This study examined the relationship between total body fat percentage (TBFP), trunk fat percentage (TFP), and urinary flow rate (UFR) using NHANES data (2011–2018) from 10 521 participants aged 18–59. Multivariable-adjusted regression models assessed associations between dual-energy X-ray absorptiometry-measured fat percentages and UFR. Results showed that increased TBFP and TFP were inversely associated with UFR (TBFP: β = −0·013, 95 % CI [–0·018, −0·007]; TFP: β = –0·014, 95 % CI [–0·018, −0·009]). Stratified analyses revealed gender differences: in males, higher TBFP and TFP correlated with lower UFR (TBFP: β = –0·011, 95 % CI [−0·019, −0·004]; TFP: β = –0·016, 95 % CI [−0·022, −0·009]), but this association was absent in males aged 18–35. In females, both TBFP and TFP were consistently linked to reduced UFR across all age groups (TBFP: β = −0·016, 95 % CI [−0·023, −0·008]; TFP: β = −0·012, 95 % CI [−0·019, −0·006]). These findings suggest that body and trunk fat accumulation negatively impact UFR, with stronger effects in females and age-dependent variations in males. Further research should explore mechanisms behind these disparities to guide targeted interventions.