Vitamin D and the Athlete: Current Perspectives and New Challenges

Abstract

<h2>Abstract</h2> <p><a href="/terms/vitamin-d/" class="term-link" data-slug="vitamin-d" title="Vitamin D">Vitamin D</a>, a fat-soluble secosteroid hormone, has received escalating attention in sports medicine due to its multifaceted role in musculoskeletal health, immune function, and potentially athletic performance. The present review critically examines the prevalence of vitamin D insufficiency in athletic populations, the mechanistic basis for its effects on skeletal muscle, and the evidence for supplementation as a performance-enhancing intervention.</p> <p>Epidemiological data consistently indicate that vitamin D insufficiency (serum 25-hydroxyvitamin D [25(OH)D] 50 nmol/L) is surprisingly prevalent among athletes, with reported rates of 30–80% depending on geographic location, skin pigmentation, and season. Indoor athletes and those training at high latitudes are at particular risk [1]. The consequences of deficiency extend beyond bone health to include impaired <a href="/terms/type-ii-muscle-fiber/" class="term-link" data-slug="type-ii-muscle-fiber" title="type II <a href="/terms/muscle-fiber/" class="term-link" data-slug="muscle-fiber" title="muscle fiber">muscle fiber</a>">type II muscle fiber</a> function, reduced maximal strength and power, altered immune competence, and elevated injury risk.</p> <p>Observational and interventional data suggest that optimizing serum 25(OH)D to 75–125 nmol/L may support improvements in muscle function, testosterone synthesis, and inflammatory regulation. However, the evidence base for a direct performance-enhancing effect in already-sufficient athletes remains inconclusive, and supplementation studies in insufficient athletes have produced the most compelling results [2].</p> <p>This review identifies key knowledge gaps and practical challenges, including variability in assay methodology, the importance of baseline status stratification in future trials, and the need for individualized supplementation approaches. Recommendations for monitoring and supplementation in athletic populations are provided.</p> <h3>References</h3> <p>[1] Owens DJ, Allison R, Close GL. Vitamin D and the athlete. <em>Sports Med</em>. 2018;48(Suppl 1):3–16.</p> <p>[2] Dahlquist DT, Dieter BP, Koehle MS. Plausible ergogenic effects of vitamin D on athletic performance and recovery. <em>J Int Soc Sports Nutr</em>. 2015;12:33.</p>

Introduction

<h2>Introduction</h2> <p>For much of the twentieth century, <a href="/terms/vitamin-d/" class="term-link" data-slug="vitamin-d" title="vitamin D">vitamin D</a> was conceptualized primarily as a regulator of calcium and phosphate homeostasis, essential for the mineralization of bone and the prevention of rickets and osteomalacia. The discovery of the vitamin D receptor (VDR) in virtually every tissue of the body, including skeletal muscle, cardiac muscle, immune cells, and the brain, has fundamentally revised this understanding [1]. It is now recognized that vitamin D functions as a pleiotropic hormone, influencing gene expression in hundreds of tissues and participating in the regulation of cell differentiation, apoptosis, immunomodulation, and metabolic function.</p> <p>The primary source of vitamin D in humans is cutaneous synthesis from 7-dehydrocholesterol upon exposure to ultraviolet B (UVB) radiation. Dietary sources—oily fish, fortified dairy products, and egg yolks—contribute meaningfully but typically insufficiently to maintain adequate serum status without regular sun exposure. Vitamin D undergoes sequential hydroxylation in the liver (to 25(OH)D, the principal circulating form used to assess status) and in the kidney (to 1,25(OH)₂D, the biologically active form) [2].</p> <p>For the athletic community, vitamin D has gained attention on multiple fronts. The high prevalence of insufficiency even in elite athletes suggests that this population is not immune to the inadequate sun exposure and dietary patterns that characterize modern life. Moreover, the VDR's presence in skeletal muscle and its role in regulating genes associated with myogenesis, fiber type composition, and contractile function raises the possibility that vitamin D status directly modulates athletic capacity [3].</p> <p>The present review was undertaken to synthesize current knowledge regarding the consequences of vitamin D status for athletic populations, the mechanisms underpinning any relationship between vitamin D and muscle function, and the practical considerations for monitoring and supplementation in this context.</p> <h3>References</h3> <p>[1] Holick MF. Vitamin D deficiency. <em>N Engl J Med</em>. 2007;357(3):266–281.</p> <p>[2] Hossein-nezhad A, Holick MF. Vitamin D for health: a global perspective. <em>Mayo Clin Proc</em>. 2013;88(7):720–755.</p> <p>[3] Hamilton B. Vitamin D and human skeletal muscle. <em>Scand J Med Sci Sports</em>. 2010;20(2):182–190.</p>

