Dietary glycemic index, glycemic load, and risk of incident breast cancer in postmenopausal women.

Insulin and insulin-like growth factor-I (IGF-I) are associated with increased risk of breast cancer in several studies. Circulating concentrations of insulin increase with dietary consumption of high glycemic index foods, which, in turn, may influence IGF-I levels or activity, but the relevance of such dietary patterns for breast cancer risk is unclear. We investigated whether consumption of carbohydrates with high dietary glycemic index would predict risk of postmenopausal breast cancer among 63,307 United States women in the Cancer Prevention Study II Nutrition Cohort. From baseline in 1992, participants 40-87 years of age and free from cancer and diabetes, were followed for 5 years; 1442 incident breast cancer cases were documented. Diet was assessed at baseline by a validated 68-item food frequency questionnaire from which we calculated dietary glycemic index and glycemic load. Dietary glycemic index and load were not associated with increased risk of postmenopausal breast cancer (rate ratio = 1.03; 95% confidence interval, 0.87-1.22 and rate ratio = 0.90; 95% confidence interval, 0.76-1.08, respectively) after adjustment for multiple breast cancer risk factors. Associations were not modified by body mass index, physical activity, hormone use, or stage of disease. Future evaluations of glycemic index and breast cancer risk may be strengthened by longer follow-up, more complete dietary information, and measurement of plasma insulin and IGF-I levels.