Diet in relation to breast cancer etiology has been studied widely, but results have remained inconsistent. Various dietary components including fruits, vegetables, and meat have been implicated through their effects on inflammation. Using data from the Iowa Women's Health Study we examine prospectively the association between the dietary inflammatory index (DII) and breast cancer incidence.

DII scores were computed based on baseline dietary intake assessed by a validated 121‐item food frequency questionnaire in a cohort of 34 700 women, aged 55–69 years at recruitment in 1986 and followed for incident breast cancer. During the 25‐year follow‐up period (1986–2011), 2910 incident breast cancer cases were identified. We used Cox proportional hazards regression to estimate multivariable hazard ratios (HRs) and 95% confidence intervals (CI). We found positive associations between DII scores and breast cancer risk (HR for DII_tertiles: T₃ vs T₁ = 1.11; 95% CI: 1.00, 1.22), with stronger associations in obese women (HR for DII_continuous: 1.05 per unit increase in DII; 95% CI: 1.02, 1.12; HR for DII_tertiles: T₃ vs T₁ = 1.35; 95% CI: 1.10, 1.66, p‐value for interaction = 0.02).

A proinflammatory diet, as indicated by higher DII scores, appears to increase the risk of developing breast cancer, especially in obese postmenopausal women.