The endogenous insulin secretion was suppressed during insulin therapy in NIDDM patients.

Urinary C-peptide excretion (U-C-peptide) was measured in order to examine the changes in endogenous insulin secretion after the beginning of insulin therapy. U-C-peptide in the 24-h urine (TU-C-peptide) of non-insulin-dependent diabetes mellitus (NIDDM) patients was measured on the 6,7,8th (period-A), 13,14,15th (period-B) and 20,21,22nd day (period-C) after admission. The TU-C-peptide of NIDDM patients, newly receiving insulin therapy from the 9th day, decreased to 58.7 +/- 6.2% (mean +/- SEM) of its basal level (period-A, 19.8 +/- 3.8 nmol/day) at period-B and remained at the same level at period-C, although U-C-peptide in the urine collected between 04.00 h and 06.30 h did not decrease significantly. Their plasma C-peptide levels from 2-5 h after the ingestion of a mixed meal at period-C decreased significantly compared with those at period-A. On the other hand, the TU-C-peptide of NIDDM patients who continued therapy with insulin or sulfonylureas in the outpatient clinic at period-B and -C did not decrease significantly from the basal level at period-A, although fasting plasma glucose decreased to the same level in both groups of patients. These results suggest that the endogenous insulin secretion after food ingestion is suppressed during insulin therapy in NIDDM patients.