About two thirds of the body weight is water with the majority being within the cells. Water is the main transport mechanism within the body for carrying oxygen, nutrients and hormones to the cells and removing waste products. The waste products are eliminated from the body through the water in sweat, urine and feces. Water also regulates osmotic pressure controlling the proper electrolyte balance and acid/base balance. Water lubricates and cushions. It acts as the body's main cooling system.
The requirement for water depends on the weight and age of the individual, but the average adult needs about 2 quarts of water a day to maintain water balance within the body . The balance is maintained when intake of water matches output of body fluids. The main output for water is urine, however, some is lost in the feces and some through exhaled air. Insensible perspiration, which cannot be seen, is a significant source of body water loss. Sweat losses increase greatly with exercise or hot environmental conditions.
The major source of water is fluid intake. Solid foods also contribute through their water content and through metabolism of these foods for energy. Fat, CHO and protein produce water known as metabolic water when broken down for energy.
Normal levels of body water are maintained through kidney function. Loss of body water results in conservation by the kidneys while increased consumption leads to the kidneys ridding the body of excess water. Your body usually lets you know when you need water by thirst. In normal conditions, thirst is usually a good guide to body water needs and is effective in restoring body water to normal. Thirst is not a good indicator of body needs during exercise and especially when exercising in hot weather.