Differential and Dose-Dependent Redistribution of Vitamin D Metabolites After Acute High-Intensity Exercise in Mixed Martial Arts Athletes and Untrained Men: Pilot Study

Background/Objectives: Acute high-intensity exercise may transiently alter circulating vitamin D metabolites. However, the effects of training status, supplementation dose, and vitamin D-binding protein (VDBP) on the exercise-induced redistribution and bioavailability of vitamin D remain unclear. This pilot study examined whether training status and vitamin D3 supplementation dose modulate vitamin D status, anaerobic performance, and acute post-exercise changes in vitamin D metabolites and VDBP. Methods: Thirty-four healthy young men participated, comprising 22 mixed martial arts (MMA) athletes and 12 untrained controls. MMA athletes received either 3500 or 6000 IU/day of vitamin D3 for four weeks, while untrained participants received 3500 IU/day or a placebo. Before and after the intervention, participants performed a supramaximal anaerobic exercise protocol (3 × 30 s Wingate tests). Blood samples were collected at rest and 30 min and 24 h post-exercise to assess vitamin D metabolites, VDBP, bioavailable and free 25(OH)D3, interleukin-6, and lactate. Results: Supplementation with 3500 IU/day significantly increased serum 25(OH)D3 in untrained men (p = 0.003) but not in MMA athletes. In contrast, 6000 IU/day increased 25(OH)D3 in MMA athletes to a sufficient concentration (p = 0.001) and improved maximal power (+7.5%), mean power (+4.9%), and total work (+5.0%). Acute exercise increased circulating vitamin D metabolites in trained athletes but reduced them in untrained men supplemented with vitamin D. Conclusions: The efficacy of vitamin D3 supplementation and the acute exercise-induced vitamin D responses appear to be training-dependent. A daily dose of 6000 IU is more effective in achieving vitamin D sufficiency and performance benefits in MMA athletes, whereas 3500 IU is sufficient for untrained men.