Weight Loss During Marathon Improves Running Time

Study Suggests Less Fluid Intake May Improve Performance for Marathoners

Reviewed by Laura J. Martin, MD on December 20, 2010

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.

Fluid Intake and Running Time

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.

Take-Home for Runners

Noakes has a simple take-home for distance runners: "Drink to thirst," he says. "That is all you need to know."

But he points out that's very individual. "For some that will be 800 milliliters per hour (28 ounces); for others it will be 100 milliliters per hour (4 ounces)."

Noakes blames the sports drink industry in the U.S. for persuading runners to overdrink during a race. "The sports drink industry in the U.S. unfortunately promotes the false idea that drinking to thirst is not adequate. That is why athletes in the U.S. are more likely to overdrink than athletes in any other country."

Second Opinion

William Roberts, MD, medical director for the Twin Cities Marathon in Minneapolis-St. Paul, Minn., reviewed the study for WebMD but was not involved in the research.

''Fast runners are fast despite their body weight and fluid status," he says. "Would they individually be faster at minus 2% [body weight] compared to 0% or minus 6% is hard to judge from this study and others, but as a group the faster runners are carrying less weight at the end of the race."

But he does agree with Noakes on the important point: "Drink to thirst probably works for most and 750 milliliters per hour [25 ounces] may be too much fluid for runners who do not sweat very much."

"Slow runners probably sweat less and have more time to drink, so they really need to pay attention to the risk of drinking too much."

He points out that ''there is no mention of medical outcomes so no conclusions about the safety of losing body weight can be made."

Show Sources


Timothy Noakes, MD, Discovery Health Professor of Exercise and Sports Science, University of Cape Town, South Africa.

Zouhal, H. British Journal of Sports Medicine, published online Dec. 21, 2010.

William Roberts, MD, director, University of Minnesota St. John's Hospital  Family Medicine Residency; medical director, Twin Cities Marathon, Minneapolis-St. Paul, Minn.

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