The truth about marathon running and heat stroke

Team RW
·2-min read
Photo credit: David McNew - Getty Images
Photo credit: David McNew - Getty Images
  • New study suggests younger, faster runners are more likely to suffer from heat stroke

  • There are more cases of heat stroke when races start in mild weather but become hot, rather than races that are hot throughout

As anyone who ran the 2018 London Marathon will tell you, running 26.2 miles in sweltering heat is a tall order. But other than the temperature on the day, what other factors affect the chances of runners suffering from heat stroke?

That’s the subject investigated in a new study on runners at the Boston Marathon. Looking at records between 2015 and 2019, the researchers found 51 cases of heat stroke. Interestingly, younger, faster runners were most at risk of heat stroke. While this might seem counter-intuitive, given that slower runners will be out in the heat for longer, it actually makes sense. The biggest factor in heat stroke is the heat you generate yourself – and the faster you move, the more heat you create.

While dehydration is often linked to heat stroke, hard evidence in support of this notion is thin on the ground. The data from the Boston marathons doesn’t tell us how much each of the runners with heat stroke had drunk, but 18 of the 51 were given intravenous fluids, while 24 were seemingly given nothing at all. It’s hard to draw any firm conclusions from that, but it does suggest heat stroke may not be primarily linked with dehydration.

Then there’s the final factor: the weather. It’s generally accepted that temperatures in excess of 21C present an elevated risk of heat stroke. However, the Boston research found there were significantly more cases of heat stroke in years when the weather started warm and then got hotter, rather than years when it was fairly hot throughout. Why might this be? One reason is that runners tend to run more conservatively in the heat. Another is that if a race heats up in the second half, that coincides with the time when runners are already working hard – thus creating more of their own heat – and, therefore, may push some into heat stroke.

What can runners, take from this? When you check the weather forecast, keep the trajectory in mind. If it feels mild at the start but you know it’s about to get significantly more toasty, consider reining in your pace slightly. In a marathon, that’s generally a good idea, anyway.

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