Glucose tolerance, plasma insulin levels and insulin sensitivity in elderly patients

The oral glucose tolerance test (OGTT) and the intravenous glucose tolerance test (ivGTT) were carried out in 23 elderly patients (mean age 79 years) and in 14 healthy young adults (mean age 28 years). The ivGTT was followed immediately by a second injection of glucose, this time with insulin added. The percentage rate constants for the disappearance of glucose alone (KG, % min-1) or with added insulin (KG+I, % min-1) were calculated. Seventeen of the elderly patients had a blood glucose value at 120 min in the OGTT greater than or equal to 7.7 mmol/1 (140 mg/100 ml) or had KG less than or equal to 1% min-1 and 13 were 'diabetic' by both criteria. This was associated with a sluggish pattern of insulin release and with diminished insulin sensitivity (KG+I). Since these changes could reflect impaired inhibition of hepatic glucose output, a preliminary attempt was made to assess gluconeogenic capacity. After the injection of substrate levels of L-alanine, plasma levels of glucose and alanine both rose but fell more slowly in the elderly than in young adults, suggesting impaired transmembrane transport of both substances in old people. After injection of alanine, plasma insulin rose to a lesser extent and glucagon to a greater extent in the elderly than in young adults.