Renal Function. The ability of the kidney to conserve sodium decreases with age in response to varying and thus lower intake of salt decreases with age. The ability of apparently healthy older individuals to adapt by decreasing urinary sodium when fed very low sodium diets (in the range of 0.23 g [10 mmol]/day) has been shown to be much slower than the adaptation seen in younger individuals; however, with time, older individuals were able to adapt and reduce urinary sodium excretion to levels less than 10 mmol/day (Epstein and Hollenberg, 1976). In a study in which individuals over 40 years of age were compared with race-, sex-, and body weight-matched controls below 40 years of age, short-term loading via intravenous saline administration demonstrated distinct age-related differences in sodium excretion, which included excreting significantly more sodium during the night than the younger control subjects (Luft et al., 1980, 1982, 1987).

The clinical significance of this impaired response may be considerable when older individuals must quickly adapt to the reduced sodium intakes that are often seen during illnesses or following surgery. The result of a rapid decrease in sodium and fluid intake is a reduction in extracellular fluid volume, which is clinically manifested as a decrease in circulating blood volume. In clearance studies of apparently healthy younger and older subjects, older subjects had a reduced ability to reabsorb sodium at the distal tubule compared with their younger counterparts (Macias-Nuñez et al., 1978). Possible mechanisms by which distal tubule function is affected include development of interstitial fibrosis (Macias-Nuñez et al., 1980) or loss of functioning nephrons. In addition, the hormonal