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Exercise and Avoiding Dehydration

QUESTION:

I tend to sweat a lot when I exercise. How much extra water do I need to drink to avoid dehydration?

ANSWER:

Exercise increases our daily fluid needs. We can lose large amounts of water during exercise, mostly in the form of sweat. This loss can be as much as 8 cups or more in the first hour of intense exercise if our surroundings are hot and humid. Maintaining your hydration level is not only important to exercise performance and safety, but to the ability to recover properly from a bout of exercise. This is especially true for individuals who may be performing two exercise sessions in one day because starting off the second bout of exercise dehydrated can be detrimental.

Ensuring that you are adequately hydrated prior to exercise will start you off on the right foot. However, it's important to continue to consume fluid during exercise as well as after! Because it's difficult to measure actual water loss, we estimate the amount of water loss by looking at the loss in body weight during exercise. We can use this method because body weight loss during an exercise session is mostly due to water loss. By measuring the water lost during exercise, we can determine how much water we need to drink to bring us back to a state of pre-exercise hydration.

Water weighs about 1g per milliliter or 2.2 lbs per liter. This translates into about 2 cups of fluid for every pound of body weight lost. (As some of the ingested fluid will be lost to urinary excretion, we increase this to 3 cups of fluid per pound lost to compensate.) By weighing in prior to your exercise and then again after, you can estimate how much extra water you need to drink to replenish your lost fluid stores. Below is a summary of recommended guidelines for fluid ingestion pre, during and post exercise.

Before Exercise† consume:

400 - 600 mL (14 to 21 oz) of fluid 2-3 hrs before exercise

During Exercise† consume:

150 - 350 mL (6 to 12 oz) of fluid at 15- to 20-minute intervals

Following Exercise consume:

720 mL (24 oz) of fluid per pound of body weight loss

Useful conversion: ~240 mL = 8 oz = 1 cup

†	Before and During exercise recommendations are from the joint position stand entitled "Nutrition and Athletic Performance - Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine".

Our expert, Dr. Sharon E. Griffin, holds a B.S., M.S., and Ph.D. in the areas of exercise science/physiology. She also holds a second M.S. degree in Nutrition and is a licensed nutritionist and an ACSM certified health and fitness instructor.