Lifetime grain consumption and breast cancer risk.

We evaluated individual grain-containing foods and whole and refined grain intake during adolescence, early adulthood, and premenopausal years in relation to breast cancer risk in the Nurses' Health Study II. Grain-containing food intakes were reported on a baseline dietary questionnaire (1991) and every 4 years thereafter. Among 90,516 premenopausal women aged 27-44 years, we prospectively identified 3235 invasive breast cancer cases during follow-up to 2013. 44,263 women reported their diet during high school, and from 1998 to 2013, 1347 breast cancer cases were identified among these women. Cox proportional hazards regression was used to estimate relative risks (RR) and 95 % confidence intervals (95 % CI) of breast cancer for individual, whole and refined grain foods. After adjusting for known breast cancer risk factors, adult intake of whole grain foods was associated with lower premenopausal breast cancer risk (highest vs. lowest quintile: RR 0.82; 95 % CI 0.70-0.97; P trend = 0.03), but not postmenopausal breast cancer. This association was no longer significant after further adjustment for fiber intake. The average of adolescent and early adulthood whole grain food intake was suggestively associated with lower premenopausal breast cancer risk (highest vs lowest quintile: RR 0.74; 95 % CI 0.56-0.99; P trend = 0.09). Total refined grain food intake was not associated with risk of breast cancer. Most individual grain-containing foods were not associated with breast cancer risk. The exceptions were adult brown rice which was associated with lower risk of overall and premenopausal breast cancer (for each 2 servings/week: RR 0.94; 95 % CI 0.89-0.99 and RR 0.91; 95 % CI 0.85-0.99, respectively) and adult white bread intake which was associated with increased overall breast cancer risk (for each 2 servings/week: RR 1.02; 95 % CI 1.01-1.04), as well as breast cancer before and after menopause. Further, pasta intake was inversely associated with overall breast cancer risk. Our results suggest that high whole grain food intake may be associated with lower breast cancer risk before menopause. Fiber in whole grain foods may mediate the association with whole grains.