How Much Water Should You Drink When You Exercise?

Last Editorial Review: 8/7/2017

Ask the experts

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?

Doctor's response

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.

What are the symptoms of dehydration?

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:

Although hyponatremia is a serious condition that should be avoided, some sports medicine professionals believe we may have gone overboard in our warnings to athletes to make sure they drink enough. In a study conducted at the Boston Marathon, 62 out of 488 runners who consented to have their blood tested after the race had hyponatremia, and three of them had critical hyponatremia. The incidence was highest in (1) the leanest runners, (2) those who ran slowest (more than four hours completion time), (3) those who consumed more than 3 liters of fluids during the race, and (4) those who consumed fluids every mile. The slowest runners are probably susceptible because they run slow enough to consume every drop of water in the cup, whereas faster runners typically spill fluid because they run so fast, and they also may skip water stations to maintain speed. The lesson here is that one must drink to maintain fluid balance but not so much that fluid needs of the body are exceeded.

Both hyper- and hypohydration are conditions that can and should be prevented. I suggest weighing yourself nude before and after your workouts to determine how much fluid you lose and then replace those fluids as per the NATA guidelines. A liter water of water weighs 2.25 pounds and so you can make your calculations based on that information.


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Robert Bargar, MD; Board Certification in Public Health & General Preventive Medicine


"Etiology, clinical manifestations, and diagnosis of volume depletion in adults"