Introduction
The pathogenesis of Type 2 diabetes has been studied extensively. In the course of this work it has become abundantly clear that by the time hyperglycaemia develops, reductions in both insulin sensitivity and beta-cell function have already occurred [1]. While it is clear that hyperglycaemia is associated with both insulin resistance and beta-cell dysfunction, there has been much debate over the past few decades regarding the relative importance of these two abnormalities. Many groups have suggested that insulin resistance is the primary abnormality and that beta-cell dysfunction is a late event that arises from the prolonged, increased secretory demand placed on the beta cell by insulin resistance [2, 3]. In contrast, others have suggested that reduced beta-cell function, manifest as decreased insulin release, is a prerequisite for the progression from NGT to hyperglycaemia [4, 5, 6].
This disagreement over the relative importance of insulin resistance versus beta-cell dysfunction is partly due to the fact that evaluation of these parameters has frequently been done in isolation. This approach does not take into account the fact that glucose homeostasis, as is the case for most endocrine systems, is vitally dependent on a feedback system. Once consideration is given to the presence of a tightly regulated feedback system incorporating the beta cell and the insulin-sensitive tissues, it becomes abundantly clear that reductions in both insulin sensitivity and beta-cell function are present early in the course of the development of Type 2 diabetes.
This review will focus on the importance of insulin resistance and beta-cell dysfunction in the pathogenesis of Type 2 diabetes. In the course of the discussion, it will be apparent that changes in beta-cell function are both an early and critical component in the pathogenesis of the hyperglycaemia of Type 2 diabetes.