Are specific serum insulin levels low in impaired glucose tolerance and type II diabetes?: measurement with a radioimmunoassay blind to proinsulin, in the population of Wadena, Minnesota.

It has been suggested that serum insulin levels in subjects with recently diagnosed type II diabetes have been overestimated, and that after correction for proinsulin, true insulin levels are depressed rather than elevated. We tested this possibility in a cross-sectional study of a population-based sample of 328 adults living in Wadena, a Minnesota community in which residents are of northern European background. Specificity of insulin measurements was provided by an antibody blind to proinsulin and its major metabolite. Oral glucose tolerance and liquid mixed-meal (Ensure-Plus) tests were performed on separate days. Mean insulin levels before and 90 minutes after the mixed meal were as follows. Among 302 randomly ascertained adults not previously known to have diabetes, both fasting and postmeal levels in subjects with impaired glucose tolerance (IGT) and newly identified type II diabetes were equal to or greater than levels in subjects with normal glucose tolerance (fasting: normal 52 pmol/L, IGT 78, new type II 87; postmeal: 317, 565, and 406, respectively). The fasting insulin to glucose ratio was significantly increased in IGT and new type II diabetes subjects. Among 26 established (previously known) type II diabetic subjects not taking insulin, fasting levels were elevated and postmeal levels were normal in absolute terms (75 and 328), but were normal or low with respect to plasma glucose. Relationships among the groups were not materially changed by adjustment for body mass index (BMI), sex, age, or blood pressure. There was marked overlap of individual insulin levels from group to group. In summary, randomly selected adults in Wadena with IGT or asymptomatic diabetes showed, on average, elevated insulin levels, but physician-diagnosed diabetes was associated with relative diminution of serum insulin. In this population, the current view of insulin resistance in "early" diabetes was supported by insulin-specific measurements.