Background: Increased body-mass index (BMI) is associated with increased prostate cancer incidence and death. However, an ‘obesity paradox’ exists where patients with BMI have longer survival once diagnosed with cancer. We investigated the influence of BMI on overall survival of veterans with metastatic hormone-sensitive prostate cancer (mHSPC) and metastatic castration-resistant prostate cancer (mCRPC). Methods: Two cohorts were identified in the US Veterans Health Affairs. The mHSPC cohort included patients diagnosed with de-novo mHSPC from 2011-2021. The mCRPC cohort was treated with abiraterone or enzalutamide from May 2011 to June 2017. We determined BMI at time of treatment and weight loss during the one year prior. BMI was categorized as ≤20, >20-25, >25-30 or >30 kg/m2. We used Kruskal-Wallis testing for ordinal variables and Cox hazard models to assess the association between BMI, age, Charlson comorbidity index, PSA level at diagnosis, Black race, weight loss, and survival. Results: We identified 5019 veterans with mHSPC and 8159 with mCRPC. Mean BMI was 27.2 (SD 5.9) in mHSPC and 28.2 (SD 5.8) in mCRPC. Mean age was 75.7 years (SD 9.8) in mHSPC and mean age of 74.6 (SD 9.3) in mCRPC. Median overall survival was 25.6 months (95% CI 24.6-26.6) in mHSPC and 20.5 months (95% CI 19.9-21.1) in mCRPC. There were significant differences in age, Black race, and PSA at diagnosis across BMI strata with BMI>30 having the youngest age, lowest percentage of Black race, lowest PSA, and longest survival in mHSPC and mCRPC (see Table). In unadjusted analyses, veterans with BMI>30 had lowest risk of death with Hazard Ratio (HR) 0.66 (95% CI 0.60-0.71) in mHSPC and 0.60 (95% CI 0.56-0.64) in mCRPC compared to BMI>20-25. In adjusted analyses, veterans with BMI>30 had lowest risk of death with adjusted hazard ratio (HR) 0.89 (95% CI 0.81-0.97) in mHSPC and HR 0.77 (95% CI 0.72-0.83) in mCRPC compared to BMI>20-25. Conclusions: Increased BMI is associated with younger age at diagnosis, lower PSA, and longer survival in both mHSPC and mCRPC. Further studies between BMI and prostate cancer are warranted to understand these complex interactions.

*Krusal-Wallis.

#Log-Rank.