Insulin secretory function in type 2 diabetes: Does it matter how you measure it?

The following estimates of insulin secretory function have been used widely to evaluate the role of pancreatic β-cells in the pathogenesis and treatment of patients with type 2 diabetes (2DM): (i) Homeostatic Model Assessment (HOMA)-β, a calculation based on fasting plasma glucose and insulin concentrations; (ii) post-glucose acute insulin response (AIRg), the increment in insulin concentration measured in the 5 min after intravenous glucose; and (iii) ΔI/ΔG, the ratio of the increment in plasma insulin concentration (I)/increment in plasma glucose concentration (G) 30 min after the oral administration of 75 g glucose. Experiments based on these approaches have led to a widely held point of view that that the natural history of 2DM is characterized by a progressive increase in the magnitude of hyperglycemia, secondary to an inexorable decline in pancreatic β-cell function: the greater the increase in plasma glucose concentration, the greater the impairment of the ability of the pancreas to secrete insulin. In the present review, theoretical questions are raised as to the physiological validity of these estimates of insulin secretory function and experimental data are presented demonstrating that hourly measurements of plasma insulin and glucose concentrations in response to mixed meals throughout an 8-h day lead to a very different point of view. Studies are also reviewed that question the 'inexorability' of the changes in insulin secretory function that have been reported. It is concluded that it may be time to challenge current conventional wisdom as to the role of the β-cell in the natural history of 2DM.