Vitamin D for Athletes: An Evidence-Based Guide to Performance, Injury Risk, and Clinical Interpretation

Introduction

Vitamin D status is frequently assessed in athletes, yet its interpretation remains a common source of uncertainty in clinical practice. Athletes often present with laboratory values that fall below population reference thresholds despite high training volumes, good general health, and no overt symptoms. This raises practical questions for clinicians and athletes alike: when does a low serum 25-hydroxyvitamin D level warrant intervention, and what outcomes can reasonably be expected from correction?

It is completely natural for an athlete to want to maximize their performance. Many athletes invest a lot in their diet and therefore also in supplements. Many of my athletic patients tell me that they take a large amount of vitamins every day. Many people have the question in their minds of whether it is worth taking a vitamin D supplement and how much. My answer to patients and those interested is that if there is no vitamin D deficiency, increasing vitamin D intake beyond normal requirements will not necessarily enhance performance.

Vitamin D for Athletes: Scope and Prevalence of Deficiency

The prevalence of vitamin D inadequacy among athletes highlights why vitamin D for athletes has become an important topic in sports medicine and performance-focused healthcare. A systematic review and meta-analysis examining 2,313 athletes found that 56% had vitamin D inadequacy (defined as serum 25-hydroxyvitamin D [25(OH)D] below 32 ng/mL (≈80 nmol/L)) [1]. More recent research confirms this remains a widespread problem, with prevalence estimates ranging from 30-56% depending on the population studied and the threshold used [2].

Important note on thresholds: Prevalence estimates depend heavily on the cut-off used. Studies variously define inadequacy as <32 ng/mL (<80 nmol/L), insufficiency as <50 nmol/L (<20 ng/mL), or deficiency as <20 ng/mL (<50 nmol/L). These different thresholds produce different prevalence figures and are not directly interchangeable across studies. For example, in Finland many laboratories report a lower reference limit of around 50 nmol/L (≈20 ng/mL). However, this value reflects the laboratory reference range derived from the general population and does not necessarily represent an optimal level for health or athletic performance.

The deficiency is particularly pronounced during winter months. In a meta-analysis of elite athletes using the 50 nmol/L threshold, approximately 30% demonstrated vitamin D insufficiency [2]. Indoor athletes face even greater risk, with studies showing significantly higher rates of inadequacy compared to outdoor athletes [1], reflecting differences in sun exposure during training.

Geographic location plays a critical role. Athletes training at latitudes ≥40°N showed a modest overall increased risk (RR 1.14), though this rose to RR 1.85 in a sensitivity analysis excluding the Middle East as an outlier [1]. In my clinical experience, vitamin D deficiency is particularly common in my home country Finland, where limited winter sunlight significantly restricts cutaneous vitamin D synthesis. That’s why vitamin D is usually included in our basic laboratory test packages. In addition, indoor sport activities carry a 19% increased risk compared to outdoor training [1].

Among para-athletes, the situation proves even more concerning. Research on elite wheelchair athletes reveals that 68.1% of samples taken throughout the year were considered insufficient, with winter prevalence reaching 74.1% [3].

Vitamin D for Athletes and Aerobic Performance

Vitamin D status demonstrates measurable associations with aerobic capacity in athletes. Studies on soccer players have documented significant correlations between serum 25(OH)D levels and VO2max, with one investigation reporting a correlation coefficient of 0.4192 (p = 0.0024) [4].

Some supplementation trials report improvements. One randomized controlled trial in Polish soccer players showed greater VO2max increases in the supplemented group (8.65 ± 3.57 mL/kg/min) compared to controls (5.03 ± 2.02 mL/kg/min, p = 0.021), along with improvements in physical working capacity-170 [4].

However, the evidence remains mixed. A recent network meta-analysis examining multiple supplements during training found that vitamin D showed limited, non-robust benefits for VO2max [5], and some well-designed controlled trials show no VO2max improvements despite successfully raising 25(OH)D levels. The variability in findings likely reflects differences in baseline vitamin D status—benefits appear most pronounced when correcting established deficiency rather than supplementing already-adequate athletes. 

