When it comes to athletic performance, how much vitamin D is enough?

As another long, sun-starved Canadian winter draws to a close, there's a roughly 40-per-cent chance that your vitamin D levels are below the recommended threshold, according to the latest figures from Statistics Canada.

Should you care?

If you have a summer of athletic adventures lined up, you might want to consider it. In addition to its long-debated potential health benefits, vitamin D has emerged in recent years as a highly touted sports supplement, with benefits such as enhancing muscle recovery and warding off missed training time because of colds and flu.

"These are both crucial for athletes," says Graeme Close, a professor of human physiology at Liverpool John Moores University in Britain who also works as a sports nutrition consultant with professional athletes and sports teams.

Indeed, there have been some intriguing hints that vitamin D has particular benefits for athletes. A 2015 preseason study of 80 Pittsburgh Steelers football players, for example, found that those with higher vitamin D levels were more likely to make the team and less likely to have a history of bone fractures.

And, earlier this year, a review in the British Medical Journal concluded that vitamin D supplementation could reduce the risk of respiratory infections. Given that 40.3 per cent of supplement-takers had at least one respiratory infection, compared with 42.2 per cent of non-supplement-takers, the benefits might seem negligible – unless you're an athlete, where success is a matter of centimetres or milliseconds.

Still, as with the links between vitamin D and general health, the case for it as a sports supplement is far from clear. And even if you're convinced, it's not easy to figure out how much is enough.

The threshold used by Statistics Canada, based on blood tests of a marker called 25(OH)D, is 50 nanomoles (nmol) per litre.

But this threshold is far from unanimous. In a paper published last year in Exercise Immunology Reviews, British researchers argued that 25(OH)D levels above 75 nmol/L conferred the best protection against respiratory infections. In contrast, a review published in the British Medical Journal earlier this year found that the immune-boosting benefits of taking extra vitamin D were most pronounced in those with very low baseline levels below 25 nmol/L. The short answer, in other words, is that no one knows.

That hasn't stopped professional sports teams from aggressively targeting high levels with supplement regimes that reach as high as 70,000 international units (IU) per week, according to Close.

The typical vitamin D pill you get at a drugstore might contain 1,000 IU, and Health Canada's dietary reference intake for adults is just 600 IU; but supplements are available that contain as much as 50,000 IU in a single tablet – an attractive option for team physicians who want to hand the pills out to their athletes once a week and ensure that they swallow it.

Such large doses may have drawbacks, as Close and his colleagues reported in Medicine & Science in Sports & Exercise in February. They compared the effects of either 35,000 or 70,000 IU of vitamin supplements per week for 12 weeks in a group of 42 professional athletes. At the higher dose, the body's attempts to break down the excess vitamin D left the athletes potentially worse off than before they started supplementing.

Racial differences in vitamin D processing present another pitfall. While some vitamin D is available from foods such as fatty fish, eggs, and fortified milk, larger amounts are produced in the body when you're exposed to ultraviolet light from the sun. People with dark skin process that sunlight differently, and tests commonly show lower levels of 25(OH)D in those populations.

The problem, Close says, is that we may be measuring the wrong thing. While 25(OH)D offers an indirect measure of vitamin D levels, a New England Journal of Medicine study in 2013 suggested that people with dark skin are able to use a higher proportion of the 25(OH)D circulating in their bloodstream compared with people with light skin. That means black athletes may be diagnosed as "deficient" when they have perfectly adequate levels.

As a result, indiscriminate supplementing isn't a good idea. While high doses of vitamin D aren't generally considered dangerous, there have been a few studies that found risks at the highest levels. For example, a Danish study of 250,000 people found 25(OH)D levels above 140 nmol/L were associated with a higher risk of dying during the follow-up period.

So how do you find the sweet spot? Close starts by checking vitamin D levels, if possible using a special test of "bioavailable" vitamin D if he's working with athletes of difference races. If that's not possible, he suggests a default of 2,000 IU per day during the winter for those in the northern hemisphere – and "sensible sun exposure," which means about 15 minutes in the midday sun, instead of supplements during the summer.

Other experts are even more conservative: Neil Walsh, an exercise immunologist at Bangor University who has researched vitamin D in athletes, suggests 1,000 IU per day in the winter only.

Ultimately, the precise link between vitamin D and athletic performance – if there is one – remains to be clarified. But Close's research reminds us that, like pretty much every other supplement, superfood, or miracle workout ever studied, the fact that some is good doesn't mean more is always better.