The belief that vitamin D can aid muscle function, increase aerobic capacity, decrease recovery time, and therefore be used as an ergogenic aid by athletes is gaining traction. 

As a result, there’s a growing number of athletes using vitamin D supplements in the hope that they will enhance performance.

Whether this is another ‘fad’ that will pass with time remains to be seen, but it's worth exploring the research and the potential benefits…

What is a vitamin?

The word vitamin comes from the suffix ‘-amine’ which means ‘a nitrogen-containing compound’ because at the time of discovery it was assumed that they were probably amines.

Given that these chemicals needed to be consumed in order for us to live, the Polish chemist Casimir Funk combined ‘amine’ with ‘vital’ and coined the term ‘vit-amine’.

The last ‘e’ was later dropped when it was discovered that, no, actually not all vitamins were ‘amines’, and it became ‘vitamin’ which stuck.

The actual definition of a vitamin is ‘an essential micronutrient’. An organic compound in food needed in small amounts for good health.

If you don’t take enough, disease can rear its head. A lack of Vitamin C? Have yourself some scurvy (a very real problem for transatlantic voyagers in the mid-19th century). Not enough vitamin B1? Beriberi disease looms.

The cause and effects of vitamin D deficiency

Vitamin D has long been recognised as necessary for our skeletal health and it performs many other functions. Its most prominent role is the regulation of calcium homeostasis and there’s a strong association between vitamin D status and bone health.

Despite its importance, a paper published in 2014 claimed that 88.1% of the world’s population had an inadequate vitamin D status, a startlingly high figure.

A deficiency can be caused by insufficient dietary intake and is more commonly associated with inadequate sunlight exposure. This is because UV light from the sun is necessary for synthesis of the vitamin in the skin.

In children, a vitamin D deficiency may cause rickets - a skeletal disease which affects growing bones by causing them to soften, resulting in skeletal deformities and pain. This was a real problem in the 1920s when industrialisation was bringing more people to cities which led to children getting less and less sunlight.

As a result, many children were growing up with bowed legs due to a weakening of the bone around their growth plates. The eventual discovery of a compound, which came to be known as vitamin D, essentially eliminated the disease as food producers rapidly began adding it to any possible medium (even things like hot dogs and beer!).

It was a welcome turnaround at the time but many countries stopped adding it to food in the 1950s when it was found that some children in the UK had too much calcium in their blood. With the role vitamin D plays in allowing the bowels and kidneys to absorb calcium, excessive vitamin D consumption ran the risk of attaining too much calcium which can then upset the heart’s rhythm.

A sharp rise in rickets has once again been seen in recent years as more and more children spend increasing time indoors and worried parents apply excessive sunscreen when they venture outside.

For adults, rickets is less of a concern as it’s a disease which only affects developing bones. However, Vitamin D, once converted into its biologically-active form - 1,25-dihydroxyvitamin D (25(OH)D) - regulates the expression of over 900 gene variants – some of which have been shown to have an impact on performance-related variables as well as health.

In short, it plays an important role in the body no matter your age.

How much vitamin D do we need?

The Endocrine Society in 2011 issued clinical practice guidelines for vitamin D stating that the desirable 25(OH)D status is >75 nmol/L to maximise the effect on calcium, bone, and muscle metabolism. A vitamin D insufficiency and deficiency are determined as 30-50nmol/L and <29 nmol/L, respectively.

Sub-optimal levels have been linked to an increased risk of acute illness, inflammatory illness, stress fractures, muscle pain and weakness, and reduced muscle performance.

It’s recommended that a sensible sun exposure of between 5 (in very fair-skinned people) and 30 (in darker-skinned people) minutes on the arms, legs and back several times a week without sunscreen should be the target and should provide sufficient vitamin D synthesis.

Because sunlight is necessary for vitamin D synthesis, anything that limits the amount or quality of sun exposure can compromise our status. So unsurprisingly, vitamin D levels are generally lower in the winter months.

At times of reduced sunlight exposure, adequate dietary intake of vitamin D is essential to health. The only problem is that vitamin D is found naturally in only a limited number of foods – fatty fish, fish liver oils, eggs and some fortified dairy products (in countries where regulations allow).

