Athletes prefer "D": it prevents injuries and can affect performance

Editorial note: This article was previously published and has been updated according to scientific and informative criteria. It is for informational purposes only and does not replace medical advice.

IN BRIEF

• Vitamin D is a fundamental hormone for regulating bone metabolism and also influences muscle function and inflammatory response.
• In athletes, 25(OH)D concentration shows clear seasonality with higher values in summer and lower values in winter.
• Randomized studies support a protective effect of calcium+vitamin D on stress fractures in high-load contexts (e.g., military recruits), while the effect on muscle performance is modest and depends on the baseline status.
• Supplementation can correct hypovitaminosis and improve some outcomes (e.g., lower limb strength or markers of muscle damage) if the baseline level is low; results are not uniform for all measures.

MAIN SECTION

Abstract: what does science say?

Vitamin D (measured as 25-hydroxyvitamin D in the blood) is essential for calcium homeostasis and bone health and has direct biological roles in muscle tissues and the immune system. Observational evidence associating lower 25(OH)D levels with a higher risk of injuries and muscle weakness is numerous; some randomized studies in specific contexts (e.g., military recruits) have shown that combined administration of calcium and vitamin D reduces the incidence of stress fractures. Meta-analyses on athletes indicate modest effects of supplementation on strength, especially of the lower limbs; outcomes vary according to baseline level, dose, and duration. Seasonality (summer peak and winter nadir) is a recurring factor; interpretation requires caution because much evidence is observational or comes from heterogeneous studies. [Summary of evidence: see Scientific Research for verified references.]

Vitamin D: definition and mechanisms relevant to sports

Vitamin D is produced in the skin under the action of UVB rays and transformed, via the liver and kidney, into the circulating form measured clinically (25-hydroxyvitamin D). In addition to its classic role in calcium regulation and bone formation, the vitamin D receptor (VDR) is also present in muscle tissue, suggesting direct mechanisms on the proliferation, differentiation, and function of muscle fibers. Recent reviews and syntheses describe how vitamin D can modulate gene activity in the myocyte, influence local energy metabolism, and participate in modulating the inflammatory response after intense exercise [1]. These biological mechanisms make a role for vitamin D plausible in reducing the risk of overuse injuries, improving strength, and accelerating muscle recovery after exercise-induced damage, although demonstrating consistent effects in healthy athletes requires more robust and homogeneous evidence [2].

Vitamin D and bone integrity: stress fractures and clinical trials

The relationship between vitamin D and the risk of stress fractures has been evaluated using different approaches. A randomized, controlled study on a large cohort of female Navy recruits showed that supplementation with calcium (2 g/d) and vitamin D3 (800 IU/d) during the training period reduced the incidence of stress fractures compared to placebo, a significant result in a context of high physical load and short follow-up [3]. Subsequent observational studies in the same population found that higher pre-diagnostic levels of 25(OH)D were associated with a lower risk of fracture, suggesting a plausible dose-response relationship [4]. These results support the hypothesis that, in conditions of rapid increase in workload (e.g., military recruitment, intense competitive phases), correcting hypovitaminosis can contribute to the prevention of overuse bone events. However, it remains crucial to distinguish different contexts: data on young and specific athletes (endurance or impact sports) are not automatically generalizable to other professional or amateur populations [3][4].

Vitamin D and muscle function: strength, power, and recovery

The relationship between vitamin D and muscle capacity has been explored in observational studies, randomized trials, and meta-analyses. Some meta-analyses on athletes show a specific benefit on lower limb strength, while the effect on upper limb strength or explosive power is less consistent [5]. Controlled trials indicate that supplementation increases serum 25(OH)D levels and, in subjects with low baseline levels, can improve some strength parameters or accelerate strength recovery after eccentric exercise or muscle damage [6]. On the other hand, other studies have not found significant differences in key performance tests, demonstrating that the effect depends strongly on the baseline status (insufficiency vs. sufficiency), the dose, the duration of supplementation, and the type of sport or measure evaluated [7]. The predominant synthesis is that correcting hypovitaminosis can provide muscular benefits in selected subgroups, but routine supplementation to improve performance in athletes already with adequate levels does not find consistent support [5][7].

