Figure 4.

Schematic diagram to illustrate the importance of expressing β-cell responsivity in relation to S_I by using the DI metric (i.e., the product of β-cell responsivity times S_I is assumed to be a constant). Left: A normal subject (state I) reacts to impaired S_I by increasing β-cell responsivity (state II), while a subject with impaired tolerance does not (state 2). In state II, β-cell responsivity is increased but the DI is unchanged, and normal glucose tolerance is retained normal; while in state 2, β-cell responsivity is normal but not adequate to compensate the decreased S_I (state 2), and glucose intolerance is developed. Right: Impaired glucose tolerance can arise due to defects of β-cell responsivity and/or defects of S_I. In this hypothetical example, subject x is intolerant due to his poor β-cell function, while subject y has poor S_I. The ability to dissect the underlying physiological defects (S_I or β-cell responsivity) allows us to optimize medical treatments.