(A) In obesity, the function of individual β cells appears to be normal and both phases of GSIS are enhanced primarily due to an increase in β cell mass. (B) In IGT, the first phase is slightly impaired because of a decrease in the size of the RRP and/or partial defect in the exocytotic process of the granules in this pool, and the second phase is only moderately reduced. (C) In T2DM, the first phase is absent because of a complete loss of the RRP and/or a complete defect in the exocytotic process. The second phase is also reduced, probably due to the decreased RP and/or disturbance of cortical actin network in T2DM. Black lines indicate dynamics of insulin secretion in normal state, whereas red lines indicate insulin secretion in disease.