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.