ABSTRACT
The prevalence of impaired glucose tolerance and type 2 diabetes increases with age. However, controversial results have been reported in regard to which has a greater influence on the deterioration of glucose tolerance with age, namely impaired insulin sensitivity or impaired insulin secretion. The conflicting results may arise mainly from differences in the evaluation of insulin secretion and insulin sensitivity, and from differences in the physical composition and the ethnicity of the study subjects. We therefore selected Japanese subjects, between 20 and 80 years of age, with normal glucose tolerance (NGT) and with a body mass index (BMI) below 25.0 kg/m2, and then examined the subject's insulin sensitivity based on the indices of a homeostasis model assessment-insulin resistance index (HOMA-IR) and ISI composite (ISI), and beta-cell function by these of HOMA-beta, AUC I/G(0-120), an insulinogenic index (deltaI30/deltaG30), and then (deltaI30/deltaG30)/HOMA-IR derived from a 75 g-oral glucose tolerance test (OGTT). The subjects were divided into the six subgroups according to sex and age, below age 30, between ages 30 and 49, and equal to and over age 50. Both HOMA-IR and the ISI showed no differences across the range of age and sex. HOMA-beta decreased with age, and AUC I/G(0-120) decreased in the elderly. No change was observed in deltaI30/deltaG30 across the age range in men, however deltaI30/deltaG30/HOMA-IR, the index of the early phase insulin secretion adjusted for insulin sensitivity, decreased with age in both men and women. These data indicated that aging itself had no effect on insulin sensitivity, while insulin secretion in both the early and late phase during the OGTT deteriorated with age even within the NGT subjects.
