Vitamin D3 Increases Overall Sporting Performance
The studies available show that vitamin D can improve overall physical performance.
As such it is imperative that as an athlete you should not be deficient in vitamin D.
LAST UPDATED: April 2019 by Ben.
Vitamin D & DOMS
Scientists have discovered that getting enough of Vitamin D3 can speed up your recovery from sessions, reduce DOMS and enhance your overall exercise performance. 
And, as athletes, it could be that you not only need more vitamin D to sustain repeated physical demands on your body but you are more likely to be deficient in it in the first place.
It is said that many conventional training habits, especially during winter, compound the lack of vitamin D available to our bodies.
This can be due to a vast array of reasons, and not just because people train indoors, people train at night, dawn, wearing skin tight compression gear etc and it all prevents your body getting the suns rays.
It is clear from studies that the impact that dwindling vitamin D has on many aspects of fitness.
Vitamin D3 is more like a steroid hormone than a vitamin in the way it acts on the body and one side-effect of low values is diminished muscle function.
It is needed by stem cells for muscle regeneration and recovery after a hard session and there is evidence it might protect immune functions during periods of intensive training.
It has also been found that a vitamin D supplement which was given to a group of swimmers, who trained predominantly indoors, seemed to offer protection against muscle injuries compared with training partners who took a daily placebo.
More than three quarters of the injuries recorded between September and March happened after a substantial drop in blood levels of vitamin D.
This left the authors to conclude;
“supplementation could prove to be an easy and affordable method to preserve bone and decrease risk of injury”. 
D and Performance
Further studies demonstrated that D3 increased the strength of indoor judo athletes. 
With growing awareness about the heightened risks of skin cancer among athletes, experts remain cautious about advocating sun exposure without sunscreen.
However, approximately 15 minutes per day with your face and arms exposed is probably enough safe for most people.
The darker your skin, the more time you will spend longer in the sun to produce the same amount of vitamin D.
Without exception, all experts agree that the skin should never be allowed to redden or burn.
Obviously times vary across the globe, certain territories in Australia for instance require only a few minutes sun exposure daily. 
Interestingly, even countries with traditionally less sunlight do only require lighter skinned people to have sunlight exposure of minutes rather than hours.
Studies also note that a few minutes of sunlight daily throughout the year is better and more effective at maintaining higher levels of vitamin D than shorter but more intensive bouts of sunbathing on vacation.
Those with darker skin could probably afford up to 25 minutes of daily exposure. 
Up to one person in three now takes a D3 supplement at some time during the year; its popularity buoyed by evidence that higher levels offer greater protection against some diseases.
It is also important to note in countries such as the UK there is not enough UVB radiation for the skin to produce vitamin D. 
Therefore a supplement is key.
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The studies demonstrate that adequate vitamin D intake is able to maintain stronger muscles by improving recovery which can translate to increased performance and a reduction of injuries.
In addition, there is further evidence suggesting that it may also protect immune function which can be reduced during periods of intense training. 
This was particularly noticeable amongst Finnish Military recruits.
Those who were low on vitamin D lost significantly more days from training than their counterparts who had higher levels.
As such, there is good reason to ensure that you continually have optimal levels of vitamin D.
 Supplemental vitamin D enhances the recovery in peak isometric force shortly after intense exercise. Barker T, Schneider ED, Dixon BM, Henriksen VT, Weaver LK. Nutr Metab (Lond). 2013 Dec 6;10(1):69. doi: 10.1186/1743-7075-10-69. PMID: 24313936
 The effects of season-long vitamin D supplementation on collegiate swimmers and divers. Lewis RM, Redzic M, Thomas DT. Int J Sport Nutr Exerc Metab. 2013 Oct;23(5):431-40. Epub 2013 Mar 8. PMID: 23475128
 The influence of winter vitamin D supplementation on muscle function and injury occurrence in elite ballet dancers: a controlled study. Wyon MA, Koutedakis Y, Wolman R, Nevill AM, Allen N. J Sci Med Sport. 2014 Jan;17(1):8-12. doi: 10.1016/j.jsams.2013.03.007. Epub 2013 Apr 23. PMID: 23619160
 Vitamin d and physical performance. Moran DS, McClung JP, Kohen T, Lieberman HR. Sports Med. 2013 Jul;43(7):601-11. doi: 10.1007/s40279-013-0036-y. Review. PMID: 23657931
 Acute Effects of Vitamin D3 Supplementation on Muscle Strength in Judoka Athletes: A Randomized Placebo-Controlled, Double-Blind Trial. Wyon MA, Wolman R, Nevill AM, Cloak R, Metsios GS, Gould D, Ingham A, Koutedakis Y. Clin J Sport Med. 2016 Jul;26(4):279-84. doi: 10.1097/JSM.0000000000000264. PMID: 26535872