You can lose about one and a half liters of fluid per hour of exercise. 

To put that in perspective, you have about 5 liters of blood total in your body. One liter is about four 8-ounce glasses of water. So, yes! You can lose about 6 glasses full of water in an hour!

You lose fluids by sweating and through your respiratory system. As you exercise and spend energy, your body creates heat. You sweat to help your body stay cool. You also lose electrolytes when you sweat. Electrolytes are important substances that are essential for proper cell function (read: your muscles won’t work without electrolytes!).

Dehydration occurs when your body loses too many fluids. One of the first symptoms of dehydration is fatigue, followed by lightheadedness, an increase in heart rate (as your body fights to work without the water it needs), muscle cramps, and headaches. As you can imagine, dehydration will significantly affect your sporting performance, but it can also be deadly. You need to hydrate your body.

To keep hydrated and feeling good during your workout:

• Drink plenty of fluids before you exercise
• Check the toilet! You know when you’ve gotten enough water when your urine is clear as water! Dark yellow urine is a sign of dehydration
• Drink, on average, ½ cup (4oz) every 15 minutes during exercising
• Take advantage of all breaks to drink up
• After exercising, drink to ensure you are fully re-hydrated

To stay hydrated, water usually does the trick (especially if it’s ice cold, because it empties from the stomach faster that way). Drinking water keeps your body in balance and hydrated, regulates your temperature, transports nutrients to cells, and removes waste products, as well as other essential functions. 

However, when you exercise for more than an hour or have a particularly intense workout, you need to replenish your electrolytes, as well.

To replenish electrolytes, you may wish to have a sports drink, such as Gatorade or Powerade. These drinks contain sodium to help your body retain fluid that would otherwise be lost through sweat. The calories in these drinks also help to restore energy. However, those calories are often in the form of high levels of sugar. Many experts recommend diluting (meaning, weaken) sports drinks with water because so much sugar is not always necessary.