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.