Nonlinear Dose-Response Association Between Exercise Intensity and Urinary Liver-Type Fatty Acid-Binding Protein in Young Men: A Piecewise Threshold Analysis

High-intensity physical exercise can induce temporary renal hemodynamic strain and subclinical tubular injury, yet the exact dose-response relationship remains poorly defined. This study investigated the relationship between exercise intensity and urinary liver-type fatty acid-binding protein (L-FABP) concentrations, a sensitive marker of proximal tubular stress, to identify an exploratory intensity threshold. We conducted an observational cohort study of 285 healthy young men undergoing a routine 20 min physical training session. Exercise intensity was quantified as metabolic equivalents of task (METs) using wearable accelerometers, and urinary L-FABP was measured pre-exercise and 30 min post-exercise. Post-exercise urinary L-FABP concentrations increased significantly from baseline (median, 1.89 vs. 0.90 µg/gCr; p < 0.001). Restricted cubic splines confirmed a nonlinear dose-response relationship (p for nonlinearity < 0.001) with an estimated breakpoint at 6.76 METs (95% CI, 5.67 to 7.12). Below 6.76 METs, exercise intensity was not significantly associated with post-exercise L-FABP (β = 0.068, p = 0.358), whereas above this threshold, L-FABP escalated rapidly (β = 0.491, p < 0.001). In conclusion, exercise intensity nonlinearly correlates with proximal renal tubular stress in young men. The 6.76 MET threshold represents an exploratory breakpoint marking an inflection zone beyond which exercise-associated subclinical renal tubular stress accelerates.