Dec. 20, 2010 -- Marathoners who lost the most body weight during a race finished more quickly, even though their weight loss exceeded the amount typically recommended for best running performance, according to a new study.
The weight loss reflects a lower level of fluid intake or a faster rate of fluid loss.
Most runners are cautioned to ''hydrate, hydrate, hydrate." But the new study suggests that might be too much of a good thing for performance results.
"The clear evidence is that drinking just to thirst is the optimum strategy," says researcher Timothy Noakes, MD, the Discovery Health Professor of Exercise and Sports Science at the University of Cape Town in South Africa.
"Drinking either more or less than to thirst impairs exercise performance," he tells WebMD in an email. "Drinking more can be fatal, as has occurred to a number of U.S. female marathon runners and soldiers."
In recent years, U.S. marathons have experienced participant deaths due to excessive intake of water, which can lead to an abnormally low level of sodium in the blood, or hyponatremia.
The study is published in the British Journal of Sports Medicine.
Noakes studied runners participating in the 2009 Mont Saint-Michel Marathon in France.
At the start, 884 marathoners agreed to participate, and 643 of those finished and were included in the study results.
The study participants -- 560 men and 83 women -- weighed in at the start of the race and then at the finish.
All were given the same pre-race guidelines: try to drink about 250 milliliters (8.5 ounces) of either water or a 4%-8% carbohydrate-containing drink every 20 minutes during the race.
On average, runners in the group overall lost 2.3% of their body weight during the race. Among sports experts, there is debate about the issue, but many experts warn that a weight loss above 2% can impair athletic performance.
But when Noakes looked at the weight loss and finishing times, he found those who lost the most ran the fastest.
The three groups, by time, and their weight loss:
Those who finished under three hours averaged a 3.1% body weight loss.
Those who finished between three and four hours lost 2.5%.
Those who took more than four hours to finish lost 1.8%.
The results echo those of other studies, Noakes writes.