Abnormal glucose homeostasis Impaired first-phase insulin secretion In patients with impaired glucose tolerance or in the early stages of type 2 diabetes, first-phase insulin release is almost invariably lost despite the enhancement of second-phase secretion. An abnormal reduction in the ability to produce first-phase insulin secretion, which is defined as the initial burst of insulin released in the first 5 to 10 minutes after the beta cell is exposed to a rapid increase in glucose (or other secretagogues).