Water and Nutrition

Definition

Water (hydrogen oxide, or H2O) is essential to life and nutritional health. Humans can live for several weeks without food, but can survive only a few days without water. Water makes up a large percentage of the body and is in muscles, fat cells, blood, and even bones.

Purpose

Every cell, tissue, and organ requires water to function properly. Water transports nutrients and oxygen to the cells, provides a medium for chemical reactions to take place, helps to flush out waste products, aids in maintaining a constant body temperature, and keeps the tissues in the skin, mouth, eyes, and nose moist.

Description

Intracellular fluid, the liquid inside individual cells, represents about two-thirds of the body's water, or about 40% of total body weight. Intracellular fluid contains both water and salts, primarily potassium, as well as enzymes and other organic molecules. A process called osmosis largely controls flow of water into and out of the cell. Body water may be lost through various mechanisms including respiration, perspiration, and urination, and must constantly be replaced. In optimal conditions, water levels would be completely balanced, and water intake would match the amount of water lost.

Water for sustaining life

The body works to maintain water balance through mechanisms like the thirst sensation. When the body requires more water, the brain stimulates nerve centers in the brain to encourage a person to drink in order to replenish water stores.

The kidneys are responsible for maintaining homeostasis of the body water (i.e., water balance) through the elimination of waste products and excess water. Water is primarily absorbed through the gastrointestinal tract and excreted by the kidneys as urine. Water intake can vary widely on a daily basis, influenced by such factors as access to water, thirst, habit, and cultural factors. The variation in water volume ingested is dependent on the ability of kidneys to dilute and concentrate the urine as needed. There is a reservoir of water outside of the bloodstream that can replace or absorb excess water in the blood when necessary.

For a normal adult, a minimum daily intake of between 0.74–0.84 quarts (700–800 mL) is required to meet water losses and maintain the body's water balance. To protect against dehydration and developing kidney stones, greater water consumption (between 1.5–2 qt./day or 1.4–2 L/day) is advised. The exact amount of water necessary depends on body size, age, climate, and exertion level. Water needs are increased by:




Hydration: Benefits of hydrating the body





Hydration: Benefits of hydrating the body

Daily adequate intake of water

Age

Approximate daily intake of water (cups)*

Children 0-6 mos.

3

Children 7-12 mos.

3⅓

Children 1-3 yrs.

Children 4-8 yrs.

7

Boys 9-13 yrs.

10

Girls 9-13 yrs.

9

Boys 14-18 yrs.

14

Girls 14-18 yrs.

10

Men 19≥ yrs.

15½

Women 19≥ yrs.

11½

Pregnant women

13

Breastfeeding women

16

*Includes water contained in food, beverages, and drinking water

SOURCE: Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies Press, 2005. https://doi.org/10.17226/10925 (accessed April 19, 2018).

Avoiding some beverages, such as coffee, tea, alcohol, and caffeinated soft drinks, may reduce the risk of dehydration. These beverages are all diuretics (substances that increase fluid loss). Water in foods, especially fruits and vegetables, is a great source of fluid and contributes to overall fluid intake. Fruits and vegetables can contain up to 95% water, so eating a well-balanced diet is a good way to ensure proper hydration.

Role in weight loss

Beyond its role in general health, water can play a major role in maintaining body weight through a program of caloric restriction. Foods that contain large amounts of water, such as fruits and vegetables, have low energy density, and so may produce sensations of satiety without high caloric intake.

Several published studies showed interesting patterns of food intake based on the water composition of foods. In one, subjects were given either food containing a high concentration of water, such as a soup of a stew, or the same solids prepared as a casserole, with water to accompany the meal. Although in each case the total amount of both solids and water were the same, subjects ingested fewer calories when the water was incorporated into the food source. In a related study, advising people to eat foods with low energy density—that is, foods containing higher concentrations of water—was a more successful weight-loss strategy than attempts to limit portion size.

Another study evaluated the effects of pre-loading water before a meal. Subjects were asked to drink water before eating. Although subjects claimed that the quantity of water ingested had filled them up and they had no appetite, the amount of food actually consumed after the pre-load was no different from that eaten by members of the control group. Although these studies are not definitive, they do indicate that foods with a high concentration of water, such as soups, stews, or salads, may be useful in weight-loss programs by providing satiety with low levels of energy intake.

Precautions

The body does not store excess water, unlike it does with other nutrients. With physical exertion, water requirements increase; therefore, fluid replacement during exercise is critical. The longer the duration and the more physical exertion athletes put into their exercise, the more fluid they lose during workouts. To keep the body working at its best, it is essential to replenish lost fluid after workouts, and to stay well hydrated during exercise.

The body can accommodate extreme changes in water intake when the brain and kidneys are functioning normally. It is usually possible for a person to consume enough water to maintain blood volume and electrolyte balance in the blood. However, if a person is unable to consume enough water to equal excessive water loss, dehydration may result.

Complications

Failure to maintain adequate water levels can lead to dehydration. Sodium deficiency (hyponatremia) and water imbalances (dehydration) are two of the most serious and widespread deficiencies in the world. These electrolyte deficiencies tend to arise from excessive losses from the body, such as during prolonged and severe diarrhea or vomiting. Diarrheal diseases are a major world health problem and are responsible for about a quarter of the ten million infant deaths that occur each year. Nearly all of these deaths occur in impoverished parts of Africa and Asia, where they result from contamination of the water supply by animal and human feces.

Dehydration

The initial symptoms of dehydration are thirst and dry mouth, followed by lightheadedness and dizziness. Sweating and the output of urine both decrease. If water intake continues to fall short of water loss, dehydration worsens.

