For example, on a cool, non-exercising day, the average person loses the equivalent of 9 to 12 cups of water. Breathing alone accounts for about 1 cups of this loss. During strenuous exercise, the loss can be 2 quarts per hour or more. The consequences of this leakage can be significant. In addition, day to day dehydration contributes to constipation and cramps and may worsen asthma , dental disease , kidney stones , and urinary-tract infection and increase the risk of colds and even cancer .
Severe dehydration can cause serious heat illnesses.
A deficit of three quarts can make blood volume and blood pressure plummet, potentially causing delirium , fainting , disorientation or a heart attack. The major reason we do not properly hydrate, besides drinking water being boring, is that our thirst mechanisms are not finely tuned. Unlike other warm blooded animals, we do not experience thirst until we are down a quart or so. And as we age, this biological weakness gets worse as our thirst signaling mechanisms become less sensitive. This is reflected in the fact that dehydration is one of the top ten conditions that send older folks to the hospital.