Healthful and Unhealthful Plant-Based Diets and Risk of Breast Cancer in U.S. Women: Results from the Nurses' Health Studies

Plant-based diets have been associated with lower risk of various diseases, including type 2 diabetes, cardiovascular disease, and other cardiometabolic risk factors. However, the association between plant-based diet quality and breast cancer remains unclear.

We prospectively followed 76,690 women from the Nurses' Health Study (NHS, 1984–2016) and 93,295 women from the NHSII (1991–2017). Adherence to an overall plant-based diet index (PDI), a healthful PDI (hPDI), and an unhealthful PDI (uPDI) was assessed using previously developed indices. Cox proportional hazards models were used to estimate HR and 95% confidence intervals (CI) for incident invasive breast cancer.

Over 4,841,083 person-years of follow-up, we documented 12,482 incident invasive breast cancer cases. Women with greater adherence to PDI and hPDI were at modestly lower risk of breast cancer [(HR_{Q5 vs. Q1}, 0.89; 95% CI, 0.84–0.95); (HR_{Q5 vs. Q1}, 0.89; 95% CI, 0.83–0.94)]. We observed significant heterogeneity by estrogen receptor (ER) status, with the strongest inverse association between hPDI and breast cancer observed with ER-negative tumors [HR_{Q5 vs. Q1}, 0.77; 95% CI, 0.65–0.90; P_trend < 0.01]. We also found an inverse association between extreme quintiles of healthy plant foods and ER-negative breast cancer [HR, 0.74; 95% CI, 0.61–0.88; P_trend < 0.01].

This study provides evidence that adherence to a healthful plant-based diet may reduce the risk of breast cancer, especially those that are more likely to be aggressive tumors.

This is the first prospective study investigating the relation between healthful and unhealthful plant-based dietary indices and risk of total and subtype-specific breast cancer.