Hydration refers to how the human body takes in and maintains a steady level of water in its tissues and organs down to the cellular level. Hydration describes having a balance of fluids (water homeostasis) taken in and released. When fluid balance is not maintained, a person is said to be dehydrated. People also can become overhydrated from excessive fluid intake; this condition is sometimes called water intoxication. Overhydration can lead to hyponatremia, a condition in which the level of sodium in the blood plasma is too low. This condition can cause symptoms such as swelling of brain tissue, seizures, and, in some cases, death. Hydration is important in the area of fitness and exercise because usually people perspire (sweat) when exercising. Sweating lowers the fluid level in the body and can lead to dehydration.


Adequate hydration is essential for survival. Water is distributed throughout three compartments in the body: inside the cells (intracellular), in the tissue (interstitial), and in the bloodstream (intravascular). These compartments contain different amounts of electrolytes that must remain balanced for organs and systems in the body to function correctly. Humans can live for only a week without water and can survive less time if they are stranded in a desert or other hot, dry environment. Water is necessary for vital body processes for the following reasons:

Since water is essential to a number of vital bodily processes, adequate hydration is necessary for survival and an important aspect of good health.


Total body water varies in relation to age, gender, and amount of body fat. Adult males have approximately 60% water content, and adult females have 50%. Most newborns have about 80% water and infants an estimated 77%. The elderly have 46% to 52% water in their bodies. An increase in body fat causes a decrease in the percent of fluid content because fat does not contain significant amounts of water, which is a good reason to maintain a normal height-to-weight ratio. Regular exercise helps to maintain a healthy ratio.

All people lose water through urination, elimination of solid wastes, sweating, and exhaling water vapor. They must replace this water through their diets. The Food and Nutrition Board of the Institute of Medicine (IOM) recommended in 2004, and maintained through 2017, that adult men take in about 125 oz. (3.7 L, about 15 cups) of fluids daily and that women take in about 91 oz. (2.7 L, about 10 cups) to replace lost water. The amount varies somewhat based on age and activity level. These recommendations are for total fluid intake from both beverages (80%) and food (20%). The World Health Organization (WHO) recommendations are slightly different: 74 oz. (2.2 L) for women (162 oz. [4.8 L] for pregnant women) and 98 oz. (2.9 L) for men. Highly active adults and those living in very warm climates need more fluid; the WHO recommends 152 oz (4.5 L) per day for both men and women in these conditions.


Water in the body comes from two sources: fluids taken in through the mouth and the metabolism in food. Fluids taken by mouth are absorbed throughout the stomach and digestive tract. Water obtained by metabolism arises as a by-product of the breakdown of nutrients in food. The amount of water produced depends on the composition of the nutrient: 1 g of carbohydrate yields 0.60 g of water when metabolized, compared to 0.41 g of water for 1 g of protein and 1.06 g of water for 1 g of fat.

The normal daily turnover of water in healthy adults is between 5% and 10%. The main way the body regulates its water balance is by varying urination in response to a wide variation in water intakes and losses. Almost half of all water loss in the body occurs through the lungs and skin; in an moderate climate, average adults lose about 44 oz. (1.3 L) of fluid per day in their urine, 11 oz. (0.3 L) through the lungs, 32 oz. (0.9 L) through the skin, and 3 oz. (0.09 L) in fecal matter. Illness such as a stomach virus can cause an imbalance in fluids and lead to dehydration.


Precautions for maintaining proper hydration include understanding that drinking too much water can be as harmful as too little. Too much fluid in the body can lead to overhydration or water intoxication and eventual hyponatremia. People whose work or exercise patterns require them to pay particular attention to their level of hydration may want to consider learning one or more of the following methods for measuring fluid loss during exercise:

Other precautions for maintaining healthy hydration include dressing appropriately for exercise or heavy work, particularly in hot, humid weather; keeping a supply of sports drinks or water readily available; and consulting a specialist in nutrition or sports medicine for advice about hydration.


The American Council on Exercise (ACE) makes the following recommendations about water intake before, during, and after exercising or working outdoors:

The American College of Sports Medicine (ACSM) has recommended that preparing for adequate hydration during exercise should be tailored to each athlete (or worker) rather than tied too rigidly to general guidelines. This is because changes in electrolyte balance and amount of sweat vary greatly from one individual to the next. The ACSM also recommends drinking small amounts of fluid throughout the duration of exercise instead of consuming large amounts before or after a workout. A 2015 report indicated that hydration has been too emphasized in sports performance; however, the authors confirmed that losing more than 3% of body weight can cause performance or health problems for athletes.


