Next to air, water is the substance most necessary for survival. Most everything in the body occurs in a water medium. Although people can live without vitamins and minerals for extended periods, death results within a few days without water.Water makes up about 60% of the body’s weight. Every cell in the body is bathed in water of the exact composition that is best for it. Even tissues that are not thought of as “watery” contain large amounts of water. For example, water makes up about 75% of brain and muscle tissues; bone tissue and fat tissue are about 20% water. As a rule the bodies of men contain more water than the bodies of women. Men have more muscle tissue, and muscle tissue holds more water than fat tissue, which is more prominent in the bodies of women.

Water performs many functions. It is vital to digestion and metabolism because it acts as a medium for chemical reactions in the body. It carries oxygen and nutrients to the cells through blood, regulates body temperature through perspiration, and lubricates the joints. It removes waste through sweat and urine, protects the fetus, and assists in respiration by moistening the lungs to facilitate intake of oxygen and excretion of carbon dioxide. It also assists with constipation relief and provides satiety, thus serving as a deterrent to the over consumption of food.

The average adult consumes and excretes about 96 ounces of water a day. Approximately 600 calories of energy are removed with the evaporation of each liter of perspiration. For this reason, people who have a high fever or who are exposed to high temperatures, hot climates, or strenuous physical activity considerably increase their need for more calories, as well as liquids. Long airplane flights also demand extra fluid intake; a traveler can lose about 1.5 liters (6 cups) of water during a 3-hour flight. The dehumidified air in an airplane is so dry that it induces excessive, although often unnoticed, perspiration and evaporation. Although most water intake comes from beverages, solid foods also make a significant contribution. Most fruits are more than 80% water, meats are 50% water, bread is 33% water, and butter is approximately 15% water.

Some beverages and foods can increase the need for water. Alcoholic beverages, tea, and coffee contain water but can have a contradictory effect on the body. An ounce of pure alcohol requires 8 ounces of water to be metabolized. Caffeinated beverages stimulate the adrenal glands and serve as diuretics, increasing water output and the need for water. The sugar and sodium found in many beverages and foods place an extra burden on the body because they require water to be dissolved, used, and excreted.

How much water should you drink? People are advised to drink six to eight 8 ounce glasses of fluids per day, regardless of thirst. Although thirst is the body’s first sign of dehydration, the thirst mechanism is not always reliable, especially during exercise and illness. A person can drink just enough fluid to quench a thirst but not enough to satisfy the body’s needs. One way to determine proper water consumption is to check the urine. A trickle of deep yellow, cloudy urine that lasts only a few seconds indicates more water is needed. A flow of pale yellow or clear urine for 10 seconds means that a sufficient amount of water is being consumed.

People on high protein diets need more water than they thirst for to provide the kidneys with enough water to flush out the waste products of protein metabolism. Under normal circumstances, too much water cannot be consumed because the body is efficient at getting rid of what it does not need. However, water consumption should occur through out the day. A sudden drinking binge in a short period early in the day will not satisfy the body’s needs later; the excess will be excreted by the kidneys.