Are You Drinking Too Much Water?
Pop quiz: When should you drink water when training for, or running, a race?
A. As often as you can.
B. On a schedule, based on how much you sweat.
C. When you’re thirsty.
If you said A or B, you’re a few years behind the times—but you’re not alone. About half of runners use one of those strategies when training and racing for distances from 5K to the marathon, according to recent survey results published in the British Journal of Sports Medicine.
You may have been told you’re already dehydrated by the time the urge to take a sip kicks in. But the latest research suggests it’s best to use thirst as your guide, says lead author James Winger, M.D., a sports medicine physician at Loyola University Health System in Maywood, Ill.
The old fluid rules
It’s not surprising that runners would be confused. Over the past decade or so, advice on what to drink and when has changed based on both exercise science and sports-drink marketing.
Runners were once told to drink a preset number of ounces per mile or per hour, Dr. Winger says, but that didn’t account for individual differences in sweat rate. More recently, expert guidelines advise a personalized approach. The American College of Sports Medicine recommends weighing yourself before and after a run to estimate your fluid losses, then drinking enough to stay within 2 percent of your starting weight.
A better way to measure hydration
But there are a few problems with these strategies, Dr. Winger says. For one, body weight isn’t actually a very precise measure of hydration, after all.
The better measure is something called blood osmolatity, a ratio of how much fluid vs. how many other particles (like sodium) are in your plasma. A part of your brain called the hypothalamus is charged with keeping your osmolality in balance.
When you sweat enough that your osmolality increases by more than 1 or 2 percent—meaning that you have slightly more particles than fluid—your hypothalamus triggers your pituitary gland to release a hormone called vasopressin. This tells your kidneys to soak up water that would have otherwise become urine, sending it to your bloodstream to replace what you’ve lost.
If your osmolality increases another 1 to 2 percent or so (the exact amount varies by person), your hypothalamus sends a signal to your conscious brain: It’s time to take a drink. That’s when you become aware of being thirsty. “Thirst is literally older than humanity—it’s an inborn process that has been present for millions if not billions of years, and it’s pretty fine-turned,” says Dr. Winger.
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Deadly dangers of drinking too much
If you drink more than your body’s telling you, you risk a dangerous drop in blood sodium concentration called hyponatremia. When you down too much water, your body starts shuttling it from your blood to your body tissue cells to maintain the right osmolality—which becomes a problem for brain cells, packed tightly inside your skull.
Brain swelling from hyponatremia can cause confusion and convulsions, and has killed a handful of marathon runners in previous years. Studies suggest between 0.3 percent and 13 percent of marathon finishers have some evidence of its effects. Stats on dehydration are harder to come by, in part because there is no standard definition for the term. What’s known is that while almost all runners lose some body weight while running, “no one has ever documented a death due solely to dehydration during an athletic competition,” Dr. Winger says.
Do sports drinks help? Debatable
Whether drinking sports drinks instead of water can save you from hyponatremia is also controversial. Of course, they’re marketed as a way to replenish lost sodium. But drinking too much of them still dilutes your blood, Dr. Winger says, because their sodium concentrations (about 30mmol/L) are closer to that of water (zero) than of blood (136-145 mmol/L).
A recent re-analysis of Gatorade-funded data by Timothy Noakes, a leading sports-science researcher at the University of Cape Town, concurs. The amount you drink, rather than what you drink, accounts for 95 percent of the changes in your blood sodium levels, he found. His study found that there might be a slight difference between sports drinks and water, but it’s not clear how significant the variation is. (There are other reasons to rely on sports drinks for longer sweat sessions. Read musclemorphosis.com for you.)
What’s more, it’s perfectly normal to lose body weight while you exercise, and—unless taken to extremes—it doesn’t seem to hurt your performance, Dr. Winger says. A meta-analysis of previous research also published earlier this year in the British Journal of Sports Medicine found cyclists who lost up to 4 percent of their body weight didn’t experience any declines in speed, and that those who drank only when they were thirsty performed the best.
A recent look at 650 marathoners by Noakes found that the fastest finishers lost the most weight. This adds to a pile of data on those who win marathons, triathlons, and other endurance events. Many drop 3 to 4 percent of their body weight, or more, during the event. That doesn’t prove that these athletes perform better because they’re dehydrated—but does suggest that this level of weight loss didn’t slow them down, Dr. Winger says.
Your super-simple hydration plan
Chances are, you’re not trying to win a marathon. But even if you’re just going out for a half-hour or hour-long run, it’s still best to rely on your body’s cues to know when to chug, says Eric Goulet, Ph.D., of Sherbrooke University in Canada, the lead author of the cycling paper. Other expert groups, including the International Marathon Medical Directors Association and USA Track & Field, now advise athletes to follow their thirst as well.
The strategy works when you’re off the path or track too, say Drs. Winger and Goulet. Drinking water to quench your thirst throughout the day helps you start your run in the perfect state of fluid balance.
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