Vitamin D and Muscle Function

<h2><a href="/terms/vitamin-d/" class="term-link" data-slug="vitamin-d" title="Vitamin D">Vitamin D</a> and Muscle Function</h2> <h3>Genomic Mechanisms</h3> <p>The vitamin D receptor (VDR) is expressed in skeletal muscle and acts as a ligand-activated transcription factor following binding by 1,25-dihydroxyvitamin D. Upon activation, the VDR-ligand complex translocates to the nucleus and modulates the transcription of target genes relevant to muscle physiology. Among the VDR's reported genomic targets are genes involved in myosin heavy chain isoform expression, satellite cell proliferation, and mitochondrial function [1]. Muscle biopsies from vitamin D-deficient individuals have shown altered fiber type distributions, with a disproportionate reduction in type II (fast-twitch) fiber <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a>—the fiber type most critical for explosive power and high-force production.</p> <h3>Non-Genomic Mechanisms</h3> <p>In addition to transcriptional regulation, 1,25-dihydroxyvitamin D activates rapid, non-genomic signaling cascades through membrane-associated VDRs. These include stimulation of mitogen-activated protein kinase (MAPK) pathways, modulation of intracellular calcium flux, and activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling—a pathway central to <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> and <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> [2]. These non-genomic pathways may partly explain the relatively rapid improvements in muscle function observed in some supplementation studies, which precede the slower timescale of genomic-mediated effects.</p> <h3>Observational Evidence</h3> <p>Cross-sectional studies in both general and athletic populations have documented <a href="/terms/concentric-contraction/" class="term-link" data-slug="concentric-contraction" title="positive">positive</a> associations between serum 25(OH)D concentrations and measures of muscular strength, power, and physical performance. Vitamin D insufficiency has been associated with increased odds of muscle weakness, elevated biomarkers of <a href="/terms/muscle-damage/" class="term-link" data-slug="muscle-damage" title="muscle damage">muscle damage</a>, and greater susceptibility to musculoskeletal injury, including stress fractures [3]. In elite ballet dancers and professional footballers, lower vitamin D status was associated with more frequent musculoskeletal injuries over the course of a competitive season.</p> <h3>References</h3> <p>[1] Ceglia L, Harris SS. Vitamin D and its role in skeletal muscle. <em>Calcif Tissue Int</em>. 2013;92(2):151–162.</p> <p>[2] Bischoff-Ferrari HA. Relevance of vitamin D in muscle health. <em>Rev Endocr Metab Disord</em>. 2012;13(1):71–77.</p> <p>[3] Lappe J, et al. Calcium and vitamin D supplementation decreases incidence of stress fractures in female navy recruits. <em>J Bone Miner Res</em>. 2008;23(5):741–749.</p>

