Drink to avoid dehydration during exercise or hydrating too much
How much should I drink to avoid dehydration during exercise?
It’s the first lesson you learn about exercising in the heat: if you don’t drink enough, you’ll get dehydrated, and that will force you to slow down. By the time you feel thirsty, we’re told, it’s already too late. But some exercise physiologists, led by South African researcher Tim Noakes, have reached an alternative conclusion. It’s not being dehydrated that actually slows you down, Noakes argues; rather, it’s letting yourself get thirsty that signals to your brain to put the brakes on.
The idea that drinking according to thirst isn’t enough to replenish your sweat losses is supported by plenty of research. For example, a 2007 study by researchers at the Gatorade Sports Science Institute found that experienced runners who were allowed to drink as much as they wanted during a 75-minute run replaced only about 30 percent of the fluid they lost by sweating. The conclusion the Gatorade scientists drew from this study is that we should plan to drink much more than thirst alone dictates—but Noakes disputes this interpretation.
The initial studies linking dehydration with impaired performance date back to World War II, as researchers sought to give U.S. military forces fighting in desert or jungle conditions an edge over their enemies. Since then, dozens more studies have shown that if you dehydrate someone (by administering a diuretic, for example), their exercise performance will suffer. Similarly, when subjects aren’t allowed to drink during prolonged exercise, their performance is decreased. Based on these studies, researchers concluded that sweating out more than about 2 percent of your total body weight will slow you down.
But the subjects in all these studies are not just dehydrated; they’re also forced to become thirsty. None of the studies show that subjects who drink freely according to thirst (and thus fail to replace their sweat losses) perform any worse than subjects who drink enough to replace all their sweat. Both thirst and the slowdown that eventually follows are the body’s way of protecting itself in advance from damaging dehydration, Noakes argues. The flaw in the conventional studies is illustrated by a 2006 study in which subjects were told in advance whether or not their fluid intake would be restricted during a 50-mile cycling trial. When they knew they wouldn’t be able to drink freely, the subjects biked more slowly right from the start of the trial, long before any physical effects of dehydration could have had an impact.
In Noakes’s “central governor” theory, the brain monitors signals from various parts of the body with the goal of reducing exercise intensity before the body is in any danger of damaging itself. As fluid levels drop—but before they reach the point at which performance would be compromised—the central governor responds by initiating thirst and reducing intensity. In this picture, you won’t slow down unless you actually feel thirsty, no matter how much fluid you’ve lost. Studies of endurance athletes show that the thirst mechanism varies greatly between individuals: some drink very little during races, while others drink a lot. Interestingly, the fastest finishers tend to also be the most dehydrated—a finding that lends support to Noakes’s argument.
Until a few years ago, few researchers paid attention to Noakes’s ideas. But the potentially fatal dangers of drinking too much—which were first pointed out by Noakes in the 1980s but ignored for nearly two decades—have caused a re-evaluation of hydration guidelines. Most authorities still recommend aiming to limit sweat losses to less than 2 percent of your body weight, but the consensus is no longer as strong. In that light, it’s worth considering the advice Noakes gave in a 2007 article in Medicine & Science in Sports & Exercise: “Drink according to the dictates of thirst. If you are thirsty, drink; if not, do not. All the rest is detail.”
Is it possible to hydrate too much?
In 2007, 28-year-old mother of three Jennifer Strange collapsed and died in her Sacramento home after taking part in a contest—“Hold your Wee for a Wii”—sponsored by a local radio station. The goal was to drink as much water as possible without going to the bathroom, with the winner earning a Nintendo Wii. Strange’s death was blamed on a condition called hyponatremia, sometimes known as water intoxication. Simply put, she had drunk so much that the levels of sodium in her blood were diluted, causing dangerous—and in this case fatal—swelling of the brain.
Over the last decade, medical directors at major marathons have sounded the alert about the dangers of overdrinking, thanks to about a dozen deaths at running races attributed to hyponatremia since the condition was first identified at the Comrades ultra-marathon in South Africa in 1981. Because the condition was so obscure, medical teams often mistook the symptoms for dehydration and pumped more fluids into collapsed runners, making it worse. Now that race personnel are aware of the dangers, these mistakes are less likely to occur.
Still, the condition is far more common than most people realize, since its first stages may not produce any obvious symptoms. Researchers at the University of London recruited 88 volunteers taking part in the 2006 London Marathon to give pre- and post-race blood samples in order to measure sodium levels. To their surprise, 11 of the volunteers (or 12.5 percent) developed asymptomatic hyponatremia, as indicated by abnormally low sodium levels—a particularly high number considering the cool and wet conditions of that year’s race, which discouraged excess drinking. As expected, the runners who developed hyponatremia drank more frequently during the race (typically every mile) compared with those who stayed healthy (typically every two miles).
