Known as the ‘Sunshine Vitamin’, Vitamin D is vital for strong bones and a healthy immune system. But how does it affect athletic performance and how can you benefit from supplementing it into your diet?
Vitamin D is a fat soluble vitamin that plays an important role in the absorption of Calcium and Phosphorus and promotes normal immune function. It is not naturally found in high quantities in foods; rather, it is primarily produced in the skin as a response to exposure to sunlight. This has led to it being known as the ‘Sunshine Vitamin’.
The term ‘Vitamin D’ refers to several different forms of this vitamin. The two main forms important to humans are: Vitamin D2, which is made by plants, and Vitamin D3, which is made by human skin when exposed to sunlight.
Although Vitamin D is categorised as a vitamin, it is actually a secosteroid hormone. Most vitamins act as antioxidants but the functions of Vitamin D are quite different. Due to the presence of Vitamin D receptors in many tissues, Vitamin D has wide-ranging effects throughout the body.
The major source of Vitamin D for humans is casual exposure of the skin to sunlight. Season, time of day, length of day, cloud cover, air pollution, skin melanin content and sunscreen are among the factors that affect UV radiation exposure and subsequent Vitamin D synthesis. Vitamin D production from sunlight operates on a negative feedback loop (which means that it stops producing when there are sufficient amounts). It is recommended in a fair skinned person that 20 to 30 minutes of sunlight exposure on the face and forearms, two to three times per week, is sufficient to achieve adequate Vitamin D levels in the summer.
Unfortunately in the UK, solar radiation is negligible throughout October to April due to the geographic latitude, which means we need to get our Vitamin D elsewhere. Without supplementing or dipping into the summer Vitamin D stores, athletes have to rely on diet and artificial exposure to UVB radiation throughout this period. The lack of quality dietary sources results in ongoing catabolism of internal stores and Vitamin D status (the body’s available stores of Vitamin D) can fall significantly.
This could cause Vitamin D deficiency, which is when the body does not have enough to properly absorb the required levels of Calcium and Phosphate. Mild to moderate Vitamin D deficiency can lead to bone pain and weakening of the bones which could also, in turn, make you more likely to fracture a bone if you had a fall.
In a group of professional football players, winter Vitamin D status was found to fall to less than 50% of their previously recorded summer values. This is of particular concern because individuals engaged in outdoor training would be expected to have a higher Vitamin D status due to increased exposure to sunlight. The figure below shows seasonal variation in Vitamin D status from a large group of British males.
Vitamin D3 and Athletic Performance
Any kind of micronutrient deficiency is going to have a negative effect on performance levels, but what about supplementing Vitamin D3 to normalise or even increase the amounts being processed in our bodies?
Many studies have been conducted on the effects of supplementing Vitamin D3. One study from Graeme Close’s Liverpool Research Group showed that with an eight-week dose of 5,000 IU/day, Vitamin D3 was able to increase ten-meter sprint times and vertical jump height compared to placebo. But the experimental sample consisted of five youth team football players (after dropouts). The most consistent increases in performance were seen in 1RM (Rep-Max) bench press and 1RM back squat. It must be noted, however, that external training and diet were not controlled or recorded for across the eight-week study. Given the small sample size, there is a possibility that any performance improvements were due to chance or differences in diet and activity levels, which were not controlled.
However, another study conducted by Bruce Hamilton for the Asian Journal of Sports Medicine concluded that the maintenance of Vitamin D at supra-physiological levels results in enhanced muscle development and performance. His study suggests that maintaining optimum levels of Vitamin D is vital to maintaining peak performance in athletes, especially during the winter months where deficiency is more likely to occur. More research is needed to shed light on any further reaching effects of Vitamin D.
Vitamin D and Supporting the Immune System
There is a clear link between Vitamin D status and immune function but this has only recently been highlighted. A Group from Loughborough looked into the association between Vitamin D status and the incidence, severity and duration of upper respiratory tract infections (URTI) during a 16-week winter training period in 225 male and female athletes. Results showed that those with a higher Vitamin D status presented significantly fewer URTI episodes.
This study followed a cohort and looked for associations between biomarkers. This kind of approach can give us insight into potential mechanisms at play here, but it can be difficult to draw a true cause-and-effect relationship. This is encouraging evidence, but more research is needed to determine how far reaching Vitamin D is when related to other aspects of the immune system.