Epidemiology and natural history of breast cancer. Implications for the body weight-breast cancer controversy.

The mechanisms of differentiation of the mammary gland apparently can explain the contradictory findings on the association of breast cancer with excess body weight. Excess weight may be related to the initiation of breast cancer in premenopausal women through its effect on menstrual cycles and on progesterone secretion and to the promotion of breast cancer in postmenopausal women through its effect on estrogen metabolism. Although body weight is unlikely to be as strong a risk factor for breast cancer as it is for endometrial cancer, it may have a greater importance from a public health point of view, because the ratio of the incidence of breast cancer to that of endometrial cancer is 4.5 in whites and 6 in blacks. Thus, more studies seem warranted. It is not possible, however, to rule out the view that the reported correlation between excess weight and breast cancer is attributable to failure to adjust for potential confounders, such as dietary fat. New insights may come from the combined assessment of weight, different types of dietary fat, and reproductive history factors known to be involved in the natural history of breast cancer. For example, in the Women's Intervention Nutrition Study (WINS), the survival rate of breast cancer patients on a 20 per cent fat diet is being compared with that of breast cancer patients of similar weight keeping their usual 35 to 40 per cent fat diet. This investigation will show whether dietary fat influences the rate of progression (metastasis) and of promotion (occurrence of cancer in the other breast) independent of body weight.