Association Between Plant-Based and Animal-Based Diet Indices and Breast Cancer Risk in the Southern Community Cohort Study (SCCS)

Diet is a potentially modifiable risk factor for breast cancer. While evidence suggests potential benefits of plant-based diets for cancer prevention, many studies lack racial and socioeconomic diversity. We examined the association of plant-based and animal-based diet indices and risk of breast cancer within the Southern Community Cohort Study (SCCS), a prospective cohort study. Data from 42,845 women among whom 1,401 were diagnosed with breast cancer during a median 12.8 years of follow-up were utilized in the analyses. We applied three previously developed plant-based diet indices—the overall plant-based diet index (PDI), healthful PDI (hPDI), and unhealthful PDI (uPDI)—developed by Satija et al.. These indices scored individuals based on intake of 18 food groups categorized as healthful plant foods, unhealthful plant foods, or animal foods using baseline dietary data from a self-administered FFQ. Dietary intake of the 18 food groups was ranked into quintiles based on the distribution in the study population and given a positive or reverse score of 1 to 5 to later create three versions of plant-based diet indices (PDI, hPDI, and uPDI) and one animal-based index. Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for breast cancer risk by quartiles of dietary scores with adjustments for age, energy intake, recruitment site, race and ethnicity, education, income, insurance, smoking status, NSAIDs use, hormone therapy use, age at menopause, age at menarche, age at first birth, family history of breast cancer, physical activity, body mass index (BMI), and alcohol, and separately stratified by socioeconomic indicators. Women in the fourth quartile of the unhealthful plant-based diet score compared to the first quartile had a decreased risk of breast cancer, though confidence intervals were imprecise and included the null, HR 0.86 (95% CI:0.72, 1.02) Notably, associations appeared stronger among participants with lower socioeconomic status. Women with income ≤ $25,000 had stronger associations than those with higher income for the unhealthful plant-based diet score; HRs (95% CIs) were 0.80 (0.65, 0.97) and 1.07 (0.76, 1.52), respectively. Among those with lower education (≤ high school graduation), the HR for high unhealthful plant-based diet index was 0.77 (95% CI: 0.61, 0.97), while the HR was 0.95 (95% CI: 0.72, 1.27) for individuals with vocational, technical, business training, college, or college graduation, and 1.24 (95% CI: 0.63, 3.46) for those with graduate-level education. No substantial associations with breast cancer were observed for the PDI, hPDI, or animal-based indices. We observed that higher scores on an unhealthful plant-based diet index were associated with a reduced risk of breast cancer, primarily among individuals with lower income or education. These associations were unexpected and need to be confirmed in other large prospective cohort studies in diverse populations. Jessica Sainyo, Susan E. Steck, Monique J. Brown, Jiajia Zhang, Brie Turner-McGrievy. Association between plant-based and animal-based diet indices and breast cancer risk in the Southern Community Cohort Study (SCCS) [abstract]. In: Proceedings of the 18th AACR Conference on the Science of Cancer Health Disparities; 2025 Sep 18-21; Baltimore, MD. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2025;34(9 Suppl):Abstract nr A061.