Cutting the Fat Won't Cut It
A major new study discounts the protective benefits of a low-fat diet
Sticking to a low-fat diet isn't easy: It means salad dressing with more vinegar than oil, ballgames but no peanuts, and summer afternoons without any ice cream. Yet for years, many people have been forgoing such pleasures in the belief that cutting fat automatically cuts the risk of heart disease and cancer, too. Not so, says research published last week in the Journal of the American Medical Association. Women who trimmed the fat from their diets were just as vulnerable to colon cancer, breast cancer, and heart disease as women who did not.
The message? A low-fat diet isn't equivalent to a healthful diet, says Marcia Stefanick, a physiologist at Stanford University's Prevention Research Center, who helped run the government-sponsored study. Some 49,000 women between ages 50 and 79 were divided into two groups and followed for an average of about eight years as part of the Women's Health Initiative. One group was instructed to cut fat intake to 20 percent of total calories and to eat at least five daily servings of fruits and vegetables and six of grains. The other women were left to eat as they pleased. In the end, both groups had about the same occurrence of colorectal cancer, stroke, and heart disease. A slight difference in the rate of breast cancer among the lower-fat-diet women might be explained by chance alone. The results most likely apply to men, too, researchers say.
Whoa. They're hardly a green light to go on a junk-food binge, though, researchers caution. For one thing, the women on the diet didn't hit their target; they whittled fat intake just to 29 percent of calories--from about 35 percent--by the end of the sixth year of the study. Moreover, the recommended diet made no distinction between "good" unsaturated fats and "bad"saturated fats and trans fats, whose importance to heart health has been recognized since the data-gathering started. And since all the women in the study were eating fairly healthfully beforehand, it's possible that the small changes in vegetable and grain consumption by the dieting group weren't big enough that any benefits registered. Rather than focus on total fat intake, Stefanick advises, go easy on foods containing saturated fats and trans fats and eat more vegetables and fruits and whole grains.
The findings on heart disease and colorectal cancer seem pretty bulletproof--though women on the low-fat diet did experience fewer polyps in the colon, a risk factor for cancer. But many researchers see promise in the breast cancer results: a small, 9 percent drop in incidence among those on the low-fat diet and a bigger decline among those who started out with the highest fat consumption. For now, says Rowan Chlebowski, a medical oncologist at the Los Angeles Biomedical Research Institute and one of the study authors, "I'd say we have a signal that this might play a role, but we don't know for sure." His previous research suggests that women who have had breast cancer can significantly reduce their risk of a relapse if they stick to a low-fat diet. The women in that study were more successful at reducing their fat calories. They also lost more weight than the women in this study--who ended up only about a pound lighter than their average starting weight of nearly 170 pounds.
That could be significant. Long-term health may depend more on achieving a healthy body weight and getting regular exercise than on cutting out fat, says Tim Byers, an epidemiologist at the University of Colorado Health Sciences Center in Denver. Overweight people who "lower fat but don't control calories can only make tiny changes to [their] chronic disease risk," he says. Until the links between disease and diet are fully understood, there are other ways to protect yourself: Get your cholesterol and blood pressure checked, and schedule that colonoscopy and mammogram. No matter what you eat, says Byers, a long life means knowing early where the problems lie.
Are We Winning?
Americans are older and fatter than ever, but cancer deaths have dropped for the first time in over 70 years: from 557,271 in 2002 to 556,902 in 2003, according to the latest figures analyzed by the American Cancer Society. Better prevention, screening, and treatment explain the trend--which may not continue. Experts expect 564,830 cancer deaths this year. Thank smoking, obesity, and lack of exercise.
This story appears in the February 20, 2006 print edition of U.S. News & World Report.
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