For years, it seemed, runners couldn't get enough to drink. They carried fluids as they ran, stashed bottles along the road for long runs, and threw back multiple cups at every aid station during races. Then a few high-profile cases of hyponatremia (overhydration and a diluting of blood-sodium levels), including a young woman who died during the 2002 Boston Marathon, struck fear in the hearts of guzzlers. Suddenly, runners went from worrying about not drinking enough to worrying about drinking too much.
If you're confused, and maybe even a little afraid that erring to one extreme or the other could have dire consequences, join the club. The guys in the white lab coats don't even agree. The International Marathon Medical Directors Association (IMMDA) released its long-awaited hydration guidelines, which concluded that runners should, simply, drink when thirsty. "The new scientific evidence says that thirst will actually protect athletes from the hazards of both over- and underdrinking," says the IMMDA announcement.
This shockingly easy prescription counters years of advice from organizations such as the American College of Sports Medicine (ACSM), and even the IMMDA itself. In fact, as this issue went to press, the ACSM and other groups were re-examining their hydration guidelines. So as the hottest, sweatiest, most humid days of summer approach, what's a runner supposed to do about the drinking issue? We talked to experts on both sides of the great hydration debate to bring you the most up-to-date thinking on how much fluid runners really need.
The Over and Under of Hydration
There's no denying that a serious shortage of fluids can cause problems. At the tail end of every marathon or ultra, there are always several runners staggering into the medical tents, suffering from nausea, diarrhea, and weakness caused by dehydration. "It's pretty common for athletes to hit at least one or two percent dehydration during endurance events," says Craig Horswill, Ph.D., senior research fellow at the Gatorade Sports Science Institute. "The body's temperature-regulating mechanism is affected even at one percent dehydration."
It is, therefore, Horswill's (and Gatorade's) opinion that even small amounts of dehydration should be avoided because it will affect performance. He, as well as the ACSM, recommends staying less than two percent dehydrated. For a 130-pound woman, that would mean losing no more than 2.6 pounds of fluid (and ideally, much less) during any run.
But other experts point to the fact that most elite marathoners typically finish—and win—races significantly dehydrated because they don't take the time to drink much along the course. "Would they perform even better if they drank more? I doubt it," says Lewis Maharam, M.D., medical director of the ING New York City Marathon and chairman of the board of governors for the IMMDA. "There is no evidence that you have to replace 100 percent of lost fluid during a race."
At the other end of the spectrum, overhydrating can be even more dangerous than not drinking enough. Hyponatremia occurs when your fluid intake exceeds your rate of fluid loss from sweating, which results in low blood-sodium levels. Symptoms—nausea, disorientation, muscle weakness—can be similar to dehydration. Giving additional liquids to hyponatremic runners only exacerbates the problem by diluting their blood-salt levels even more, which can lead to coma and, in the worst cases, death.
Experts hypothesize that hyponatremia has become more of an issue in the past few years because so many beginning runners are attempting marathon and ultramarathon distances. This means people are on the course for five or six hours, or even more. "Slower, back of the pack runners who aren't sweating as much don't need to replace so much fluid," says William O. Roberts, M.D., past president of ACSM and long-time medical director of the Twin Cities Marathon. Women, smaller runners, slower runners, and those who are not as well trained face the greatest risk of hyponatremia.
"Based on the lab data we have, women tend to over drink and they typically have a lower sweat rate than men," says Horswill. The idea that every runner needs to down a cup at every aid station can be a dangerous one, warns Dr. Maharam, because such a rigid fluid-replacement strategy doesn't account for differences in body size, running pace, terrain, climate, metabolic rate, and sweat rate.
More: Hydration Basics