Drinking Fluids Doesn't Cool Runners
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
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
This suggests "that pacing in the early part of the race is an important
strategy in the avoidance of exertional heat illness," the researchers
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,