You probably already know that you need to drink something after you exercise. When you work up a good sweat, it just makes sense to replace those fluids to keep your body humming along. But what, when and how often should you drink? That's a bit more of a mystery. We sat down with personal trainer and running coach Meghan Kennihan, from Train with Meghan for the straight scoop on getting re-hydrated after exercise.

What should you drink?

"Most of the time, water is all you need to re-hydrate your body," says Kennihan, while emphasizing that the type and amount of hydration you need will vary based on the length of your workout and the outside temps. "People who exercise for more than 45 minutes or in very hot temperatures may need a beverage containing electrolytes such as potassium and sodium."

That's because when you sweat, you lose more than just water from your body. You also lose the nutrients — such as potassium and sodium — that your body needs to do important jobs like regulating your heartbeat or contracting your muscles. If you just sweat a little bit, you can probably replace the fluid lost with water and your body can correct the imbalance. But when you sweat a lot, you lose electrolytes faster than your body can replace them. That's when it's time to reach for something that contains both fluids and electrolytes.

But Kennihan stressed the need to read the labels before you chug that sports drink. "Look for one that has no more than 14 grams of carbohydrates and no more than 50 calories in a serving," she advises, as many are just "sugar bombs" that may sabotage your fitness.

How much should you drink?

man pouring water on himself after exercise What, when and how often you need to drink has a lot to do with your workout, your body, and the weather. (Photo: stezzon/Shutterstock)

The American Council on Exercise suggests basic guidelines for drinking water before, during and after exercise:

  • Drink 17 to 20 ounces of water 2 to 3 hours before you begin exercising.
  • Drink 8 ounces of water 20 to 30 minutes before you start exercising or while you are warming up.
  • Drink 7 to 10 ounces of water every 10 to 20 minutes during exercise.
  • Drink 8 ounces of water within 30 minutes after you finish exercising.

The unscientific answer is to drink to quench your thirst and then drink a little more. "The thirst mechanism is typically delayed and can be an inaccurate indicator of dehydration," says Kennihan. "You'll have to monitor your urine to determine whether or not you've had enough," she adds.

For those who want a little more guidance on their hydration needs, Kennihan recommends weighing yourself before and after exercise. "For each pound lost during activity, drink 24 ounces of fluid," she notes. She also suggests doing these weigh-ins on both easy days and after more intense exercise, so that you know what your hydration needs are in various situations. Knowing your numbers can help prevent both dehydration and it's equally detrimental alter-ego, hyponatremia. That's a rare condition, usually found in athletes, when the sodium levels in your body are dangerously low because your body holds on to too much water.

How long should you continue re-hydrating?

So your workout is over, you downed your water/electrolyte drink and you're feeling pretty good. Do you need to continue thinking about your re-hydration throughout the day or are you good to go? "A good indicator for how long you should re-hydrate after exercise is your urine," says Kennihan. If your urine is darker than pale/light yellow, or if you have not urinated a few hours after exercise, Kennihan notes that you need to keep taking in fluids because you're likely still dehydrated.

Other signs of dehydration

winded female runner taking a break from exercise Know the signs of dehydration, which can include dizziness or feeling lightheaded. (Photo: lzf/Shutterstock)

In addition to continued thirst and changes in urine, there are other symptoms that can point to dehydration. According to the American Academy of Family Physicians, they include:

  • Dizziness or lightheadedness
  • Nausea or vomiting
  • Muscle cramping
  • Dry mouth
  • No sweating
  • Fast, pounding heartbeat

If you experience weakness or confusion, get medical help right away.

What about salt tablets?

You may have heard that salt tablets are a good way to replace electrolytes during a workout. While this is true, it's also important to note that salt tablets only replace sodium and chloride, not the other minerals that you may lose when you sweat. But for some athletes, particularly endurance athletes who work out for several hours at a time, salt tablets combined with water and other electrolyte replacement solutions may be a good fit.

Kennihan only recommends salt tablets to her athletes who will be working out for two hours or longer. In those cases, she suggests taking them once every hour after the first or second hour of exercise, depending upon the heat and humidity of the day and how much you will be sweating.

When in doubt, it's always a good idea to run it by your doctor.