Regardless of whether diet is related to the occurrence of breast cancer, if postdiagnosis diet were related to risk of recurrence or survival, then dietary modifications might assist in breast cancer treatment. Several studies have examined dietary fat intake before breast cancer diagnosis in relation to survival by following up the cases from breast cancer case control studies; results have been inconsistent. Because the original interest was the relation of diet to breast cancer incidence, the investigators were at pains to assess diet before diagnosis.
This approach is unsatisfactory, however, if the question is whether diet after diagnosis has an influence on survival, because women may make major changes in their diets after receiving the diagnosis. In one study of diet after diagnosis (albeit in the 1 to 5 months immediately after the diagnosis), no association was seen between dietary fat intake and survival. In a larger study, diet was assessed both before and after breast cancer diagnosis. Fat intake after diagnosis was not associated with survival.
However, higher protein consumption, mainly from poultry, fish, and dairy sources, was related to a better prognosis, even after controlling for protein consumption before diagnosis. In the same study, neither alcohol consumption nor vitamin A intake was associated with survival. A randomized trial of a low-fat intervention diet among breast cancer patients is under way. However, other aspects of diet also merit further examination in relation to breast cancer survival.
Summary of Diet and Breast Cancer Risk
The role of specific dietary factors in breast cancer causation is not completely resolved. Enthusiasm for the hypothesisthat dietary fat intake is responsible for the high breast cancer rates in Western countries was based largely on the weakest form of epidemiologic evidence - ecologic correlation studies. Results from prospective studies do not support the concept that fat intake in middle life has a major relationship to breast cancer risk during up to 14 years of follow-up. High energy intake in relation to physical activity, which accelerates growth and the onset of menstruation during childhood and leads to weight gain in middle life, contributes substantially to breast cancer risk.
These effects of energy balance clearly account for an important part of international differences in breast cancer rates. Although the responsible constituents are not clear, considerable evidence suggests that low intake of vegetables modestly increases the risk of breast cancer. The possibility that vitamin A or other compounds in vitamin A-rich foods are protective deserves further consideration.
Alcohol intake is the best-established specific dietary risk factor for breast cancer, and studies demonstrating that even moderate alcohol intake increases endogenous estrogen levels provide a potential mechanism, thus supporting a causal interpretation. Hypotheses relating childhood and adolescent diet to breast cancer risk decades later will be more difficult to test unless novel data sources or methods for measuring diet in the distant past are developed, or unless radiographic, histologic, or other markers of breast cancer risk are found and validated for use as intermediate markers of breast cancer.
Nevertheless, available evidence is strong that breast cancer risk can be reduced by avoiding weight gain during adult years, limiting alcohol consumption, and consuming an abundant amount of vegetables and fruits. Some evidence suggests that replacing saturated fat with monounsaturated fat may reduce risk of breast cancer, and this will reduce risk of coronary heart disease.
Walter C. Willett, Beverly Rockhill, Susan E. Hankinson, David J. Hunter and Graham A. Colditz
W. C. Willett: Harvard Medical School, Boston, Massachusetts; Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts
B. Rockhill: Harvard Medical School, Brigham and Women’s Hospital, Boston, Massachusetts
S. E. Hankinson: Departments of Medicine and Epidemiology, Harvard Medical School and Harvard School of Public Health, Boston Massachusetts
D. J. Hunter: Departments of Epidemiology and Nutrition, Harvard School of Public Health, Boston, Massachussetts
G. A. Colditz: Department of Medicine, Harvard Medical School, Boston, Massachussetts