Patients often ask me if it is worth taking an excess of vitamin D to maximize performance. In practice, taking excess vitamin D is not necessarily beneficial for performance, but only for correcting a deficiency. However, it is good to remember that at least in Finland, vitamin D is the most common vitamin that the general population is deficient in, assuming that the patient eats a mixed diet. Therefore, taking a vitamin D supplement is generally recommended. Among some of my vegetarian patients, I also occasionally encounter deficiencies in other vitamins, most notably vitamin B12.

Muscle Strength and Power

The relationship between vitamin D and muscle strength presents a complex picture. A systematic review examining one-repetition maximum (1RM) tests found that vitamin D3 was associated with positive impacts on muscle strength, with improvements ranging from 1.37% to 18.75% across studies [6].

Notably, one study of elite ballet dancers receiving 2,000 IU daily (≈50 µg/day) daily for four months demonstrated an 18.75% increase in isometric quadriceps contraction strength compared to controls (p = 0.030), while the control group showed negligible change of 0.32% [7].

A more recent meta-analysis of 318 athletes (166 supplemented, 152 control) found that while overall strength measurements did not reach statistical significance (SMD 0.18, 95% CI: −0.02 to 0.37, p = 0.08), there was a significant increase specifically in quadriceps contraction (SMD 0.57, 95% CI: 0.04 to 1.11, p = 0.04) [8].

Interestingly, evidence suggests differential effects on upper versus lower body strength. One meta-analysis highlighted that vitamin D supplementation significantly increased lower body muscle strength but not upper body strength or muscle power [9]. The physiological mechanism underlying this difference remains unclear.

In my experience, patients who participate in strength sports are generally very aware of vitamin D supplements. This group seems to be one of the most nutritionally aware patients I encounter. They are also often very aware of how vitamin D deficiency may negatively affect strength levels.

Stress Fractures and Bone Health

The connection between vitamin D status and stress fracture risk represents one of the most clinically significant findings. Stress fractures affect approximately 6.5-9.7% of athletes across different sport disciplines, with prevalence reaching 20% in some populations [10]. However, vitamin D status is only one of multiple risk factors for stress fractures, alongside training load, biomechanics, nutrition, and bone geometry.

A landmark study examining 802 NCAA Division I athletes demonstrated that correcting low serum vitamin D levels was associated with substantially reduced stress fracture risk. The rate of stress fracture was 12% higher in athletes who remained vitamin D deficient compared to those who improved their status to ≥40 ng/mL (≥100 nmol/L) [11]. Similarly, athletes with persistently low vitamin D showed a 12% higher stress fracture rate compared to those maintaining normal levels [11].

Even more striking, a pilot study of high-risk collegiate athletes found that vitamin D3 supplementation reduced stress fracture incidence from 7.51% to 1.65% (p = 0.009) [12]. Among cross-country runners specifically, the reduction proved statistically significant.

Research on fifth metatarsal stress fractures in athletes revealed that those with 25(OH)D levels of 10 ng/mL (≈25 nmol/L) faced 5.1 times higher risk, while those at 20 ng/mL (≈50 nmol/L) had 2.9 times higher risk compared to athletes with adequate levels [13].

The mechanism involves vitamin D’s essential role in bone mineralization and calcium regulation. Low serum vitamin D decreases intestinal calcium absorption, elevating parathyroid hormone levels and subsequently activating osteoclasts that break down bone collagen matrix [13].

The most common stress fractures in my patients are in the lower extremities, mostly in the foot and shin. However, from time to time, back pain in a young athlete raises the suspicion of a stress fracture. Stress fractures often show up quite poorly on X-rays, especially in the early stages, so I usually recommend MRI imaging due to its greater sensitivity, but also in young athletes because X-rays can cause unnecessary radiation exposure. I usually start vitamin D supplements for young people suffering from stress fractures if they are not already taking them. 

Immune Function and Respiratory Infections

Vitamin D’s immunomodulatory effects hold particular relevance for athletes, who face increased respiratory infection risk during intense training periods. A study of 225 endurance athletes over 16 weeks found that 38% had inadequate or deficient vitamin D status at baseline, increasing to 55% by study end [14].

