Although for nearly 60 years people have been urged to decrease their consumption of saturated fats to prevent heart disease, there has been surprisingly little scientific evidence that doing so actually decreases the risk of coronary heart disease events. But a new study by researchers at Harvard School of Public Health (HSPH) provides the first conclusive evidence from randomized clinical trials that people who replace saturated fat in their diet with polyunsaturated fat reduce their risk of coronary heart disease by 19 percent, compared with control groups of people who do not.

By systematically reviewing a large group of randomized clinical trials and conducting a pooled meta-analysis of these studies, the HSPH team found that increasing the intake of polyunsaturated fats as a replacement for saturated fats could significantly reduce the rate of heart attacks and cardiac deaths in the population. The study appears in the March 23 issue of the open-access journal PLoS Medicine.

Over the past several decades, the food industry has reduced the amount of saturated fat in many products, and the public has reduced the amount of saturated fat in its food consumption. However, there has been a wide variation in the types of nutrients that have replaced this saturated fat. For example, in many products saturated fats were replaced with trans fats, which have since been determined to be detrimental. And in the overall American diet, saturated fat was generally replaced with increased consumption of refined carbohydrates and grains.

“The specific replacement nutrient for saturated fat may be very important,” said lead author Dariush Mozaffarian, assistant professor in the Department of Epidemiology at HSPH and the Department of Medicine at Harvard Medical School. “Our findings suggest that polyunsaturated fats would be a preferred replacement for saturated fats for better heart health.”

Results from prior individual randomized controlled trials of saturated fat reduction and heart disease events were mixed, with most showing no significant effects. Other trials focused only on blood cholesterol levels, which are an indirect marker of risk. Large observational studies also generally have shown no relationship between saturated fat consumption and risk of heart disease events. For example, earlier this month, researchers from HSPH and Children’s Hospital Oakland Research Institute performed a pooled meta-analysis of prior observational studies and found no evidence that overall consumption of saturated fat was related to risk of coronary heart disease or stroke events. (To access the abstract.)

Some of these mixed findings may relate to the absence of prior focus on the specific replacement nutrient for saturated fat. In other words, was saturated fat replaced primarily with carbohydrates, monounsaturated fats such as in olive oil, or polyunsaturated fats such as in most vegetable oils?

Mozaffarian and his HSPH colleagues Renata Micha and Sarah Wallace performed a systematic review and meta-analysis of all randomized controlled trials through June 2009 in which participants specifically increased their polyunsaturated fat consumption as a replacement for saturated fat, and in which coronary heart disease events were documented. Eight trials met the inclusion criteria, totaling 13,614 participants with 1,042 coronary heart disease events.

The meta-analysis of the trials showed that increasing polyunsaturated fat consumption as a replacement for saturated fat reduced the risk of coronary heart disease events by 19 percent. For every 5 percent increase (measured as total energy) in polyunsaturated fat consumption, coronary heart disease risk was reduced by 10 percent. This is now just the second dietary intervention — consuming long-chain omega-3 fatty acids is the first — to show a reduction in coronary heart disease events in randomized controlled trials.

Currently, the Institute of Medicine guidelines recommend that a range of 5 to 10 percent in energy consumption come from polyunsaturated fats. In addition, some scientists and organizations have recently suggested that consumption of polyunsaturated fats (largely omega-6 fatty acids) should actually be reduced, due to theoretical concerns that such consumption could increase coronary heart disease risk.

The results from this study suggest that polyunsaturated fats from vegetable oils may be an optimal replacement for saturated fats, an important finding for dietary guidelines and for when food manufacturers and restaurants are making decisions on how to reduce saturated fat in their products. The findings also suggest that an upper limit of 10 percent of energy consumption from polyunsaturated fats may be too low, as the participants in these trials who reduced their risk were consuming about 15 percent of their energy from polyunsaturated fats.

Support for this study was provided by the National Heart, Lung and Blood Institute, the National Institutes of Health, and a Searle Scholar Award from the Searle Funds at the Chicago Community Trust.

The study is publicly available on the PLoS Medicine Web site.

David Fanning to receive the Goldsmith Career Award