How much you drink while running should be determined by the duration of your run, the weather, and how much you sweat.
Runners should consume 6-8 ounces of water before non-intense training runs lasting less than an hour. For longer or more intense runs, such as speed workouts and races, hydrate with a sports drink containing sodium (to help maintain fluid balance) and carbohydrate, to help fuel your activity and prevent “hitting the wall,” or running out of glycogen (stored carbohydrate).
During your longer run, aim to consume no more than 15 grams of carbohydrates every 15 minutes starting at the one-hour mark. Small amounts of fluid, consumed frequently are absorbed better than large amounts consumed all at once.
To estimate your fluid needs, weigh yourself before and after you run and and take note of the weight you’ve lost, then drink enough to return to your pre-run weight. This will give you a sense of how much to drink during and after your next run. You don’t have to exactly match your sweat loss but you’ll have a reasonable guideline. Don’t drink beyond your sweat-rate loss, as you may risk a dangerous condition known as hyponatremia, or low sodium balance.
Finally, pay attention to your body’s signals that you need to hydrate, but keep in mind that thirst may not kick in until you are already 3.5% dehydrated, so you may need to “remind” yourself to drink before you feel thirsty. Like many athletes, you may do better with a strategic plan for fueling and hydrating, such as 6-8 ounces every 15-20 minutes. Keep in mind that you will need to drink more on warm, humid days than in cool, dry conditions.
Heidi Skolnik is a sports nutritionist at the Women’s Sports Medicine Center at Hospital for Special Surgery. She worked with the NY Giants Football team for 18 years and has consulted to professional teams for 25 years including the NFL, MLB, NBA, MLS, as well as Olympic, collegiate, high school and recreational athletes. Heidi is a contributing advisor to Men’s Health magazine and is co-author of Nutrient Timing for Peak Performance; the right food, the right time, the right results (Human Kinetics, 2010).