C-peptide levels as a criterion in treatment of maturity-onset diabetes.

C-Peptide levels, both basal and glucose-stimulated, were contrasted in diet-controllable vs. nondiet-controllable maturity-onset diabetic patients. Fasting and glucose-stimulated plasma C-peptide values in diet-controlled diabetics were similar to those in normal subjects. Diabetic patients who were not solely diet controllable could be differentiated into two subgroups. One diet failure subgroup was truly insulin dependent, in that insulin treatment could not be stopped for even a brief period. In these patients, fasting C-peptide levels were very low, in the same range as in the majority of juvenile-onset diabetic patients. In the second diet failure subgroup, it was possible to manage patients without insulin therapy for varying periods of time, although most patients eventually needed insulin. Fasting C-peptide levels in this second subgroup were also in the same range as those in normal subjects. However, their C-peptide responses to glucose were significantly blunted. Measurement of C-peptide may furnish a useful criterion for the categorization of maturity-onset diabetic patients. Furthermore, C-peptide levels help considerably in the selection of treatment modalities for these patients. Low C-peptide levels identify those patients who require insulin treatment, whereas high C-peptide levels in insulin-treated patients suggest the possibility of discontinuing insulin.