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Drinking Water During Exercise

How Much Water Should I Drink When I Work Out?
  -- By Dean Anderson, Personal Trainer and Becky Hand, Licensed and Registered Dietitian
The Well-Watered Exerciser
Except for oxygen, there’s nothing your body needs more than an adequate supply of water. And the more you exercise, the more important it is to drink the right amount of water before, during, and after your workouts. Dehydration can make it hard to get the most out of your workout, and in extreme situations, can even be dangerous to your health. However, drinking too much water at the wrong time can also hinder performance.

So what’s an exerciser to do? How do you know how much you need to drink, when to drink it, and when you might need something other than plain water, like a sports drink? Obviously, people differ a lot in body size, how much they sweat, the type and amount of exercise they do, and the climate in which they exercise. All these factors make one-size-fits-all recommendations on drinking water ineffective or even dangerous. An approach used by competitive marathoners in hot, humid weather isn’t going to make much sense for you if you’re jogging or walking for 30 minutes on a treadmill in an air-conditioned gym (or vice-versa). Therefore, a competitive athlete could benefit greatly from an assessment of individualized fluid, electrolyte and energy needs by a Registered Dietitian with experience in sports nutrition.

Do you need to worry about drinking during exercise?
The good news is that staying hydrated during exercise really isn’t complicated for most people. If your typical exercise session is around 60 minutes or less, and doesn’t involve vigorous activity outdoors in hot, humid weather, you probably don’t need to interrupt your exercise session for a drink unless you prefer to. A healthy, average-sized person can produce as much as 32 oz of sweat during an hour of moderate to vigorous indoor exercise. That may feel and look like a lot of sweating, but it shouldn’t be enough to cause problems unless you’ve been seriously short-changing yourself on fluid intake prior to starting your exercise. You can tell whether that’s a potential problem by checking your urine color before exercise. If it’s dark yellow with a strong urine smell, it’s a good idea to have a cup or two of water 30-60 minutes before you start exercising. If it’s clear to light yellow, it should be fine to just rehydrate gradually after your exercise session without worrying about stopping to drink during the middle of it.

According to the American College of Sports Medicine, dehydration is likely to start affecting exercise performance when sweating causes you to lose 2% or more of your normal (hydrated) body weight. That's more than 51 ounces, or a little over 3 pounds, for an "average" person of 160 pounds. At this level of mild dehydration, you’ll probably be a little thirsty (though many people don’t experience thirst until they’re already dehydrated), and you may start to feel as if you have to work significantly harder to maintain your performance level. As dehydration gets progressively more severe, you may start to feel lightheaded, uncoordinated, or have muscle cramps. If you continue, you may start experiencing the symptoms of heat exhaustion, and that can progress to heat stroke, which is potentially fatal and needs immediate medical attention.

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:

Drink Water How Much? When?
Before Exercise 8-16 oz At least 15 minutes before workout
During Exercise 4-8 oz Every 15-20 minutes
After Exercise 16-24 oz per pound* lost As soon as possible

*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.

Likewise, when you sweat heavily for an extended period, you’re not losing just plain water. You’re also losing a significant amount of sodium, which needs to be maintained within a certain range to avoid potentially serious problems like exercise-induced hyponatremia (a serum sodium concentration <130 mEq/L), which can also occur if you drink too much plain water, in a short time period. It isn’t very easy to give yourself hyponatremia: An otherwise healthy person would have to combine hours of heavy sweating with drinking significantly more water than they’re actually losing during the exercise and not replacing the lost sodium. You can avoid this by making sure your water intake during and immediately after exercise is equal to the amount you lose (see above about weighing yourself).

For particularly long endurance workouts OR exercise sessions in hot and humid weather, one may need more than the Tolerable Upper Intake Level for sodium (2,300 milligrams daily). That's why commercial sports drinks are often recommended for athletes taking part in such endurance events. Make sure the sports drink contains about 120–170 milligrams of sodium per 8-ounce serving. If you are looking for a less expensive option for the commercial sports drink, try this simple recipe. You may also need to replace lost sodium by consuming high-sodium foods and beverages following the event. High sodium food choices could include: canned soups, beans and vegetables; boxed side dishes like macaroni and cheese, potato mixes, or rice blends; Oriental style entrees with soy sauce; pizza; jarred spaghetti sauce; salted nuts; and regular cheese.

Sources
American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine. "Nutrition and Athletic Performance." 109 (2009): 509-527.

Ivy, J.L., Costill, D.L., Fink, W.J., Lower, R.W. "Influence of caffeine and carbohydrate feedings on endurance performance." Medicine and Science in Sports and Exercise. 43 (2011).

Sawka M.N., Burke L.M., Eichner E.R., Maughan R.J., Montain S.J., Stachenfeld N.S. Exercise and Fluid Replacement." Medicine and Science in Sports and Exercise. 39 (2007): 377-390.

Wedro, Benjamin C. "Heat Exhaustion," accessed July 2011. MedicineNet.com.