Walter Willett: The Facts on Diet and Breast Cancer

I hear that a low-fat diet reduces the chance of having breast cancer. Does that include all fat or only saturated fats?

Walter Willett: The belief that dietary fat is related to the risk of breast cancer developed because women in Japan and some other countries where dietary fat is low also had low rates of breast cancer. However, the difference in rates of breast cancer could have been due to many other aspects of diet and lifestyle that varied among these countries.

More recently, many large follow-up studies have looked at the relationship between dietary fat and breast cancer and have consistently found little or no relationship. Also, in a very large randomized trial, the Women's Health Initiative, women assigned to a low-fat diet experienced rates of breast cancer not statistically different from that of women assigned to follow their usual diet. Thus, the available evidence does not support any substantial relationship between fat intake during midlife or later and the risk of breast cancer. Although the data are limited, some evidence suggests that high intake of red meat and high-fat dairy products during adolescence or premenopausal years may increase the risk of breast cancer before menopause.

In contrast to the disappointing data on dietary fat, there is now clear evidence that weight gain during adult life increases the risk of breast cancer—and that weight loss reduces risk. Weight gain is related to excessive calories from any source, including dietary fat and dietary carbohydrate. Replacing calories from fat with calories from carbohydrate will make little difference.

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