Rules of hydration and body temperature regulation
Drink plenty of water when exercising outdoors in heat
July 3, 2015
Exercising 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 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.
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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?
Experts say to drink 16 ounces of water an hour and a half to an hour before and after a run or exercising outdoors.