Vitamin D Status and Supplementation and the Functional Outcomes of Human Musculoskeletal Tissues: A Stratified Systematic Review.

Background and Aim: This systematic review examined associations between vitamin D status and/or supplementation and functional outcomes (e.g., strength, performance, falls), with secondary reporting of biomechanical endpoints of human muscle, tendon, and bone. Methods: Following PRISMA guidelines, we reviewed human studies published from 1997 to 2024 that evaluated vitamin D supplementation/intake or serum 25-hydroxyvitamin D (25[OH]D) status in relation to functional outcomes and reported at least one biomechanical endpoint (e.g., strength-related properties, stiffness/tensile characteristics, bone mineral density [BMD], tendon outcomes). Eligible studies were English or Persian, conducted in humans, and included a vitamin D exposure and functional outcomes with biomechanical reporting. We excluded non-human and in vitro studies, conference abstracts without peer-reviewed full text, reviews, and studies lacking either vitamin D exposure or relevant functional/biomechanical outcomes. Results: Across eligible studies, higher vitamin D status and/or supplementation was generally associated with better muscle-related functional outcomes in older or clinical populations, particularly when baseline deficiency was present. Findings in athletic populations were mixed, with many studies showing null or inconsistent effects on strength and performance. Evidence suggested potential benefits for tendon repair in select contexts, and fracture risk reduction in specific populations; however, effects varied by baseline vitamin D status, participant characteristics, and outcome definitions. Large contemporary trials in broadly vitamin D-replete adults commonly reported no benefit for BMD or fracture outcomes. Conclusion: Vitamin D supplementation appears most beneficial in older/clinical groups with deficiency, with improvements observed in selected functional outcomes (notably balance/falls). In broadly replete adults, evidence does not support meaningful benefits for BMD or fractures, and in elite athletes, performance effects are inconsistent and often null. Overall, findings support correction of deficiency rather than universal supplementation for performance enhancement.