New study gives marathon runners a reason to take in less fluids on the course

Much has been written about the dangers of hyponatremia, a serious condition that can be caused by taking in too many fluids during the marathon.  But many runners still worry about getting dehydrated out on the course.

Now a new study in the British Journal of Sports Medicine suggests that the fastest marathon runners actually drink less and get more dehydrated, with little negative impact.

Dr. Timothy Noakes, a professor of exercise and sports science at the University of Cape Town, led the study.  Noakes has run more than 70 marathons and ultramarathons, and has been researching hyponatremia for years.

Dr. Noakes' most recent study looked at 643 marathon finishers at the 2009 Mont Saint-Michel Marathon in France.  The participants were weighed both before and after the race.  All were encouraged to take in about 8 ounces of water every 20 minutes.

The runners who finished below three hours lost, on average, 3.1% of their body weight .  Runners who finished between three and four hours lost 2.5%, and runners who finished in more than four hours lost 1.8%.

While some research states that losing more than 2% of your body weight can have a negative effect on your performance, Noakes' study would suggest this isn't the case, at least not for elite athletes.

Dr. Noakes says that the most important lesson we should learn from the study is to drink to thirst, and not beyond.  He blames the U.S. sports drink industry for encouraging runners to drink too much.  

"This messaging has promoted the concept that any dehydration that occurs during exercise impairs exercise performance and increases the risk for potentially adverse outcomes," he said.

Whether or not drinking too much makes you slower isn't really proven.  But because slower runners spend significantly more time out on the course, they are also the most likely to suffer from hyponatremia. 

Noakes' study should help send the message that you don't need to down two cups of sports drink at every water stop.  While hyponatremia can kill you, getting slightly dehydrated probably won't slow you down.