Best parts of Christie Aschwanden’s book, “Good to Go”:
"We do lose electrolytes through sweat, but even when you exercise continuously for many hours, you will simply correct any losses via your normal appetite and hunger mechanisms. (You’ve already experienced this if you’ve ever had a hankering for a salty snack.) One small study of cyclists and triathletes found that it didn’t really matter whether they drank plain water, a sports drink or a milk-based beverage after an hour of hard exercise. As long as they drank some liquids along with a meal, they restored their fluid levels just fine.
The problem with this model of hydration is that it overlooks basic physiology. It turns out, your body is highly adapted to cope with losing multiple liters of fluid, especially during exercise. When you exercise, you lose fluid and salts through sweat, and that translates into a small change in what’s called your “plasma osmolality” — the concentration of salts and other soluble compounds in your blood. You need enough fluid and electrolytes in your blood for your cells to function properly, and this balance is tightly regulated by a feedback loop.
When you sweat, your brain senses the corresponding rise in plasma osmolality and directs the release of antidiuretic hormone (ADH), which prods the kidneys to activate aquaporins, which are like tiny straws that poke into the kidneys to draw water back into the blood. It’s a pathway to conserve water. As your body reabsorbs water, your plasma osmolality returns to normal, your brain senses the change, and it shuts down ADH. This feedback loop is finely tuned to keep plasma osmolality in a safe range. Even a tiny drop in electrolytes will activate this system to keep your fluid balance in check. People always worry they’re going to be dehydrated when the reality is, it’s much easier to over- hydrate because our bodies are so good at conserving water. Being a little dehydrated is not a bad thing. Our bodies can handle it.
Athletes who develop hyponatremia during exercise usually get there by drinking too much because they’ve been conditioned to think they need to drink beyond thirst. Even if you don’t drink anything (which she does not recommend), your blood sodium levels will rise in response to sweat losses, and as a result, your body will shift fluid into the blood to maintain your fluid balance.
The same feedback loop that calls in the aquaporins also activates your thirst. “You don’t have to drink above thirst — you’ll be fine!” she says. Just as sleepiness is your body’s way of telling you that it’s time to sleep, thirst is how your body ensures that you seek fluids when you need them. No one tells you to sleep before you’re tired, and unless you’re in a situation where you can’t drink for a prolonged period, there’s no sense in drinking before you feel thirsty either. Your body is a finely tuned machine that that is capable of adapting to changing conditions, and it’s not usually necessary to try to outsmart it.
You can also forget those pee charts that look like paint swatches for urine, and ignore anyone who says that yellow pee is a sign that you need to drink more water. If you think about hydration from the standpoint of what’s going on inside your body, it’s easy to see why urine hue isn’t helpful. The color of your pee is essentially just a measure of how concentrated your urine is. If it contains more waste than water, it looks dark, and if it’s mostly water, it’s light or almost clear. But that’s not what’s important. What you really want to know is what’s going on in your blood, and your urine can’t tell you that. Dark pee might mean that you’re running low on fluid, but it could also mean that your kidneys are keeping your plasma osmolality in check by conserving water. Very light or clear urine just means that you’ve drunk more water than your body needs, and that’s not necessarily a good thing, especially right before an athletic event.
If our bodies are so good at adapting to moderate fluid loss and letting us know when we need to drink, why are there still so many messages out there urging us to drink before we feel thirsty?
An obvious explanation for this is that most of what we hear about hydration comes from companies and researchers with a vested interest in making it all seem complex and highly scientific. The current guidelines from the ACSMand the National Athletic Trainers’ Association have been updated to warn about hyponatremia, but they still promote the ideas that thirst is a poor indicator of hydration and that more than a 2 percent body weight loss should be avoided. The ACSM, NSCA and NATA all receive funding from sports drink makers, as do some of their members. If staying hydrated were as simple as just drinking to thirst, you wouldn’t need expert advice or scientifically formulated products like Gatorade.
From a biological perspective, it’s hard to imagine that the human body is so delicate that it can’t function properly without scientists (or football stars) swooping in with calculators to tell us how to keep it running properly. “You have to trust your body,” Knepper says. Humans have evolved to survive exercising without chugging water or sports drink on some rigid schedule. “You get clues about what you need if you listen to your own body,” he says. “You don’t have to know chemistry to survive.”
After examining the science, I can’t help thinking we’ve made hydration unduly complicated. I take my dog running with me most of the time, and I’ve never measured the color of her pee or forced her to drink (as if I could). I make sure she has regular access to water, but she doesn’t always take it. At times, she won’t drink at all during a long run, and on those occasions, she always goes straight to her water dish when we get home and slurps until she’s satisfied. I’ve never had to give her an emergency IV for low fluid levels. If drinking to thirst is good enough for her, it’s probably good enough for me too.”
Although hydration guidelines instruct athletes to drink according to how much weight they’re losing through sweat and respiration, “drinking according to the dictate of thirst throughout a marathon seems to confer no major disadvantage over drinking to replace all fluid losses, and there is no evidence that full fluid replacement is superior to drinking to thirst,” the study’s authors wrote. Furthermore, athletes who lose the most body mass during marathons, ultramarathons, and Ironman triathlons are usually the most successful, which suggests that fluid losses are not as tightly linked to performance as sports drink makers claim. Instead, the results imply that there must be some tolerable range for dehydration that doesn’t impair performance.
Hoffman’s guidelines also recognize that dehydration is rarely a cause of heat illness and say that most muscle cramps are not caused by electrolyte loss. (The latest science on cramps suggests that they have more to do with neuromuscular fatigue than with hydration or electrolytes, Hoffman says.)”