To explore the age-related changes in tubular function and in the renal reserve (RR) a total of 98 healthy subjects were studied while on free living conditions. Enrolled people were divided into 3 groups: group A (n = 40, age range 5-18 years, Na intake 80 +/- 20 mM/day, protein intake 1.30 +/- 0.76 g/kg BW); group B (n = 34, age range 19-60 years, Na intake 110 +/- 12 mM/day, protein intake 1.32 +/- 0.75 g/kg BW), and group C (n = 24, age range 61-89 years, Na intake 159 +/- 12 mM/day (p less than 0.01 vs. A and B), protein intake 1.26 +/- 0.23). Glomerular filtration rate (GFR) (inulin), renal plasma flow (p-aminohippurate), the amount of filtrate and sodium delivered from the proximal tubule (lithium clearance), endogenous creatinine clearance, and predicted creatinine clearance were measured. The RR was evaluated after a meat meal (providing 2 g/kg BW of proteins) by subtracting baseline GFR from the peaking postprandial GFR. GFR was age-related and was identical in groups A and B and significantly lower in group C (p less than 0.0001). The drop in GFR averaged 7% per decade in the age range 61-89 years. The RR was not statistically different in groups A and B and increased significantly in group C (p less than 0.0001). In groups A and B the filtration fraction was constant after the meat meal and significantly increased over baseline GFR in group C (p less than 0.01), while the percentage of filtration capacity utilized at rest was lower in group C (p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)