Low-Fat Diets and the Heart

Are Low-Fat Diets Dead?

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Beginning in 1977, both the US government and the American Heart Association (AHA) spent over 30 years urging Americans to limit the amount of fat they consume in their diet, to no more than 25 - 35% of daily calories. This recommendation, which was quietly dropped in 2010, was based on the theory that, since dietary fat increases cholesterol levels, reducing fat consumption would lower the risk of atherosclerotic cardiovascular disease.

Despite these longstanding recommendations, the scientific evidence that strictly limiting dietary fat lowers the risk of atherosclerosis is - and always has been - quite weak.

Government Recommendations on Dietary Fat

The formal recommendation that all people should restrict the total amount of fat in our diet was first made through Congress by the McGovern Commission which, in 1977 after a series of hearings on diet and health, published the first edition of the Dietary Goals for the United States. At the time it was known that eating saturated fats make cholesterol levels increase, and therefore (it was assumed), eating saturated fats would cause coronary artery disease (CAD). (This assumption has not held up in subsequent studies.)

Even in 1977 scientists knew that not all fats are “bad,” and indeed that certain fats are essential for good cardiovascular health. But the McGovern Commission was determined to limit all fat intake, both to reduce heart disease and (it was incorrectly assumed) to fight obesity.

They feared they would just confuse the public by trying to convey a relatively complex message that most fats should be avoided, but some fats are desirable. So, the official message became to avoid fats altogether, and instead to rely largely on carbohydrates for the majority of our caloric intake.

This low-fat-high-carb dogma prevailed for most of the next 40 years, with the imprimatur of the both US government and the AHA.

Dietary Fats and the Heart - The Evidence

Despite these longstanding and vociferous recommendations in favor of a low-fat diet, subsequent studies have failed to find any compelling evidence that higher levels of total dietary fat cause CAD. Here are a few of the more notable studies in this regard:

The 20-year Nurses Health Study, a cohort study involving 80,000 women, showed no correlation between the risk of heart disease and dietary fat. A subsequent meta analysis of several cohort studies similarly showed no association between dietary fat and heart disease or death.

In the most rigorous randomized trial ever conducted to study dietary fat, the Women’s Health Initiative randomized over 48,000 women to a low-fat-diet (and used intense behavior modification to reduce daily fat intake to 20% of total calories, and to increase the consumption of grains and vegetables) or to a control group that received only “usual” dietary education (this control group consumed 37% of their diet from fat).

After 8 years, there was no reduction in the the risk of CAD in the low-fat group. In fact, the trend was for higher risk. Other randomized trials have similarly failed to show a benefit to a low-fat diet.

Additional studies have failed to show a reduced risk of cancer with low-fat diets, or that that low-fat diets are associated with less obesity.

In summary, after several decades of study, there is no convincing evidence that reducing total dietary fat to less than 30 - 35% of daily calories lowers the risk of heart disease, cancer or obesity.

What About the Ornish Diet?

The Ornish diet, and other variants of ultra-low fat diets, famously claim to not only prevent CAD, but also to reverse it. These diets are far more rigorous in their restriction of dietary fats - especially from animal sources - than are the low-fat diets recommended by the AHA. Proponents of Ornish-type diets claim, correctly, that the studies that have failed to show a benefit with AHA-type diets do not necessarily apply to their far more fat-restrictive diets.

However, the claims that Ornish-type diets are proven effective are themselves based on imperfect data that does not hold up well to objective scrutiny. The hypothesis that a very-low-fat vegetarian diet prevents or reverses heart disease has been neither convincingly proven nor disproven, although it is a hypothesis that deserves more study.

The Bottom Line

The recommendation that everyone should eat a low-fat diet was from the beginning based on a flawed theory, and on a conscious decision to sacrifice precision for the sake of simplifying the message. After more than three decades of attempting to confirm that low-fat diets reduce heart disease, clinical trials did not support longstanding recommendations that everyone should be on a fat-restricted diet.

Current guidelines from the government and the AHA no longer specify a low-fat diet, but but still recommend severely limiting saturated fats, and using low-fat dairy products. In February, 2015, the Dietary Guidelines Advisory Committee (DGAC, the committee that reviews nutritional science every five years on behalf of the US government), published its latest report. In that report, any recommendation for a low-fat diet is conspicuously absent. Instead, the DGAC says , “… dietary advice should put the emphasis on optimizing types of dietary fat and not reducing total fat.”

At least in regard to total dietary fat, the official dietary guidelines finally reflect the science.

Sources:

Oh K, Hu FB, Manson JE, et al. Dietary fat intake and risk of coronary heart disease in women: 20 years of follow-up of the nurses’ health study. Am J Epidemiol 2005; 161:672.

Skeaff CM, Miller J. Dietary fat and coronary heart disease: summary of evidence from prospective cohort and randomised controlled trials. Ann Nutr Metab 2009; 55:173.

Howard BV, Van Horn L, Hsia J, et al. Low-fat dietary pattern and risk of cardiovascular disease: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA 2006; 295:655.

Ornish D, Scherwitz L, Billings J, et al. Intensive lifestyle changes for reversal of coronary heart disease Five-year follow-up of the Lifestyle Heart Trial. JAMA 1998; 280: 2001–2007

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