(P19-039-23) Planetary Health Diet Index and Risk of Total and Subtypes of Breast Cancer in the Nurses’ Health Studies

Objectives: Current global food systems threaten human health and environmental sustainability. In 2019, the EAT-Lancet commission defined a reference 'planetary health diet', and outlined a combination of food groups and ranges of food intakes that would optimize human health and the environment. We aim to prospectively evaluate the association between the Planetary Health Diet (PHD) Index and the incidence of breast cancer.

Methods: We followed 68,254 participants in the Nurses’ Health Study (NHS; 1986-2016) and 93,287 in Nurses’ Health Study II (NHSII; 1991-2017). The PHD index was calculated based on validated food frequency questionnaires administered every 4 years, with a higher score representing better adherence to planetary health diet. Hazard ratios and 95% confidence intervals (CI) were estimated using multivariable-adjusted Cox regression models adjusted for reproductive, hormonal, and anthropometric factors (height and BMI at age 18 y); race; neighborhood socioeconomic status; total alcohol intake; total caloric intake; and physical activity.

Results: During 3,892,069 person-years of follow-up, we documented 9,949 breast cancer cases (6,037 NHS, 3,912 NHSII). Women in the highest, compared with the lowest, PHD quintile (Q) were at lower breast cancer risk (HR_{Q5 vs. Q1} = 0.87; 95% CI: 0.82, 0.93; P-trend < 0.001). With further adjustment for weight change, results were slightly attenuated but remained statistically significant (HR_{Q5 vs. Q1} =0.90; 95% CI: 0.84, 0.96; P-trend=0.004). Although heterogeneity by estrogen receptor (ER) status was nonsignificant (p=0.10), the strongest association between PHD and breast cancer was observed for ER-negative tumors (HR_{Q5 vs. Q1} =0.80; 95% CI: 0.68, 0.95; P-trend=0.01). No significant associations were observed by molecular subtypes.

Conclusions: Our findings indicate that high adherence to a PHD was associated with lower breast cancer risk, especially for ER-negative tumors. In light of the increasing global burden of breast cancer, our results seem to be clinically relevant for breast cancer prevention, and the PHD is an attainable and sustainable objective. Nonetheless, further studies evaluating this index are necessary.

Funding Sources: This study was supported by grants UM1 CA186107, U01 CA176726, P01 CA87969, and R01 CA50385 from the National Institutes of Health, and Ramon Areces Foundation.