What is type 2 diabetes?

Type 2 diabetes mellitus (T2DM) is a common metabolic disorder characterized by chronic hyperglycaemia. It is associated with reduced life expectancy owing to a greater risk of microvascular and macrovascular complications. The best-known predictors of increased diabetes risk are elevated fasting plasma glucose, elevated 1- and 2-hour plasma glucose concentrations after an oral glucose tolerance test, obesity and evidence of impaired insulin action. However, the mechanisms by which people with impaired fasting glucose and/or abnormal glucose tolerance ‘progress’ to overt T2DM are not completely understood. Moreover, T2DM is defined in a ‘negative’ sense (hyperglycaemia not accounted for by autoimmune destruction of islet cells or other known causes). Therefore it is important to remember that first, T2DM is a heterogeneous group of hyperglycaemic disorders with multiple different mechanisms; and second, defining such a complex disease affecting many different metabolic pathways simply in terms of hyperglycaemia may overlook the pathogenesis of the condition. In this review, we explore some of the mechanisms that underlie T2DM, and attempt to remind readers that hyperglycaemia is only one of its characteristics.