KEY TERMS
Dehydration—
A condition of water loss caused by either inadequate intake of water or excessive loss of water as through vomiting or diarrhea.
Diuretic—
An agent or drug that eliminates excessive water in the body by increasing the flow of urine.
Electrolyte—
Any of several chemicals dissolved in blood and other body fluids that are capable of conducting an electric current. The most important electrolytes in humans and other animals are sodium, potassium, calcium, magnesium, chloride, phosphate, and hydrogen carbonate.
Homeostasis—
An organism's regulation of body processes to maintain internal equilibrium in temperature and fluid content.
Hyperhydration—
Excess water content of the body.
Hyponatremia—
Inadequate sodium levels in the body, possibly caused by loss of sodium through perspiration, diarrhea, or vomiting, and replacement of fluids with water that does not contain adequate electrolytes.

Causes of dehydration may include:

Dehydration is often accompanied by a deficiency of electrolytes—sodium and potassium in particular. Water does not move as rapidly from the reservoir inside of the cells into the blood when electrolyte concentration is decreased. Blood pressure can decline due to a lower volume of water circulating in the bloodstream. A drop in blood pressure can cause lightheadedness or a feeling of impending blackout, especially upon standing (orthostatic hypotension). Continued fluid and electrolyte imbalance may further reduce blood pressure, causing shock and damage to many internal organs including the brain, kidneys, and liver.

Consumption of plain water is usually sufficient for mild dehydration. However, when both water and electrolyte losses have occurred after vigorous exercise, electrolytes must be replaced—sodium and potassium in particular. Most individuals do not require sports drinks, but athletes or people undergoing rigorous workouts may add a little salt to their drinking water or consume drinks like Gatorade during or following exercise to replace lost fluids. Individuals with heart or kidney problems should consult a physician regarding the replacement of fluids after exercise.

Hyperhydration

Although water intake is normally very safe, excessive water intake, also known as hyperhydration, can occur, and may be fatal. Drinking large amounts of water does not typically lead to overhydration if the kidneys, heart, and pituitary gland are functioning properly. An adult would have to drink more than 2 gallons per day (7.6 L per day) to exceed the body's ability to excrete water. Excessive water intake can lead to dilution of the sodium levels in the body, causing hyponatremia. Overhydration occurs primarily in individuals whose kidneys do not function normally, such as in patients with kidney, heart, or liver disease. It is sometimes seen in infants who may ingest too much water, either because they are given only water to drink or because excessive water is used to dilute infant formulas. Water intoxication may also result from severe vomiting or diarrhea in which the fluid is replaced with water without replacing the electrolytes. Rarely, athletes who have undergone very great exertion may perspire excessively, and, if the fluid loss is replaced with water without electrolytes, may experience water intoxication. Although this is very rare, it did occur at the 2007 London Marathon, when temperatures were unseasonably warm and over 5,000 runners needed to be treated on site. More than 70 runners were taken to the hospital for treatment and one first-time marathoner, 22 years of age, died from hyperhydration. Voluntary hyperhydration has been reported and has been known to be fatal. On occasion, hyperhydration has been reported as part of school hazings.

Symptoms of water intoxication are similar to those of dehydration: muscle cramps, confusion, nausea, slurred speech, and disorientation. Because of this, athletes may mistake water intoxication for dehydration and drink even more water after toxicity has appeared. The goal of rehydration is to drink just enough water to replace the amount lost to perspiration. Forcing fluids can be dangerous. While sports drinks replace electrolytes, they may also provide a high level of calories. For people exercising to lose weight, an appropriate amount of water has been advocated as the most appropriate method of rehydration.

See also Dehydration ; Hydration .

QUESTIONS TO ASK YOUR DOCTOR

Resources

BOOKS

Speakman, Elizabeth, and Norma Jean Weldy. Body Fluids and Electrolytes: A Programmed Presentation. 8th ed. London: Mosby, 2001.

Workman, M. Linda. Introduction to Fluids, Electrolytes and Acid-Base Balance. London: W. B. Saunders, 2000.

PERIODICALS

Beck, L. H. “The Aging Kidney. Defending a Delicate Balance of Fluid and Electrolytes.” Geriatrics 55, no. 4 (2000): 26–28, 31–32.

Gray, R. W., et al. “Increasing Preload Volume with Water Reduces Rated Appetite but Not Food Intake in Healthy Men Even with Minimum Delay Between Preload and Test Meal.” Nutritional Neuroscience 6, no. 1 (2003): 29–37.

Norton, G. N., A. S. Anderson, and M. M. Hetherington. “Volume and Variety: Relative Effects on Food Intake.” Physiology & Behavior 87, no. 4 (2006): 714–22.

Sawka, M. N., and S. J. Montain. “Fluid and Electrolyte Supplementation for Exercise Heat Stress.” American Journal of Clinical Nutrition 72, no. 2S (2000): S564–S572.

Stiefel D., and A. Petzold. “H2O Coma.” Neurocritical Care 6, no. 1 (2007): 67–71.

WEBSITES

Centers for Disease Control and Prevention. “Water and Nutrition.” http://www.cdc.gov/healthywater/drinking/nutrition/index.html (accessed April 23, 2018).

ORGANIZATIONS

Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600, amacmunn@eatright.org, http://www.eatright.org .

Food and Nutrition Information Center, National Agricultural Library, 10301 Baltimore Ave., Rm. 105, Beltsville, MD, 20705, (301) 504-5414, Fax: (301) 504-6409, fnic@ars.usda.gov, http://fnic.nal.usda.gov .

Crystal Heather Kaczkowski, MSc

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