Water is the most essential component of the human body as it provides an important role in the function of cells. Important functions of water include transportation of nutrients, elimination of waste products, regulation and maintenance of body temperature through sweating, maintenance of blood circulation and pressure, lubrication of joints and body tissues, and facilitation of digestion.
More than half of the human body is composed of water, and it is impossible to sustain life without it.
Exercise produces an elevation in body temperature, which depends on the intensity and duration of exercise, environmental conditions, clothing worn, and metabolic rate. In order to get rid of the excess heat, your body secretes sweat, which is primarily composed of water and electrolytes such as sodium.
The evaporation of sweat is the primary mechanism of heat loss during exercise. Exercise can lead to substantial water and electrolyte loss from sweat leading to dehydration and, in cases of excessive fluid intake, hyponatremia (low sodium in the blood). However, considerable variability exists from person to person with regard to sweat loss. Therefore, the fluid and electrolyte requirements needed for the athlete are variable from person to person as well. If water and electrolytes are not replaced from these losses, the athlete will have a decrease in performance and perhaps an adverse effect on his or her overall health.
Thirst is a signal that your body is headed toward dehydration. Therefore, it is important to drink before you feel thirsty and to drink throughout the day. Thirst is not a good indicator of hydration and should not be used to monitor hydration status. One way to check your hydration status is to weigh yourself before and after exercise. The before-exercise measurement is best as a nude weight first thing in the morning after urinating. Comparing your body weight before and after exercise can be used to estimate your sweat loss and your fluid requirements. Any weight loss is likely from fluid loss, so drinking enough to replenish these losses will maintain hydration.
The table below shows us that over a one percent loss in body weight indicates dehydration and over five percent indicates serious dehydration. These fluid losses need to be replaced.
% Body Weight Change
Well Hydrated -1 to +1%
Minimal Dehydration -1 to -3%
Significant Dehydration -3 to -5%
Serious Dehydration > -5%
Another way to check hydration status is the urine color test. A large amount of light-colored urine means you are well hydrated. The darker the color, the more dehydrated you are.
Dehydration is the loss of fluids and salts essential to maintain normal body function. Dehydration occurs when the body loses more fluids than it takes in.
Dehydration can lead to:
- Muscle fatigue
- Loss of coordination
- Inability to regulate body temperature
- Heat illness (e.g., cramps, heat exhaustion, heat stroke)
- Decreased energy and athletic performance
Moderate caffeine intake does not affect hydration status or urine output. However, alcohol will increase your urine output and decrease hydration. Enhancing palatability of a fluid will help to encourage fluid consumption. This can be done with proper flavoring, proper salt (sodium) content and drinking a cold beverage (15-21 degrees Celsius).
Carbohydrates within a sports beverage help to replenish your sugar (glycogen) stores and electrolytes help to accelerate rehydration. Sports beverages for use during prolonged exercise should generally contain four to eight percent carbohydrate, 20-30 meq/L of sodium, and 2-5 meq/L of potassium. The need for carbohydrates and electrolytes within sports beverages increases with prolonged activity.
Carbohydrate consumption helps to sustain and improve exercise performance during high-intensity exercise longer than one hour as well as lower-intensity exercise for longer periods. You should ingest one-half to one liter of a sports drink each hour to maintain hydration. Also, sports drinks should not exceed a carbohydrate concentration of eight percent.
HYDRATION BEFORE EXERCISE
Check your hydration status before exercise because there is a wide variability in fluid needs for each person.
- Drink 16-20 fluid ounces of water or sports beverage at least four hours before exercise.
- Drink 8-12 fluid ounces of water 10-15 minutes before exercise.
Consuming a beverage with sodium (salt) and/or small meal helps to stimulate thirst and retain fluids.
HYDRATION DURING EXERCISE
- Drink 3-8 fluid ounces of water every 15-20 minutes when exercising for less than 60 minutes.
- Drink 3-8 fluid ounces of a sports beverage (5-8 percent carbohydrate with electrolytes) every 15-20 minutes when exercising greater than 60 minutes.
Do not drink more than one quart/hour during exercise.
HYDRATION GUIDELINES AFTER EXERCISE
Obtain your body weight and check your urine to estimate your fluid losses. The goal is to correct your losses within two hours after exercise.
Drink 20-24 fluid ounces of water or sports beverage for every one pound lost
Overhydration, also called water intoxication, is a condition where the body contains too much water. This can result in behavioral changes, confusion, drowsiness, nausea/vomiting, weight gain, muscle cramps, weakness/paralysis and risk of death.
In general, overhydration is treated by limiting your fluid intake and increasing the salt (sodium) that you consume. If overhydration is suspected, you should see your doctor for appropriate lab tests and treatment. You should not consume more than one liter per hour of fluid.
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Reprinted with permission of the American College of Sports Medicine. Copyright © 2011 American College of Sports Medicine. This brochure is a product of ACSM’s Consumer Information Committee.
Fotografía de Darwin Bell, usada bajo licencia de Creative Commons