Acute-phase insulin secretion and glucose tolerance in young and aged normal men and diabetic patients

A defect in the early phase of insulin secretion has been proposed as a major determinant of the impaired glucose homeostasis of adult onset diabetes. Since the process of aging is also associated with a deterioration of glucose tolerance, we have measured the acute phase of insulin secretion and glucose disposal rate (KG) after a 20 g glucose pulse in 11 normal, healthy men greater than 65 yr and compared it with the response in 11 nondiabetic men 20-31 yr. The mean fasting plasma sugar levels of the aged subjects (107 mg/dl), tended to be higher than that of the young (94 mg/dl). Comparison of Kg provided clear separation (P less than 0.01) of the old (mean Kg = 0.93) from the young population (mean Kg = 1.95). The acute insulin response to the glucose pulse when expressed either as the area above basal for the 10 min post stimulation or as the mean encremental increase above basal at 3, 4 and 5 min (delta 3-5 IRI) was the same in both timing and magnitude for the old and young subjects. Although our aged subjects had clearly abnormal kg's their acute phase of insulin secretion in response to glucose was normal. Seven noninsulin requiring diabetics with fasting plasma glucose levels greater than 140 mg/dl similarly studied had abnormal kg's (mean = 0.55) but markedly attenuated early release of insulin. Therefore, even though in the diabetics a deficiency of acute phase insulin secretion may have contributed to their impaired kg, in our aged subjects the lack of correlation between kg and a deficiency of acute phase insulin release suggests that factors other than an impairment in the early phase of insulin secretion are responsible for their glucose intolerance or, that there is a dissociation between the biologic and immunologic activity of insulin in these aged subjects.