Urinary C-Peptide: A Simple Measure of Integrated Insulin Production with Emphasis on the Effects of Body Size, Diet, and Corticosteroids*

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Abstract
C-Peptide is secreted from the β-cell in equimolar quantities with insulin. Since a fraction of C-peptide is excreted in the urine, measurement of C-peptide in timed urine collections is a simple indirect measure of integrated insulin production. Normal subjects were studied to determine the effects of diet and oral prednisone on urinary C-peptide excretion. In subjects on a defined diet, there is a positive correlation of urinary Cpeptide with body weight. When insulin production is increased after oral prednisone, there is also a positive correlation with body mass index and percent ideal body weight. Prednisone increases plasma glucose, immunoreactive insulin, and serum and urinary C-peptide levels beginning 8–12 h after oral administration. This effect of prednisone is most marked in the postprandial state. Diets high in carbohydrate and protein result in significantly more insulin production, as measured by urinary Cpeptide, than isocaloric diets with low protein or carbohydrate composition.