The Thirst Connection
The concept of drinking according to thirst may seem too simple to be an accurate barometer of fluid needs. In fact, for years runners have been urged to drink ahead of their thirst — the message being that by the time you feel thirsty, you're already on the road to dehydration. Despite the controversy, there is increasing scientific evidence to support the notion that thirst is actually the ideal way to gauge hydration needs.
Thirst is the basic physiological instinct that the body uses to maintain normal thickness of body fluids. "Humans evolved the thirst mechanism over millennia," says Timothy D. Noakes, M.D., a professor of exercise and sports science at the University of Cape Town, South Africa, and author of The Lore of Running. "It is the only system used by all other creatures on this earth. Why should it not also be ideal for humans?"
Dr. Noakes and his colleagues recently completed a study looking at the connection between thirst and sports performance. They found that drinking less than what thirst dictated resulted in a two percent drop in cycling performance during an 80 kilometer time trial, and that drinking more than thirst dictated did nothing to enhance performance. "We concluded that if you drink according to the dictates of thirst, your performance will be optimized," says Dr. Noakes.
The IMMDA has found the latest body of research on thirst so compelling that in May at a meeting in Barcelona, the group dismissed its own advice that runners stay ahead of dehydration by using an ounces-per-minute fluid-replacement strategy and instead strongly endorsed thirst in its groundbreaking new fluid recommendations. "We're used to hearing that thirst follows too far behind what you really need, but that doesn't hold true scientifically," says Dr. Maharam. "Your body's thirst mechanism is giving you real-time feedback on your internal fluid balance."
This feedback can be especially important when running on steamy August days. According to Dr. Noakes's research, your body will respond to the heat by increasing your thirst. And on the flip side, when you aren't sweating as much or losing as much fluid, your thirst will guide you to drink less.
Dr. Maharam suggests listening to your instincts. "If you come up on a water station and you're ambivalent about downing a cup, you're not thirsty and you don't need to drink," he says. For instance, having a dry mouth—which can be the result of nerves or heavy breathing—doesn't necessarily mean you're thirsty. "But if you see the water at the station and crave it," says Dr. Maharam, "then you're truly thirsty and should have a drink."
Fears of hyponatremia have also fueled controversy over how much sodium runners need to replace along with fluids. According to Dr. Roberts, the need for salt is exaggerated, since the average runner actually loses very little salt during a one- or two-hour run. "As long as you have some salt in your diet, there is probably not a huge need to have salt in your fluids," he says.
The new IMMDA guidelines, however, still favor sports drinks over water when running longer than 30 minutes because they contain both carbohydrate (for energy) and electrolytes. A recent study done at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts, recommends using beverages containing electrolytes to help delay the development of hyponatremia, because the sodium in such drinks can assist in maintaining healthy blood sodium levels.
When it comes to determining your own hydration needs, it's still important to remember that you are an experiment of one, since there is such individual variation in sweat rates among runners. "Thirst is simple and it's based on good, strong research," says Dr. Maharam. But he adds that if you feel you can't rely on thirst alone, you might want to determine your own sweat rate (see "Know Thy Sweat Rate"). This will give you an estimate of your fluid losses during a run so that you can calculate your own rate of fluid replacement.
Another way to assess your level of hydration is to pay attention to the color of your urine. If it's totally clear, you may be drinking too much. If it looks dark—like iced tea—you're definitely not drinking enough. Your bathroom scale can also help. If you gain any weight on a run, you're taking in too many fluids, but if you lose more than two percent of your body weight on a single outing, you probably need to drink more.
In the end, much of the hydration debate comes down to listening to your body—a concept quite familiar to runners. So when you're thirsty or sweating by the bucket-full on a long run, go ahead and drink. But when you're not sweating heavily or you just don't feel the urge to drink, pass by that next water stop.
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