Objective. Many extremely preterm infants develop hyperglycemia in the first week of life during continuous glucose infusion. The objective of this study was to determine whether defective insulin secretion or resistance to insulin was the primary factor involved in transient hyperglycemia of extremely preterm infants.

Methods. A prospective comparative study was conducted in appropriate-for-gestational-age preterm infants <30 weeks of gestational age with the aim specifically to evaluate the serum levels of proinsulin, insulin, and C-peptide secreted during transient hyperglycemia by specific immunoassays. Three groups of infants were investigated hyperglycemic (n = 15) and normoglycemic preterm neonates (n = 12) and normal, term neonates (n = 21). In addition, the changes in β-cell peptide levels were analyzed during and after intravenous insulin infusion in the hyperglycemic group. Data were analyzed using analysis of variance and analysis of variance for repeated measures.

Results. At inclusion, insulin and C-peptide levels did not differ in hyperglycemic subjects and in preterm controls. Proinsulin concentration was significantly higher in the hyperglycemic group (36.5 ± 3.9 vs 23.2 ± 0.9 pmol/L). Compared with term neonates, proinsulin and C-peptide levels were higher in normoglycemic preterm infants (23.2 ± 0.9 vs 18.9 ± 2.71 pmol/L and 1.67 ± 0.3 vs 0.62 ± 0.12 nmol/L, respectively). During and after insulin infusion in hyperglycemic neonates, plasma glucose concentration fell and proinsulin and C-peptide levels were lowered (18.4 ± 7.6 and 20.7 ± 4.5 pmol/L, respectively).

Conclusion. These data suggest that 1) preterm neonates are sensitive to changes in plasma glucose concentration, but proinsulin processing to insulin is partially defective in hyperglycemic preterm neonates; 2) hyperglycemic neonates are relatively resistant to insulin because higher insulin levels are needed to achieve euglycemia in this group compared with normoglycemic neonates. These results also show that insulin infusion is beneficial in extremely preterm infants with transient hyperglycemia.

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