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Cysteine: a conditionally essential amino acid in low-birth-weight preterm infants? ^1,2,3,4

¹ From the Department of Pediatrics, Division of Neonatology (MAR and JBvG), Mass Spectrometry Laboratory (GV), Erasmus MC-Sophia Children's Hospital, University Medical Center, Rotterdam, the Netherlands; the Department of Pediatrics, Amphia Hospital, Breda, the Netherlands (RHTvB); the Department of Pediatrics, Vlietland Hospital, Vlaardingen, the Netherlands (HMAdB); and the Department of Pediatrics, Deventer Hospital, Deventer, the Netherlands (ACMD)

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.

Key Words: Amino acid requirements • indicator amino acid oxidation • nutrition

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