Vitamin D is a fashionable supplement to have in your cupboard. A quick online scout of popular sites sees headlines such as…
“Vitamin D the most underrated vitamin on the planet”
“Get lean by taking vitamin D – it raises testosterone and elevates metabolism”
“D is for domination” (personal favourite)
So, does it live up to the hype?
Here, we will briefly critique the current evidence regarding vitamin D supplementation and athletic performance.
A quick primer on vitamin D…
Vitamin D is a fat-soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement. Although vitamin D is categorised as a vitamin, it is actually a secosteroid hormone. Most vitamins act as anti-oxidants or cofactors in enzymatic reactions, however, the functions of vitamin D are significantly 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 (stops producing when there is sufficient amounts), therefore vitamin D toxicity from UVB radiation is not typically a concern. It is recommended in a fair skinned person, that 20-30 minutes of sunlight exposure on the face and forearms, 2-3 times per week is sufficient to achieve sufficient vitamin D levels in the summer.
Unfortunately for those of us in the UK, solar radiation is negligible throughout October to April due to the geographic latitude – that means we need to get our vitamin D elsewhere. Athletes who do not supplement or expose themselves to artificial UVB radiation have to rely on diet and summer vitamin D stores 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. In a group of professional soccer playerswinter vitamin D status was found to fall to <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 cohort of British males.
The physiological effect of vitamin D is dependent on the available stores within your body. Below is a table of the most widely regarded classifications, these different ranges for vitamin D status continue to be debated.
*Optimal is a concentration ≥40 ng/ml which has been suggested by researchers based on individuals who live habitually in a sun-rich environment. This value is not recognised by either of the leading organisations, and despite its biological plausibility there is little quantitative data to support its use.
How much do we need?
The current recommended dietary allowance for all adults aged 18-70 years old is 600 IU/day. Data from the (US) National Health and Nutrition Examination Survey (NHANES 2005-2006) estimated vitamin D intakes to be well below the RDA when supplements were not included. Average intake levels for males and females from foods alone ranged from 204-288 IU/day and 144-276 IU/day respectively. From these reports it is clear to see why vitamin D supplementation has become so popular. The Endocrine Society has proposed that 1500-2000 IU/day may be required to consistently maintain adequate vitamin D status throughout winter. Anecdotally, it is fairly common to see people supplement with anywhere from 2000-5000 IU/day without having an assessment of serum 25(OH)D concentrations (vitamin D status). A recent systematic review and meta-analysis found vitamin D3 to be more effective at raising serum concentrations than vitamin D2 and should therefore be considered the superior form for supplementation.
Toxicity from vitamin D3 supplementation is rare, and data from human studies suggest that prolonged intakes up to 10,000 IU/day have not been associated with adverse effects. However, the current tolerable upper intake level (UL) for vitamin D as set by the Institute of Medicine is 4000 IU/day. Although the UL is not a recommended level of intake, it is an estimate of the highest level of intake which carries no appreciable risk of adverse health effects.
Following that overview of vitamin D we are now left with two key questions…
- Can supplementation with vitamin D3 increase vitamin D status and maintain summer concentrations in athletes?
- Does exhibiting a higher vitamin D status confer additional benefit to athletic performance?