None of the runners in the study suffered any ill effects, but the high prevalence of low sodium levels suggests that many runners are still aiming to drink as much as possible along the route. Those who take more than four hours to complete a marathon are thought to be at higher risk, because they have more time to ingest water. Even though sports drinks have sodium in them, there’s no evidence that they’re less likely to cause hyponatremia. The best solution is simply to avoid drinking too much—no more than about eight ounces every 20 minutes, according to some experts. Or according to others, only when you’re thirsty.
What ingredients do I really need in a sports drink?
If you’re an old-school type who thinks plain water is all you need, consider this puzzling fact: rinsing your mouth with a drink containing carbohydrates will boost your athletic performance, even if you don’t swallow and can’t taste the carbs.
Of course, it’s not just carbohydrates that you find in sports drinks these days. The latest offerings feature a bewildering array of formulations aimed at different sports and levels of activity, along with high-tech additives that purport to improve everything from alertness to metabolism. But you should be wary of the hype surrounding these magic ingredients. The core of any sports drink remains simple, says University of Guelph researcher Lawrence Spriet. Here are the three key ingredients, in order of importance:
• FLUIDS: The first point is simple: “If you’re engaging in physical activity you’re going to lose fluids,” says Spriet, who also serves as chair of the Gatorade Sports Science Institute’s Canadian advisory council. Although the precise link between dehydration and performance is still a topic of debate, it’s clear that letting yourself get thirsty during exercise will compromise your performance.
• CARBOHYDRATES: The second element is carbohydrates, which are typically found in sports drinks in the form of glucose or other easily digested sugars. The goal is to maintain blood-sugar levels and replace glycogen stores in hard-working muscles, which is essential in bouts of exercise lasting longer than an hour. Sports drinks traditionally contain about 6 percent carbohydrate, about half the level of a typical juice or soft drink. That’s about as high a concentration as your stomach can handle before absorption is slowed down, according to a recent study by University of Birmingham researcher Asker Jeukendrup in the journal Nutrition & Metabolism. Newer formulations such as Gatorade’s G2 have cut the carb level to 3 percent, which is more appropriate for people who aren’t pushing hard for hours at a time and thus don’t need the extra calories.
Scientists have long been puzzled about why carb-filled drinks also seem to help in shorter bouts of exercise, when energy stores shouldn’t be an issue. Researchers at the University of Birmingham published a study in 2009 showing that cyclists performed better in a time trial if they rinsed their mouth with a drink containing either glucose or a tasteless carbohydrate called maltodextrin but saw no improvement from rinsing with an artificially sweetened drink. Brain scans showed that glucose and maltodextrin made the reward centers in the subjects’ brains light up while artificial sweetener didn’t, suggesting that our mouths have previously unknown carbohydrate sensors. Sports scientists have now begun advising elite athletes to rinse and spit out sports drinks as they approach the end of grueling endurance races, when their stomachs may be unable to handle drinking anything.
• SALTS: Electrolytes, which replace the salts lost in sweat, are thought by some researchers to play a role in muscle cramping but are more relevant to post-exercise recovery for most people. “You have to be working really, really hard for the salt to matter,” Spriet says. Gatorade makes a little-known product called GatorLytes, which is essentially a sachet of salts that you add to regular Gatorade to up its electrolyte content—but it’s available only to high-level sports teams, since typical athletes simply don’t need it.
For the average person working out at the gym for an hour or less at a time, there’s no need to drink anything other than water. If you prefer a sports drink, choose one with a smaller amount of carbohydrate (3 percent is better than 6 percent), or simply dilute a standard sports drink with water. And don’t put your faith in the magical claims of high-tech additives in some sports drinks—because beyond the three ingredients listed above the science gets a lot weaker. In fact, Gatorade’s relaunch of its product line in the United States coincided with the decision to disband its U.S. scientific advisory panel in 2009. The new line boasts specialized formulations such as vitamin C to perk you up in the morning, B vitamins to help you metabolize energy, theanine to improve focus, antioxidants to “protect your body,” and so on. Spriet is unimpressed. “Everyone wants to make things more complicated, but there’s a reason the basic formulation hasn’t changed in years,” he says. “It’s fluid, sugar, and salt. That’s all it is—and it works!”