Seasonality and monitoring: the circannual rhythm in athletes

Several studies conducted on professional athletes show a clear seasonality of 25(OH)D values, with higher average values in summer and a decrease in winter. A cohort of footballers and skiers monitored over the long term showed a significant circannual rhythm of vitamin D, also correlated with endocrine indicators such as testosterone and cortisol, suggesting that photoperiod and training load interact in influencing serum levels [8]. Surveys of university and professional groups confirm that athletes who train predominantly indoors or who live at latitudes with reduced UVB radiation during winter have a higher risk of insufficiency [9]. This temporal profile justifies the practice of monitoring levels at critical times of the year (late winter/early spring) to identify subjects at risk of hypovitaminosis and evaluate possible targeted correction [8][9].

Supplementation and clinical outcomes: what studies show

The randomized literature on athletes and high-risk populations reports heterogeneous results. Trials with moderate-to-high dosages (e.g., 2,850–5,000 IU/d for several weeks) generally normalize 25(OH)D in insufficient subjects, but the effect on clinical performance measures is variable [6]. Meta-analyses of RCTs specific to athletes indicate an improvement in lower limb strength but not a uniform effect on all performance measures [5]. In military contexts, the calcium+vitamin D combination has shown a reduction in stress fracture cases; however, applicability to different sports requires caution. Studies on exercise-induced muscle damage have found reductions in some markers of inflammation and pain perception in some trials, while others have not confirmed these results, indicating that methodological variability and setting influence the conclusions [3][10][6].

Observational evidence vs. causal evidence

It is important to distinguish between associations and causality. Observational studies document correlations between low 25(OH)D levels and a higher risk of injuries or poor performance, but they cannot alone prove that the deficiency is the direct cause. RCTs offer more robust evidence for causality but are often conducted in selected populations, with different outcomes and short follow-up. For this reason, the totality of the evidence must be interpreted probabilistically and contextually: most of the beneficial effects documented by experimentation are observed when supplementation corrects a true underlying deficiency [3][5].

DOMS and inflammation: what we know

Some controlled clinical studies have evaluated the effect of vitamin D on muscle damage induced by eccentric exercise and on inflammatory markers. Some RCTs have reported decreases in CK, IL-6, or reduced pain perception in treated groups compared to placebo, especially when supplementation started from insufficient levels; other research, however, has not shown clear benefits, so the issue remains open and probably dependent on dose, duration, and the population studied [10][6].

PRACTICAL SECTION

What it means in practice

For those who practice sports at a competitive or amateur level, the main practical implications are: (1) recognizing the seasonality of levels and measuring 25(OH)D at sensitive times of the year (e.g., late winter); (2) correcting hypovitaminosis with targeted interventions under clinical supervision, especially in athletes with risk factors (indoor training, dark skin, vitamin D-poor diet); (3) considering that supplementation can reduce the risk of some bone injuries in high-load contexts and can improve some muscle parameters if the baseline level is low, but it is not a guarantee of generalized performance enhancement; (4) avoiding excessive dosages without medical supervision, as hypervitaminosis D has potential risks. These indications are not prescriptive but aim to guide evaluations based on evidence and individual context [3][5][8][9].

KEY POINTS TO REMEMBER

• Vitamin D is a modulator of bones, muscles, and inflammatory response; the reference clinical measure is 25(OH)D.
• There is a marked seasonality in serum values: summer peak and winter minimum in many athletes. [8]
• Combined supplementation with calcium reduced stress fractures in a large RCT on female recruits. [3]
• Meta-analyses indicate a potential benefit on lower limb strength, but effects on other performance measures remain uncertain. [5]
• The decision to supplement should be based on laboratory monitoring and individual clinical evaluation.

