The acute Effect of high- and moderate-intensity endurance exercise on serum concentrations of CC16, SP-D, and the CC16/SP-D ratio in active healthy adult men

Abstract
Background and Purpose: Previous studies have shown that increased ventilation during endurance exercise leads to evaporation of the airway epithelial lining fluid (ELF) and imposes shear stress on the respiratory system. These conditions can compromise airway epithelial integrity and cause epithelial cell injury. However, the effect of exercise intensity on airway epithelial injury markers, including club cell protein 16 (CC16), surfactant protein-D (SP-D), and the CC16/SP-D ratio, remains unclear. Therefore, this study aimed to investigate the acute effects of high- and moderate-intensity endurance exercise on CC16, SP-D, and the CC16/SP-D ratio in healthy adult men.
Methods: Twenty active healthy men (age, 22 ± 3 years; weight, 75 ± 8 kg; height, 1.82 ± 0.06 m; BMI, 23 ± 1.86 kg.m2) were randomly assigned to a high-intensity endurance group (HE, n = 10) or a moderate-intensity endurance group (ME, n = 10). Participants performed a single 20-minute continuous treadmill session at 85–90% or 65–70% of maximal heart rate (MHR), respectively. The initial treadmill speed was set at 9 km/h for the HE group and 7.5 km/h for the ME group. Blood samples (5 mL) were collected from the antecubital vein before (baseline) and one hour after the intervention. Independent t-tests were used to compare age, anthropometric characteristics (weight, height, BMI), lean body mass, muscle mass, and distance covered between HE and ME groups. Analysis of covariance (ANCOVA) was applied to compare study variables between groups, and paired-sample t-tests were used for within-group comparisons.
Results: Based on within-group comparisons, serum CC16 (P = 0.008) and the CC16/SP-D ratio (P = 0.006) increased significantly after high-intensity endurance exercise, whereas SP-D levels did not show a significant change compared to baseline (P = 0.064). In the ME group, no significant changes were observed in CC16 (P = 0.759), SP-D (P = 0.072), or the CC16/SP-D ratio (P = 0.121) after the intervention. Between-group comparisons using ANCOVA indicated significant differences in CC16 (P = 0.021) and the CC16/SP-D ratio (P = 0.025) post-intervention, while SP-D levels showed no significant difference (P = 0.446).
Conclusion: The results of the present study demonstrate for the first time that high-intensity endurance exercise elevates serum CC16 levels and the CC16/SP-D ratio without affecting SP-D concentrations. Moderate-intensity endurance exercise does not influence airway epithelial integrity markers. Further research involving larger and more diverse populations is warranted to confirm these results, clarify underlying mechanisms, and explore their clinical and physiological implications.