Reevaluation of urine C-peptide as measure of insulin secretion.

Urine C-peptide (UCP) has been proposed as a measure of insulin secretion, because insulin and C-peptide are consecreted in equimolar concentrations by the pancreatic beta-cell. The validity of this approach was tested by comparing insulin secretion rates, calculated by application of a two-compartmental analysis of peripheral C-peptide concentrations, with UCP excretion rates. Insulin secretion and UCP excretion with subjects on a mixed diet were simultaneously measured over a 24-h period in 13 patients with noninsulin-dependent diabetes mellitus and in 14 matched nondiabetic control subjects. The fraction of secreted C-peptide that was excreted in the urine (fractional C-peptide excretion) showed considerable intersubject variability in the diabetic (11.3 +/- 1.6%, range 3.9-20.8) and control (8.0 +/- 1.7%, range 1.1-27.9, P = .07) subjects (means +/- SE). UCP clearance demonstrated a similar degree of variability and was not significantly different (P = .07) between diabetic (23.8 +/- 3.0 ml/min) and control (16.5 +/- 2.7 ml/min) subjects. In control subjects, the 24-h insulin secretion rate correlated more closely with the fasting insulin secretion rate (r = .97, P = .0001), fasting C-peptide (r = .81, P = .0005), and fasting insulin (r = .80, P = .0005) concentrations than with the 24-h UCP excretion rate (r = .62, P = .02). Similar results were obtained in the diabetic patients.(ABSTRACT TRUNCATED AT 250 WORDS)