Nutrition plays an essential role in the athletes’ performance and health. However, the focus is often placed on some variables such as energy consumption or adequate intake of macronutrients (proteins, carbohydrates, and fats), leaving out the important role of other elements such as vitamins.

 The origin of vitamin D

Vitamin D is one of the main vitamins and plays an important role for our immune and cardiovascular system as well as our muscles; its deficiency is associated with numerous pathologies such as cardiovascular, rheumatoid, muscle weakness or osteoporosis.¹ Vitamin D can be synthesised through the contact of ultraviolet-B (UVB) rays from the sun with the skin. These UVB rays convert the precursor 7-dehydrocholesterol, which is present in the skin, into D3 (cholecalciferol). Cholecalciferol can also be obtained from food (salmon, sardines, milk and fortified cereals, or egg yolk), increasing its absorption with foods high in fat. It is then when this molecule is transported to the liver to become 25(OH)D, the storage form of vitamin D, being able to be activated in the renal tubules to 1.25(OH)2D thanks to the parathyroid hormone.

 Vitamin D and athletes

Although it has been suggested that the recommendations regarding ‘optimal’ levels of vitamin D should be re-established, with the current recommendations, most of the population would present vitamin D deficiency, as confirmed by a systematic review that included studies performed in countries all around the world.² Athletes also seem to have generally low vitamin D values, although these may depend on the type of sport (lower in indoor sports), moment during the season (lower during the winter), the country (lower at higher latitudes) and skin type (less in people with dark skin).³ Low sun exposure, therefore, seems to be one of the main factors associated with vitamin D deficiency in athletes, although a study carried out in more than 500 elite Dutch athletes showed that less than 15 % met the intake recommendations for vitamin D.⁴

This data is particularly alarming in the field of sports performance, especially considering the important function of vitamin D at a multi-systemic level. For example, a study performed with American football players and published in the prestigious American Journal of Sports Medicine showed that athletes with lower amounts of vitamin D were more prone to suffer an injury, including bone fractures.⁵ Also, another study showed that low levels of vitamin D were associated with an increased risk of disease.⁶

That is why it has been suggested that vitamin D supplementation could be beneficial to prevent the deficiency of this micronutrient in athletes. In fact, vitamin D supplementation can reduce the number of athletes who take insufficient levels of vitamin D to less than 30 % – while without supplementation this number reaches up to 85 % -.⁴  Nevertheless, controversy exists as to whether vitamin D supplementation has a direct effect on a physical test performance. A study led by Dr. Graeme L. Close showed that vitamin D supplementation improved sprint and jump ability for athletes.⁷ However, the same research group showed in another study that, although vitamin D supplementation for 6 weeks was sufficient to increase the concentration of this micronutrient in all participants (half of them were vitamin D deficient before the study), it did not improve any performance variables.⁸  Similarly, two recent systematic reviews that included up to 13 studies have shown that, although supplementation is effective in reducing vitamin D deficiency in athletes, it does not induce significant improvements in muscle strength.^9,10 Nevertheless, another recent study showed that in cases of deficiency (as it occurs, for example, during winter), it could indeed be beneficial for sprint performance.¹¹

Conclusions

In summary, vitamin D is involved in important functions for the health of our organism, being relevant in the field of sports. However, evidence suggests that there is a high prevalence of vitamin D deficiency in athletes, and that it can have significant negative consequences at a multi-systemic level (ranging from a weakened immune system to an increased risk of injury). Vitamin D supplementation has been shown to be safe and effective in reducing the deficiency of this micronutrient in most athletes. For this reason, although there is not enough evidence to unanimously support the benefits of vitamin D supplementation on physical performance (e.g. strength or sprint tests), weighing the damages of having low levels of vitamin D (especially in the case of elite athletes who have to endure high training loads), and the absence of adverse effects with supplementation even at moderate/high doses, vitamin D supplementation may be recommended.

Pedro L. Valenzuela

References

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