Considerations for Athletes

<h2>Considerations for Athletes</h2> <h3>Prevalence of Insufficiency in Athletic Populations</h3> <p>Despite expectations that physically active individuals with outdoor training habits might maintain adequate <a href="/terms/vitamin-d/" class="term-link" data-slug="vitamin-d" title="vitamin D">vitamin D</a> status, epidemiological surveys consistently reveal high prevalence of insufficiency and deficiency among competitive athletes. A <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> of vitamin D status in athletes reported that approximately 56% of athletes had 25(OH)D concentrations below 75 nmol/L, with the highest rates of deficiency observed in athletes training predominantly indoors (gymnasts, wrestlers, swimmers, basketball players) and those with darker skin pigmentation [1].</p> <p>Seasonal variation is a critical confounding factor, with vitamin D status typically lowest in late winter and early spring in temperate latitudes. Athletes who train indoors year-round or who use sunscreen consistently during outdoor training may fail to generate adequate cutaneous vitamin D even in sun-rich environments.</p> <h3>Impact on Injury Risk and Immune Function</h3> <p>Beyond direct effects on muscle performance, vitamin D status influences athletic health through several additional pathways. Vitamin D is a key regulator of immune function, modulating innate and adaptive immune responses and supporting mucosal barrier integrity. Athletes in heavy training blocks experience transient immunosuppression, and sufficient vitamin D status may attenuate this vulnerability to upper respiratory tract infections during periods of high training load [2].</p> <p>The role of vitamin D in bone health is of particular relevance to athletes in weight-bearing sports and those with high rates of stress fracture. Vitamin D insufficiency impairs intestinal calcium absorption and increases parathyroid hormone secretion, accelerating bone resorption and potentially compromising bone mineral density over time.</p> <h3>Assessment Challenges</h3> <p>Accurate assessment of vitamin D status requires measurement of serum 25(OH)D by validated assay methods. However, considerable variability exists between commercially available assay platforms, complicating both individual monitoring and cross-study comparisons. The optimal target range for athletes may also differ from that established for general health, warranting sport medicine-specific guidance [3].</p> <h3>References</h3> <p>[1] Farrokhyar F, et al. Prevalence of vitamin D inadequacy in athletes. <em>Sports Med</em>. 2015;45(3):365–378.</p> <p>[2] He CS, et al. Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes. <em>Exerc Immunol Rev</em>. 2013;19:86–101.</p> <p>[3] Close GL, et al. Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK. <em>J Sports Sci</em>. 2013;31(4):344–353.</p>

Practical Guidelines

<h2>Practical Guidelines</h2> <h3>Testing and Monitoring</h3> <p>Routine assessment of serum 25(OH)D concentration is recommended for all competitive athletes, particularly those at elevated risk of insufficiency. Testing should ideally be performed at the end of winter (when status is likely at its nadir) and at the end of summer (when status is typically highest) to characterize seasonal variation for individual athletes. Athletes with deficiency (25(OH)D 50 nmol/L) or insufficiency (50–75 nmol/L) should be considered for supplementation regardless of their primary sport [1].</p> <h3>Supplementation Dosing</h3> <p><a href="/terms/vitamin-d/" class="term-link" data-slug="vitamin-d" title="Vitamin D">Vitamin D</a>₃ (cholecalciferol) is the preferred supplemental form due to its greater efficacy in raising and sustaining serum 25(OH)D compared to vitamin D₂ (ergocalciferol). For athletes with confirmed insufficiency, supplementation with 1,000–4,000 IU/day of vitamin D₃ is generally appropriate and safe, with higher doses (up to 10,000 IU/day for limited periods) sometimes warranted in cases of frank deficiency under medical supervision [2].</p> <p>Supplementation should ideally be taken with the largest meal of the day, as the fat-soluble nature of vitamin D enhances absorption in the presence of dietary fat. Response to supplementation should be confirmed by re-testing serum 25(OH)D after 8–12 weeks.</p> <h3>Target Serum Concentrations</h3> <p>For athletes, a target serum 25(OH)D of 75–125 nmol/L is widely recommended, representing a level at which most musculoskeletal and immune benefits are likely to be realized without increased risk of hypercalcemia or other adverse effects. Concentrations above 250 nmol/L have been associated with potential toxicity, though this threshold is rarely approached with recommended supplementation doses [3].</p> <h3>Practical Strategies Beyond Supplementation</h3> <p>Safe sun exposure (10–30 minutes of midday sunlight on unprotected skin) during UVB-rich months can contribute meaningfully to vitamin D status. Dietary optimization through consumption of oily fish, eggs, and fortified foods may provide additional incremental benefit, though dietary sources alone are rarely sufficient to achieve target concentrations.</p> <h3>References</h3> <p>[1] Owens DJ, et al. Vitamin D and the athlete. <em>Sports Med</em>. 2018;48(Suppl 1):3–16.</p> <p>[2] Heaney RP. Guidelines for optimizing design and analysis of clinical studies of nutrient effects. <em>Nutr Rev</em>. 2014;72(1):48–54.</p> <p>[3] Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. <em>Am J Clin Nutr</em>. 1999;69(5):842–856.</p>