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Vitamin D3 | Benefits, Deficiencies, Guidance 

Overview of Vitamin D and Synthesis 

Science in Sport Vitamin D3 is formulated with 5000iu vitamin D3 per tablet. Vitamin D is a fat-soluble vitamin with two compounds in this group, ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3).  

Vitamins D2 and D3 can be thought of as prohormones, in that they have no significant bioactivity until converted into their active form (1,25[OH]D). 

The beneficial effects of vitamin D depend on the subsequent availability of blood 25[OH]D*; therefore, the goal of supplementation should be to correct any deficiencies and raise blood 25[OH]D levels to normal values. 

Vitamin D3 supplementation is thought of to be more effective at raising blood 25[OH]D levels compared to vitamin D2 (1), meaning that this specific form is the one to look for when choosing a supplement. 

  *25[OH]D is what vitamin D2 and D3 are first converted to in the liver 

Benefits of Vitamin D3 | Why Supplement with Vitamin D3? 

Through exposure to the sun and synthesis via the skin, our body can naturally produce vitamin D3. However, vitamin D3 deficiency is common in both the general public and athletic populations (2) 

This may be due to:

  • Living at far northern or southern latitudes 
  • Training indoors 
  • Wearing protective clothing 
  • The application of sun-screen 
  • Consciously avoiding sun exposure 

All of these factors reduce the sun exposure required for the body to synthesize vitamin D3, therefore it may be beneficial to supplement it.  

Supplementing with vitamin D3 has been linked to a number of health benefits such as: 

  • Optimising muscle function 
  • Remodelling, maintaining bone health  
  • Strengthening the immune system and minimizing infection risk 


Muscle Function 

 One of the functions of training is to provide a stimulus to the working muscle, causing stress on the muscle and muscular damage. The process of repairing and remodelling the muscle results in beneficial adaptations and improved physical performance. 

It has been shown that supplementing with vitamin D3 has the potential to influence muscle remodelling (3). Additionally, it has been suggested that vitamin D deficiency has a negative impact on muscular performance (4) and improved exercise performance has been seen alongside vitamin D3 supplementation (4, 5). 


Bone Health 

Vitamin D supports calcium absorption and utilisation with the body, meaning that vitamin D supplementation may have a role to play in bone health. Bone is an active tissue in the body and high-impact, loading type activities provide the stimulus to maintain bone health, promoting peak bone mineral density. However, athletes undertaking non-weight-bearing exercise, such as cycling, are at an increased risk of low bone mineral density and subsequent poor bone health, with proposed beneficial effects of vitamin D supplementation on bone density (6). 


Immune Function 

Vitamin D also plays an important role in the immune system, with the activation of some immune cells being regulated by circulating vitamin D levels (3) 

Exercise can increase the chances of illness and the prevalence of upper respiratory tract infections (URTI) is higher in overreached endurance athletes (7).   

In endurance trained populations, vitamin D deficiencies resulted in more episodes of upper respiratory tract infections and more URTI symptom days (8). Additionally, vitamin D3 supplementation has been shown to prevent decreases in IL-5 compared to a placebo trial (9). Given that IL-5 is a cell of the immune system which protects against viruses and bacterial infections, this finding supports the notion that vitamin D3 may play a role in immune function support. 


Vitamin D at a glance:

  • Vitamin D is a fat-soluble vitamin and vitamin D3 can be synthesised through exposure to sun or supplemented in the diet 
  • As sun exposure is key in vitamin D3 synthesis seasonal deficiencies maybe evident. Particular attention should be paid to athletes training indoors, in far northern latitudes or during winter months
  • Vitamin D plays a role in muscle repair and remodelling, bone health and immune function
  • Athletes often experience low vitamin D levels and benefits of supplementation are seen when correcting deficiencies, with dosages in the range of 2000-5000iu per day used in the cited research 


  1. Friedl, C., & Zitt, E. (2017). Vitamin D prohormone in the treatment of secondary hyperparathyroidism in patients with chronic kidney disease. International journal of nephrology and renovascular disease10, 109.
  2. Close, G. L., Leckey, J., Patterson, M., Bradley, W., Owens, D. J., Fraser, W. D., & Morton, J. P. (2013). The effects of vitamin D3 supplementation on serum total 25 [OH] D concentration and physical performance: a randomised dose–response study. Br J Sports Med47(11), 692-696.
  3. Owens, D. J., Allison, R., & Close, G. L. (2018). Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports Medicine, 1-14.
  4. Close, G. L., Russell, J., Cobley, J. N., Owens, D. J., Wilson, G., Gregson, W., … & Morton, J. P. (2013). Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function. Journal of sports sciences31(4), 344-353.
  5. Wyon, M. A., Koutedakis, Y., Wolman, R., Nevill, A. M., & Allen, N. (2014). The influence of winter vitamin D supplementation on muscle function and injury occurrence in elite ballet dancers: a controlled study. Journal of science and medicine in sport17(1), 8-12.
  6. Neal, S., Sykes, J., Rigby, M., & Hess, B. (2015). A review and clinical summary of Vitamin D in regard to bone health and athletic performance. The Physician and sportsmedicine43(2), 161-168.
  7. Hausswirth, C., Louis, J., Aubry, A., Bonnet, G., Duffield, R., & Le Muer, Y. (2014). Evidence of disturbed sleep patterns and increased illness in functionally overreached endurance athletes. Medicine and science in sports and exercise.
  8. He, C. S., Handzlik, M. K., Fraser, W. D., Muhamad, A. S., Preston, H., Richardson, A., & Gleeson, M. (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.
  9. Barker, T., Martins, T. B., Hill, H. R., Kjeldsberg, C. R., Henriksen, V. T., Dixon, B. M., … & Weaver, L. K. (2012). Different doses of supplemental vitamin D maintain interleukin-5 without altering skeletal muscle strength: a randomized, double-blind, placebo-controlled study in vitamin D sufficient adults. Nutrition & metabolism9(1), 16.

Written By

Ben Samuels

Ben is a Performance Nutritionist at Science in Sport