Running in the heat: Rules of hydration and body temperature regulation |

Running in the heat: Rules of hydration and body temperature regulation

Greg Jaeger competes in the running portion of the half-Ironman triathlon Sunday
Austin Colbert

— Training in heat can have performative benefits, since the body will be able to better adjust to changes in internal body heat. But without the proper preparation and risk management, aerobic exercise can stress the body, sometimes to a fatal degree.

“The big danger of course is that, without the right hydration, your core levels get so high that your internal organs overheat,” said Marietta Roberts, fitness director at Old Town Hot Springs. “If they overheat too much, they can stop working completely.”

The National Athletics’ Trainer’s Association published a dehydration study performed in both the lab and the field. During the lab tests, researchers found that dehydration reduces the overall water volume in the body, which in turn reduces central blood volume and blood flow to the skin.

To compensate for this, the subject’s heart rate increases in an attempt to increase the blood flow to the skin, since the body’s thermoregulation is a balance of metabolic heat production and the exchange of heat with the environment. For every 1 percent body mass loss, the subject’s core body temperature and heart rate increased by .12 to .25 degrees Celsius and three to five BPM, respectively.

When the body cannot thermoregulate quickly enough to keep the body cool enough, motor units shut down.

During the field study, researchers had runners with different levels of hydration run to attain the same finishing time. They found that when the more hydrated runners ran faster, they had a lower body core temperature than the dehydrated runners. Furthermore, uncontrollable external factors like humidity and air flow caused the core body temperature and heart rate to increase at a rate faster per percent body mass loss than in the lab.

Overall, the study found that physiological strain due to dehydration and higher body core temperature decreased performance.

Despite these risks, athletes are not inclined to stop all aerobic exercise on hot summer days. They have races to run, miles of road to bike and a body to maintain.

“The number one rule is to hydrate in this type of weather,” said Josh Boles, owner of Twisted Trails Running Co. in Steamboat Springs. “During a race, you don’t want to skip a water spout. Also, you have to adjust your pace to the heat, or else the rate of dehydration accelerates.”

Dehydration exhibits a variety of physical symptoms, including prune fingers, a dry tongue and yellow-to-orange-colored urine. In extreme situations, athletes can get light-headed, have strong headaches and see flashing, dark spots.

The old jargon surrounding hydration advised athletes to drink as much water as they could. Now, experts advise drinking water with the expectation of losing up to 2 percent body weight and never gaining weight during aerobic exercise.

But how much water does that mean?

Roberts tells runners to drink 16 ounces of water an hour and a half to an hour before and after the run. For endurance runners especially, she suggests carrying a water bottle and drinking some every 10 minutes, known as the “sip-and-carry” method.

“For runs longer than 90 minutes, the body needs more than just water since electrolytes start to rapidly decrease after that,” Roberts said. “Runners should start with water for the first 90 minutes, then drink Gatorade or Powerade or take a salt tablet after 90 minutes so the body system can work naturally.”

To reach Liz Forster, call 970-871-4374, email or follow her on Twitter @LizMForster

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