The secretion rate of insulin (SR-I) of 50 normal subjects was calculated from the 24-h integrated concentration of insulin (IC-I), the peripheral metabolic clearance of insulin (pMCR-l), and the mean fractional hepatic insulin extraction (fhMCR-l) that was derived from our data. fhMCR-l was determined as the difference in the molar secretory rate of C-peptide (SR-C) and the molar peripheral clearance of insulin (pMCRI × IC-I) divided by SR-C. The IC-I in our 50 subjects was 1.19 ± 0.38 ng/ml and the IC-C was 2.93 ± 0.58 ng/ml. Based on these data, the fhMCR-l was 0.40 and the SR-I was estimated to be 54.8 ± 18.0 U/24 h.

The 24-h urinary C-peptide excretion (U-C), 44.9 ± 20.4 μg/24 h, had a statistically significant correlation with SR-I (r = 0.838, P < 0.0001), while the IC-I correlated significantly with the 24-h urinary C-peptide/g of creatinine (r = 0.823, P < 0.0001). The U-C may thus serve as a practical method for estimating the SR-I.

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