Adherence to Healthy and Unhealthy Plant-Based Diets and Risk of Breast Cancer Overall and by Hormone Receptor and Histologic Subtypes Among Postmenopausal Women

OBJECTIVES: Previous studies on vegan and vegetarian diets have reported beneficial effects on health outcomes by reducing low-grade systemic inflammation. However, few studies have assessed the incremental decrease of animal foods and how the quality of plant foods (healthy and less healthy) is linked with favorable health outcomes. There is still limited data on breast cancer (BC) in this regard. We aimed to assess the association between long-term adherence to a plant-based diet and BC risk among postmenopausal women. METHODS: Women from the E3N (Etude Epidémiologique auprès de femmes de la Mutuelle Générale de l'Education Nationale) cohort (n = 65,574, mean age = 52.8 years) were followed from 1993 to 2014. Incident BC cases were identified, validated, and classified into receptor and histological subtypes. Scores for healthful (hPDI) and unhealthful (uPDI) plant-based diets were developed using self-reported dietary intake at baseline (1993) and during follow-up (2005), and studied as a continuous variable and in quintiles. Multivariable Cox regression models were used to estimate BC hazard ratios (HR) and 95% confidence intervals (CI). RESULTS: Over a mean follow-up of 21 years, there were 3,968 incident BC cases. Adherence to hPDI was associated with lower BC risk (HR(Q5 vs. Q1), 0.86; 95% CI, 0.77–0.95, P(trend) < 0.001), whereas adherence to uPDI was associated with increased BC risk (HR(Q5 vs. Q1), 1.20; 95% CI, 1.08–1.33, P(trend) < 0.001). Adherence to hPDI was inversely associated with ER-positive, PR-positive, PR-negative, and ER/PR positive BC, and both ductal and lobular carcinomas (HR(1-SD increase), 0.92; 95% CI, 0.89–0.96, 0.93; 95% CI, 0.89–0.97, 0.94; 95% CI, 0.89–0.99, 0.93; 95% CI, 0.89–0.97, 0.92; 95% CI, 0.89–0.96, and 0.92; 95% CI, 0.85–1.00, respectively). Adherence to uPDI was associated with increased ER-positive BC and lobular carcinoma (HR(1-SD increase), 1.05; 95% CI, 1.01–1.08 and 1.12; 95% CI, 1.04–1.22, respectively). CONCLUSIONS: Our findings support that adherence to a healthy plant-based diet may reduce the risk of BC. In contrast, an unhealthy plant-based diet may increase BC risk, further emphasizing the importance of diet quality regarding cancer prevention. FUNDING SOURCES: Ministère de l'enseignement supérieur de la recherche et de l'innovation.