Vitamin D deficiency in athletes: causes and links with training load and pharmacology
Vitamin D is not just a 'vitamin for bones'. It acts as a hormone that affects calcium metabolism, muscle function and immunity. Paradoxically, deficiency is often found even in well-trained athletes. The editorial team explains which features of sporting life lower vitamin D levels and how certain drugs affect it.
How vitamin D is formed and works
The main source of vitamin D for humans is synthesis in the skin under the action of ultraviolet B. From 7-dehydrocholesterol, cholecalciferol (D3) is formed, which in the liver is converted into 25-hydroxyvitamin D - it is exactly this that is measured in a test. The active form, calcitriol, is formed mainly in the kidneys.
Calcitriol binds to the vitamin D receptor (VDR) and regulates the absorption of calcium and phosphorus in the intestine. VDR receptors are also found in muscle and immune cells, which explains the interest of sports science in this vitamin.
Little vitamin D comes from food: fatty fish, egg yolks, fortified products, mushrooms grown under UV light. For most people without supplements, diet does not cover the need in the absence of sufficient sun.
In severe deficiency, adults develop osteomalacia - a disturbance of bone mineralization - as well as secondary hyperparathyroidism, which increases bone loss. Milder deficiency is linked with an increased risk of stress fractures and, possibly, worse muscle function.
Why athletes often have a deficiency
A systematic review and meta-analysis by Farrokhyar and co-authors (2015) showed that insufficient vitamin D levels in athletes occur often - in a significant proportion of those examined. The frequency was higher in winter and spring, in indoor sports and at high latitudes.
The main cause is a lack of sun. Gymnasts, basketball players, volleyball players, hockey players, swimmers in indoor pools and combat sport athletes train mainly indoors. Early and evening training, clothing and sunscreen at outdoor sessions also limit synthesis.
Geography is of key importance. At the latitudes of Ukraine in the cold season the sun is too low, and there is almost not enough ultraviolet B for synthesis in the skin. During this period the level depends on the stores accumulated over the summer and on nutrition.
Darker skin contains more melanin, which absorbs ultraviolet. Therefore athletes with dark skin who live and train at high latitudes need to spend more time in the sun for the same synthesis, and their risk of deficiency is higher.
Body composition, nutrition and energy deficiency
Vitamin D is fat-soluble and is partly deposited in adipose tissue. In people with obesity the level of 25(OH)D in the blood is on average lower, and the need for supplements is higher. For strength-sport athletes with a large body mass, for example super-heavyweight weightlifters, this is relevant.
Restrictive diets, in particular vegan ones without fortified products, as well as low-fat diets during 'cutting', reduce the intake and absorption of fat-soluble vitamins. In athletes with low energy availability, vitamin D deficiency is often combined with a disturbance of bone metabolism.
- training indoors or in early/late hours;
- living at high latitudes, winter season;
- dark skin;
- excess fat mass;
- restrictive diets and low energy availability;
- diseases of the intestine, liver, kidneys.
Intestinal diseases with impaired absorption - celiac disease, inflammatory bowel diseases - as well as stomach surgeries reduce the absorption of vitamin D. Liver and kidney diseases impair its conversion into active forms.
So deficiency in an athlete is not only a question of sun. Often it is a combination of factors that should be assessed together, especially if there is a history of stress fractures.
Drugs that affect vitamin D
Glucocorticoids with prolonged systemic use reduce calcium absorption and disrupt vitamin D metabolism, and also directly suppress bone tissue formation. Athletes receive them for injuries and asthma; systemic use in the competition period is regulated by WADA rules.
Anticonvulsants (phenytoin, carbamazepine, phenobarbital) and rifampicin accelerate the hepatic metabolism of vitamin D, lowering its level. The antifungal ketoconazole and some antiretroviral drugs also affect this metabolism.
| Drug or group | Effect on vitamin D / bones |
|---|---|
| Systemic glucocorticoids | Less calcium absorption, suppression of osteoblasts |
| Anticonvulsants, rifampicin | Accelerated breakdown of vitamin D in the liver |
| Orlistat, bile acid sequestrants | Reduced absorption of fat-soluble vitamins |
| Ketoconazole | Suppression of vitamin D activation enzymes |
Orlistat, cholestyramine and other agents that reduce fat absorption lower the assimilation of fat-soluble vitamins. They are sometimes used for weight loss, and in such cases the doctor usually takes into account the need for supplements.
Regarding anabolic steroids and weight-loss drugs, which are widespread in bodybuilding, there are no reliable data on a direct effect on vitamin D. However, their frequent combination with extreme diets and indoor training makes monitoring the level appropriate.
Does deficiency affect results
The link of vitamin D with bones is proven best. A low level is associated with a higher risk of stress fractures, especially in recruits and runners. Studies among military personnel showed that calcium and vitamin D supplements can reduce the frequency of stress fractures.
Regarding strength, power and endurance the data are contradictory. Some small studies showed an improvement in muscle function after correcting a pronounced deficiency, but in people with a normal level additional doses gave no advantages.
Immune function is another area of interest. Meta-analyses point to a small reduction in the risk of acute respiratory infections with daily supplement intake, especially in people with a deficiency, although the magnitude of the effect is debated.
The IOC consensus on supplements classifies vitamin D among agents that may be useful to athletes in case of deficiency, and recommends determining the need by a test rather than taking it 'just in case' in large doses.
Editorial conclusions
Vitamin D deficiency in athletes is a common phenomenon, linked primarily with a lack of sun: indoor training, the winter season, high latitudes and dark skin.
Body composition, restrictive diets, intestinal diseases and some drugs (glucocorticoids, anticonvulsants, orlistat) additionally lower the level.
The best-proven effect of deficiency is on bone health and the risk of stress fractures. One should not expect a 'strength gain' from supplements in people with a normal level.
We also advise reading our articles on the prevention and diagnosis of vitamin D deficiency, on stress fractures in athletes and on calcium in an athlete's diet.
References
- Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930.
- Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med. 2018;48(Suppl 1):3–16.
- Farrokhyar F, Tabasinejad R, Dao D, et al. Prevalence of vitamin D inadequacy in athletes: a systematic-review and meta-analysis. Sports Med. 2015;45(3):365–378.
- Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med. 2018;52(7):439–455.
- Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947.
- Buckley L, Guyatt G, Fink HA, et al. 2017 American College of Rheumatology guideline for the prevention and treatment of glucocorticoid-induced osteoporosis. Arthritis Rheumatol. 2017;69(8):1521–1537.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