An observational winter-training cohort reported that athletes with very low 25(OH)D (<30 nmol/L; <12 ng/mL) experienced more upper respiratory tract infection (URTI) symptom-days than those with higher levels (>120 nmol/L; >48 ng/mL). The deficient group experienced a median of 9 URTI symptom days compared to just 1 day in the optimal group (p = 0.040) [15]. Symptom severity scores followed similar patterns: deficient athletes scored 102 compared to 43 in optimal-status athletes (p = 0.013) [15].

A randomized controlled trial in taekwondo athletes demonstrated that 5,000 IU/day (≈125 µg/day) vitamin D3 supplementation during four weeks of winter training increased serum 25(OH)D by 255.6% and showed a negative correlation between vitamin D status changes and total URTI symptoms (r = −0.435, p = 0.015) [16].

The mechanism involves vitamin D’s regulation of both innate and adaptive immunity. Vitamin D deficiency correlates with lower salivary immunoglobulin A secretion and reduced pro-inflammatory cytokine production by monocytes and lymphocytes [17].

While these athlete-specific findings are suggestive, it’s worth noting that general-population meta-analyses on vitamin D and acute respiratory infections show more mixed results, and the evidence for universal respiratory infection prevention remains debated. However, several studies suggest that the association may be strongest in individuals with vitamin D deficiency, who appear to experience higher rates of respiratory symptoms compared with those with adequate vitamin D status [14][15][16].

For many athletic patients, illness interrupts the competition season. I often advise patients to take a vitamin D supplement to avoid deficiency and unnecessary illness, which may help support immune function. The most common diseases patients face are respiratory infections. Most are flu-like symptoms that go away on their own. However, occasionally patients require medical evaluation. 

Optimal Vitamin D Levels for Athletes

Interpretation of serum 25(OH)D depends on the cut-off used and the clinical context. Many athlete studies define insufficiency as <50 nmol/L (20 ng/mL), which is also the normal population reference limit in most laboratories in Finland, while others use higher thresholds such as <32 ng/mL, which materially changes prevalence estimates.

Sports medicine discussions increasingly reference athlete-specific targets, with some practitioners recommending serum levels above 32 ng/mL (80 nmol/L) and preferring levels above 40 ng/mL (100 nmol/L) [13]. The NCAA study on stress fractures used ≥40 ng/mL (100 nmol/L) as a meaningful threshold for bone health in that specific cohort, though this should not be interpreted as a universal “optimal performance” standard for all athletic outcomes [11].

Some evidence suggests potential benefits at levels between 40–60 ng/mL (100–150 nmol/L) for certain outcomes, though definitive performance-optimizing thresholds remain debated. For immune function specifically, one breakpoint for URTI risk appears around 95 nmol/L (38 ng/mL), with athletes below this threshold showing increased infection episodes [18].

Supplementation Protocols

So exactly how much vitamin D supplementation should you take? When deficiency is confirmed, vitamin D supplementation for athletes represents an effective intervention to restore physiological levels and reduce selected health risks. In sports medicine reviews and practice discussions, commonly used daily vitamin D3 doses for athletes fall in the 2,000–6,000 IU range (approximately 50–150 µg), adjusted to baseline level, season, and follow-up testing [13].

For athletes with established deficiency (<20 ng/mL; <50 nmol/L), higher initial doses may be warranted. One protocol uses 50,000 IU weekly (≈1,250 µg) for 8 weeks, followed by maintenance dosing [11]. For insufficiency (20–39 ng/mL; 50–97.5 nmol/L), 30,000 IU weekly (≈750 µg) for 8 weeks has been employed [11].

The tolerable upper limit set by the Institute of Medicine is 4,000 IU/day (≈100 µg/day) for individuals aged 9 years and older, though the no-observed-adverse-effects level extends to 10,000 IU/day (≈250 µg/day) [4]. Very high intermittent dosing may increase catabolic pathways (such as 24-hydroxylation), potentially blunting returns, though specific thresholds for this effect require further clarification.

A systematic review of supplementation studies in elite athletes found dosages ranging from 2,000–7,143 IU/day (≈50–179 µg/day), with an average of 4,959 IU/day (≈124 µg/day) [4]. In the included athlete trials, adverse events were rarely reported, with only one participant noting constipation across multiple studies, though this should not be interpreted as definitive evidence of universal safety at all doses. 

