Cysteine: a conditionally essential amino acid in low-birth-weight preterm infants?

Abstract

Background: Cyst(e)ine can be synthesized de novo from methionine and serine and is, therefore, a nonessential amino acid in human adults. Several studies have suggested that cyst(e)ine might be a conditionally essential amino acid in preterm infants because of biochemical immaturity. No data are available on cyst(e)ine requirements in low-birth-weight (LBW) preterm infants.Objective: The aim was to determine cyst(e)ine requirements in LBW infants with gestational ages from 32 to 34 wk, measured 1 mo after birth with the use of the indicator amino acid oxidation technique.Design: LBW infants were randomly assigned to 1 or 2 of the 5 formulas containing graded cystine concentrations (11, 22, 32, 43, or 65 mg cyst(e)ine/100 mL) and generous amounts of methionine. After 24-h adaptation, cyst(e)ine requirement was determined by 13CO2 release from [1-13C]phenylalanine in expired breath. 13CO2 enrichment was measured by isotopic ratio mass spectrometry.Results: Cyst(e)ine requirement was determined in 25 LBW infants with a mean (±SD) gestational age of 33 ± 1 wk and birth weight of 1.78 ± 0.32 kg. Fractional oxidation of [1-13C]phenylalanine did not differ between the 5 groups.Conclusions: There is no evidence for limited endogenous cyst(e)ine synthesis in 4-wk-old LBW preterm infants born at gestational ages from 32 to 34 wk. It is safe to conclude that the cyst(e)ine requirement is <18 mg · kg−1 · d−1 providing generous amounts of methionine and that cyst(e)ine is probably not a conditionally essential amino acid in fully enterally fed LBW preterm infants born at 32–34 wk.