Control of non-insulin dependent diabetes is not correlated with endogenous insulin secretion

C-peptide was determined in seventy-one patients with non-insulin dependent diabetes mellitus (NIDDM) before and after a standard 500-calorie breakfast. Whereas in the normal-weight and obese controls the fasting C-peptide was 1.7-2.2 and postprandial maximum 6.0-6.6 ng/ml, in NIDDM the fasting level was only 2.4 +/- 1.5 ng/ml in spite of hyperglycemia of 234 mg/dl, and increased after breakfast to only 3.9 +/- 1.9 ng/ml. Fasting and postprandial levels of C-peptide correlated among themselves but did not correlate with age, duration of diabetes, body mass index, fasting or postprandial glycemia or--in the insulin-treated group--with the dose or duration of the treatment. There was no difference in glycemia between the subgroups of patients with the fasting C-peptide 5.9 +/- 1.7 and 1.0 +/- 0.2 ng/ml. No differences in any parameter were found between patients treated with insulin and with sulphonylureas.