Understanding New Findings on Low-Fat Diets

After the recent news that a low-fat diet does little to reduce the risk of cancer, heart disease or stroke for older woman, many people are understandable confused about the benefits of giving up the foods they loved. Dr. Walter Willett of Harvard School of Public Health talks to Michele Norris about what the findings mean.

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ROBERT SIEGEL, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris. After the recent news that a low-fat diet does little to reduce the risk of cancer, heart disease, or stroke for older women, many people are understandably confused about the benefits of giving up the foods they love. What's the point of swapping French fries and juicy steaks for lettuce and whole grains if it doesn't help ward off deadly disease?

To help interpret these findings, we turn to Dr. Walter Willett. He's the Chair of the Department of Nutrition at the Harvard School of Public Health. He's also the author of the book Eat, Drink, and be Healthy. Professor Willett, thanks so much for being with us.

Dr. WALTER WILLETT (Harvard School of Public Health): Glad to join you.

NORRIS: Were you surprised by these results?

Dr. WILLETT: The results really were very consistent with what we've seen emerging in our long-term studies of diet and health in women. We really haven't found the percentage of calories from fat to be related to heart disease or cancer. So it's not a surprise.

NORRIS: But that seems to be counterintuitive. It seems to debunk many of the popular theories of today.

Dr. WILLETT: Well, during the 1990s a dominant theory and the national advice had been all fat is bad and we should reduce it as much as possible and instead load up on complex carbohydrates. In fact, there was never really much evidence to support that and this new study, I think, really provides three strikes against the idea of the low-fat diet.

NORRIS: So three strikes against the benefit of a low-fat diet, but one of the key issues here seems to be the role that different kinds of fat can play in disease prevention. All fats aren't equal.

Dr. WILLETT: That's really the important point and people should not interpret these findings to run out and load up on sausage and cheeseburger and French fries. For heart disease in particular, the type of fat is very important. And it's become even more clear with the recognition that trans fat is an important part of the problem. So replacing trans fats and saturated fats in the diet with healthy types of fats, like unsaturated and mono-unsaturated fat, really provides you with a double win and a substantial reduction in risk of heart disease.

For cancer, as well as heart disease, it's very clear that the total amount of calories is very important, whether they're from fat calories or from carbohydrate calories and there had been a belief that it was just fat calories that count. So avoiding overweight can substantially reduce the risk of heart disease and particularly for postmenopausal breast cancer as well. Even losing a few pounds after menopause can have an important benefit for breast cancer risk.

NORRIS: Now at this point, we should all know the difference between trans fats and polyunsaturated fats. Not all of us do, however, so could you just sort of walk us through this?

Dr. WILLETT: Sure. The major sources of saturated fat we've known for quite a while. Those come with red meat and high-fat dairy products. Trans fats are a little more complicated because they are from partially hydrogenated vegetable oils.

It used to be that margarine was the major source, but many margarines now are trans-free and you need to look for that on the label. But, deep-fried fast foods are very high in trans fat and many commercial baked products are also high in trans fat. The healthy forms of fat come as liquid vegetable oils, nuts and seeds, and of course, things made with liquid vegetable oils.

NORRIS: So olive oil, for instance.

Dr. WILLETT: Olive oil would be a very healthy type of fat, but so is canola oil, corn oil, and soybean oil.

NORRIS: What kind of advice will doctors now give their patients? I mean, if people read this and they were advised by their physician to either reduce or cut bad fats out of their diet, what will their doctors tell them now?

Dr. WILLETT: Well, I think doctors should convey to patients that diet is very important, that the type of fat is very important and the type of carbohydrate also makes a big difference. Replacing refined starches and sugar with whole grain, high fiber kinds of carbohydrates can help reduce heart disease rates and diabetes risk. The reality is that most doctors don't have enough time in their busy practices to go into detail. But you know, this is our body and it's worth taking a few hours to understand how to really take care of it.

NORRIS: Dr. Willett, thank you very much.

Dr. WILLETT: It was a pleasure.

NORRIS: Dr. Walter Willett is the chair of the Department of Nutrition at the Harvard School of Public Health. He is also author of the book, Eat, Drink, and be Healthy.

You can read the Woman's Health Initiative studies at our Web site, NPR.org.

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