Water is also formed with carbon dioxide when food is metabolized. Water is lost through urine, skin, breath and the feces. The use of protein as an energy source speeds up the body's hydration during exercise. The amount of water lost through sweat increases with exercise intensity. As water is lost, core temperature begins to increase.
Weather plays an important role as well.
More humid environments prevent the evaporation of water from the skin and cause an increase in core temperature. As a result, metabolic and cardiovascular functioning are affected. It makes sense then to assure that hydration is occurring prior to and even during activity. Generally, the more intense the activity and the more humid the environment, the more water will be needed. However, great amounts of fluid taken prior to an activity may cause stomach upset.