Background: The underlying mechanism linking the decline in exercise capacity with renal dysfunction remains unclear. Urinary liver-type fatty acid-binding protein (L-FABP) levels reflect the degree of peritubular capillary blood flow, an important factor for renal dysfunction with aging. The aim of this study was to examine the relationship between exercise capacity and urinary L-FABP levels. Methods: This was a cross-sectional study of 187 middle-aged and older individuals (aged 50–83 years) without chronic kidney disease (CKD). We assessed urinary L-FABP levels, peak oxygen consumption (V ˙ O 2peak), and grip strength. Results: Urinary L-FABP levels inversely correlated with both V ˙ O 2peak (rs = −0.349) and grip strength (rs = −0.485). When the participants were divided into four groups according to the median values of aerobic fitness and muscular strength (V ˙ O 2peak and grip strength), urinary L-FABP levels were the highest in participants with lower levels of aerobic fitness and muscular strength (2.95 ± 1.43 μg/g creatinine) and the lowest in the participants with higher levels of aerobic fitness and muscular strength (1.33 ± 0.76 μg/g creatinine). The difference between the two groups was significant (P < 0.001). Conclusion: Our results demonstrate that both V ˙ O 2peak and grip strength were inversely associated with urinary L-FABP levels in middle-aged and older individuals without CKD. This suggests that a decline in exercise capacity is associated with a reduction in peritubular capillary blood flow, providing a novel insight into the underlying mechanism linking the decline in exercise capacity to the development of renal dysfunction.
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