical Nutrition. 2010.  "A meta-analysis of prospective epidemiologic studies showed that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD."

A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease. Archives of Internal Medicine. 2009. "Insufficient evidence (less than or equal to 2 criteria) of association is present for intake of supplementary vitamin E and ascorbic acid (vitamin C); saturated and polyunsaturated fatty acids; total fat; alpha-linolenic acid; meat; eggs; and milk" They analyzed prospective studies representing over 160,000 patients from 11 studies meeting their rigorous inclusion criteria, and found no association between saturated fat consumption and heart attack risk.

The Questionable Role of Saturated and Polyunsaturated Fatty Acids in Cardiovascular Disease. Journal of Clinical Epidemiology. 1998. Dr. Uffe Ravnskov challenges the diet-heart hypothesis simply by collecting all the relevant studies and summarizing their findings.

Where's the Disconnect?

The first part of the diet-heart hypothesis states that dietary saturated fat raises the cholesterol/LDL concentration of the blood. The second part states that increased blood cholesterol/LDL increases the risk of having a heart attack. What part of this is incorrect?

There's definitely an association between blood cholesterol/LDL level and heart attack risk in certain populations, including Americans. MRFIT, among other studies, showed this definitively, although the lowest risk of all-cause mortality was at an average level of cholesterol.

So we're left with the first premise: that saturated fat increases blood cholesterol/LDL. Could this hypothesis be less well supported than it appears?  The data that are used to support it come almost exclusively from short-term feeding studies (<3 and="" association="" between="" blood="" consumption="" effect="" fat="" found="" habitual="" have="" here="" how="" information="" is="" lipids.="" little="" long="" months="" most="" nbsp="" observational="" on="" p="" persists="" saturated="" studies="" surprisingly="" this="">

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