Background: Results of observational studies of the association between dietary fat and risk of invasive breast cancer have been inconsistent. In the Women's Health Initiative dietary modification (DM) randomized trial designed to lower fat intake, the intervention was not associated with a statistically significant reduction of overall breast cancer risk. However, the DM association with risk of ductal carcinoma in situ (DCIS) of the breast, a putative breast cancer precursor, has not been reported. Methods: A total of 48, 835 postmenopausal women, ages 50-79 years at enrollment, with no breast cancer history and ≥32% of total energy intake from fat, were randomly assigned either to a dietary intervention (n=19, 541) designed to reduce total fat intake to 20% of energy and to increase vegetable, fruit, and grain consumption, or to a comparison group (n = 29, 294). Cox proportional hazards models were used to estimate HRs and 95% confidence intervals for the association between the intervention and DCIS risk. Results: During 18.7 years (median) cumulative follow-up, including intervention (~8.7 years) and post-intervention phases (~13.0 years), 817 DCIS cases were ascertained. No evidence of an association between the DM intervention and DCIS risk was observed overall, or by trial phase (intervention and postintervention). Similarly, no associations were found in subgroups defined by potential risk factors for DCIS. Conclusions: DMaiming to reduce fat intake was not associated with altered risk of DCIS. Impact: These results do not provide evidence of an association between dietary fat reduction and the risk of DCIS among postmenopausal women.
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