A study of serum free C-peptide responses to oral glucose load in diabetic patients: With special reference to types of diabetes and methods of treatment.

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Abstract
Serum free C-peptide immunoreactivities (serum free CPR) during a 100 g oral glucose tolerance test (OGTT) were measured in 21 patients with insulin-dependent diabetes mellitus (IDDM, with abrupt onset and ketosis-prone), 57 insulin-treated patients with noninsulin-dependent diabetes mellitus (INIDDM, with gradual onset and not ketosis-prone), 39 oral hypoglycemic agent-treated patients with noninsulin-dependent diabetes mellitus (ONIDDM) and 9 healthy young men for control study. Although the fasting blood glucose value of the INIDDM group was not significantly different from that of the IDDM and ONIDDM groups, the free CPR response at each interval during OGTT in the INIDDM group was significantly higher than that in the IDDM group and lower than that in the ONIDDM group. The sum of serum free CPR during OGTT (.SIGMA. serum free CPR) was negatively correlated to the duration of insulin treatment either in bivariate or multivariate analysis in INIDDM patients. Using 9.5 ng/ml as an index, all .SIGMA. serum free CPR values in the ONIDDM group were above this index, whereas all the values except 1 in the IDDM group were below it. The values in the INIDDM were scattered within the ranges of the other 2 groups. The insulinogenic index .DELTA. serum free CPR/.DELTA. blood glucose (30 min-fasting) of the ONIDDM group was significantly lower than that of normal subjects, although .SIGMA. serum free CPR values were not significantly different. Residual pancreatic .beta.-cell function in INIDDM patients is lower than that in ONIDDM patients and is negatively correlated to the duration of insulin treatment in INIDDM patients. Measuring serum free CPR may be a discriminative method for establishing insulin dependency in insulin-treated patients. Impairment of early insulin secretion after the oral glucose load is a distinguished characteristic of diabetic patients.