Table of Contents


Water is hydrogen oxide and it is composed of two molecules of hydrogen and one molecule of oxygen. It has a molecular weight of 18.016 and is the most universal solvent known.


Water is the most universal solvent known. In the human body, it is capable of dissolving simple elements, ions and large organic molecules. Because of water's ability to maintain these materials in solution, the various body chemicals are capable of undergoing reactions that would not be possible in other forms.

Because water is a liquid, it can be carried through the circulatory system, reaching to all cells in the body.


Water is the most common compound in the human body, although the percentage of body water will vary from individual to individual, depending on age, gender, and general body composition. Newborn infants are about 78% body water, but this drops to

Daily adequate intake of water

AgeApproximate daily intake of water (cups)*
Children 0—6 mos.3
Children 7—12 mos.3 1/3
Children 1—3 yrs.5K2
Children 4—8 yrs.7
Boys 9—13 yrs.10
Girls 9—13 yrs.8-9
Boys 14—18 yrs.14
Girls 14—18 yrs.9-10
Men 19>yrs.15½
Women 19> yrs.11½
Pregnant women12-13
Breastfeeding women16

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

SOURCE: Adapted from the Dietary Reference Intakes Table, Food and Nutrition Board, Institute of Medicine, National Academies

(Illustration by GGS Information Services/Thomson Gale.)

65% by one year of age. Although the adult percentages are often quoted as 60% for males and 55% for females, this is strongly influenced by the amount of body fat present in the body. Since fat cells contain very little water, higher levels of body fat will reduce the overall percentage of water.

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. Flow of water into and out of the cell is largely controlled by osmosis. The outermost layer of an animal cell is the cell membrane, and water can flow through the membrane from areas of low salt concentration to areas of high salt concentration. The remaining water is in the form of extracellular fluid that includes blood and cerebrospinal fluid. The most common ion of the extracellular fluid is sodium. Body water may be lost through various mechanisms including respiration, perspiration, and urination, and must constantly be replaced. Under the best circumstances, water levels will be completely balanced, and the intake will match the amount of water lost.

Because water can be moved through the body rapidly, people have used diuretics to give the illusion of weight loss. Diuretcis, both drugs and diuretic herbs, promote loss of water through the kidneys. Water loss is at best transient, and has no real benefit in terms of either health or physical appearance.


Dehydration —A condition of water loss caused by either inadequate intake of water or excessive loss of water as through vomiting or diarrhea.

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.

Molecular weight —The total of the atomic weights of the atoms in a molecule.

Pre-loading —Administering in advance, such as drinking water prior to exercise that is likely to cause water loss.

Water intoxication—A condition caused by excessive water in the body, related to hyponatremia.

Beyond its role in general health, water can make 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 with low 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.

The second study evaluated the effects of preloading 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 indiate 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.


Failure to maintain adequate water levels can lead to dehydration. While this may be the result of various diseases, the initial symptoms are thirst and dry mouth. followed by lightheadedness and dizziness

Although water intake is normally very safe, excessive water intake, also known as hyperhydration, can occur, and may be fatal. Excessive water intake can lead to dilution of the sodium levels in the body, causing hyponatremia. This condition 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 extertion may perspire excessively, and, if the fluid loss is replaced with water without electrolytes, may experience water intoxication. Althoug this is very rare, it did occur at the 2007 London Marathon, when temperatures were unseasonably warm that over 5,000 runners needed to be treated on site. Over 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, althletes 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.


Weight loss programs should target body fat; however, some weight-loss remedies, in an attempt to show prompt results, have incorporated diuretic drugs. These may lead to loss of body water, with the risk of dehydration.

Parental concerns

Adolescents and teen-agers should be aware of the hazards associated with hyperhydration. Children of this age may be at risk both of excessive water intake after athletics, and also as part of school hazing rituals.


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Baby Milk Action. 34 Trumpington Street, Cambridge, CB2

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