C-Peptide Suppression during Insulin Infusion in the Extremely Preterm Infant Is Associated with Insulin Sensitivity

Context: Little is known about the individual response of glucose-regulating factors to administration of exogenous insulin infusion in extremely preterm infants. Objective: To evaluate longitudinal serum concentrations of insulin, C-peptide, and plasma glucose levels in a high-frequency sampling regimen in extremely preterm infants treated with insulin because of hyperglycemia. Design: Prospective longitudinal cohort study. Setting: Two university hospitals in Sweden between December 2015 and September 2016. Patients and Intervention: Serum samples were obtained from nine extremely preterm infants, gestational age between 22 (+3) and 26 (+5) weeks (+ days), with hyperglycemia (plasma-glucose >10 mmol/L) at the start of insulin infusion, at 12, 24, and every 24 hours thereafter during ongoing infusion, and 12, 24, and 72 hours after the end of insulin infusion. Main outcome measures: Longitudinal serum concentrations of insulin and C-peptide and plasma glucose levels. Results: During insulin infusion, the serum C-peptide concentrations decreased compared with at start of infusion (P = 0.036), and then increased after ending the infusion. Individual insulin sensitivity based on the nonfasting plasma glucose/insulin ratio at the start of insulin infusion correlated with the initial decrease in serum ΔC-peptide_{[after 12h]} (P = 0.007) and the degree of lasting decrease in serum ΔC-peptide_{[after end of infusion]} (P = 0.015). Conclusion: Exogenous insulin infusion suppressed the C-peptide concentration to individually different degrees. In addition, the effect of insulin infusion on β cells may be linked to individual insulin sensitivity, where a low insulin sensitivity resulted in a more pronounced decrease in C-peptide during insulin infusion.