Fitness Q&A by Richard Weil

My trainer is always telling me to stay hydrated. How much water should I drink when exercising? What will happen if I drink too much?


The National Athletic Trainers' Association recommends the following hydration guidelines for exercise:

    1. Two to three hours pre-exercise: 17 to 20 fluid ounces of water or sports drink.

    2. Ten to 20 minutes pre-exercise: 7 to 10 ounces of water or sports drink.

    3. During exercise: Fluid replacement should approximate sweat and urine losses and at least maintain hydration at less than 2% body weight reduction. This generally requires 7 to 10 ounces of water or sports drink every 10 to 20 minutes. Include carbohydrates in the beverage if the exercise is intense or lasts more then 45-50 minutes. Water alone will suffice, and save calories, if the exercise is moderate or less than 45-50 minutes.

    4. Post-exercise: Athletes should weigh themselves nude before and after workouts to learn how much weight is lost from sweat (water and salt) and then ingest fluid equal to 150% of the weight loss, ideally within two hours, and no more than four to six hours after the event. Including sodium in the drink allows fluid volume to be better conserved and increases the drive to drink, and carbohydrate in the drink will improve the rate of intestinal absorption of the fluid as well as replenish glycogen stores in the muscles and liver.

    5. The volume of fluid in the stomach is critically important for proper hydration. Maintaining 12 to 20 ounces of fluid in the stomach will optimize gastric emptying and prevent dehydration. Concentrations of 4% to 8% of carbohydrates should be used if they are included in the fluid (for example, Gatorade and other energy drinks). Concentrations higher than 8% slow the rate of fluid absorption, while 6% to 8% concentrations are optimal for hydration and performance.

The effects of dehydration can be profound. They are:

    1. A loss of just 1% to 2% of body weight begins to compromise cardiovascular, body temperature regulation, and muscular function, and can lead to decreases in aerobic power. For example, heart rate rises an additional three to five beats per minute for every 1% of body weight loss.

    2. Muscle endurance and maximal aerobic power decreases when 3% to 4% of body weight is lost. Slightly more than 2% loss of body weight can result in as much as a 35% to 48% reduction in physical work capacity.

    3. Dehydration of greater than 3% of body weight increases the risk of developing exertional heat illness (heat cramps, heat exhaustion, or heat stroke). Heat illness is common in sports and can occur after just one hour of intense exercise in the heat.

As for drinking too much, there has been a great deal of discussion, and some controversy, recently, over the issue of over-hydration during athletic events and the risk of abnormally low sodium (hyponatremia). It started in the public press some five or six years ago when runners at a marathon presented at the finish line with what appeared to be dehydration. The medical people confused the symptoms of hyponatremia with dehydration (the symptoms are similar), and so they rehydrated the runners with intravenous fluids in the medical tent, then rushed them to the emergency room where they subsequently died, not from dehydration, but from over-hydration.

Hyponatremia is an electrolyte disorder where not enough sodium (salt) is present in the body fluids outside the cells. As a result, water enters the cells in an attempt to balance the concentration of salt outside the cells, and the cells swell as a result of the excess water. Most cells can accommodate the swelling, but brain cells cannot because the skull confines them. Brain swelling accounts for most of the symptoms of hyponatremia, which include:

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