INTRODUCTION: It has been proposed that serum free 25-hydroxyvitamin D (25-(OH)D) may better reflect vitamin D action than total 25-(OH)D based on the free hormone hypothesis. Vitamin D plays an active role in muscle inflammatory response, protein synthesis, and regulation of skeletal muscle function. Strenuous muscle contractions during exercise may induce mechanical muscle damage. Thus, monitoring of psychophysical stress markers in athletes may help to tailor training/competition and recovery regimes to optimize performance. Therefore, the aim of our study was to check the relationship between free 25-(OH)D levels and blood count, iron, ferritin, testosterone (T), cortisol (C), creatine kinase (CK), liver enzymes (ALT, AST) and high-sensitivity C-reactive protein (hs-CRP) in professional football players. METHODS: Professional male football players (n=35, 26.8±4.8 yrs, body fat 17.6±3.2%) participated in the study. Total 25-(OH)D was assessed using LM-MS/MS. Free 25-(OH)D concentration was evaluated by calculation, using serum albumin and vitamin D binding protein (VDBP) levels. The following biochemical parameters using standard automated technique were assessed: blood count, iron, ferritin, T, C, CK, ALT, AST and hs-CRP. The T/C ratio was calculated by dividing the two hormone levels both expressed in nmol/l. Multiple regression was performed to analyse association between VDBP, free, bioavailable 25-(OH)D and biochemical, hormonal parameters. Statistically significant models were adjusted for age, body mass and body fat. A statistical level of p< 0.05 was accepted. RESULTS: Our study showed decreased serum total 25-(OH)D levels (26.1±4.3 ng/ml) in 63% of the professional players. In studied football players, there was significant correlation between serum VDBP concentrations and cortisol levels (p<0.001). There was no correlation between total 25-(OH)D levels and studied variables. Serum free 25-(OH)D inversely associated with hs-CRP (p<0.028) and cortisol (p<0.034) concentrations. Serum bioavailable 25-(OH)D significantly correlated with hs-CRP (p<0.017) levels. CONCLUSION: Based on the results of our study, we concluded that to check some of psychophysical stress parameters (hs-CRP, cortisol) in athletes measuring free, bioavailable 25-(OH)D may provide a better index of vitamin D status that total 25-(OH)D levels in such situations. Furthermore, seems justified to monitor the levels of vitamin D, cortisol and testosterone in professional football players could be decisive in the development of more specific supplementation and injury prevention strategies.
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