Urinary excretion rate of C-peptide in fed and fasted obese humans

Abstract. The aim of the study was to evaluate the reliability of urinary excretion rate of C-peptide as a marker of B-cell function during fasting. Ten obese subjects of both sexes fasted for 5 days. Diurnal serum C-peptide was collected before and on the 5th day; morning serum samples (for glucose, insulin and C-peptide) and 12-h urine samples (7.00 to 19.00 h) were collected daily. Body weight decreased from 138.7 ± 15.9 to 132.9 ± 15.6 kg. Morning glucose, insulin (–40%) and C-peptide (–50%) fell significantly throughout the study. Mean diurnal C-peptide values were 2.19 ±0.69 nmol/l before and 0.60 ±0.19 nmol/l after fasting (P< 0.0001) and its secretion rate was 909.4 ± 297.9 and 244.4 ± 83.9 nmol/12 h (P< 0.005), respectively. Excretion rate of C-peptide fell progressively from basal (11.2 ± 4.2 nmol/12 h) to a nadir value of 1.3 ± 0.8 nmol/12 h (P< 0.0005); similarly, the C-peptide to creatinine clearance ratio fell from 0.062 ± 0.035 to 0.028 ± 0.015 (P< 0.05). These results indicate that fasting modifies renal metabolism of C-peptide thus creating several complications in the quantitative interpretation of urinary levels as an index of its secretion rate from the B-cell.