Purpose: Plant-based diets have been associated with lower risk of various diseases, including type 2 diabetes, cardiovascular disease and other cardiometabolic risk factors and are recommended for both health and environmental benefits. However, the association between plant-based diet quality and breast cancer remains unclear. Therefore, we examined the associations of plant-based diet quality with risk of total and subtypes of breast cancer.

Methods: We followed 74,516 women from the Nurses' Health Study (NHS, 1984-2016), and 93,915 women from the Nurses' Health Study II (NHSII, 1991-2017), without cancer at baseline. Incident invasive breast cancer cases were confirmed with medical records, and subtypes were determined by tissue microarray data and pathology reports. We assessed an overall plant-based diet index (PDI) from dietary data collected using repeated semi-quantitative food-frequency questionnaires (FFQ), by assigning positive scores to plant foods and reverse scores to animal foods. We also investigated a healthful PDI (hPDI) where healthy plant foods (whole grains, fruits/vegetables, nuts/legumes, oils, tea/coffee) received positive scores, while less healthy plant foods (juices/sweetened beverages, refined grains, potatoes/fries, sweets) and animal foods received reverse scores. We also assessed an unhealthful PDI (uPDI), assigning positive scores to less healthy plant foods, and reverse scores to healthy plant foods and animal foods. Multivariable hazard ratios (HRs) and 95% confidence intervals (CI) were calculated with Cox proportional hazards models using data pooled across cohorts, adjusting for breast cancer risk factors.

Results: Over 4,193,915 person-years of follow-up, we documented 11,281 incident invasive breast cancer cases. Women with greater adherence to PDI were at modestly lower risk of breast cancer [HR Q5vsQ1 0.92; 95% CI 0.86, 0.99; p trend=0.02]. Furthermore, greater adherence to a hPDI was associated with a decreased risk of overall breast cancer [HR Q5vsQ1 0.90; 95% CI 0.85, 0.96, p trend=<0.001]. We observed significant heterogeneity by estrogen receptor (ER) status (p=0.01), with the strongest inverse association between hPDI and breast cancer observed with ER-negative tumors [HR Q5vsQ1 0.75; 95% CI 0.64, 0.89; p trend=<0.001]. Moreover, higher uPDI was positively associated with a higher risk of ER-negative breast cancer [HR Q5vsQ1 1.21; 95% CI 1.02, 1.44; p trend=0.02] (p for heterogeneity=0.03).

Conclusions: This study provides evidence that adherence to a plant-based dietary pattern and, specifically, its healthful version, may reduce the risk of breast cancer, especially those that are more likely to be aggressive tumors. In contrast, a low-quality plant-based diet may be associated with an increased risk of ER-negative breast cancer.

Citation Format: Andrea Romanos-Nanclares, Walter C. Willett, Bernard Rosner, Laura C. Collins, Frank B. Hu, Estefania Toledo, A. Heather Eliassen. Healthful and unhealthful plant-based diets and risk of breast cancer in U.S. women: Results from the Nurses' Health Studies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 837.