Pre-Run HydrationIf 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. It's not a good idea to run with a hangover because you'll most likely be dehydrated when you start running.
An hour before you start your run, try to drink about 16 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 RunThe current advice about running and hydration is very simple -- try to drink to thirst. Scientific evidence says that drinking when you're thirsty can help prevent underhydrating (which can lead to dehydration) and overhydrating, which can lead to hyponatremia (low blood salt level due to abnormal fluid retention).
If you're looking for a general rule of thumb for fluid consumption during your runs: You should take in 4 to 6 ounces of fluid every 20 minutes during your runs. Runners running faster than 8-minute miles should drink 6 to 8 ounces every 20 minutes. 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). The carbohydrates and electrolytes in the sports drink also help you absorb the fluids faster.
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. However, if you're running in a race, you shouldn't have to carry your own fluids because they should have water stops on the course.
Determining Your Sweat RateThe 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. Note the weather conditions on that day, and keep in mind that you may need to adjust your consumption if the conditions are different. You can do the sweat rate test on another day to see how different conditions affect your sweat rate.
Post-Run HydrationDon'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.
Source: Maharam, Lewis, MD, et al. "IMMDA's Revised Fluid Recommendations for Runners and Walkers" 5/6/2006