New Studies: Low-Fat Diets Don't Prevent Cancers

Three studies of post-menopausal women show low-fat diets don't prevent heart disease, breast cancer or colon cancer. The report appears in Wednesday's Journal of the American Medical Association. Two years ago, the same studies showed that hormone replacement therapy didn't prevent disease.

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From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

There's some surprising news about low-fat diets in tomorrow's Journal of the American Medical Association. Three studies of post-menopausal women show low- fat diets don't prevent heart disease, breast cancer, or colon cancer. Two years ago, the same studies showed that hormone replacement therapy didn't prevent disease. NPR's Patricia Neighmond reports.

PATRICIA NEIGHMOND reporting:

These studies looked at nearly 50,000 women over the age of 50; about half the women received nutritional counseling recommending they lower the amount of fat in their diet, the other half ate their usual diets. After eight years, researchers found no significant difference between the two groups in their risk of heart disease, breast cancer, or colon cancer. Researchers caution this doesn't mean a low fat diet doesn't provide benefit. There are a number of caveats in the studies. First, heart disease.

Epidemiologist Barbara Howard says women were told only to reduce the overall amount of fat in their diet.

Dr. BARBARA HOWARD (Epidemiologist): They weren't told, well, try to cut the saturated fat, replace it with the good fats, with the polys (sic). So the polys dropped. So you see, you sort of, you didn't drop the bad fat that much, and, you dropped one of the good fats. And so the net result was there wasn't much change in saturated fat. That led to very small change in LDL, and therefore, very little improvement of heart disease.

NEIGHMOND: Polyunsaturated fats help lower bad cholesterol. These are fats from vegetable oils, nuts and fish. Had the women eaten more of this type of fat, Howard says, there might have been bigger benefits. Saturated fats are the most dangerous ones, she says, and whether they know it or not, most Americans have benefited from less of this type of fat in food.

Dr. HOWARD: When you go to McDonald's or any fast food restaurant now, things are all fried in non-lard, they're not animal fats. Years ago, they were all lard. Lard was the thing that was used in all of these, or Crisco. An Oreo cookie, years ago, was made with Crisco. Now it's made with an unsaturated fat. Then came the lower fat dairy products, so that a lot of times people, even passively, wind up with less than whole milk, less cream.

NEIGHMOND: As for a low fat diet and a risk of breast cancer, again there was no significant difference among women. Researcher Ross Prentice says there were differences among certain groups of women. Those who participated most actively in nutrition classes saw a greater decrease in breast cancer risk. So did those who ate diets with the highest amount of fat to begin with.

Dr. ROSS PRENTICE (Biostatistician): The group with higher than, say, 35%, they made a bigger dietary change. They had more room to get down to the dietary goals and they also show stronger evidence of a reduction in breast cancer risk.

NEIGHMOND: In addition, Prentice says, following these women for a longer period of time might well show differences in breast cancer risk. So too, when it comes to colon cancer. Epidemiologist Shirley Beresford says there was a significant decrease in polyps in the colon and rectum among women who maintained a low fat diet. And in longer follow up, she says, researchers may well see a decrease in colorectal cancer itself.

In addition, Beresford adds that the dietary changes in this study were made among women over the age of 50.

Dr. SHIRLEY BERESFORD (Epidemiology, Washington State University): It would not be an unreasonable inference to say these women had been consuming high levels of fat in their diet for maybe 40 years. You know, maybe the influence of the past 40 years can't be eradicated in even a long intervention study that goes on for about eight years.

NEIGHMOND: Healthy, low fat, high fiber diets may be most beneficial, says Beresford, when started as early in life as possible. Researchers will continue to follow all these women and their diets and incidence of disease for another five years.

Patricia Neighmond, NPR News.

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