How Much to Drink During a Marathon

The 2011 Chicago Marathon on Sunday marks the beginning of the fall marathon season in the United States, culminating on Nov. 6 with the New York City race. In those two events alone, more than 80,000 runners will attempt to cover the 26.2-mile marathon distance. But two newly released studies suggest that there are reasons to be concerned about some of the racers’ readiness. The studies show that a worrying large percentage of distance runners may not know how to drink.

Some runners may be drinking too much water or other fluids. Others may be taking in too little. And a disconcerting majority don’t seem to be concerned about whether they are drinking a safe amount at all, according to the new reports.

Attitudes and expert guidelines about how much fluid people should drink during prolonged endurance events have changed drastically in the past 15 years. A 1996 Position Stand from the American College of Sports Medicine concluded that “athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.” Many of us who ran a marathon in the 1990s were cautioned to “stay ahead” of our thirst, with the warning that by the time we felt thirsty, we would be clinically dehydrated. (Formal definitions of dehydration vary, but most experts agree that losing more than 3 percent of your body weight can be considered dehydration.)

But in the past few years, several marathoners died as a result of drinking too much, a dangerous condition called hyponatremia, or water intoxication. Before then, hyponatremia, marked by low blood sodium levels, had been unheard of in marathon fields. Twenty years ago, a typical marathon racer strode fast and drank little. But as the event gained popularity, finishing times rose. Slower runners generally sweat less, and many have been told to drink copiously. If you ingest more fluid than you lose through sweating or urination, however, you dilute your blood’s sodium levels. Osmosis then draws water from the blood into body cells to equalize sodium levels, and those cells swell. If the cellular bloating occurs in the brain, it can be fatal.

Most experts have now begun advising marathon runners to drink less. They’ve focused on marathoners because hyponatremia is uncommon in events that last less than four hours or so (at least for middle-of-the-pack and slower competitors). Recent guidelines from the International Marathon Medical Directors Association explicitly say to drink only when you’re thirsty.

But the new studies show that many marathon runners are not paying heed. In one of the reports, which appears in the current issue of Sports Health, researchers surveyed 419 men and women who were training for the Chicago Marathon. Most were in their late 30s or early 40s, and they had been running, on average, for 10 years. A third were training for their first marathon, and 17 percent had run a single marathon before.

A majority of the 419 runners reported a notable nonchalance about proper hydration. Almost 65 percent responded that they were “not at all” concerned about keeping themselves properly hydrated during the upcoming race. Notably, when asked specifically whether they worried about the possibility of developing hyponatremia, 63 percent said that they were not.

A second survey, conducted by researchers at Loyola University Medical Center and published in June in the British Journal of Sports Medicine, reached the starker conclusion that almost half of runners may be drinking too much during their races. This survey, which recruited experienced runners from the Chicago area, asked the respondents directly about how they hydrate and why.

Just half of the runners surveyed by the Loyola researchers reported drinking only when they felt thirsty, the yardstick now recommended by most sports experts. The others drank according to a preset schedule of some kind, and almost 10 percent told the researchers that they drank “as much as possible.”

“There is still a widespread misconception that you have to quote-unquote ‘stay ahead’ of your thirst,” said Dr. James Winger, a professor of family medicine and lead author of the study. “That idea is contrary to science, and it is dangerous.”

But as his study’s results indicate, it remains surprisingly pervasive.

Even those athletes who reported in the survey that they drank when thirsty weren’t typically responding to expert recommendations, Dr. Winger says. Most would have liked to drink more, but had “experienced gastrointestinal distress,” he says, a condition that seems to be more persuasive, so far, than science at preventing athletes from overdoing fluids.

The lesson of his and other studies, Dr. Winger is quick to point out, is not that endurance athletes should avoid hydrating. “The lesson is that you should drink only when you need to, when you’re thirsty,” he says. “That is the best way to protect yourself against hyponatremia” and also against dehydration. “Thirst is a very reliable indicator” of your body’s actual hydration status, he says.

“Now we just have to persuade” those 80,000 or so runners toeing the line at Chicago, New York and other marathons in the next month “to listen to it.”