Low-fat diets are not enough.

Since early February the news media has given enormous attention to an eight-year study of 49,000 women ages 50 to 79--by far the largest of its kind--which found that a low-fat diet did not reduce the risk of breast, cancer, colorectal cancer, and heart disease. The subjects assigned to a significantly reduced fat intake had the same rates of these diseases, heart attacks, and strokes as those without any dietary restrictions.

The study was part of the Women's Health Initiative of the National Institutes of Health; the researchers expect the colon and heart disease risk results to be the same in a similar age demographic for men. The study found that women who were randomly assigned to follow a low-fat diet ate significantly less fat over the next eight years. The women were not trying to lose weight, and their weights remained fairly steady. LDL cholesterol, which raises heart disease risk, turned out to be slightly lower in the low-fat group, but the difference was not significant enough to make a difference in heart disease risk.

Women were told to aim for a diet that had 20% of its calories from fat; with the help of a nutritionist most got the percentage down to 24 in the first year. That number was 29 by the final year of the study. The control group ate 35% of their calories as fat, and by the end of the study their dietary fat content was 37%. The two groups consumed about the same number of calories.

The relationship between diet and health is undoubtedly more nuanced than we have become accustomed to thinking. But these results do not mean that people should abandon low-fat diets.

For starters, it remains possible that eight years is simply not enough time to see big health gains, and that low-fat diets must be adopted as a lifestyle choice throughout our lives. The other key point is what the women ate. It's unclear whether they consumed more fruits and vegetables or simply made up for fat with more protein or carbohydrate from refined grains.

The widely-recommended Mediterranean diet, for example, is low in saturated fats (like butter), low in trans fats (such as margarine and hydrogenated oil), and high in certain natural oils (mainly olive oil). The women in the study reduced all kinds of fat. Heart disease researchers, along with much of the public, have already moved on from the notion that controlling total fat is the answer to prolonged good health.

It's also important to realize that diet alone is not enough to stay healthy. This is something the present study makes abundantly clear, at least given an eight-year period of improved dietary habits. Our nationwide chronic health problems require a less sedentary lifestyle, a point that readers of this publication have known for a long time. Obviously, cessation of smoking is another necessary step. Indeed, the researchers never had plans to suggest that low-fat diets were protective against cancer. And to a certain extent, genetics determine whether someone develops a certain disease. The quantity of food a person eats could of course be another factor.

Still, the comprehensive nature of this large, federal study has researchers--and the public--questioning popular assumptions about diet and obesity/overweight. And a few notions fairly embedded in the popular conscious did not hold up at all over the course of the eight years. Specifically, the Atkins idea that a high-carbohydrate, low-fat diet leads to weight gain, higher insulin and blood glucose levels, even more diabetes, was not supported by the study results. Conversely, an unrestricted caloric intake but lower overall fat did not result in significant weight loss either. Perhaps the best strategy remains the most natural: eating when hungry in six small sessions daily, fruits and vegetables in abundance, and regular exercise.

(JAMA, 2006, Vol. 295, No. 6, pp. 629-666)