title: 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.
  
  
  METHODS
  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 (13)CO(2) release from [1-(13)C]phenylalanine in expired breath. (13)CO(2) 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-(13)C]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.
authors.affiliation: Department of Pediatrics, Division of Neonatology, Erasmus MC-Sophia Children's Hospital, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Maaike A
authors.initials: MA
authors.lastname: Riedijk
authors.forename: Ron H T
authors.initials: RH
authors.lastname: van Beek
authors.forename: Gardi
authors.initials: G
authors.lastname: Voortman
authors.forename: Henrica M A
authors.initials: HM
authors.lastname: de Bie
authors.forename: Anne C M
authors.initials: AC
authors.lastname: Dassel
authors.forename: Johannes B
authors.initials: JB
authors.lastname: van Goudoever
mesh_terms: D000601:Amino Acids, Essential / Q000008:administration & dosage / Q000378:metabolism; D001944:Breath Tests; D002247:Carbon Isotopes; D003545:Cysteine / Q000008:administration & dosage* / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005260:Female; D005865:Gestational Age; D006801:Humans; D007227:Infant Nutritional Physiological Phenomena / Q000502:physiology*; D007230:Infant, Low Birth Weight / Q000254:growth & development / Q000378:metabolism*; D007231:Infant, Newborn / Q000254:growth & development*; D007234:Infant, Premature / Q000254:growth & development / Q000378:metabolism; D008297:Male; D008715:Methionine / Q000008:administration & dosage / Q000378:metabolism; D009751:Nutritional Requirements; D010084:Oxidation-Reduction
chemical_list: D000601:Amino Acids, Essential; D002247:Carbon Isotopes; D008715:Methionine; D003545:Cysteine