The American College of Sports Medicine has issued the following position stand regarding fluid replacement:
1. Drink 500 ml (16 oz) of fluid 2 hours before exercise.
2. Drink early and often, especially during exercise sessions lasting longer than 1 hour.
3. Cool fluids are more readily absorbed than room temperature drinks.
4. For exercise sessions lasting less than one hour, plain water is fine. But for longer sessions/races, try and consume between 30 to 60 grams (120 to 240 calories) of carbohydrate with some added sodium for each hour of exertion.
5. Aim to consume between 1/2 and 1 liter of fluid per hour. By choosing sports drinks you can ensure that each liter consumed provides adequate carbs and sodium to fuel your performance for races as long as the marathon. For example, 20 oz. of Gatorade has 34 grams of carbohydrate and 270 mg sodium.
Athletes with faster metabolism/more lean body mass may wish to add one gel (Powerbar, Gu) per hour. One gel has about 100 calories/25 grams of carbs. And some flavors have added caffeine.
These recommendations are especially important to keep in mind when racing, as even mild dehydration (less than 2% body weight, or 3 lbs in the 150-lb athlete) has been shown to reduce aerobic performance and increase perceived effort. Two percent of body weight is not a lot. It is equivalent to 32 oz of fluid, which you can easily sweat off in an hour of running in warm weather.