So, during periods of reduced UV light exposure, dietary intake may not be enough for many individuals and supplementation may be beneficial or even necessary.

The big question that follows is: How much should you take?

How much vitamin D you need depends on many factors, including age, race, latitude, season, sun exposure, sunscreen usage, clothing and more. Recommendations from the Institute of Medicine (IOM) suggest that an average daily intake of 400–800 IU, or 10–20 micrograms, is enough for the vast majority of individuals.

However, some studies have shown that the daily intake needs to be higher than that if you aren’t being exposed to sun. For instance, if you live far north or south of the equator and have a limited exposure to adequate sunlight during certain times of the year.

One study of healthy adults showed that a daily intake of 1120–1680 IU was needed to maintain sufficient blood levels (see paper here). In the same study, individuals who were vitamin D deficient needed 5000 IU to reach adequate blood levels.

The only way to know if you actually need to take a vitamin D supplement is to have your blood 25-OH vitamin D levels measured.

All things considered, a daily vitamin D intake of 1000–4000 IU, or 25–100 micrograms, should be enough to ensure optimal blood levels in most people. 4000 IU is considered the safe upper limit according to the IOM and will only be needed if you’re deficient to begin with.  

Despite this, up to 10,000 IU per day has not been shown to cause harm to healthy individuals but that’s not to say it should be done.

Whilst the idea of vitamin D overdose and toxicity is outdated, it is associated with dangerously high levels of calcium in the blood, as seen in the aforementioned British children in the 1950s. This response would typically only be seen in individuals who have accidentally or intentionally taken extremely high doses of vitamin D for long periods of time, such as 50,000–1 million IU/day for months (for extra reading, this paper discusses the updated perspective).

Image credit: Luis Graterol via Unsplash (copyright free).

Does vitamin D enhance athletic performance?

So how are vitamin D and performance related? It became increasingly clear to me that vitamin D in athletes hasn’t been as well-studied as it has in non-athletes/the general population. The research is still emerging and the existing studies are a little conflicting.

The first thing to establish is that it doesn’t appear that the requirements of athletes are any different from those of the general population. As it stands, there’s no evidence supporting an above ‘normal’ intake.

When working with athletes however it’s worth recognising that their vitamin D sufficiency will vary depending on season, training location, skin colour and sport (whether it is predominantly indoor or outdoor).

Basketball players and African American athletes appear to be at greatest risk for vitamin D insufficiency and deficiency presumably due to the time spent indoors, differences in diet, and darkened skin colour.

Having said that, a recent study suggests that African Americans possess lower amounts of vitamin D binding protein, so a greater proportion of bioactive vitamin may be available despite lower serum 25(OH)D levels.

Older adults, whose skin isn’t as efficient at producing vitamin D, and obese people, whose excess fat stores trap the vitamin, are also more likely to be deficient.

Vitamin D has been ear-marked as a potential ergogenic aid because of indications that, on top of its part in bone health, it also plays a role in non-skeletal functions including skeletal muscle growth, immune function, inflammatory modulation and athletic performance.

Force and power output

Vitamin D has been shown to increase force and power output of skeletal muscle tissue, perhaps by increasing the sensitivity of calcium binding sites in the muscle cells leading to an enhanced muscular contraction. There’s further evidence that vitamin D supplementation might also potentially increase both size and number of Type II muscle fibres (see papers here and here).

These findings have mainly been supported in mobility-limited elderly (mostly women) and are yet to be properly tested in the athletic population but they offer an insight into the potential athletic benefits.

One randomized placebo-controlled study took a bash at looking at vitamin D status and its effects upon force and power production in an athletic population. They used an 8-week long intervention of either receiving 5000 IU/day of vitamin D or a placebo.

The vitamin D group had a significant increase in serum 25(OH)D levels and a significant improvement in both their 10-metre sprint times and vertical jump when compared to the placebo group. Promising stuff and a step in the right direction.

Unfortunately, the study used just 10 soccer players. It’s hoped that similar studies using larger populations will follow.

Maximal oxygen uptake

Vitamin D was also regularly linked to maximal oxygen uptake (VO2max). It’s known that vitamin D receptors are present in the cardiac muscle and blood vessels indicating that 25(OH)D concentration might influence VO2max via the ability to transport and utilize oxygen within the blood to various tissues.

