Vitamin D is a fat soluble vitamin which is slightly more unusual than the others as its main source is not from the diet but from sunlight. With the exception of oily fish, a rich source of vitamin D3, few dietary sources of vitamin D are available.

Although Vitamin D is available from food sources, it is unlikely we will meet our Vitamin D requirements from food alone. The main source is from sunlight. The sun’s rays react with a protein within our skin to produce the active form of Vitamin D. Therefore, in the summer months, it is likely the general population will obtain sufficient Vitamin D from spending time outdoors. It is also important to note that at certain times of the year the sun is not high enough in the sky or strong enough so the skin can manufacture vitamin D. Another added problem for older individuals is as we age our skin also becomes less efficient at manufacturing the vitamin.

An inadequate vitamin D status has been associated with an increased risk of stress fracture, total body inflammation, infectious illness, impaired muscle function. It has a number of roles. It has been long known that it is an essential part of bone health along with calcium, but in more recent times research has focused on another couple of areas.

Supplementation with vitamin D may also enhance skeletal muscle function which is important not only for athletes but as we get older in maintaining muscle mass and our ability to carry out functions of everyday life as we get older.

The Role of Vitamin D

Bone Health

Vitamin D is essential to calcium metabolism, increasing calcium absorption for optimal bone health, which is relevant to all athletes, but particularly those participating in sports with a high risk of stress fracture.

Immunity

An important factor that can limit athletic performance is poor immune health. Infections reduce ability to train and compete. Vitamin D has long been known to improve immune health in a variety of populations. This could have important Research comparing individuals with sufficient levels to insufficient or deficient levels of 25(OH)D has shown that it helps to prevent injury, promote larger type II muscle fibre size, reduce inflammation, reduce risk of acute respiratory illness enhance functional rehabilitation, thereby optimizing recovery and acute adaptive responses to intense training through reduced inflammation and increased blood flow.

Muscle Functioning

Emerging evidence from integrative biology studies suggest that maintaining serum 25[OH]D concentrations > 50 nmol/L may be beneficial to maintain skeletal muscle and potentially to help further growth and remodelling. Supplementation of vitamin D-inadequate men with 4,000 IU D3/day has been shown to improve muscle force recovery following a high volume session of eccentric lower limb contractions (Owens et al., 2015; 2017).

There are implications for athletes who, for various reasons (high training load, making weight, restricting nutrients such as carbohydrates), can compromise their immune function. It has been shown that, supplemental vitamin D to increase serum 25[OH]D was sufficient to reduce infection risk during a 16- week period of winter training (He et al., 2013). 

Emerging evidence also suggests that oral spray vitamin D may provide an accelerated route of absorption compared with capsules and may be advantageous in those with gastrointestinal malabsorption (coeliac disease). Owing to the fat soluble nature of vitamin D, oral sprays containing this micronutrient typically contain a carrier substance as well as other fatty acids (such as α-tocopherol and oleic acid), which increases the ability of vitamin D into the into circulation around the body (Todd et al. 2016).

The interest in vitamin D research and its potential effects on body systems is evolving. The discussion around vitamin D during the Covid-19 pandemic has also added to the research questions and debate around it. It is clear that clinically low vitamin D concentrations are not helpful to many aspects of health for the general public and athletes but it may also influence athletic performance. Low level supplementation has been advised through the wintertime to avoid deficiencies.

He, C.S., M. Handzlik, W.D. Fraser, A. Muhamad, H. Preston, A. Richardson, and M. Gleeson (2013). Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes. Exerc. Immunol. Rev. 19:86-101.

Owens, D.J., D. Webber, S.G. Impey, J. Tang, T.F. Donovan, W.D. Fraser, J.P. Morton, and G.L. Close. (2014). Vitamin D supplementation does not improve human skeletal muscle contractile properties in insufficient young males. Eur. J. Appl. Physiol. 114:1309-1320.

Owens, D.J., A.P. Sharples, I. Polydorou, N. Alwan, T. Donovan, J. Tang, W.D. Fraser, R.G. Cooper, J.P. Morton, C. Stewart, and G.L. Close (2015). A systems-based investigation into vitamin D and skeletal muscle repair, regeneration, and hypertrophy. Am. J. Physiol. Endocrinol. Metab. 309:E1019-E1031.

Owens, D.J., J.C. Tang, W.J. Bradley, A.S. Sparks, W.D. Fraser, J.P. Morton, and G.L. Close (2017). Efficacy of high-dose vitamin D supplements for elite athletes. Med. Sci. Sports Exerc. 49:349-356.

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Todd JJ, McSorley EM, Pourshahidi LK, Madigan SM, Laird E, Healy M, Magee PJ. Vitamin D3 supplementation in healthy adults: a comparison between capsule and oral spray solution as a method of delivery in a wintertime, randomised, open-label, cross-over study. Br J Nutr. 2016 Oct;116(8):1402-1408.