Seasonal changes in free 25-(OH)D and vitamin D metabolite ratios and their relationship with psychophysical stress markers in male professional football players

Introduction: Novel markers of vitamin D status are currently being investigated, including free 25-(OH)D (25-(OH)D(F)) and the vitamin D metabolite ratio (24,25-(OH)(2)D(3):25-(OH)D(3); VMR). The VMR may provide additional functional information on vitamin D metabolism in athletes. Therefore, the main objective of the current study was to evaluate 25-(OH)D(F), bioavailable 25-(OH)D (25-(OH)D(B)), VMR, and psychophysical stress markers during different training periods over a half-season. The second aim was to assess the association between vitamin D binding protein (VDBP), total and free 25-(OH)D, VMRs, and psychophysical stress markers in professional football players. Moreover, we examined the relationship between 25-(OH)D(3) and vitamin D metabolites (24,25-(OH)(2)D(3), 3-epi-25-(OH)D(3)) to determine if training loads in different training periods influenced the vitamin D metabolome. Methods: Twenty professional football players were tested at six different time points across half a year (V1—June; V2—July; V3—August; V4—October; V5—December; V6—January). Results: Analyses indicated a significant seasonal rhythm for VDBP, and total 25-(OH)D (25-(OH)D(T)), 25-(OH)D(B), 24,25-(OH)(2)D(3), 3-epi-25-(OH)D(3), 25-(OH)D(3):24,25-(OH)(2)D(3), and 24,25-(OH)(2)D(3):25-(OH)D(3) VMRs throughout the training period. No correlation was detected between 25-(OH)D(T), 25-(OH)D(B), 25-(OH)D(F), vitamin D metabolites, VMRs, VDBP, and ferritin, liver enzymes (aspartate transaminase [AST] and alanine transaminase [ALT]), creatine kinase (CK), cortisol, testosterone, and testosterone-to-cortisol ratio (T/C) in each period (V1-V6). However, there was a strong statistically significant correlation between 25-(OH)D(3) and 24,25-(OH)D(3) in each training period. Conclusion: In conclusion, a seasonal rhythm was present for VDBP, 25-(OH)D(T), 25-(OH)D(B), vitamin D metabolites (24,25-(OH)(2)D(3), 3-epi-25-(OH)D(3)), and VMRs (25-(OH)D(3):24,25-(OH)(2)D(3), 25-(OH)D(3):3-epi-25-(OH)D(3)). However, no rhythm was detected for 25-(OH)D(F) and markers of psychophysical stress (ferritin, liver enzymes, CK, testosterone, cortisol, and T/C ratio). Moreover, the relationships between free and total 25-(OH)D with psychophysical stress markers did not demonstrate the superiority of free over total measurements. Furthermore, training loads in different training periods did not affect resting vitamin D metabolite concentrations in football players.