Vitamin D helps the body to absorb calcium. Its active form is the calcitriol, which acts binding to specific receptors, the VDR, present in most cells of the human body, including the cells in the skeletal muscles.
Skeletal muscles are one of the major muscular types in the body. They appear usually attached to bones, by the collagen fibres that we know as tendons.
Bone cells produce vitamin D from a precursor, the 25(OH)D, whose free levels in serum are used as an indicative of the body vitamin D status. The calcitriol stimulates protein synthesis in muscular tissue, increasing the number and diameter of type II fibre muscle cells. Consequently, it contributes to muscle contraction velocity and strength.
Vitamin D deficiency is associated with degeneration of type II muscle fibres, causing a loss of strength and decrease in physical performance. Therefore, the importance of adequate vitamin D levels for the athletic performance.
In sedentary subjects has been seen a correlation between 25(OH)D levels and VO2max, although results are not clear in professional athletes. It looks that at a certain fitness level there is no such relation between vitamin D levels and VO2max improvements. Its effects could be linked to an increase in the haemoglobin capacity to transport oxygen.
Additionally, vitamin D has also other important functions, affecting bone mineralization, hormones production and the normal function of the nervous, immune and cardiovascular systems.
Athletes have higher bone mineral density, or BMD levels, than sedentary people. Although it doesn´t mean that they are not susceptible to vitamin D deficiency. But the questions that arises is: Do we need vitamin D supplementation to improve our performance?
Most studies in the literature have been carried out in elderly or inactive subjects, where vitamin D supplementation lowered the risk of bone fractures and improved muscle performance. In athletes there is no clear consensus, although it could improve lower limbs muscle strength, but not affect muscle power of upper limbs or sprinting capacities. Maybe the effects seen in old subjects is because they already had low vitamin D levels.
Levels in the range of 20-30 ng/mL indicate vitamin D insufficiency, and deficiency when they are lower than 20 ng/mL. A meta-study of the literature found that as many as 56% of athletes had low vitamin D levels, especially in the winter season. Also, it was more common in areas of low insolation.
But there is also a genetic factor related to the vitamin D binding protein, or VDBP, whose gene varies greatly among racial groups.
As a conclusion, it looks that vitamin D supplementation is not useful in active and healthy individuals trying to improve its athletic performance. Another thing is when there is an insufficiency/deficiency of vitamin D levels, as often in the elderly, that as we commented have an important effect on health and performance. In this case vitamin D supplementation, could probably be recommended, after looking for a medical check-up.
More studies are needed to clarify the role of vitamin D in sports performance. Meanwhile eat healthy. If you want to increase your ingestion of vitamin D, you can try to include foods with high levels of vitamin D: fatty fish, like tuna, sardines or salmon; egg yolks, especially if eggs come from outdoor raised chicken, and foods fortified with vitamin D, such as some dairy products, certain cereals and orange juice among others.
Vitamin D, Skeletal Muscle Function and Athletic Performance in Athletes-A Narrative Review.
Książek A, Zagrodna A, Słowińska-Lisowska M
Nutrients. 2019 Aug 4;11(8). pii: E1800. doi: 10.3390/nu11081800.