The best way for me to stress the importance of proper hydration for runners, is to explain how dehydration affects the body.  Running causes the body to lose fluid from exhaling moisture and sweating.  If fluid losses are not replaced by water or sport drinks, the concentration of solutes in the blood becomes too high, leading to dehydration and thickening of blood.  In order to maintain adequate blood flow to working muscles and vital organs the heart rate must increase, which places strain on the cardiovascular system. When excessive fluid loss occurs, the blood volume and blood pressure falls.   As blood volume depletes, hormones are released signaling the kidneys to reabsorb and recirculate water. When the body needs water, less is excreted and more is retained causing impaired heat dissipation from the skin and core temperature to rise, eventually leading to heat illnesses.
            The chart below shows the signs and symptoms of dehydration based on the amount of fluid lost.  When a certain percentage of dehydration occurs the body cannot continue normal physiologic functions.  This can be devastating to a runner’s performance and most importantly their health.

A runner can sweat out six cups of fluid in an hour.  Just think about how much you drink in a day, not including pop or alcohol beverages.  With that in mind, are you drinking enough fluids? 

Also, for severe dehydration it may be necessary to hospitalize the person and restore fluid balance through intravenous fluid replacement.   In order to prevent dehydration, drink before, during, and after your runs. Drink before running and make sure you have access fluids if running longer than 30 minutes. During longer runs, some of your fluid intake should include a sports drink to replace electrolytes.