If you don't take care of hydration, then nothing else matters. Months of great training, good nutrition, and carbohydrate loading - none of them matter if you get dehydrated.
During exercise, muscles produce tremendous amounts of heat. These muscles can only keep working if that heat is removed efficiently. Blood flowing through the muscles during exercise picks up the heat that is being produced and carries it away from the muscles. Also, more blood is flowing to the skin where small capillaries have dilated, giving us that reddish glow, allowing heat to be transported to the surface of the body. During exercise in hot weather, most of the muscle-generated heat is removed from the body by the evaporation of sweat. Consequently, loss of water from the body is necessary to prevent over-heating.
Most runners are surprised to learn how much water they lose through sweat loss during running. Typical sweat loss during running under average environmental conditions is about 1.5 quarts of water per hour. This is equivalent to six 8-oz. cups of water per hour to replace the water loss. Some people lose significantly more sweat than this and some lose less.
A good way to determine your own personal sweat rate it is to weigh yourself on an accurate scale before and after a bout of exercise. Typical weight loss during an hour of running (without any fluid consumption) is about three pounds. This is the weight of 1.5 quarts of water. Of course, some of this weight loss is from the utilization of fat and carbohydrate stored in the body. However, the amount used during an hour of exercise is only a few ounces. Over 90 percent of the change in your body weight during exercise is due to water loss.
Most people can tolerate water loss up to about two percent of a their body weight before dehydration impairs exercise performance. For a 150 lb. person that equals three pounds, or 1.5 quarts of sweat loss. Consequently, dehydration is usually not a problem during exercise events lasting 30 to 45 minutes or less.
A smart runner has the terms prehydration, hydration, and rehydration burned into their brain. To maintain a good training program, a runner should start their workout well hydrated, stay adequately hydrated during exercise, and rehydrate after exercise.
Prehydration
To anticipate your water needs before exercise, plan ahead and drink about two cups of water or other fluid two hours before you plan to exercise. This allows time for your body to take up and retain the water it needs and to lose any extra water by forming urine that you can void before exercise. Then, 5 to 15 minutes before the start of exercise, drink about two cups of water or a sports beverage. When you do this, your stomach works like a built-in, time-release water bottle that gradually releases fluid into small intestine where it can be absorbed into the blood and delivered throughout the body where it is needed. Also, keep in mind that you can't "hyper-hydrate" by drinking lots of extra water the day before an event.
Hydration During Exercise
Most runners don't drink enough during exercise. It can be a challenge to determine how much fluid you need during exercise. The longer the bout of exercise, the more important it is to take care of hydration.
If you have tested yourself for sweat loss by weighing yourself before and after exercising under average conditions, then you have a pretty good idea about how much you need to drink to replace those losses during exercise. For every pound of the anticipated weight loss, you need to drink two 8 oz. cups of fluid. For most people, this means drinking one to two cups of fluid every 15 minutes.
If the conditions are more hot and/or more humid than usual, you should plan to moderate the running pace and drink more fluid than usual. Otherwise, you can over heat your engine (muscles) and pay dearly for it with poor performance and feeling miserable.
If weather conditions are extremely humid, cooling the body during exercise becomes very difficult. The evaporation of sweat from the skin becomes very limited, greatly reducing the effectiveness of this major cooling system. During hot humid weather, moderate your pace and listen carefully to your body to prevent over heating.
Rehydration After Exercise
Because athletes can handle moderate dehydration, they usually finish a bout of exercise somewhat dehydrated. To come back strong for a workout the following day, you need to re-hydrate adequately during the next 12 to 24 hours. This can be monitored by making sure that weight lost during exercise is replaced by adequate fluid intake. Again, for every pound of weight loss you need to drink two 8 oz. cups of water or other fluid. Since your body will not hold on to all of the water you consume (some goes out through urine production, unnoticeable sweat losses, and breath), you actually need to drink a bit more than this. Some experts recommend that you drink three 8 oz. cups of fluid to replace each pound of weight loss.
A commonly used technique to tell if you're drinking enough water is to make sure that your urine is not too dark in color. If you are well hydrated, urine color should be light yellow to clear. This technique can be complicated by the use of vitamin supplements or fortified foods that are high in riboflavin (vitamin B-2). Riboflavin has a bright yellow color that can make the urine very yellow even in a well hydrated person.
Summary
Prehydrate, hydrate, rehydrate
C. Alan Titchenal, PhD, CNS
Department of Human Nutrition, Food & Animal Sciences
CTAHR, University of Hawaii