The incidence of breast cancer in the United States began climbing steadily in the early 1970s, and is now the highest ever seen in human history. Nearly 50,000 American women die of the disease every year. In the face of this tragedy, a great deal of attention has been given to genetics, but the presence of the breast cancer susceptibility gene, called BRCA-1, only accounts for at most 5 percent of breast cancers.
Exercise is very important to breast cancer risk. In fact, women who exercise (walk) for four hours per week lower their risk by 33 percent. And women who exercise more than that lower their risk even further. 
But diet, it turns out, is even more important. . .
What We Know
Death rate from breast cancer in the United States: 22.4 (per 100,000)
Death rate from breast cancer in Japan: 6.3 (per 100,000)
Death rate from breast cancer in China: 4.6 (per 100,000)
Primary reasons for difference: People in China and Japan eat more fruits and vegetables and less animal products, weigh less, drink less alcohol, and get more exercise than people in the United States.
Breast cancer rate for women in Italy who eat a lot of animal products compared to women in Italy who don't: 3 times greater 
Breast cancer rate for women in Uruguay who eat meat often compared to women in Uruguay who rarely or never eat meat: 4.2 times greater 
Breast cancer rate for affluent Japanese women who eat meat daily compared to poorer Japanese women who rarely or never eat meat: 8.5 times greater 
Impact on breast cancer risk for adult women who are 45 pounds overweight: Double 
American women who are aware that there are any dietary steps they can take to lower their chances of developing breast cancer: 23 percent 
American women with less than high school educations who are aware that there are any dietary steps they can take to lower their risk of developing breast cancer: 3 percent 
American women who believe that mammograms prevent breast cancer: 37 percent 
 Thune, I., et al., "Physical Activity and the Risk of Breast Cancer," New England Journal of Medicine 336 (1997): 1269-75.
 Decarli, A, et al., "Macronutrients, Energy Intake and Breast Cancer Risk. . . ," Epidemiology 8 (1997):425-28; see also Wynder, E., et al., "Breast Cancer: Weighing the Evidence for a Promoting Role of Dietary Fat," Journal of the National Cancer Institute 89 (1997):766-75; Nicholson, A, "Diet and the Prevention and Treatment of Breast Cancer," Alternative Therapies 2 (1996):32-8; and Outwater, J., et al., "Dairy Products and Breast Cancer. . . ," Medical Hypotheses 48 (1997):453-61.
 Ronco, E., et al., "Meat, Fat, and Risk of Breast Cancer: A Case Control Study from Uruguay," International Journal of Cancer 65 (1996):328-31.
 Hirayama, T., "Epidemiology of Breast Cancer with Special Reference to the Role of Diet," Preventive Medicine 7 (1978):173-95.
 Huang, Z., et al., Journal of the American Medical Association (1997), cited in 'Weight Gain Increases Risk of Breast Cancer," Associated Press, November 4, 1997.
 Bamard, N. D., et al., "Beliefs about Dietary Factors in Breast Cancer among American Women, 1991-1995," Preventive Medicine 26 (1997): 109-13.
 Hirayama, T., "Diet and Cancer," Nutrition and Cancer 1 (1979a):67-81. See also Colditz, G. A., et al., "Diet and Lung Cancer: A Review of the Epidemino-logic Evidence in Humans,” Archives of Internal Medicine 147 (1987): 157-60; and International Journal of Cancer 78 (1998): 430-6, cited in “fruits, Carrots May Reduce Lung Cancer Risk,” Reuters, November 25, 1999.
(‘The Food Revolution: how your diet can help save your life and our world’, by John Robbins)