The normal glucose tolerance continuum in obese youth: evidence for impairment in beta-cell function independent of insulin resistance.

Normal glucose tolerance is expressed over a wide range of glucose concentrations. Whether there is a continuum of risk for developing type 2 diabetes mellitus even when the 2-h plasma glucose is still within this normal range is uncertain. Oral glucose tolerance tests were performed in 407 obese normal glucose tolerance youth (4-20 yr) to examine the relationship between variations in 2-h plasma glucose levels and beta-cell responsiveness. Individuals were grouped by 2-h plasma glucose levels as follows: 1) less than 100 mg/dl, 2) 100-119 mg/dl, and 3) 120-139 mg/dl. Subsequent analysis stratified each 2-h plasma level by insulin sensitivity index. Increased 2-h glucose level was associated with a progressive increase in glucose between 0 and 30 min (P < 0.05). The Delta (0-30 min) insulin did not vary significantly across levels, thus resulting in a decreased insulinogenic index (P < 0.02). This pattern was observed at every level of insulin sensitivity (P < 0.02). These data translated to an unfavorable (leftward) shift in the insulin feedback system for increasing 2-h glucose level (P < 0.005). Increased 2-h plasma glucose within the range of normal glucose tolerance in obese youth is associated with a specific impairment of beta-cell responsiveness distinct from the deterioration of insulin sensitivity.