Use of Urinary C-Peptide to Estimate Insulin Secretion during Starvation*

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Abstract
The usefulness of measurements of urinary Cpeptide excretion in indirectly assessing integrated insulin secretion during starvation was studied in eight obese subjects during a 72-h fast. Blood and urine samples were collected at 12-h intervals for measurement of insulin and C-peptide immunoreactivity. After 60 h, serum insulin and plasma C-peptide levels declined 47% and 37%, respectively, and the values were highly correlated (r = 0.8; P < 0.001). By 72 h, urinary C-peptide excretion had declined to 70% of the level in the first 12-h period. The urinary clearance of C-peptide was not altered by starvation. A highly significant correlation was found between urinary C-peptide and C-peptide secretory rate (P < 0.001). The molar ratio of plasma C-peptide to insulin remained constant during the fasting period. These data indicate that basal insulin secretion can be added to the list of physiological conditions in which /3-cell secretion can be effectively evaluated by urinary Cpeptide measurement.