From Christine Luff,
Your Guide to Running & Jogging.
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Staying hydrated is critical to your running performance and, more importantly, for preventing heat-related illnesses. Dehydration in athletes may lead to fatigue, decreased coordination, and muscle cramping. Other heat-related illnesses, such as heat exhaustion and heat stroke, have even more serious consequences. Runners need to pay attention to what and how much they’re drinking before, during and after exercise.

Pre-Run Hydration

If you're doing a long run or race (more than 8 to 10 miles), it's important to make sure you're well-hydrated during the few days leading up to your long run. You know you're well-hydrated if you void large volumes of pale urine at least six times a day. In the days leading up to your long run (or race), drink plenty of water and nonalcoholic fluids. Not only does alcohol dehydrate you, but it can also prevent you from getting a good night's sleep.

An hour before you start your run, try to drink 16 to 24 ounces of water or other non-caffeinated fluid. Stop drinking at that point, so that you can void extra fluids and prevent having to stop to go to the bathroom during your run. To make sure you're hydrated before you start running, you can drink another 4 to 8 ounces right before you start.

Drinking on the Run

Here's a general rule of thumb for fluid consumption during your runs: You should take in 6 to 8 ounces of fluid every 20 minutes during your runs. During longer workouts (90 minutes or more), some of your fluid intake should include a sports drink (like Gatorade) to replace lost sodium and other minerals (electrolytes).

If you don't have access to water on your running routes, you'll have to carry your own fluids with you. Here are some fluid carriers that you can use to hold your fluids while you run.

Determining Your Sweat Rate

The above guidelines are the basic rules of thumb, but it's important to remember that everyone's fluid needs vary. Some people sweat more than others.

To determine how much liquid to take during a run or race, you need to know your sweat rate, and that can vary between 1 to 4 quarts per hour. Weigh yourself nude before a timed training run, and then again after. One pound of weight loss equals 1 pint of water loss. Calculate your sweat rate and use this to determine your fluid needs during a run or race. For example, if you lose 2 pounds during an hour run, that's 2 pints or 32 ounces. Thus, you need 8 ounces of water or sports beverage every 15 minutes.

Post-Run Hydration

Don't forget to rehydrate with water or a sports drink after your run. You should drink 20 to 24 fl oz. of water for every pound lost. If your urine is dark yellow after your run, you need to keep rehydrating. It should be a light lemonade color.