When intravenous glucose is used as a stimulus to insulin secretion, the response in normal subjects is biphasic. It is suggested that the first phase of insulin release is a from a readily releasable pool of synthesized insulin and that secretion is markedly diminished or lost. Inadequate first phase insulin release result in hyperglycaemis, and thus greater stimulus to second phase secretion, and hence second phase secretion is exaggerated. This situation persists until the more severely hyperglycaemic stage of the disease is reached, when second phase response also becomes impaired.
Thus it is now widely accepted that both insulin resistance and diminished insulin secretion contribute to the pathogenesis of type 2 diabetes, although the molecular basis of neither mechanism is fully understood.