How to Hijack Your Physiology to Become a Smarter Sweater in the Heat

Often, as spring progresses toward summer, runners warn me that they won’t do well because they aren’t good at running in heat. 

My answer is always the same. “You’re going to have to prove that.” I then tell them of a trainee who, after accepting that challenge, shattered her 5K PR in an 87°F road race. Admittedly, it was an evening race, with a lot of shade. But what really made the difference was that she’d taken time to acclimate. 

One way to do this is simply by embracing the heat as the days warm from 65°F to 70°F, 75°F, and beyond. Don’t hide from it by running at dawn, but use it to make yourself gradually more heat tolerant. In my PR years, I would wind up running in late afternoon, with the sun not only hitting me, but reflecting back at me from white-painted buildings that flanked one of my favorite running routes. Hot? What’s that? I loved it. Especially because pretty much everyone else was melting.

That’s one way to adjust.

But if needed, you can do it a lot more quickly, says Lawrence Armstrong, a heat researcher at the University of Connecticut. Only a few days of focused heat acclimatization, he says, can produce an amazing array of physiological adaptations, all designed to make your body sweat more efficiently.

Increase Your Sweat Reservoir

One is that your plasma volume increases. This may increase your weight by a couple of pounds, but don’t worry about it: that’s an extra liter of fluid your body is able to sweat away without dehydrating. “You have a greater reservoir to work with,” Armstrong says.

At the same time, your sweat becomes less salty, conserving sodium. That is useful, says Matt Harber of Ball State University, Muncie, Indiana, because conserving sodium helps you maintain proper fluid balance, even as you sweat.

You also start to sweat more profusely, and earlier on in the workout. 

That used to be seen as a sign of being out of shape, says Yannick Molgat-Seon, an exercise physiologist at the University of Winnipeg, Manitoba. “But it’s actually a good thing,” he says — an indicator that your body is anticipating what’s soon to come and taking preemptive measures to deal with it. Harber agrees. “Better sweaters are in better shape,” he says.

Optimize Heat Output 

Another change is that your body diverts more blood to the skin. 

That has two effects: One is that it allows sweat glands to increase their output. But when you are running, 75-80% of the energy expended by your muscles winds up as heat, and the more easily your blood can carry this heat to the skin where your sweat can eliminate it, the less there is to accumulate in your muscles.

Pump More Blood 

Finally, your heart rate during exercise lowers. 

On first blush, that doesn’t sound like a good thing — after all, you want as much blood as possible circulating to your muscles (for running) and your skin (for cooling). But it turns out that slowing the heart rate at any given exertion level allows the heart to fill more completely between beats. That actually allows it to pump more blood per minute, not less.

Daniel Lieberman, the evolutionary biologist and exercise researcher of “Born to Run” fame, thinks these adaptations are holdovers from our distant past. Early humans, he says, came from the African savannas where they evolved to hunt and forage in midday heat, when it was too hot for less heat-tolerant predators.

We no longer have to avoid becoming lunch to a lion, but deep in our DNA, he suggests, we retain the genes that allowed our ancestors to survive. All we need to do is to reawaken them when needed.

10 Days from Heat Wimp to Heat Champ

I once lived in Texas, and have visited it several times. It’s a place where summer heat can set in with a vengeance, as though someone flipped a switch, and one rule of thumb I picked up from the local runners is that it takes about 10 days of running in heat to adjust. 

Brett Ely, a 2:38 marathoner who is now an assistant professor of exercise science at Salem State University, Massachusetts, agrees. She once helped the Army develop a protocol for training soldiers for deployment to Iraq, where temperatures can exceed 110°F. Eight to 14 days, she says, is all you need.

Nor do you need to spend extended amounts of time each day in heat. All the way back in 1963, a research team led by someone named A.R. Lind found that there was no advantage to extending heat-training workouts beyond 100 minutes. An hour is probably good enough.

Heat-training workouts don’t even need to be all that hard. In a 1991 paper in Sports Medicine, Armstrong found that all that’s really needed is to run at something more than 50% of VO2max — basically an easy jog.

I myself have found that you can assist the process by measures unrelated to running, such as being sparing in your use of air conditioning, especially in your car. Sitting in traffic with the windows down and the air conditioning off isn’t the same as running intervals on the track, but heat is heat, and if you can get comfortable doing that, it will help.

You also don’t need to bake yourself every day. My runner took two months to adapt for her PR at 87°F, and another old study, this one by someone named J.T. Fein, bears her out. Training once every three days for a month, Fein found, is every bit as effective as training daily for ten days. What seems to matter is getting in about 10 heat-training sessions, overall — just as my Texan friends taught me, all those years ago. 

Caveats  

There are, of course, caveats. 

Be aware of the symptoms of potentially dangerous overheating. A sudden sensation of feeling chilled, even though it’s hot, is a red-flag warning to “stop now.” You should also check to make sure you’ve not stopped sweating, an even stronger warning of incipient heatstroke. Nausea on a hot day is another sign you may have crossed into the danger zone.

“Start gradually,” Ely says. “If you were going to high altitude, you wouldn’t do a hard workout on the first day. You’d ease into it.”

But, don’t just write off summer as too hot. Embrace the heat (cautiously) and see what surprises might lurk in your ancient savanna-hunter genes.