Premenopausal dietary carbohydrate, glycemic index, glycemic load, and fiber in relation to risk of breast cancer.

Carbohydrate intake, glycemic index, and glycemic load have been hypothesized to increase risk of breast cancer by raising insulin levels, but these associations have not been studied extensively. The insulin response to dietary carbohydrate is substantially greater among overweight women than among leaner women. Although fiber intake has been hypothesized to reduce the risk of breast cancer, data from early adult life are lacking. We examined dietary carbohydrate, glycemic index, glycemic load, and fiber in relation to breast cancer risk among 90655 premenopausal women in the Nurses' Health Study II aged 26-46 years in 1991. Diet was assessed with a food frequency questionnaire in 1991 and 1995. During 8 years of follow-up, we documented 714 incident cases of invasive breast cancer. Dietary carbohydrate intake, glycemic load, and glycemic index were not related to breast cancer risk in the overall cohort. However, the associations differed by body mass index (BMI): among women with BMI < 25 kg/m(2), the multivariate relative risks for the increasing quintiles of carbohydrate intake were 1.00 (referent), 0.87, 0.77, 0.66, and 0.62 [95% confidence interval, 0.40-0.97; P, test for trend = 0.02]; and among women with BMI >or=25 kg/m(2), the corresponding relative risks were 1.00 (referent), 1.30, 1.35, 1.50, and 1.47 (95% confidence interval, 0.84-2.59; P, test for trend = 0.14; P, test for interaction = 0.02). Similar interaction with BMI was observed for glycemic load, but not for glycemic index. Intakes of total fiber and different types of fiber were not appreciably related to breast cancer risk. Our findings suggest that the associations between carbohydrate intake or glycemic load and breast cancer risk among young adult women differ by body weight. Our data do not support a strong association between fiber intake and breast cancer risk.