65% by one year of age. Although the adult percen-
tages 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 con-
trolled byosmosis. 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 issodium. 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 useddiureticsto 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.
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 sensa-
tions of satiety with low caloric intake.
Several published studies showed interesting pat-
terns of food intake based on the water composition of
foods. In one, subjects were given either food contain-
ing 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 concentra-
tions of water, was a more successful weight-loss strat-
egy than attempts to limit portion size.
The second 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 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.Daily adequate intake of waterApproximate daily intake of
Age water (cups)*
Children 0–6 mos. 3
Children 7–12 mos. 3
Children 1–3 yrs. 5½
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 women 12–13
Breastfeeding women 16
*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(^13)
(Illustration by GGS Information Services/Thomson Gale.)
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.
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 replace-
ment 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.
Waterintoxication—Aconditioncausedbyexcessive
water in the body, related to hyponatremia.
Water