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 heatstroke, have even more serious consequences. Runners need to pay attention to what and how much they’re drinking before, during and after exercise.
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 pass 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 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 4 to 6 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 NUUN or 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 such as the Amphipod Hydration Belts! When running in the race, water and sports drink should be on course, but never expect it as it is best to continue to have your hydration belt with you as well and full of your favorite fluids.
Don't forget to re-hydrate with water or a sports drink after your run. You should drink 20 to 24 fl oz. of water for every pound lost. Post run hydration should begin immediately after you run to start to replace those essential electrolytes and nutrients lost. Also, if you are able to get out of the sun & heat as well to begin the body cool down as well (if not doing a small run to cool down from a workout). Keep replenishment fluids going all day event when not working out or after workout.