Aftercare includes calculating how much body water was lost during physical activity or exercise and then replacing fluid and electrolyte losses. The amount of fluid lost can be measured by comparing the person's weight after exercise with the person's weight before exercise and replacing the weight lost on a one-for-one basis—that is, 34 oz. (1 L) of fluid for each kilogram of weight lost, or 2 c. (0.5 L) for each pound.

The ACSM states that “The speed with which rehydration is needed and the magnitude of fluid electrolyte deficits will determine if an aggressive replacement program is merited.” Emergency fluid or electrolyte replacement may be given either orally or intravenously.


Many complications can result when individuals do not drink enough fluids to match the amount that is used. Dehydration (too little water in the body) and water intoxication (too much water) can occur.

Water loss (dehydration)

Risk factors for loss of water in the general population include the following:

Elderly people are at increased risk of inadequate hydration for several reasons:

Water intoxication

Risk factors for water intoxication (overhydration) include the following:

A drug designed to encourage urination in people who accumulate excess fluid such as people who have high blood pressure or a heart condition.
Substances in the body that can conduct electricity. They are essential in the normal functioning of body cells and organs.
An abnormally low level of sodium in blood plasma. It can result from water intoxication.
Lymphatic system—
Organs and tissues in the body that help filter out toxins or unwanted materials.
A nearly clear liquid that makes up part of the blood.
Drinking fluid prior to exercise or heavy work in order to maintain proper hydration during the activity.
Psychogenic polydipsia—
A psychiatric disorder in which patients consume excessive amounts of water, sometimes going to great lengths to obtain it from any source possible.
Water homeostasis—
A condition of adequate fluid level in the body in which fluid loss and fluid intake are equally matched and sodium levels are within normal range.
Water intoxication—
A condition that results from taking in more fluid than is needed to replace fluid losses from sweating, digestion, and other body processes. It is also called overhydration or hyperhydration.


The results of adequate hydration are basic good health, rapid recovery from any diarrheal disease, and enjoyment of athletic activity without risk of dehydration and eventual heat-related illness.

Healthcare team roles

  • How much fluid should I take in per day given my age, activity level, and geographical location?
  • How can I tell if I am adequately hydrated?
  • What are the health risks of dehydration?
  • What are the health risks of overhydration?

Caregiver concerns

Caregiver concerns include monitoring the fluid intake of elderly patients to make sure it is adequate and monitoring the fluid intake of infants to make sure they are not overhydrated.

See also Dehydration .



“Dehydration.” The Merck Manual of Diagnosis and Therapy, Section 6, edited by R. S. Porter. New York: Andesite Press, 2015.

Knoop, Kevin J., et al., editors. Atlas of Emergency Medicine, 4th ed. New York: McGraw-Hill Education, 2016.

Meyer, Flavia, and Zbigniew Szygula. Fluid Balance, Hydration, and Athletic Performance. Boca Raton: CRC Press, 2015


American College of Sports Medicine (ACSM). “American College of Sports Medicine Position Stand: Exercise and Fluid Replacement.” Medicine and Science in Sports and Exercise 39 (February 2007): 377–90.

Wall, Bradley A., et al. “Current Hydration Guidelines Are Erroneous: Dehydration Does not Impair Exercise Performance in the Heat.” British Journal of Sports Medicine 49 (2015): 1077–83.


American College of Sports Medicine. “Selecting and Effectively Using Hydration for Fitness.” http://www.acsm.org/docs/brochures/selecting-and-effectively-usinghydration-for-fitness.pdf (accessed November 11, 2016).

American Council on Exercise. “Fit Facts: Healthy Hydration.” http://www.acefitness.org/fitfacts/fitfacts_display.aspx?itemid=173 (accessed November 14, 2016).

Water U.K. “Water and Health.” http://www.water.org.uk/consumers/water-and-health (accessed November 14, 2016).


American College of Sports Medicine, 401 West Michigan Street, Indianapolis, IN, 46202-3233, (317) 6379200, Fax: (317) 634-7817, http://www.acsm.org .

American Council on Exercise, 4851 Paramount Drive, San Diego, CA, 92123, (858) 576-6500, Fax: (858) 576-6564, (888) 825-3636, resource@acefitness.org, http://www.acefitness.org .

Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA, 30333, (800) 232-4636, cdcinfo@ cdc.gov, http://www.cdc.gov .

World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, 41 22 (791) 2111, Fax: 41 22 (791) 3111, info@who.int, http://www.who.int/en/ .

Rebecca J. Frey, PhD
Revised by William A. Atkins, BB, BS, MBA
Revised by Teresa G. Odle, BA, ELS

  This information is not a tool for self-diagnosis or a substitute for professional care.