In my clinical experience, vitamin D overdose is extremely rare. Most guidelines consider 4,000 IU/day (≈100 µg/day) to be the tolerable upper intake level for long-term use, while doses up to 10,000 IU/day (≈250 µg/day) are generally not associated with adverse effects in healthy adults [19][20]. Reported cases of toxicity typically involve sustained intakes exceeding 40,000–100,000 IU/day (≈1,000–2,500 µg/day) over weeks or months [21]. Importantly, the clinical consequences of vitamin D toxicity are primarily related to hypercalcemia, which results from increased intestinal calcium absorption rather than a direct toxic effect of vitamin D itself [20][21].

Vitamin D3 (cholecalciferol) demonstrates superior efficacy compared to vitamin D2 (ergocalciferol). Studies show vitamin D2 less effective at improving muscle strength, while vitamin D3 produces more consistent benefits [6]. Vitamin D3 maintains higher serum concentrations for longer periods and more reliably produces beneficial outcomes [13].

One practical benefit of vitamin D supplementation that I find convenient is that you don’t have to take it every day, because as it is fat-soluble, it is stored in adipose tissue and the liver, so occasional forgetfulness is not harmful. Vitamin D is also relatively inexpensive in Finland and is available in grocery stores, so you don’t have to go to the pharmacy to get it.

Summary

Vitamin D plays an important role in bone health, muscle function, and immune regulation, which explains why it has attracted increasing attention in sports medicine. Research shows that low vitamin D levels are relatively common among athletes, particularly during winter months, at northern latitudes, and in athletes who train primarily indoors. While vitamin D status has been associated with aerobic capacity, muscle strength, and reduced risk of stress fractures, the overall evidence suggests that supplementation is most beneficial when correcting a true deficiency rather than enhancing performance in already sufficient individuals.

From my clinical perspective, vitamin D assessment should be interpreted in context. Athletes with low levels may benefit from supplementation, particularly when deficiency is confirmed or when risk factors such as limited sun exposure are present. Adequate vitamin D levels may support bone health, reduce the risk of stress fractures, and potentially contribute to better immune resilience during periods of intense training. However, vitamin D supplementation should not be viewed as a universal performance-enhancing strategy.

In practice, maintaining adequate vitamin D status through sensible supplementation is often reasonable, especially in northern countries my home country Finland where sunlight exposure is limited for much of the year. At the same time, clinicians should recognize that vitamin D is only one piece of the broader picture of athletic health, which also includes appropriate training load, nutrition, recovery, and overall medical care.

References

1. https://pubmed.ncbi.nlm.nih.gov/25277808/
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9596536/
3. https://sportsmedicine-open.springeropen.com/articles/10.1186/s40798-024-00756-y
4. https://journals.sagepub.com/doi/10.1177/23259671231220371
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC12663695/
6. https://pubmed.ncbi.nlm.nih.gov/27379960/
7. https://www.sciencedirect.com/science/article/abs/pii/S1440244013000558
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC11163122/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC10570740/
10. https://www.mdpi.com/1648-9144/57/3/223
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC8008136/
12. https://www.dovepress.com/vitamin-d3-supplementation-and-stress-fractures-in-high-risk-collegiat-peer-reviewed-fulltext-article-ORR
13. https://pmc.ncbi.nlm.nih.gov/articles/PMC8342187/
14. https://pubmed.ncbi.nlm.nih.gov/23977722/
15. https://digitalcommons.wku.edu/ijesab/vol10/iss1/75/
16. https://pmc.ncbi.nlm.nih.gov/articles/PMC6164435/
17. https://pmc.ncbi.nlm.nih.gov/articles/PMC9539769/
18. https://pmc.ncbi.nlm.nih.gov/articles/PMC5790847/
19. https://www.researchgate.net/publication/50891070_The_2011_Dietary_Reference_Intakes_for_Calcium_and_Vitamin_D_What_Dietetics_Practitioners_Need_to_Know
20. https://www.ncbi.nlm.nih.gov/books/NBK56070/
21. https://pmc.ncbi.nlm.nih.gov/articles/PMC3046611/