Here comes the sun
With the return of warmer weather, runners know that temperature and humidity are the two factors they need to check before heading out the door (instead of temperature and wind chill in the winter). But that’s not the full story. Environmental physiologists have long known that sunlight itself – not just warm air – makes runners more likely to overheat, thanks to the effects of solar radiation. But no one had ever measured just how much of a difference sunlight makes.
That’s what researchers at Himeji Dokkyo and Hyogo universities in Japan set out to find out, in a study published earlier this year in the European Journal of Applied Physiology. They asked volunteers to cycle to exhaustion at a brisk pace in a room at 30 C and 50 per cent humidity, with artificial solar radiation ranging from 0 to 800 watts per square metre, which corresponds roughly to noon under a cloudless sky at the latitudes of most major Canadian cities.
The results were striking. With no sunlight, the subjects lasted an average of 46.3 minutes before giving up; with full sunlight, they lasted just 22.5 minutes despite the identical air temperature and humidity. Between the two extremes, radiation corresponding to thin clouds produced a time of 42.9 minutes, while even thick clouds resulted in 29.8 minutes. The message is clear: be aware of how much (or how little) cloud cover is in the sky, as well as time of day, before heading out for a long run. And on the sunniest days, look for opportunities to run early in the morning or in the evening.
Of course, that doesn’t mean you should avoid sunlight entirely. Aside from the fact that sunshine is, well, nice, it’s an important source of vitamin D. In a recent issue of Exercise Immunology Review, researchers from Bangor University and Loughborough University in Britain make the case that maintaining healthy levels of vitamin D in your blood – which they peg at 75 nmol/litre – helps ward off colds and f lu.
To achieve those levels, they recommend taking 1,000 IU per day in supplements during the winter, and aiming for about 15 minutes of unprotected sunlight (in shorts and a T-shirt) daily between the hours of 10 a.m. and 3 p.m. during the summer. For most people, that’s enough to boost vitamin D levels without risking sunburn.