Spangler et al. (1984) reported that as many as 25% of nursing home patients may be chronically dehydrated. However, these findings do not prove that there is an age-related change in fluid regulation, since the results may have been complicated by a restricted access to fluids or by the prevalent use of medications that alter body fluids. The extent of dehydration in healthy, active older individuals has been debated. One study by Phillips et al. (1984) reported normal hydration in elderly subjects, while another study by the same group found elevated baseline sodium concentrations and plasma osmolalities in healthy older subjects (Crowe et al., 1987).
Miller (1987) recently reviewed potential mechanisms for the occurrence of body fluid disturbances during the normal aging process. Lindeman et al. (1960) found that renal concentrating capacity in response to dehydration decreases with age, becoming evident between approximately 45 and 50 years of age. Rowe et al. (1976) substantiated this observation in men after 12 h of dehydration.
The regulation of plasma sodium also appears to be affected by the normal aging process. Epstein and Hollenberg (1976) studied the renal response to sodium restriction in individuals from 18 to 76 years of age. Renal sodium excretion decreased by 50% after 18 h in subjects younger than 30 years, after 24 h in subjects between 30 and 60 years of age, and after 31 h in subjects older than 60.
Impaired fluid and electrolyte balance in the elderly also may be due to an inability to detect changes in body hydration. Phillips et al. (1984) compared thirst perception between a group of young men and a group of men 67 to 75 years of age. The subjects were dehydrated for approximately 24 h, until both groups had a similar decrease in body weight. Following dehydration, the older subjects were not as thirsty as the younger subjects, based on their responses to a visual analog thirst scale, despite a greater increase in plasma osmolality.
Increased secretion of ADH in response to osmotic stimuli and decreased secretion in response to hypovolemic stimuli occur with aging (Bevilacqua et al., 1987; Ledingham et al., 1987). During water restriction, Phillips et al. (1984) found a greater increase in ADH in older subjects, despite a similar loss of plasma volume. Helderman et al. (1978) infused hypertonic saline into young and older individuals and found a greater release of ADH into the plasma of older subjects compared with that in the plasma of the young subjects. This increased responsiveness of ADH is believed to compensate for the reduced sensitivity of the kidneys of older subjects to ADH.
Baseline concentrations of atrial natriuretic factor (ANF) increase with increasing age (Wambach and Kaufmann, 1988; Yamasaki et al., 1988). The consequences of these changes in ANF regulation on body fluid responses