Running in the kind of hot weather we’re
having these days can be fun. It can also be uncomfortable, exhausting, depleting. In extreme cases it’s even dangerous. Seriously, how do you exercise in the heat?

Part of the answer is obvious. Dress lightly. Run early in the morning, which is usually the coolest time of day. Run in shade when possible. (Try the bike path.) Drink plenty of fluids. But in terms of maintaining fitness, and certainly of being competitive in a race, the key is to train your body to handle hot-weather exertion.

When I started running regularly, I was working summers in Atlanta. I got mentally accustomed to running in hot weather – actually I ran in the early afternoon -- and after a few weeks I became physically adapted to the effort.

Smith College exercise science professor Jim Johnson, a regular contributor to these pages, confirms my impressions. “I believe acclimatization is the key to exercise in the heat,” said Jim in a conversation last week. He suggests that it takes at least two weeks of exercising in the heat to condition yourself for a hot-weather race, and he emphasizes a further point: “More intense exercise is better than simply taking long runs or bike rides.”

The New York Times must have tapped my phone. A day after my conversation with Jim, the Times ran a story on exercising in hot weather, echoing Prof. Johnson’s words almost exactly. The Times writer noted that shorter bursts of intense activity seem to be more valuable than longer, milder exercise. She also wrote that to get thoroughly accustomed to the challenges of hot weather, you should eventually be able to sustain a workout for 90 minutes or more.

I’d recommend that much time if you’re training for a marathon, but if your goal is a personal best in the Tuesday night Northampton cross-country race, I think an hour’s hard workout twice or three times a week, plus a couple of easier days, is enough. Include a few short, quick intervals.

Summarizing the science behind these recommendations, Jim Johnson observes that as you become accustomed to working out in the heat, your body adapts by increasing the plasma volume (i.e. the liquid part) of your blood. More of your blood gets closer to your skin, which is how you lose heat through convection, and your body learns to sweat earlier, which means you get rid of excess heat sooner via evaporation.

Incidentally, one way you can run better in hot weather is to be short. Short people generally enjoy a higher ratio of skin surface area to body weight and so can get rid of heat faster. I’m 5 feet, 10 inches tall, which is not very short, but those summers in Atlanta helped.

As for hydration, here’s advice from years of talking with exercise scientists: During a workout, it’s ideal to drink at least half a cup of water every 15 or 20 minutes. Added sugar of some sort may be useful; according to a lot of research, your body absorbs a six percent carbohydrate solution faster than plain water. Gatorade is six percent carbohydrate; so is apple juice diluted 50-50 with plain water. Soft drinks are more like 12 percent – too high. And most people’s bodies absorb colder liquids faster than warmer ones.

Years ago I was told to take salt tablets during or after hot-weather exercise. That was a mistake. Sweat is hypotonic; i.e., the concentration of salt in your sweat is lower than it is inside your body. The one big exception is if you’re competing in an Ironman triathlon or other extremely long endurance event. In contests lasting nine, 10, 12 hours or more, some athletes who drink only water suffer hyponatremia, which can be fatal. These are the athletes who need electrolyte replacement drinks during a race.

I share Prof. Johnson’s skepticism about high-tech fabrics that supposedly wick away moisture from your skin. I find some high-tech synthetic fiber T-shirts uncomfortable in hot weather. Occasionally I run shirtless, but usually in very hot weather I wear an old thin white cotton T-shirt that easily lets perspiration escape. Light-colored clothing absorbs less heat than dark.

A hat helps keep the sun’s radiant heat off your head. On the other hand, your body can lose up to 40 percent of its excess heat through your head, so not wearing a hat may help.

Inevitably, specific aspects of adapting to heat vary with the individual. You can figure out what works for you.

John Stifler has taught writing and economics at UMass and has written extensively for running magazines and newspapers. He can be reached at jstifler@umass.edu