Vitamin D is produced in the skin in response to sunlight, and can also be obtained in smaller quantities from the diet. Vitamin D has many roles in the body: it helps bones become (and stay) strong, muscles contract properly, blood clot, and the immune system work correctly.
Based on this, it certainly seems plausible that a lack of vitamin D in the body might impair sports performance. If muscles cannot work properly an athletes speed, power and / or endurance may be impacted. Or, if an athlete becomes ill because their immune system is weakened they may not fulfil their performance potential due to missed training days.
Some degree of vitamin D deficiency is thought to exist in most northern hemisphere populations, particularly in winter, due to lack of sunlight. In fact, the UK government recommends healthy adults supplement with 10ug of vitamin D daily between October and March. This has driven research into whether vitamin D supplementation can improve sports performance, either by correcting vitamin D deficiency or by providing some additional benefit even in individuals who are not vitamin D deficient.
To see if giving an individual ‘extra’ vitamin D can improve sports performance, studies have been performed where sports performance has been compared between individuals who have been given ‘extra’ vitamin D (either by an oral vitamin D tablet or by giving them exposure to mock sunlight to enable the skin to make vitamin D), and those who haven’t. Three UK based studies looked at this in active men in their 20’s in the UK winter. At the start of each of these studies, most of the men had low levels of vitamin D in their body. Giving ‘extra’ vitamin D fixed this, however it did not seem to improve sports performance: sprints, 1.5 mile runs, upper and lower body strength and explosive jumps were tested in one or more of the studies, but no difference was observed between those who had received ‘extra’ vitamin D and those who hadn’t.
Having said this, the men in these studies had very different sporting abilities: for example, some people were very fast at running and some were very slow. A bit like trying to find a needle in a haystack, if you have a group of people who are very different it can be hard to detect any impact that adding something like vitamin D has on performance, if the impact is only very small. For most of us, if the effect is small we might not want to bother with it anyway. However, for an elite athlete the difference between winning and losing can be milliseconds or centimetres, and so small effects can be very relevant! In addition, it might be that only when an individual is severely deficient that ‘extra’ vitamin D helps their sports performance – and no one in these studies was considered severely vitamin D deficient. More studies are needed to test all these possibilities.
In conclusion, based on studies that have been done to date, increasing vitamin D levels in the body does not appear to improve sports performance. However, more studies are needed to confirm this.