Myth: Drink eight glasses of water a day.
Truth: You do need a healthy dose of hydration daily, but how much is an individual thing. "The eight glasses a day is totally arbitrary," says Susan Yeargin, Ph.D., assistant professor of athletic training at the University of South Carolina. "Everybody, especially athletes, has different needs." The Institute of Medicine guidelines are more specific, recommending 91 ounces per day for women and 120 for men. But the institute notes that "the vast majority of healthy people adequately meet their hydration needs by letting thirst be their guide."
Myth: Pee clear to be hydrated.
Truth: Clear urine is a bit excessive. "As long as it is a pale yellow, like lemonade, you're hydrated," says Yeargin. If it's completely clear, it just means you're full to the brim; what's going in is coming out. On the other hand, if your pee is the color of apple juice or darker, or particularly smelly, you need to drink up.
Myth: Caffeine dehydrates you.
Truth: While caffeine provides a performance-boosting edge, it also acts as a diuretic, right? Not exactly. "Recent research shows that caffeine doses between 250 and 300 milligrams—about two cups of coffee—will minimally increase urine output for about three hours after consuming it," says Yeargin, "But the research also shows that exercise seems to negate those effects. If you run within one to two hours of drinking coffee, you don't pee more."
Most likely, during exercise, blood flow shifts toward your muscles and away from your kidneys, so urine output isn't affected, Yeargin explains. In addition, if you always have a latte in the morning or a soda at lunch, your body is acclimated to the caffeine, so its effect, on both your physiology and performance, is minimal.
Myth: Thirst isn't a good hydration tool.
Truth: Thirst is definitely a very strong predictor of hydration needs—and some experts would argue it's the only one you need. "Our thirst mechanism is pretty accurate," says Yeargin. "But it's always a good idea to have some other methods to ensure you're hydrated." Knowing your sweat rate is one way to track your needs, particularly for long runs, says Doug Casa, Ph.D., professor of kinesiology at the University of Connecticut and COO of the Korey Stringer Institute.
To calculate your sweat rate, weigh yourself naked before and after an hour run. Keep track of how many ounces you consume during the run and factor that into your calculation postrun. Every pound you lose translates to about 16 ounces of fluid. "Your goal isn't to match your sweat rate," says Casa, "but you should try to get as close as is comfortably possible. For some runners, that may mean replacing two-thirds of the fluid they sweat during the run." He adds that you shouldn't try to consume more fluids than you've lost.
Myth: Pure water is best for hydration.
Truth: Although water is a great way to hydrate, it may not be the best choice in all situations. For an easy, hour-long run on a coolish day, sipping water is fine. But if you're running 10 miles on an August morning and are a salty sweater (you have white salt streaks on your face or clothes postrun), you need to ingest some sodium as well. "Salt helps you retain water," says Yeargin. "You're less likely to pee it out."
A sports drink, such as Gatorade, and water enhanced with electrolytes, like Nuun, are good options; taking high-dose salt tabs before a run is less so. "There's no way to 'preload' with sodium to negate sodium loss," says Yeargin. "You just pee out anything you don't use."
Myth: You can't drink too much.
Truth: "You absolutely can drink too much," says Casa, "and it can be deadly." Too much water can cause symptomatic hyponatremia, a condition where the sodium levels in the blood become dangerously low. Although Casa estimates that fewer than one percent of marathoners develop symptomatic hyponatremia, certain groups are more prone to it, including smaller runners; those who finish marathons in more than four hours; and those who do a significant amount of walking and running in cooler weather (when your sweat rate isn't as intense as it is on warm days). "For recreational runners, the best way to prevent hyponatremia is to listen to your thirst," says Casa.
Myth: Drinking lots of water is a good way to "detox."
Truth: "There is no evidence that excess water makes your body more clean," says Dr. Stanley Goldfarb, M.D., a professor of medicine in the Renal, Electrolyte, and Hypertension Division at the University of Pennsylvania's Perelman School of Medicine. "If anything, drinking too much water can slightly impair the ability of the kidneys to filter blood." He adds that the only people who should drink more water with a focus on their kidneys are those who have had kidney stones.
Myth: Staying hydrated eliminates your risk of heat stroke.
Truth: Heat stroke is a life-threatening condition where your body temperature rises above 104 degrees. Dehydration can make you more prone to it. "People who are dehydrated are hotter," says Casa. In fact, in a study published in the Journal of Athletic Training, Casa determined that for every one percent of body mass lost through sweat, your body temperature increases by half a degree, "which makes hydration hugely important for preventing heat stroke," he says.
But there are still a number of other factors that play a role. Body size, exercise intensity, fitness level, and age as well as humidity and air temperature can affect who does or doesn't develop heat stroke, says Casa. Certainly staying hydrated is a good call and can reduce your risk, but paying attention to the whole picture is a better predictor.