Acidification capacity of the kidneys and aging

The authors studied the acidification capacity of the kidneys in 60 healthy subjects aged 18-70 years after a single load of NH4Cl in a dose of 0.1 g/kg. The acidification load was followed by a significant increase in NH4+ excretion in the first five hours afterwards in young individuals (18-30 years). In subjects aged over 50, changes in NH4+ excretion were nonsignificant under these conditions. Titratable acid excretion rose significantly after the given acidification load in subjects aged 18-60 years; in older subjects it no longer increased significantly. Changes in titratable acid excretion displayed a significant correlation to the renal excretion of phosphates. The findings indicate that the diminished capacity of older subjects to increase titratable acid excretion after an acute NH4Cl load is due to an insufficient decrease in the tubular resorption of phosphates. Renal capacity for adequate reduction of the urine pH after a NH4Cl load was unimpaired.