Hydration refers to the human body's ability to take in and maintain a steady level of water in its tissues and organs down to the cellular level. It describes a condition of fluid balance (water homeostasis) when adequate fluid levels are maintained. An imbalance caused by too little fluid intake results in dehydration. It is also possible for people to 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. Severe hyponatremia can result in neurological symptoms, including swelling of brain tissue, seizures, and in some cases death.
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). Each compartment contains differing amounts of electrolytes that must remain balanced in order for body organs and systems to function correctly. Humans can live for only a week without water, less if they are stranded in a desert or another hot, dry environment. Water is necessary for several vital body processes. Water:
A person's total body water content varies in relation to age, gender, and amount of body fat. Adult males have approximately 60% water content; adult females have about 50%; newborns and infants an estimated 80% and 77%, respectively; and the elderly have between 46%–52%. An increase in body fat causes a decrease in the percentage 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 such a healthy ratio.
People, whether they are healthy or not, lose water through urination, elimination of solid wastes, sweating, and breathing out water vapor. This water must be replaced through the diet. The Food and Nutrition Board of the Institute of Medicine (IOM) recommends that relatively inactive adult men should take in about 125 oz. (3.7 L, about 15 cups) of fluids daily and that women should take in about 91 oz. (2.7 L, about 10 cups) to replace lost water. 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; 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 of food. Fluids taken by mouth are absorbed throughout the digestive tract, primarily in the small intestine, but also in the colon. Water obtained by metabolism arises as a byproduct of the oxidation (breakdown) of the various 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%–10%. Variations in urine output are the main method by which the body regulates its water balance 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 a temperate climate, the average adult loses about 44 oz. (1.3 L) of fluid per day in the 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.
Proper hydration is about balance. Though severe hyponatremia is very rare, too much fluid intake can be as harmful as too little. Too much fluid in the body can lead to overhydration or water intoxication and, if left unchecked, hyponatremia. Normal blood sodium levels are between 135 and 145 mEq/L. Hyponatremia occurs when sodium in the blood drops to less than 135 and becomes severe when levels fall below 125. It would be very difficult and uncomfortable to reach this level under normal circumstances. Hyponatremia may occur in marathon runners or other endurance athletes replacing their fluids only with water (no sodium). People whose work or exercise patterns require them to pay close 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 water readily available; and consulting a specialist in nutrition or sports medicine for individualized advice about hydration.
The American Council on Exercise (ACE) makes the following recommendations about water intake before, during, and after exercising or working outdoors:
In 2007, the American College of Sports Medicine (ACSM) recommended that preparation for adequate hydration during exercise should be tailored to each athlete (or worker) rather than tied too rigidly to general guidelines, because the changes in electrolyte balance and amount of sweat varies 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.
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 their 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.
Complications can result when fluids expended from the body are not accurately replaced. Dehydration (too little water in the body) and water intoxication (too much water) can occur.
Risk factors for loss of water in the general population include:
Several factors may contribute to an increased risk of inadequate hydration in the elderly, including:
Risk factors for water intoxication (overhydration) include:
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.
A family doctor can order a yearly urine and blood test for people who do not need to monitor their level of hydration closely. Athletes and those who perform heavy labor outdoors in hot weather may wish to consult a nutritionist or specialist in sports medicine to evaluate their need for closer monitoring of hydration levels or an individual plan for maintaining adequate hydration.
See also Adult nutrition ; Dehydration ; Detoxification diets ; Electrolytes ; Metabolism ; Sodium ; Sports nutrition ; Water and nutrition .
Dunford, Marie. Fundamentals of Sport and Exercise Nutrition. Champaign, IL: Human Kinetics, 2010.
Isaac, Jeff. Outward Bound Wilderness First-Aid Handbook. 4th ed. Guilford, CT: Falcon Guides, 2013.
Knoop, Kevin J., et al., eds. Atlas of Emergency Medicine. 4th ed. New York: McGraw-Hill Education, 2016.
Moorman III, Claude T., Donald T. Kirkendall, and Ruben J. Echemendia, eds. Praeger Handbook of Sports Medicine and Athlete Health. 3 vols. Santa Barbara, CA: Praeger, 2011.
Rich, Brent E., and Mitchell K. Pratte. Tarascon Sports Medicine Pocketbook. Sudbury, MA: Jones and Bartlett, 2010.
Smolin, Lori A., and Mary B. Grosvenor. Nutrition for Sports and Exercise. 2nd ed. New York: Chelsea House, 2010.
American College of Sports Medicine (ACSM), et al. “American College of Sports Medicine Position Stand: Exercise and Fluid Replacement.” Medicine and Science in Sports and Exercise 39, no. 2 (February 2007): 377–90.
Dalal, S., E. Fabbro, and E. Bruera. “Is There a Role for Hydration at the End of Life?” Current Opinion in Supportive and Palliative Care 3, no. 1 (March 2009): 72–78.
Dundas, B., M. Harris, and M. Narasimhan. “Psychogenic Polydipsia Review: Etiology, Differential, and Treatment.” Current Psychiatry Reports 9, no. 3 (June 2007): 236–41.
Montain, S. J. “Hydration Recommendations for Sport 2008.” Current Sports Medicine Reports 7, no. 4 (July– August 2008): 187–92.
Noakes, T. D. “Hydration in the Marathon: Using Thirst to Gauge Safe Fluid Replacement.” Sports Medicine 37, no. 4–5 (April–May 2007): 463–66.
Rogers, I. R., and T. Hew-Butler. “Exercise-Associated Hyponatremia: Overzealous Fluid Consumption.” Wilderness and Environmental Medicine 20, no. 2 (summer 2009): 139–43.
Wotton, K., K. Crannitch, and R. Munt. “Prevalence, Risk Factors and Strategies to Prevent Dehydration in Older Adults.” Contemporary Nurse 31, no. 1 (December 2008): 44–56.
American Council on Exercise. “Fit Facts: Healthy Hydration.” January 29, 2009. https://www.acefitness.org/education-and-resources/lifestyle/blog/6675/healthyhydration (accessed April 9, 2018).
Johnston, Brian D. “Overview of Exercise.” Merck Manual for Health Care Professionals. http://www.merckmanuals.com/professional/special_subjects/exercise/overview_of_exercise.html (accessed April 9, 2018).
American College of Sports Medicine, 401 W Michigan St., Indianapolis, IN, 46202, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
American Council on Exercise, 4851 Paramount Dr., San Diego, CA, 92123, (858) 576-6500, Fax: (858) 576-6564, (888) 825-3636, email@example.com, http://www.acefitness.org .
Centers for Disease Control and Prevention, 1600 Clifton Rd. NE, Atlanta, GA, 30333, (800) CDC-INFO (232-4636), TTY: (888) 232-6348, firstname.lastname@example.org, http://www.cdc.gov .
World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, +41 22 791-2111, Fax: +41 22 791-3111, email@example.com, http://www.who.int .
Rebecca J. Frey, PhD
Revised by William A. Atkins, BB, BS, MBA