The Journal: Journal of the American Medical Association

The Studies: Three studies that looked at nearly 50,000 healthy, post-menopausal women over the course of eight years found no significant effect of a low-fat diet on reducing the risk of heart disease, stroke, breast cancer or colorectal cancer. If you dig beyond the headlines, however, you’ll realize that these findings are not as contradictory as they seem.

When the studies, which were sponsored by the National Heart, Lung and Blood Institute as part of the Women’s Health Initiative, were first designed back in the 1980s, it made sense to focus on the total amount of fat in the diet. At that time, reducing total fat seemed like the best, most likely way to reduce heart disease and certain forms of cancer. And so investigators split the participants into two groups: one that attempted to bring its total fat consumption down to 20% of calories and one that stayed up around 35% of calories from fat.

Since then, however, other research has shown that the amount of saturated fats and trans fats in the diet may be even more important, within reason, than the total amount of fat, you eat. That’s just a fancy way of saying that vegetables sautéed in olive oil are healthier for your heart than a thick juicy hamburger. More recently, epidemiologists are starting to wonder if the combination of total fat plus large amounts of heme—a natural protein that contains iron and that’s particularly abundant in red meat—serves as an important environmental trigger for colorectal cancer. And of course, there’s the issue of physical activity and exercise, which turn out to promote good health in all sorts of ways.

So even though the studies showed that lowering total fat, by itself, doesn’t work miracles in heart disease or cancer, there are signs that eating fewer of your calories from fat is still a good idea. The number of women on the low-fat regimen who went on to develop breast cancer dropped by 9%. Although that wasn’t statistically significant, it may simply be that the study didn’t last long enough or the decrease in fat consumption was too gradual for investigators to pick up an effect. (Most of the women didn’t actually get down to 20% but averaged around 29%).

In addition, the women in the low-fat group had fewer polyps and other lesions that are thought to be precancerous. So once again, perhaps their incidence of colorectal cancer wasn’t any lower because the study hadn’t lasted long enough for enough of the women to get cancer.

What it Means: The same health advice that you've been hearing all along still applies. Get more fruits and vegetables in your diet—five daily servings the size of your fist is what’s recommended but many nutrition experts believe “nine is divine.” Exercise regularly, at least 30 minutes a day most days of the week. And, if you’re old enough, be sure to get a regular mammogram, colonoscopy and heart checkup. Oh, and one other thing, the women who decreased their total fat intake didn't gain weight as they got older. In fact, some of them lost a pound or two.

—Christine Gorman

From our archive:

02/06/2006: No Miracle Diets for Heart Disease or Cancer
TIME collection: Heart Disease
TIME collection: Cancer