Whole-Body Protein Turnover in Preterm Appropriate for Gestational Age and Small for Gestational Age Infants: Comparison of [¹⁵N]Glycine and [1-¹³C]Leucine Administered Simultaneously

Abstract

ABSTRACT: Measurements of whole-body protein turnover in preterm infants have been made using different stable isotope methods. Large variation in results has been found, which could be due to different clinical conditions and/or the use of different tracers. We studied 14 appropriate for gestational age and nine small for gestational age orally fed preterm infants using [¹⁵N]glycine and [1-¹³C]leucine simultaneously, which allowed us to make a comparison of commonly used methods to calculate whole-body protein turnover. Whole-body protein turnover was calculated from ¹⁵N enrichment in urinary ammonia and urea after [¹⁵N]-glycine administration and from the ¹³C enrichment in expired CO₂ after administration of [1-¹³C]leucine. Enrichment of α-ketoisocaproic acid after [1-¹³C]leucine constant infusion was measured as a direct parameter of whole-body protein turnover. Group means for whole-body protein turnover using [¹⁵N]glycine or [1-¹³C]leucine ranged from 10 to 14 g.kg^-1.d^-1, except when using the end product method that assumes a correlation between leucine oxidation and total nitrogen excretion. We found very low ¹⁵N enrichment of urinary urea in the majority of small for gestational age infants. These infants also had a lower nitrogen excretion in urine and oxidized less leucine. Nitrogen balance was higher in small for gestational age infants (416 pm 25 mg±kg^-1d^-1) compared with appropriate for gestational age infants (374 pm 41 mg±kg^-1d^-1, p= 0.003). [¹⁵N]Glycine does not seem to exchange its label with the body nitrogen pool to a significant degree and is therefore not always suitable as a carrier for ¹⁵N in protein turnover studies in premature infants.

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