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

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 [.sup.13]C[O.sub.2] release from [[1-.sup.13]C]phenylalanine in expired breath. [sup.13]C[O.sub.2] enrichment was measured by isotopic ratio mass spectrometry. Results: Cyst(e)ine requirement was determined in 25 LBW infants with a mean ([+ or -] SD) gestational age of 33 [+ or -] 1 wk and birth weight of 1.78 [+ or -] 0.32 kg. Fractional oxidation of [[1-.sup.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.sup.-1] * [d.sup.-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