People can live for weeks without food. Without water, people will die within days. The human body is 50% to 70% water.
Water is one of the most important parts of the diet. Body fluids are mostly water. These fluids flow through arteries, veins and capillaries and carry nutrients and wastes. Fluids fill cells and the spaces between them to keep tissues healthy. Water is also needed for many chemical reactions in the body.
Water dissolves vitamins, minerals and other nutrients, helps keep joints lubricated, and helps regulate body temperature. When hot water, in the form of perspiration, evaporates from the skin it cools the body. This keeps the body's internal temperature within a very narrow range.
Adults need at least two quarts, or eight cups, of water every day for good health. Children need four to eight cups depending on age and size. The easiest way to get water into the body is to drink it. Other fluids, and even some foods, have water. These can help meet daily water needs. Milk, fruit juice and even carbonated beverages are good sources of water. Those who are unable to drink fluids for any reason, such as because of trouble swallowing, loss of consciousness, or during recovery from surgery, usually must be given fluids intravenously.
Drinks with caffeine are not as effective in meeting daily water needs. Caffeine is a diuretic, that is, a chemical that makes the kidneys excrete more water, so additional water must be consumed if one is drinking caffeinated beverages. Alcohol does not count toward meeting daily water goals.
One of the most important jobs of water is to maintain the body's fluid and salt balances. The concentrations of minerals, called electrolytes when they are dissolved in body water, are delicately balanced inside and outside the body's cells. These include sodium, potassium and chloride. Electrolytes determine how much fluid stays in cells and how much remains outside cells.
An imbalance is not healthy. Too much water can cause edema (fluid retention) or excessive urination. When the body does not have enough water to perform properly, it is called dehydration. Many medical conditions and drugs affect fluid balance. The kidneys do most of the work of controlling fluid balance. Thus, people with kidney disease often have special needs in terms of the amount they must drink.