WATER
Although humans can
live about 30 to 45 days without food, it is possible to live only 10 to 14
days without water. Water is a component of all body cells and constitutes from
50% to 60% of the body weight of normal adults. The percentage is higher in
males than females because men usually have more muscle tissue than women. The
water content of muscle tissue is higher than that of fat tissue. The
percentage of water content is highest in newborns (75%) and decreases with
age.
Body water is divided into
two basic compartments: intracellular and ex-tracellular. Intracellular fluid (ICF) is water within
the cells and accounts for about 65% of total body fluid (Figure 9-1). Extracellular fluid (ECF) is water
outside the cells and accounts for about 35% of total body fluid. Extracellular
fluid is found in the intravascular fluid (water in the bloodstream), interstitialfluid, and glandular
secretions.
Although it is a
component of all body tissues, water is the major component of blood plasma. It
is a solvent for nutrients and
waste products and helps transport both to and from body cells by way of the
blood. It is necessary for the hydrolysis of nutrients in the cells, making it
essential for metabolism. It functions as a lubricant in joints and in digestion.
In addition, it cools the body through perspiration and may, depending on its
source, provide some mineral elements (Table 9-1).
The best source of
water is drinking water, and Table 9-2 lists the Dietary Reference Intake for
Water. Beverages of all types are the second-best source. A considerable amount
is also found in foods, especially fruits, vegetables, soups, milk, and gelatin
desserts. In addition, energy metabolism produces water. When carbohydrates,
fats, and proteins are metabolized, their end products include carbon dioxide
and water (Table 9-3). See Appendix D for water content of foods.
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