Age as a determinant of renal sodium conservation in normal man

Age modifies a number of factors which determine renal sodium handling including the rate of glomerular filtration, renal hemodynamics, and the responsiveness of the renin-angiotensin-aldosterone system. This study was carried out, therefore, to examine the effects of age on the capacity of the normal human kidney to respond to restriction of sodium intake. Renal conservation of sodium and response to dietary sodium restriction was assessed in 89 healthy subjects who were free of cardiovascular, renal, or adrenal disease. The daily reduction in urine sodium which followed restriction of intake to 10 mEq. of sodium and 100 mEq. of potassium per day conformed well to an exponential function, defined by an unweighted least-squares fit. The half-time for the reduction in renal sodium excretion in subjects under 30 years was -17.6 +/- 0.7 hours, significantly faster than for subjects aged 30 to 59, who had a relatively constant half-time (23.4 +/- 1.1 hours). In subjects over 60 years of age the half-time was prolonged to 30.9 +/- 2.8 hours, significantly greater than that of the younger age group. These observations indicate that age significantly influences the kidney's capacity to conserve sodium. Age-related change must be considered in the assessment of this function in human disease.