Acute effects of exercise on the nephron index under different conditions in middle-aged males – a preliminary study

Background and Aims: The nephron index provides a non-invasive and theoretical estimate of the functional nephrons based on the assumption that serum fibroblast growth factor 23 (FGF23) concentrations correlate with phosphate excretion per functional nephron. This study aimed to investigate the effects of exercise on the nephron index under different conditions. Methods: Nine middle-aged adults performed exercise under three experimental conditions: moderate-intensity continuous exercise (MICE), high-intensity intermittent exercise (HIIE), and moderate-intensity intermittent exercise (MIIE). Blood and urine samples were collected before exercise, immediately after exercise and 60 minutes post-exercise. The nephron index was assessed using blood and urine phosphate and creatinine concentrations, serum FGF23 levels, and the estimated glomerular filtration rate. Results: A significant time effect was observed for the log-transformed nephron index (p = 0.012, ηp² = 0.42). The log-transformed nephron index increased immediately after exercise (p = 0.059) and 60 minutes post-exercise (p = 0.008) across MICE, HIIE, and MIIE conditions. Additionally, urinary fractional excretion of phosphate (defined as the ratio of phosphate to creatinine clearance) significantly increased at 60 minutes post-exercise across all conditions (p = 0.038). These findings suggest that a single bout of exercise may result in acute beneficial effects on renal microcirculation. Conclusions: These results suggest that a single bout of exercise may influence nephron responsiveness in middle-aged individuals. However, as this is a preliminary study, further investigations are warranted to confirm the physiological relevance of the nephron index under dynamic conditions.