Most of us are a long way from that level. A widely used level considered “sufficient” is 50 nmol/liter, and several studies of athletes (in particular from Graeme Close’s research group in Liverpool) have found that somewhere between half and three quarters of them fall below that threshold during the winter.
(As an aside, there’s no strong evidence that athletes have higher or lower vitamin D levels or needs than non-athletes. But athletes are more likely to notice if their physical function isn’t quite optimized.)
The tricky thing about vitamin D is that you can also get it from the sun (and from certain foods). The sun is great during the summer, but then you have to balance its benefits against the possible risk of burns and elevated long-term risk of skin cancer. I’ve often wondered if there was some ideal middle ground for sun exposure that would give you enough vitamin D without significantly raising your risk of burns and cancer.
A new review paper (freely available online) in the journal Exercise Immunology Review, from Neil Walsh at the University of Bangor, along with Michael Gleeson of Loughborough University and colleagues, takes a stab at some of these issues.
The focus of the paper is determining the vitamin D sweet spot for athletes and military personnel in order to optimize immunity. Vitamin D plays a bunch of different roles in the body—bone health, cancer prevention, blood pressure, muscle function, etc.—but one of its key contributions is to the immune system. One theory is that the reason colds and flu spike during the winter months is because that’s when we tend to be low in vitamin D.
So what is the optimal level? The authors cite evidence that those with at least 75 nmol/liter seem to have markers of stronger immune function and are less likely to contract respiratory infections. Actual intervention studies where people were given vitamin D supplements to get to that level have so far failed to lower infection rates, so the verdict definitely isn’t final at this point. We’ll see what further research shows.
Perhaps most interesting to me was this infographic included in the paper, which tries to summarize some of the practical recommendations for maintaining good vitamin D levels:
The practical takeaways: In the summer, aim for about 15 minutes of unprotected sun exposure daily, between 10 a.m. and 3 p.m. That’s useful to know for me. I do most of my exercise fairly early in the morning, when the sun isn’t strong. But I often pop out for errands or to do some shopping during the day, and I usually choose not to put on sunscreen or other sun protection for these short jaunts (except for a hat, since I have no hair to protect me!). These recommendations suggest that I should keep up that habit.
During the winter, the recommendation is to get 1,000 IU per day. You can get some of that from food (as the graphic shows), but it would be very difficult to get all of it, even in countries like Canada and the United States where food is fortified. I take a lot less than that in supplement form, but perhaps I should up my game.
This article originally appeared on Runner’s World.