Prospective study of a diabetes risk reduction diet and the risk of breast cancer.

Background Hyperinsulinemia and higher insulin-like growth factors may increase breast cancer risk. We evaluated a diabetes risk reduction diet (DRRD) and breast cancer risk. Objectives We prospectively evaluated the association between adherence to a DRRD and the incidence of breast cancer. Methods We followed 88,739 women from the Nurses' Health Study (NHS; 1980–2016) and 93,915 women from the NHSII (1991–2017). Incident breast cancer cases (n  = 11,943) were confirmed with medical records, and subtypes were determined by tissue microarray data and pathology reports. Information on diet and breast cancer risk factors was repeatedly ascertained in follow-up questionnaires. A DRRD score was derived with 9 factors: lower glycemic index of diet; lower intakes of trans fat, sugar-sweetened beverages/fruit juices, and red/processed meat; higher intakes of cereal fiber, coffee, nuts, and whole fruits; and a higher ratio of polyunsaturated to saturated fat (score range: 9–45). Multivariable-adjusted hazard ratios (MVHRs) and 95% CIs were calculated with Cox proportional hazards models. Results Being in the highest compared with the lowest DRRD adherence quintile was associated with a modestly lower breast cancer risk (MVHR_Q5vsQ1: 0.89; 95% CI: 0.84, 0.95; P -trend = 0.0002); this was attenuated after adjusting for weight change since age 18 y (MVHR_Q5vsQ1: 0.92; 95% CI: 0.87, 0.98; P -trend = 0.01). The inverse association was strongest among women with current BMI < 25 kg/m² (MVHR_Q5vsQ1: 0.89; 95% CI: 0.81, 0.98; P -trend = 0.004; P -interaction = 0.04). Among tumor molecular subtypes, the strongest inverse association was observed with basal-type tumors (MVHR_Q5vsQ1: 0.67; 95% CI: 0.45, 1.01; P -trend = 0.04). Conclusions Greater DRRD-adherence was associated with lower breast cancer risk, likely mediated by less weight gain with a DRRD; however, independently of weight change, DRRD-adherence was modestly associated with lower breast cancer risk, particularly among lean women. [ABSTRACT FROM AUTHOR]