Vitamin D3 Supplementation and Athletic Performance
Two studies from Graeme Close’s research group in Liverpool have looked at supplementation and its effects on performance, both yielding different results. In one study, thirty club-level athletes received placebo, 40000 IU/week, or 20000 IU/week of vitamin D3 for 12 weeks (January-April). Supplementation was able to correct the existing vitamin D deficiencies and increased the group average vitamin D status to ≥30 ng/ml. Despite this, supplementation had no significant effects on vertical jump, 1RM testing, or 20 meter sprints when compared to placebo. Interestingly, although the higher dose (40000 IU/week) caused a sharper increase in vitamin D status, after 12-weeks there was no difference between either dose. This suggests that to maintain adequate vitamin D status throughout winter, more is not necessarily better – this refutes a commonly held belief in those using mega-dose supplements. In a subsequent study, an 8-week dose of 5000 IU/day vitamin D3 was found to increase 10-meter sprint times and vertical jump height compared to placebo. Before we get too excited with these results, the experimental sample consisted of 5 youth team football players (after drop outs). The most consistent increases in performance were seen in 1RM bench press and 1RM back squat; however external training and diet were not recorded or controlled for across the 8-week study. Given the small sample size, there is possibility that any performance improvements were due to chance or differences in diet and activity levels, which were not controlled. Interestingly, 3 of the 5 athletes were able to achieve ‘optimal’ vitamin D status of ≥40 ng/ml following supplementation. At present there is a lack of empirical evidence that supports the role of an optimal vitamin D status. However, a 2009 review in Medicine and Science in Sport and Exercise reported that research dating back as early as 1938 suggested that a course of UVB radiation (sunbeds) resulted in improved athletic performance by increasing serum 25(OH)D concentrations >40 ng/ml. These early studies were conducted in German and Russian Olympic athletes but data are largely descriptive and adequate randomised trials are lacking.
Vitamin D and Immune Function
The link between vitamin D status and immune function has only been made very recently, but some promising evidence exists. A group from Loughborough examined 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 identified that those exhibiting a higher vitamin D status displayed significantly fewer URTI episodes. Furthermore, it was noted that when a URTI was present, athletes reduced their training load by an average of 24% - this is likely to have a negative impact on performance over time.
Again we are presented with encouraging evidence in favour of vitamin D. However, this study followed a cohort and looked for associations between biomarkers. This approach can give us insight in to potential mechanisms at play here, but a true cause-and-effect relationship cannot be drawn. A controlled trial involving vitamin D3 supplementation is currently underway from the same group of researchers and I look forward to seeing those results.
Vitamin D and Testosterone
Is there a link between vitamin D status and testosterone? It appears there might be. Cross-sectional research suggests a positive association between vitamin D and total testosterone. A more direct look at a cause and effect relationship leads us to a trial which observed overweight men consuming 3332 IU/day of vitamin D3 for 1 year. Those receiving supplementation significantly increased their total testosterone values by 25.2% - this means we can load up on Vitamin D and get swole, right? Unfortunately not, the men involved in the study started with testosterone values at the lower end of the acceptable range and supplementation caused an increase to more respectable levels. As is the case with most things nutrition, if you correct a deficiency (vitamin D) you can normalise an endocrine response (testosterone) but it will not cause a supra-physiological elevation. Sorry Bro’s!
It is important, as always to consider the context with which supplementation may be warranted. For athletes who are otherwise healthy, I’d have to ask is it necessary? The counter-argument could be… Does it appear to cause any negative or harmful effects? These could only be answers on a case-by-case basis, but from the available evidence some summary points can be made:
- Supplementation with vitamin D3 safely and effectively raises serum 25(OH)D concentrations (measurement of vitamin D status).
- Whether maintaining a higher vitamin D status has any physiological effects on athletic performance remains to be seen, and the consequence of deficiency is not fully understood.
- Although supplementation without knowledge of your serum 25(OH)D concentrations is not recommended, it seems sensible and logical to maintain vitamin D status ≥30 ng/ml. This can be achieved with a daily supplementation of 2000-4000 IU/day throughout winter months.
- Although sufficient levels of vitamin D may act to normalise testosterone, there is no evidence to suggest that mega-dosing vitamin D further enhances testosterone levels.
- Preliminary cohort data shows an association between vitamin D and immune function, and controlled trials are currently being undertaken in this area.
- Chronic mega-doses of vitamin D3 supplementation should be discouraged until comprehensive evidence shows it is safe and there are physiological benefits.
About the Author
Bio: Joe holds a B.Sc. (Honours) degree in Sport & Exercise Therapy, and is currently pursuing a M.Sc. in Nutrition Science. Through his own Performance Nutrition company, JM Nutrition, he delivers 1-to-1 nutrition consultancy to personal clients to enhance sports performance and body composition. In addition to this Joe works alongside the Fire & Rescue Service to deliver nutrition education and injury rehabilitation programs.