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Study Sees No Link Between Runners' Body Temperature and Hydration

May 10, 2007 - Drinking fluids during a race doesn't keep long-distance runners cool, a study set in the steamy Singapore Army Half-Marathon shows.

Runners are often encouraged to drink lots of water during a long-distance race. Data from earlier studies have questioned that strategy. Proof has been elusive, however, as it's hard to measure runners' body temperature while they are racing.

To solve that problem, Christopher Byrne, PhD, and colleagues at Singapore's Center for Human Performance had soldiers swallow heat sensors before running the 2003 half-marathon. The sensors, about three-quarters of an inch long and about two-fifths of an inch in diameter, send temperature readings to a small recorder strapped to a runner's lower back.

The device worked in 18 of 23 male runners. All of the runners were trained soldiers well adapted to Singapore's hot, humid weather. During the 21-kilometer race, the temperature averaged 80 degrees Fahrenheit with relative humidity peaking at 90%.

The soldiers' water intake was measured before and during the race, and they were weighed immediately before and after the race to calculate water loss.

The soldiers finished the race with times ranging from 105 to 146 minutes. None of the runners suffered heat stroke, but they did get hot. At the finish line, half had core body temperatures of more than 104 degrees Fahrenheit. One runner ended the race with a body temperature of 107 degrees Fahrenheit.

How much water the runners drank, and how much lost fluid they replaced, had no effect on body temperature. In fact, the runner that had the 107-degree body heat was the one who did the best job of fluid replacement.

Byrne says it's important to be well hydrated before running a race but that it's not necessary to force fluids.

"Listen to your body and drink if you feel thirsty," Byrne says in a news release. "But drinking several liters of water [during a marathon] will not help you run any faster."

Byrne and colleagues found that core body temperature during the first 30 minutes of the race seemed to be the most important predictor of heat strain.

This suggests "that pacing in the early part of the race is an important strategy in the avoidance of exertional heat illness," the researchers say.

The study appears in the May issue of the journal Medicine & Science in Sports & Exercise, the official journal of the American College of Sports Medicine. Byrne is a sports scientist at the University of Exeter, England.

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