Background: Sarcopenia, a severe loss of skeletal muscle mass, predicts poor outcomes in bladder cancer (BC). But, why sarcopenia occurs in BC is unknown. Our objective was to assess if diet and physical activity were the primary factors causing sarcopenia, and thus assess if sarcopenia could be addressed by lifestyle interventions. Methods: 286 patients filled out the International Physical Activity Questionnaire Long Form (IPAQ-L) and the Diet History Questionnaire II (DHQ2), had a CT abdomen/pelvis within 6 months of questionnaire administration and met inclusion criteria. The DHQ2 was converted into Healthy Eating Index 2010 scores (HEI2010). Skeletal muscle area (SM, cm²) area was measured at the L3 level using Slice-O-Matic software and divided by height (m²) to arrive at skeletal muscle index (SMI). Sarcopenia was defined as SMI < 52.4 in men and < 38.5 in women. Three raters read the images and inter-rater reliability was measured by the intra-class correlation coefficient (ICC). Associations among patient demographics, tumor characteristics, physical activity, diet quality, and body composition were examined by stratified analyses and regression models with R 3.2.3. Results: Reliability was very high, ICC=0.97. Sarcopenia was present in 71% of males and 55% of females. Key predictors of decreasing SMI included increasing age (p < 0.001), female gender (p < 0.001), and white race (p < 0.001). When adjusted for these unmodifiable patient factors, there was no association between SMI and variables such as Elixhauser comorbidity score, AJCC stage, tumor grade, and procedure type. With respect to modifiable lifestyle factors, there was no association between SMI and average weekly MET-min of physical activity level (p = 0.99) nor the daily consumption of calories (p = 0.69), protein (p = 0.28), fat (p = 0.19), or carbohydrate (p = 0.77). The HEI2010 diet score was not associated with SMI (p = 0.66). Conclusions: The three strongest predictors of decreasing muscle mass (i.e. sarcopenia) in BC patients are age, gender, and race. Modifiable risk factors, such as diet and physical activity levels, did not affect sarcopenia. Therefore, we would not expect that targeted lifestyle interventions would affect sarcopenia and improve BC outcomes.