The goals of fluid intake during exercise are to prevent dehydration from occurring and to not drink in excess of one’s sweating rate. One good way to figure out whether you need to drink something during your workout is to simply weigh yourself (without clothes) just before and after a typical workout. If your weight change is more than 2% of your starting weight, then in the future, you should plan to drink enough water during your workout to keep your post-workout weight within that 2% range. Typically, drinking a cup (8 oz) of water every 15-20 minutes will do the trick in all but the most extreme situations. While difficult to recommend a specific fluid schedule because of varying needs, this handy chart provides some basic guidelines:
*If possible, weigh yourself on the same scale, before and after exercise so you know how much to drink for rehydration.
Special Cases: Long Workouts and Hot Weather
If your workout is particularly long or your environment is hot and/or humid, just drinking plain water during your exercise may not be the best option.
Two hours of vigorous exercise can deplete the fuel supply (called glycogen) that your muscle cells use during vigorous activity. Drinking water alone won’t replenish that fuel. Assuming you can’t take a meal break in the middle of your marathon race, you may need something to drink that also contains carbohydrate for energy and to sustain performance. Commercial sports drinks containing 6% to 8% carbohydrate from various sugar sources are recommended for exercise events lasting longer than 1 hour. Higher carbohydrate amounts should be avoided because they impede the rate at which the drink leaves the stomach thereby slowing down the hydrating benefit. To estimate your carbohydrate need during sustained exercise, aim for about 30-60 grams of carbohydrate per hour of endurance exercise. Following the event, replenish your energy stores using these recommended guidelines.