专业术语—低脂食物并不能减少健康风险3（在线收听）

　　In this case, the study addressed a tricky problem. For decades, many scientists have said, and many members of the public have believed, that what people eat — the composition of the diet — determines how likely they are to get a chronic disease. But that has been hard to prove. Studies of dietary fiber and colon cancer failed to find that fiber was protective, and studies of vitamins thought to protect against cancer failed to show an effect.

　　Many cancer researchers have questioned large parts of the diet-cancer hypothesis, but it has kept a hold on the public imagination. "Nothing fascinates the American public so much as the notion that what you eat rather than how much you eat affects your health," said Dr. Libby, the Harvard professor.

　　The study found that women who were randomly assigned to follow a low-fat diet ate significantly less fat over the next eight years. But they had just as much breast and colon cancer and just as much heart disease. The women were not trying to lose weight, and their weights remained fairly steady. But their experiences with the diets allowed researchers to question some popular notions about diet and obesity.

　　There is a common belief that Americans get fat because they eat too many carbohydrates. The idea is that a high-carbohydrate, low-fat diet leads to weight gain, higher insulin and blood glucose levels, and more diabetes, even if the calories are the same as in a higher-fat diet. That did not happen here.

　　Others have said the opposite: that low-fat diets enable people to lose weight naturally. But that belief was not supported by this study.

　　As for heart disease risk factors, the only one affected was LDL cholesterol, which increases heart disease risk. The levels were slightly higher in women eating the higher-fat diet, but not high enough to make a noticeable difference in their risk of heart disease.

　　Although all the study participants were women, the colon cancer and heart disease results should also apply to men, said Dr. Jacques Rossouw, the project officer for the Women's Health Initiative.

　　Dr. Rossouw said the observational studies that led to the hypothesis about colon cancer and dietary fat included men and women. With heart disease, he said, researchers have found that women and men respond in the same way to dietary fat.

　　The most recent study follows a smaller one, reported last year, on low-fat diets for women who had breast cancer. That study hinted that eating less fat might help prevent a recurrence. But the current study, asking if a low-fat diet could protect women from breast cancer in the first place, had findings that fell short of statistical significance, meaning they could have occurred by chance.

　　Dr. Rossouw said he was still intrigued by the breast cancer data, even though it was not statistically significant. The women on low-fat diets had a 9 percent lower rate of breast cancer; the incidence was 42 per thousand per year in women in the low-fat diet group, compared with 45 per thousand per year in women consuming their regular diet.