Setting the record straight: It’s best to swap out saturated fats for healthier fats


June 15, 2017 – In a new advisory, the American Heart Association (AHA) concluded strongly that replacing saturated fats with unsaturated fats will lower the incidence of cardiovascular disease (CVD). Frank Sacks of Harvard T.H. Chan School of Public Health, lead author of the advisory, explains.

Why did the AHA think it was important to issue this advisory?

The issue of dietary fat and health is a popular topic among scientists and science writers. When studies offer contrarian conclusions, they can attract a lot of media attention. Sometimes the journalists and even the authors don’t make an effort to learn about the context of the study from all the research that has come before. Journalists rely a lot on the authors’ input—and the authors, of course, want to promote the visibility of their own work.

For instance, a couple of years ago, a well-regarded medical journal, Annals of Internal Medicine, published a meta-analysis questioning the link between saturated fat and heart disease. There was much attention by the press because of its contrarian conclusions. There were also letters to the journal about errors in the study—but this follow-up was done out of the eye of the public. The public just saw this big headline: Saturated fat is not related to heart disease.

Another example is coconut oil. It’s been touted as healthy. But the science for this opinion is not there at all. In fact, the science says that coconut oil, which has mostly saturated fatty acids, raises your LDL cholesterol, which is linked with increased risk for heart disease.

The idea behind the AHA advisory is to set the record straight on why well-conducted scientific research overwhelmingly supports limiting saturated fat in the diet to prevent diseases of the heart and blood vessels.

If the majority of scientific evidence points to the harmful effects of saturated fats, why do some studies find no link between these fats and health?

One problem is that some researchers might use methodology that is not the best or most current for evaluating effects of dietary fats. If we’re talking about decreasing intake of saturated fat, we always have to ask: What are you going to replace it with? If you cut out, say, 250 calories of saturated fat each day for health reasons, you’re likely to be hungry. So we advise replacing the saturated fat calories with something else. The choice of that something else makes all the difference in terms of the relationship to heart disease or diabetes. If the replacement is unsaturated fats, the outcome will be favorable to health. But if the replacement is refined carbohydrates and sugars, the outcome will not be good. Unfortunately, some meta-analyses that combine many studies naively look at heart disease rates in people who have low or high intake of saturated fat without considering which types of foods people consume more of when they decrease their saturated fat intake.

The fact is, several independent research groups—including my colleague Walter Willett and others at Harvard Chan School—have found that if you replace saturated fat with unsaturated fat, heart disease and diabetes rates go down. But if you replace saturated fats with carbohydrates—especially junk food carbohydrates, like refined grains or added sugars—heart disease rates don’t change because you’re swapping bad for bad. And Dr. Willett and his colleagues have gone a step further, and looked at different kinds of carbohydrate-rich food—like whole wheat bread, or wheat fiber, or whole fruits—and have found that substituting these types of foods is favorable to health.

How can the average person judge the reliability of health news about fats, especially when they hear conflicting reports?

It is understandable that many people are confused or cynical when they hear about a new study that contradicts what they thought was accepted science. One suggestion would be to look at nutritional guidelines on websites of organizations such as the American Heart Association or the American Diabetes Association. Those guidelines tend to be very carefully discussed and research is carefully evaluated, and the guidelines are a bit on the conservative side, which I think makes sense. For example, the AHA advisory statement on dietary fats that was just released was based on evaluations of more than 100 research articles plus very detailed systematic reviews of the literature from several research groups. The consistency of results gives us confidence that we are on solid ground in our recommendations.

Karen Feldscher