The case for vitamin D as a sports performance booster got a minor bump from a couple of recent studies. One showed an apparent benefit to VO2 max and 30-second sprint performance; the other saw a reduction in markers of muscle damage after a punishing 30-minute downhill running test. Both studies were placebo-controlled and blinded (meaning the subjects didn’t know whether they were getting the vitamin or the placebo). It’s enough, in other words, to make an avowed supplement skeptic wonder whether he should reconsider his skepticism.
Vitamin D has been getting hype in the sports world for well over a decade. It was originally associated with bone health, and remains a first-line recommendation for athletes with a history of stress fractures. Just last month, a study of 135 college runners from Stanford and UCLA found that 34 percent of them took vitamin D supplements, including half of those with previous bone stress injuries. But it turns out that vitamin D plays a whole bunch of other roles, including in athletically interesting areas like muscle function, muscle repair, and the immune system.
As a result, it has become what one 2018 review called “the ‘on trend’ in sports nutrition,” with sports nutritionists and pro teams issuing blanket recommendations of up 100,000 IU of vitamin D per week. For comparison, the current recommended daily allowance of vitamin D is 600 IU for adults, or 4,200 IU per week. Unlike other formerly popular supplements like vitamins C and E and resveratrol, no evidence has emerged suggesting that vitamin D blocks the fitness gains from training.
But there are a few lingering questions about vitamin D’s role as a sports supplement. One is about the difference between correcting a deficiency and boosting performance. The 2018 review, which was led by Liverpool John Moores University researcher Graeme Close, who is among the world’s leading experts on vitamin D for athletes, concludes that there’s no performance benefit from topping up beyond the level considered sufficient for general health.
Defining what’s “sufficient” is a bit of a minefield, though. Based on blood tests of a vitamin D metabolite called 25[OH]D, the Institute of Medicine considers 50 nmol/L to be “adequate.” The Endocrine Society suggests aiming for 75 nmol/L. The enthusiasts at the Vitamin D Society call for 100 to 150 nmol/L, bumping up against the Institute of Medicine’s threshold of potential toxicity, which is 150 nmol/L. (Levels of 25[OH]D are sometimes given in units of ng/mL, which are 2.5 times smaller. If you’re at 50 nmol/L, that’s 20 ng/mL.)
In the new VO2 max/sprinting study, the 28 subjects took either 6,000 IU or a placebo daily for eight weeks. That mega-dose took 25[OH]D levels from about 50 nmol/L all the way to 146 nmol/L in the experimental group, which is about as high as you want to go. The muscle damage study assigned a daily dose of 2,000 IU (or a placebo) for three weeks. In this case, the subjects were 24 ultrarunners who presumably spent a lot of time outdoors, because their baseline 25[OH]D levels were already 86 nmol/L, increasing to just over 100 nmol/L after supplementation. If going from the already-high level of 86 to 100 gives you a benefit, that makes vitamin D sound like a more-is-better performance-booster as opposed to something you just need to get enough of, like food and water.
I won’t get too deep into the nitty-gritty of these studies (they’re free to read online if you’re interested). It’s always possible to call out weaknesses: the “improvement” in VO2 max, for example, was actually no change in the vitamin D group and a decline in the control group. But I’m conscious of the fact that we tend to deploy these sorts of critiques to studies whose results we’re skeptical of, while giving a pass to studies that fit with our preconceptions. Rather than fixating on the details, it’s better to take a step back to look for broader patterns in the literature.
There have been a bunch of meta-analyses aggregating the results of vitamin D studies in athletes—but still no consensus. A 2017 meta-analysis found that vitamin D increases muscle strength, but that finding was based on just two studies with positive effects and two with inconclusive results. A 2019 meta-analysis found no benefits to muscle strength, and another 2019 meta-analysis found benefits to upper limb but not lower limb muscle strength. You can pick whichever result you prefer. The largest meta-analysis, including 13 randomized trials with a total of 532 subjects, found no improvement in athletic performance after vitamin D supplementation.
Perhaps the best illustration of the perplexities involved in studying vitamin D in athletes comes from a two-part Bangor University study from 2018. The first part involved measuring vitamin D levels in 967 military recruits and assessing their exercise performance. The tests were performed during the winter, and only 9 percent of men and 36 percent of women hit the 50 nmol/L threshold for sufficiency. While strength and power weren’t associated with vitamin D levels, time on a 1.5-mile run was: for every increment of 1 nmol/L, subjects tended to be about half a second faster on average.
In part two of the study, a subset of 137 subjects got vitamin D supplements either through a daily pill or through simulated sunlight three times a week, or else got a placebo version of the pill or the sunlight (with the ultraviolet light filtered out). Both forms of supplementation were effective, roughly doubling levels of 25[OH]D from about 40 to 80 nmol/L. But there were no effects on athletic performance.
In other words, having low vitamin D levels may be associated with lower athletic performance, but that doesn’t mean that it’s the cause, or that taking supplements will change anything. It may be that people with low levels tend to spend less time being active outdoors in the sunlight (which is the predominant natural source of vitamin D), so they’re simply less fit. Vitamin D is stored in body fat, so people with excess fat tend to have lower levels of vitamin D circulating in their bloodstream—but it could be the extra weight, not the lack of vitamin D, that hurts their performance. Or it may be that low vitamin D signals a lack of sun exposure, but other benefits of sunlight such as elevated nitric oxide levels are what influence athletic performance.
As Graeme Close’s 2018 review points out, there are also more fundamental problems with how we measure vitamin D levels. Close to 90 percent of the 25[OH]D circulating in your bloodstream is bound to a molecule called vitamin D-binding protein (VDBP). But there’s evidence that it’s the other 10 to 15 percent of the “free” 25[OH]D that has the biggest impact on health and performance. The problem is that the structure and function of VDBP can differ between ethnic groups. For example, there’s evidence that Black and Hispanic athletes can have much lower levels of total 25[OH]D without any impairment of bone health, because they still have high enough levels of free 25[OH]D. That may explain why there’s so much confusion about the “right” levels for 25[OH]D blood tests: it varies widely depending on who you are.
All of this explains why, despite the addition of two more nice-looking studies bolstering the case for vitamin D, I can’t shake my skepticism. How skeptical am I? Well, I still take a few thousand IU of vitamin D supplements per week during the winter. That’s for its putative health benefits (a whole different debate, though similarly fraught). I live in Canada, after all. I remain open to the idea that athletes should be taking bigger doses—but until the meta-analyses start pointing consistently in the right direction, I won’t be doing it myself.
Hat tip to Chris Yates for additional research. For more Sweat Science, join me on Twitter and Facebook, sign up for the email newsletter, and check out my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.