Vitamin D Supplementation and Follow-Up: Comprehensive Monitoring of Blood Markers and Performance in Runners and Non-Runners During Autumn and Winter

Introduction
Vitamin D is increasingly recognized for its roles in musculoskeletal and immune health, yet evidence on its functional impact in physically active versus sedentary populations remains limited.

Methods
We conducted a randomized trial in 47 healthy adults, stratified by physical activity (runners vs. non-runners) and supplementation status (2000 IU/day vitamin D via orodispersible films vs. no supplementation). Participants were assessed at baseline (October, T0), after 2 months of intervention (T1), and 3 months post-supplementation (T2). Outcomes included serum 25-hydroxyvitamin D [25(OH)D₃] levels, leukocyte counts, maximal oxygen consumption (VO₂max), maximal isometric force (MIF), and counter-movement jump (CMJ) performance. Diet, training load, solar irradiation, and ambient temperature were monitored longitudinally. An attempt was made to determine if gender influenced vitamin D levels.

Results
At T0, runners exhibited slightly insufficient but higher 25(OH)D₃ levels than non-runners, reflecting greater outdoor exposure. At T1, supplementation increased 25(OH)D₃ by 20.82% in runners and 28.78% in non-runners, whereas non-supplemented runners remained stable and non-supplemented non-runners decreased by 32.23%. At T2, all groups experienced significant declines, with non-supplemented non-runners approaching deficiency. Neutrophil counts fell in non-supplemented groups, while VO₂max and CMJ performance were unaffected; MIF showed modest improvement in supplemented subjects. Gender did not influence vitamin D level.

Conclusions
Two months of vitamin D supplementation effectively improved 25(OH)D₃ status and stabilized leukocyte counts in both runners and non-runners; however, discontinuation resulted in rapid declines. Despite correction of vitamin D insufficiency, no ergogenic benefits were observed in aerobic capacity or explosive power, and the modest gain in muscle strength warrants further investigation.