The human body, which is made up of between 55 and 75 percent water (lean people have more water in their bodies because muscle holds more water than fat), is in need of constant water replenishment. Consider this: Your lungs expel between two and four cups of water each day through normal breathing - even more on a cold day. If your feet sweat, there goes another cup of water. If you make half a dozen trips to the bathroom during the day, that's six cups of water. If you perspire, you expel about two cups of water (which doesn't include exercise-induced perspiration).
Ms. Dunn points out that a person would have to lose 10 percent of her body weight in fluids to be considered dehydrated, but as little as two percent can affect athletic performance, cause tiredness and dull critical thinking abilities. Adequate water consumption can help lessen the chance of kidney stones, keep joints lubricated, prevent and lessen the severity of colds and flu and help prevent constipation. "I encourage patients to drink eight to 10 cups of water each day. Those who do report that they generally feel better," notes Ms. Dunn.