Disturbances of thirst and fluid balance associated with aging.

During heat waves, significant mortality and morbidity occurs in elderly populations due to heat-stress and dehydration. The dehydration is primarily attributable to inadequate water intake caused by dysfunction of the central nervous system mechanisms controlling thirst. The phenomenon of a reduced thirst in response to dehydration in aging was first observed decades ago and has been examined extensively since. The reduced thirst and ingestive behavior have been reported consistently in response to hyperosmotic stimuli, hypovolemic stimuli and dehydration in both elderly humans and animal models of aging. There are also data to suggest that sodium appetite is reduced in aged rats, potentially indicating a common etiology. Accompanying the behavioral changes in water and sodium intake that occur with aging there are also alterations to a number of hormonal systems involved in body fluid and electrolyte homeostasis. These changes include reductions in activity of the renin-angiotensin system and increases in circulating atrial natriuretic peptide and arginine vasopressin. While there is substantial evidence to suggest that the behavioral and physiological mechanisms responsible for body fluid and sodium homeostasis are impaired in aging, the precise etiology of reduced thirst remain to be determined.