LIMITATIONS OF EVIDENCE

Much of the available research is observational and therefore subject to confounding. Randomized trials also often differ in population, dose, duration, and measured outcomes, reducing the homogeneity of results. Some specific limitations: small samples in many RCTs on athletes; short follow-up; use of different thresholds to define insufficiency/sufficiency; combination of supplements (e.g., calcium + vitamin D) making it difficult to attribute the effect to a single nutrient. For these reasons, caution must be exercised in interpretation, and absolute conclusions on causation or universal recommendations cannot be drawn. Ultimately, the distinction between observational association and causal evidence remains central and necessitates further controlled and contextualized research [3][5][6].

Editorial conclusion

Vitamin D is a relevant biological factor for bone health and muscle function in athletes. The literature suggests that monitoring and correcting hypovitaminosis can decrease the risk of some bone injuries and improve some strength parameters, especially in athletes with low 25(OH)D levels. However, the benefits on overall performance are not universally confirmed and depend on the clinical context, dose, and duration of supplementation. Practical decisions must always be based on adequate measurements and specialist advice. The path to obtaining uniform indications involves larger, standardized randomized studies for dosage and outcomes, and targeted seasonal monitoring protocols.

EDITORIAL NOTE

This article updates previously published content based on international reviews and studies. The purpose is to provide scientifically sound and informative information: it does not replace individual medical advice. For clinical or therapeutic questions, consult a healthcare professional.

SCIENTIFIC RESEARCH

1. Lappe J, Cullen D, Haynatzki G, Recker R, Ahlf R, Thompson K. Calcium and vitamin D supplementation decreases incidence of stress fractures in female Navy recruits. Journal of Bone and Mineral Research. 2008;23(5):741–749. https://doi.org/10.1359/jbmr.080102
2. Burgi AA, Gorham ED, Garland CF, et al. High serum 25-hydroxyvitamin D is associated with a low incidence of stress fractures. Journal of Bone and Mineral Research. 2011;26(8):1819–1826. https://doi.org/10.1002/jbmr.451
3. Vitale JA, Lombardi G, Cavaleri L, et al. Circannual rhythm of plasmatic vitamin D levels and the association with markers of psychophysical stress in a cohort of Italian professional soccer players. Chronobiology International. 2017;34(4):525–534. https://doi.org/10.1080/07420528.2017.1297820
4. Larson-Meyer DE, Willis KS. Vitamin D and Athletes. Current Sports Medicine Reports. 2010;9(4):220–226. https://doi.org/10.1249/JSR.0b013e3181e7dd45
5. Zhang Q, Han Q, Cao Q, et al. Effect of vitamin D supplementation on upper and lower limb muscle strength and muscle power in athletes: a meta-analysis. PLoS ONE. 2019;14(4):e0215826. https://doi.org/10.1371/journal.pone.0215826
6. Han Q, Li X, Tan Q, Shao J, Yi M. Effects of vitamin D3 supplementation on serum 25(OH)D concentration and strength in athletes: a systematic review and meta-analysis of randomized controlled trials. Journal of the International Society of Sports Nutrition. 2019;16:55. https://doi.org/10.1186/s12970-019-0323-6
7. Abrams GD, Feldman D, Safran MR. Effects of Vitamin D on Skeletal Muscle and Athletic Performance. Journal of the American Academy of Orthopaedic Surgeons. 2018;26(8):278–285. https://doi.org/10.5435/JAAOS-D-16-00464
8. Halliday TM, Peterson NJ, Thomas JJ, Kleppinger K, Hollis BW, Larson‑Meyer DE. Vitamin D status relative to diet, lifestyle, injury, and illness in college athletes. Medicine & Science in Sports & Exercise. 2011;43(2):335–343. https://doi.org/10.1249/MSS.0b013e3181eb9d4d
9. Gilsanz V, Wren TA, Mo AO, Kremer A, Sarwark JF. Vitamin D status and its relation to muscle mass and muscle fat in young women. Radiology. 2010;254(1):116–121. https://doi.org/10.1148/radiol.2242011369
10. Žebrowska A, et al. The effect of vitamin D supplementation on the muscle damage after eccentric exercise in young men: a randomized controlled trial. Journal of the International Society of Sports Nutrition. 2020;17:53. https://doi.org/10.1186/s12970-020-00386-1