Depending on who you listen to, vitamin D is said to be able to improve everything from mood to muscle function. As it’s relatively hard to get in decent quantity from real food, a supplement is advised, particularly during the colder, darker months.
What’s less known is whether vitamin D can meaningfully improve actually exercise performance – and that’s what a new study set to find out.
It followed 45 healthy adults from October to March, separating them into four groups: runners who supplemented; runners who didn’t supplement; non-runners who supplemented; non-runners who didn’t supplement.
In terms of the dosage, it was 2000 IU of vitamin D, taken daily for eight weeks. The researchers then stopped giving the dosage and tracked everyone for another 12 weeks.
In addition to testing vitamin D levels, the researchers recorded white blood counts to assess immunity status, VO2 max, countermovement jump (to assess explosive power) and maximal isometric force (to assess strength).
The runners in the study had to satisfy various criteria: 1) to have run for at least three years; 2) to run three or more times a week; 3) to run a minimum of 30 miles per week for men and 25 miles per week for women.
What did the results reveal? After eight weeks of supplementing with vitamin D, runners increased their levels by 21%. Non-runners who supplemented increased their levels even more, by 29%, though it’s worth noting their baseline level at the start of the period was slightly lower than that of the runners.
Meanwhile, non-runners who didn’t supplement saw their vitamin D levels drop by 32% during the initial eight-week period. Non-supplementing runners, by contrast, saw their levels hold steady.
Keep on taking the tablets
What happened when supplementation stopped? Vitamin D levels fell across the groups. The runners who had supplemented saw their levels drop below their initial baseline, while the non-supplemented groups ended up similarly low (or even lower). The message? Supplementation works – but you have to keep doing it as the benefits don’t hang around for long.
It was a slightly different story in terms of immunity. The supplemented groups enjoyed more stable levels of white blood cells – which fight off infection – than the non-supplemented non-runners, who saw their levels decline.
When it came to out-and-out performance, however, vitamin D was found to provide no real boost. VO2 max remained flat in runners, explosive power showed no meaningful improvement and strength went up by a tiny margin.
The message for runners? During the colder, darker months, taking 2000 IU (or more) or vitamin D seems to support your immune system. The important thing to remember is to take it consistently (daily) as the benefits don’t stick around long. In terms of athletic performance, meanwhile, the study doesn’t support the idea that vitamin D meaningfully moves the needle on things like VO2 max or strength. So use it to increase your chances of staying well, rather than a performance hack.
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Rick Pearson is the senior editor at Runner’s World UK. He’s been with the brand since 2017 and loves testing PB-friendly shoes for on and off road. Rick is a sub-three marathoner, 4-something miler and once completed 100 miles in less than 24 hours. He occasionally likes to remind people of these feats on the Runner’s World podcast, which he co-hosts.
Rick’s running CV also includes racing a steam train over 14 miles (he won, narrowly) and a horse over a marathon (he lost, comfortably).
