If each cell is to complete the reactions demanded for performance at optimal speed, movement, and endurance, the body must have adequate access to fluid. Water Balance Water Input. Water needed by the body comes from a variety of sources and is provided by food, drink, and metabolism. In addition to water itself, beverages such as milk, sports drinks, and juices contain large amounts of water. Other foods also contain rich supplies of water.
Water also is released in the body as foods are broken down and metabolized for energy.
To ensure proper hydration, fluid lost must not exceed fluid consumed. The body can lose up to half a gallon of water a day through normal perspiration, respiration, and excretion processes. During prolonged physical activity, water losses increase due to increased breathing and sweating. The body's digestive system can only absorb about 1 quart of fluid per hour, so an athlete must consume fluids before, during, and after exercise to replace fluid losses and minimize dehydration. Hot, Humid Days High temperatures increase the rate of water lost through perspiration.