Diet

Dietary influences have been proposed and examined, and recent research suggests that low-fat diets may significantly decrease the risk of breast cancer as well as the recurrence of breast cancer. Another study showed no contribution of dietary fat intake on the incidence of breast cancer in over 300,000 women. A randomized controlled study of the consequences of a low-fat diet, the Women's Health Initiative, failed to show a statistically significant reduction in breast cancer incidence in the group assigned to a low-fat diet, although the authors did find evidence of a benefit in the subgoup of women who followed the low-fat diet in a strict manner. A prospective cohort study, the Nurses' Health Study II, found increased breast cancer incidence in premenopausal women only, with higher intake of animal fat, but not vegetable fat. Taken as a whole, these results point to a possible association between dietary fat intake and breast cancer incidence, though these interactions are hard to measure in large groups of women.

In a study published in the Journal of the American Medical Association, biomedical investigators found that Brassica vegetable intake (broccoli, cauliflower, cabbage, kale and Brussels sprouts) was inversely related to breast cancer development. The relative risk among women in the highest decile of Brassica vegetable consumption (median, 1.5 servings per day) compared to the lowest decile (virtually no consumption) was 58%. That is, women who consumed the most Brassica vegetables were 58% less likely to develop breast cancer.

A significant environmental effect is likely responsible for the different rates of breast cancer incidence between countries with different dietary customs. Researchers have long measured that breast cancer rates in an immigrant population soon come to resemble the rates of the host country after a few generations. The reason for this is speculated to be immigrant uptake of the host country diet. The prototypical example of this phenomenon is the changing rate of breast cancer after the arrival of Japanese immigrants to America.