Multiple correlative studies have shown a positive correlation between VO2max and serum 25(OH)D concentration in non-athletes. However, some confounding variables were not addressed such as participant’s multivitamin and supplement intake (*face palm*).

Image credit: Rainer Bauerdick ©.

Bone health and stress fractures

Several studies provide evidence that sufficient vitamin D status is important for bone health and prevention of bone injury in athletic populations (papers here, here and here). For instance, stress fracture risk was 3.6 times higher in Finnish military recruits with blood vitamin D concentrations below 75 nmol/L (depending on what you read, this is the threshold for ‘adequate’ vitamin D status).

To add contradiction though, it appears that the current overall standpoint is that there’s limited evidence which directly links compromised vitamin D status with increased risk of severity of sports-related inflammation or injury (according to this review paper published in 2015).

Immune function

Lastly, the most recent focus has been on vitamin D and its role in immune function. The theory comes from vitamin D having the ability to turn on the gene expression of a broad spectrum of antimicrobial peptides (AMPs; i.e. key fighters in our immune response). AMPs are secreted by cells of the immune system which help defend against invading bacteria, fungi and viruses.

There’s a ton of evidence which points towards vitamin D having a protective effect against upper respiratory tract infections (URTIs). Data from 25 randomized controlled trials demonstrate that vitamin D supplementation reduced the risk of acute URTIs by more than 50 percent, especially in those with low baseline vitamin D levels.

Whilst not a direct performance-related parameter, vitamin D may play a big part in maintaining an athlete’s consistent training regime by protecting the immune system which in turn has positive outputs on their performance. An interruption of training consistency, caused by sickness, often has a significantly detrimental effect on athletes’ overall fitness.  

In addition, new evidence (published in 2020) suggests that a relationship exists between vitamin D status and vaccine response. Those with low vitamin D status at the point of initial vaccine were associated with poorer vaccine response.  

Why the focus of supplementation should always be optimisation

What does seem to come to light in the literature is the enhancement of performance or health appears to be most pronounced in individuals known to be vitamin D deficient prior to supplementation. In athletes/populations with just moderate deficiencies or adequate levels, no significant benefit from vitamin D supplementation appears to exist.

This indicates that performance benefits might only be limited to those athletes with a significant vitamin D deficiency. If you are truly deficient in vitamin D, then supplementation could be a game-changer. In these individuals it could very well be classed as an ergogenic aid.

In many ways, this principle carries over to a lot of other domains, including the use of electrolytes. People in a chronic state of low-sodium are likely to be so because they have massive sweat rates, are ‘salty sweaters’, frequent sweaters, work very manual, sweaty jobs, and/or consume a very low sodium diet (i.e. eat minimal processed foods and steer away from the saltshaker).

These individuals are more likely to enjoy the biggest ‘bang for their buck’ if/when they supplement with some electrolytes in and around their exercise or daily lives.

What’s important to highlight here is the rule which is often forgotten by athletes, and that’s that more isn’t always better (we can all name someone who believes that if some is good, more must be great).

Supplementing with anything, be that vitamin D, sodium or something else, should always be done with the intention of optimising our intake with regard to what the body actually needs. An excessive or surplus intake is often a waste at best, and at worst, it could be a detriment.

The important takeaway points 

To conclude a lengthy piece (well done if you’ve made it this far!) here are the key takeaways:  

  • Vitamin D is essential for overall health, especially our bone health
  • We get vitamin D from foods, sunlight and supplements
  • The majority of the worldwide population has an insufficient vitamin D status and around 30-40% are thought to be deficient
  • To determine vitamin D status, a blood test is required. Guidelines for vitamin D intake state that the desirable blood 25(OH)D concentration is >75 nmol/L
  • Sunlight is the best source of vitamin D and making sure that you receive adequate but sensible sunlight exposure, particularly in the summer months, is important
  • As it stands, there’s no evidence to suggest that athletes have a higher requirement for vitamin D than the general population
  • A daily vitamin D intake of 1000–4000 IU should be enough to ensure optimal blood levels in most people
  • There’s some evidence to suggest that vitamin D may play an ergogenic role across different performance-related measures. However, it’s clear that these benefits may only occur in individuals who were deficient prior to supplementation

Further reading