Renal Responses to Endurance Cycling in the Heat

Abstract

Strenuous running and cycling have been reported to alter normal kidney function. Although running protocols have been performed in both hot and cold settings, cycling protocols have been performed primarily in cool environments, thus leaving a need for a cycling protocol in the heat. PURPOSE: Observe the effect of a long distance cycling event in a hot environment on renal function and identify potential factors related to renal stress. METHODS: 45 cyclists (38 male, 7 female, 51 ± 10 y, 84.7 ± 15.6 kg) completed either a 164 km or 100 km outdoor ride in the heat (max 36.6°C, 51% RH). Pre- and post-ride: body mass was measured to determine net fluid loss, a urine sample was collected to analyze urine specific gravity (USG) and urine color (Uc), and a blood sample (50 mL) was drawn. A muscle soreness questionnaire was also completed as a surrogate for muscle damage in the legs. Serum and urine Na+, K+ and Cl- concentrations were analyzed by ion-sensitive electrodes. Serum creatinine (SCr) was measured via colorimetric assay. Paired-samples T-tests were run to analyze differences in pre- and post-ride measures. RESULTS: A net fluid loss of 1.4 ± 1.1 kg (1.6 ± 1.4% body mass loss) was observed. USG was greater post- (1.021 ± 0.007) vs pre-ride (1.015 ± 0.007; P < 0.001). Uc was greater post- (3.5 ± 1.5) vs pre-ride (2.1 ± 0.9; P < 0.001). Leg muscle soreness increased post- (1.47 ± 1.67) vs pre-ride (0.18 ± 0.58; P < 0.001). Serum Na+ (pre- 141.1 ± 2.3, post- 142.8 ± 2.9 mmol/L; P = 0.001), serum K+ (pre- 4.4 ± 0.5, post- 4.7 ± 0.6 mmol/L; P = 0.002), and urine K+ (pre- 36.6 ± 22.7, post- 95.8 ± 42.7 mmol/L; P<0.001) increased, whereas urine Na+ (pre- 96.0 ± 49.9, post- 52.1 ± 33.3 mmol/L; P < 0.001) decreased post-ride. SCr was greater post- (0.88 ± 0.21 mg/dl) vs pre-ride (0.52 ± 0.14 mg/dl; P < 0.001). CONCLUSION: Changes in electrolyte concentrations and SCr suggest kidney stress after endurance cycling in a hot environment. This impairment may be a result of dehydration and/or increased muscle damage (as indicated by increased soreness). Dehydration reduces blood volume and subsequent renal blood flow required for sufficient renal filtration. Muscle damage increases creatinine production, overloading the kidneys with waste to filter.