Hydrate Smarter: Drinking the Right Amount of Water When You Exercise

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We all know that proper hydration is important. Every cell in your body depends on water to function properly and, when you run low on H2O, systems start crashing. Most of us drink enough fluid to stave the bad stuff off, but when you exercise and start sweating, the equation gets a little more complicated.

In addition to headaches and dry-mouth, dehydration may cause lethargy, confusion, and even coma or death in extreme cases. Avoiding this isn't as simple as drinking a bunch of water (or sports drinks) while you work out. With a couple simple guidelines, though, you'll be able to better power-through your workouts without feeling hung-over afterward.


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How Much Water You Need Each Day

Obvious disclaimer here: Everyone's body is different; as such, no two people will have identical hydration needs. Further, people in hotter climates will generally need to drink more, while people in colder climates will typically need less. You may have a condition (or a simple cold) that means you need more (or less) water. With all of that in mind, then, think of these guidelines as a starting point for your personal experimentation with how much water your body really needs.


Let's set exercise aside for a second and start with your normal, day-to-day life. There is a long-held basic rule of thumb for daily hydration. It is: Half your weight (in pounds) is how many ounces of water you should consume every day. So, for example, if you weigh 180 pounds, your goal would be to drink 90 ounces of water per day.

It's simple, it's easy to remember, and it seems to work for most people. The best way to test that number, though, is to look at your pee. If it's yellow, try adding a little more water until it's just clear (remember, though, that if you're taking multivitamins, your pee will glow like Vegas no matter how much water you drink, so don't drown yourself).


Try to spread out your hydration as evenly as possible throughout your day, with the one exception being right when you wake up: at that point, you should go a little harder, since you haven't had anything to drink for eight or so hours.

How Much Water You Lose During Exercise

Here's where things get a little more complicated. Research indicates that most people underestimate how much they sweat when they work out, often by as much as 46 percent. This isn't so surprising, because much of it dries or drips off of us, but the result is that most of us don't adequately compensate for exercise. There are a ton of variables here (including temperature, fitness level, climate, weight, level of exertion, body type, and more), but the numbers we came across most often claim that the average person sweats "0.8 to 1.4 liters (roughly 27.4 to 47.3 ounces) per hour during exercise." That's a hell of a lot of fluid! Even crazier, the highest human sweat rate ever recorded was 5 liters (169 ounces) per hour. That's well over a gallon!


Averages are all fine and good, but it's better to get a sense of your own sweat rates. There are very extensive lab tests you could go through (like this guy did) to get your exact sweat rate (and loss of salts), but, for most of us, the easy way will do just fine.

  1. Get naked and weigh yourself (down to the ounce, if possible).
  2. Put your workout clothes on and exercise for exactly one hour. If at all possible, don't eat or drink anything during that hour.
  3. Strip naked again, and weigh yourself again. For every pound you lost, that corresponds to a 15.4-ounce loss of fluid. Do the math. Literally.

Note: If you absolutely must eat or drink some water during your hour of exercise, measure or weigh it as best you can, so you can subtract that amount from your weight at the end.

Okay, so let's say you generally lose 30 ounces of sweat per hour of exercise. That means you can just drink 30 ounces of fluid every hour that you exercise and you'll be good, right? Not so fast, smart guy.


Adjusting Your Daily Intake

It's not just about how much you drink during your workout. In fact, in many ways, that's much less important. See, you can't make up for sweat loss that fast. Your body's gajillion cells can't absorb water as fast as they can excrete it, and when you're engaged in strenuous exercise much of your digestive system is shut down anyway, meaning that most of that fluid stays in your stomach, just sloshing around and making you uncomfortable.


Not only that, you can't just bomb your system with water. Doing that can upset the balance of sodium in your blood (because you're losing a lot of salt through sweat). This is called "exercise-associated hyponatraemia (EAH)," also known as "water intoxication." It can cause disorientation, seizures, and death. And no, simply drinking Gatorade (or any other sugary salt-water) won't help much either, because your body can only absorb salts so quickly when exercising.

The smart way to handle this, instead, is to spread out the additional hydration you'll need throughout the day. So, if you weigh 180 pounds, and that means that you normally need 90 ounces of water per day—plus, today you worked out for an hour and lost about 30 ounces of fluid through sweat—then you need to consume 120 ounces of water today. It's a very simple equation.


That isn't to say that you shouldn't drink fluids while you exercise. Not at all. It's saying that you cannot healthily replenish the fluids you're losing as fast as you're losing them, which is why you should just factor the additional water into your daily hydration routine and spread it out so you're not shocking your system.

Pre-hydrating is especially important. In fact most marathon and triathlon coaches will tell you that getting good and hydrated isn't something you just do on the day of the race, it's something you start doing in the two days leading up to it. You want all of your cells to be nicely saturated in order to maintain optimum performance (dehydration can majorly impact the amount of work your muscles can do). In other words, you'd be best off taking more of that extra water in the morning, if not the night before you exercise.


Generally speaking, taking a more holistic approach to your hydration means that you don't need to be as concerned with maintaining a proper electrolyte-balance by supplementing your workouts with any "sports drinks" that make your intestines glow in the dark. Studies have shown that most of us lose between one and three grams of sodium per hour of exercise, which most of us are getting in our daily diets anyway. If anything, let it make you feel less guilty about having some French fries, but do yourself a favor and stay away from the sugar-water.

Image credit: Shutterstock/ostill