Effects of aging on insulin secretion.

Aging is associated with hyperinsulinemia, but reports vary on the contributions of altered insulin clearance versus insulin secretion to this phenomenon. To elucidate the role of insulin secretion in the hyperinsulinemia of aging, 10 elderly (age 66 +/- 4 yr, body mass index 25 +/- kg/m2) and 8 young (age 30 +/- 5 yr, body mass index 24 +/- 3 kg/m2) subjects were studied to determine rates of insulin secretion in response to fasting, mixed meals, and intravenous glucose administration. Insulin secretion was determined with a two-compartment model based on individual C-peptide kinetic parameters derived after bolus injection of biosynthetic human C-peptide. Basal insulin secretion rates were increased in elderly subjects (82.5 +/- 9.0 vs. 62.8 +/- 6.1 pmol.min-1.m-2; P less than .05). This was reflected in elevated serum insulin levels in elderly subjects (62.8 +/- 10.1 vs. 41.1 +/- 5.0 pM, P less than .05). During a 24-h mixed-meal profile, elderly subjects had an increase in their glucose response (P less than .01 by analysis of variance [ANOVA]) and total insulin secretion (261 +/- 28 vs. 195 +/- 22 nmol.24 h-1.m-2; P less than .05) compared with young subjects. However, the relative total increases in both glycemia and insulin secretion, calculated as a function of basal levels, were similar between the groups (both NS). To experimentally control for differences in glycemia, both groups underwent a 16.8-mM hyperglycemic clamp and a stepped intravenous glucose infusion to match glycemia. Under these steady-state and dynamic conditions, insulin secretion profiles were nearly identical (NS by ANOVA).(ABSTRACT TRUNCATED AT 250 WORDS)