Staying hydrated is one of the most important things to consider during exercise. Proper fluid intake is essential for maximal performance and safety. The more intense the activity, the more important it is to drink not only enough fluids but the “right kind” of fluids. Common causes of dehydration are excessive sweating, failure to replace lost fluids lost during exercise, exercising in dry/hot weather and inadequate fluid intake. Finding the right amount of fluids to drink depends upon a variety of factors including the length and intensity of exercise. Two simple methods one can use to determine adequate hydration levels during activity are to check urine output and the color. Generally speaking, a large amount of light-colored urine means that you are hydrated; small amounts of dark-colored urine can mean that you are dehydrated. The other way for telling if you are adequately hydrated is weighing yourself before and after exercise. It is unlikely that weight loss during the work out will be from more than water weight loss. Long term weight loss happens over time and the number will not dramatically change after working out. You should try to drink enough to replace the weight loss so that you are making sure you are rehydrating properly
To find the correct balance of fluids for exercise, the American College Of Sports Medicine suggests that “individuals should develop customized fluid replacement programs that prevent excessive (greater than 2 percent body weight reductions from baseline body weight) dehydration. The routine measurement of pre and post-exercise body weights is useful for determining sweat rates and customized fluid replacement programs.” According to the Institute of Medicine, the need for carbohydrate and electrolytes replacement during exercise depends on exercise intensity, duration, weather and differences in sweat rates. They write, “Fluid replacement beverages might contain ~20–30 meqILj1 sodium (chloride as the anion), ~2–5 meqILj1 potassium and ~5–10% carbohydrate.” Sodium and potassium are to help replace sweat electrolyte losses, and sodium also helps to stimulate thirst. Carbohydrate provides energy for exercise over 60-90 minutes. This can also be provided through energy gels, bars, and other foods.
What about sports drinks? Sports drinks can be helpful to some athletes and can be detrimental to others by upsetting the gastrointestinal system during exercise. Sports drinks can be beneficial if used more after activity as a refueling than used during activity as a “quick” fluid replacement. It is said that sports drinks can be good for athletes exercising at a higher intensity activity (60 min or more). Fluids supplying 60 to 100 calories per 8 ounces helps to supply the needed calories required for continuous performance.
Recommendations for pre-exercise hydration is mostly related to intensity of the activity being performed. Two to three hours before exercise, drink about 15-20 fl. oz. and drink 8-10 oz. 10-15 min before exercise. A good hydration goal during exercise is 8-10 oz. every 10-15 min. If exercising longer than an hour and a half, try to drink a sports drinks every 15-30 min to rehydrate and replenish some of the calories and electrolytes being lost during longer exercises. Post exercise, weigh yourself and drink 20-24 oz. of water for every pound lost. It is ok to consume sports drinks after exercise to replenish glycogen stores but should be done within two hours after exercise for greatest effect.
Contrasting views on hydration:
Link from Gatorade Sports Science Institute on Gatorade and water and the benefits.