March 17th, 2014

Studies Provide Little Support for Guidelines on Dietary Fats and Supplements

The precise cardiovascular effect of dietary fats and supplements has been the subject of heated controversy. Although there is no strong supporting evidence from clinical trials, current guidelines tend to discourage or minimize the role of saturated fats and trans fats and to encourage the intake of omega-3 polyunsaturated fatty acids. Two new studies published today help clarify some of the issues. Both studies demonstrate the shaky underpinnings of the guidelines but are unlikely to provide firm support for a new perspective on these issues.

The first study, published in Annals of Internal Medicine, is a systematic review and meta-analysis of 76 randomized and observational studies that looked at either dietary fat consumption, dietary fat biomarkers, or dietary fat supplements. Among the chief findings:

  • Omega-3 and omega-6 fatty acids: There were trends for modest benefits associated with dietary intake or supplements, but these did not achieve statistical significance. There was some evidence that people with high circulating levels of omega-3 fatty acids may have a reduced risk of coronary disease.
  • Saturated fatty acids: There was no discernible effect of total saturated fat as measured by either dietary intake or circulating biomarkers. There was a signal suggesting different effects for individual saturated fatty acids as measured by biomarkers, but the authors noted that circulating levels of these are often only poorly determined by dietary intake. There was a suggestion of benefit for saturated fats derived from milk or dairy consumption.
  • Monunsaturated fatty acids: No effect was found, either harmful or beneficial.
  • Trans dietary fats: A harmful effect was confirmed.

The authors stated that “the pattern of findings from this analysis did not yield clearly supportive evidence for current cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of saturated fats.” The conclusions are consistent with a recent editorial in Open Heart that urged revision of guidelines advocating low-fat diets, especially when saturated fats are replaced with carbohydrates or omega-6 fatty acids.

The second study, published in JAMA Internal Medicine, reports the cardiovascular outcomes of patients who participated in AREDS2 (Age-Related Eye Disease Study 2). A total of 4,203 patients with age-related macular degeneration were randomized twice, to either omega-3 fatty acids (DHA 350 mg and EPA 650 mg) or placebo and also to lutein and zeaxanthin (carotenoids found in the eye) or placebo. After 4.8 years of followup there was no significant reduction in cardiovascular outcomes in either of the treatment groups.

The authors concluded that their “results are consistent with a growing body of evidence from clinical trials that have found little CVD benefit from moderate levels of dietary supplementation.”

In an invited commentary, Evangelos Rizos and Evangelia Ntzani write that after many years of investigation it is now clear that omega-3 supplements “with daily doses close to 1 g in patients with or without established CVD shows no clear, considerable benefit.” Future trials “should focus on the remaining gaps of knowledge such as high-dose omega-3 supplementation (definitely more than 1 g daily) with various EPA/DHA ratios” in patients with high triglyceride levels. For now, omega-3 supplements should only be prescribed for patients with severe hypertriglyceridemia, “an extreme minority of the general population.”


Comments are closed.