Is cysteine considered an essential amino acid in infants?

• A. Yes
• B. No
• C. Sometimes
• D. Not sure

Answer: (A)

Infants have to rely on dietary amino acids because some internal pathways aren’t fully developed yet. Cysteine is one of those amino acids that’s not produced in sufficient amounts by newborns, especially preterm infants. The body normally makes cysteine from methionine through the transsulfuration pathway, but in infants this pathway is immature, so endogenous synthesis falls short of the needs of rapid growth and development. Because of that, cysteine is considered essential (often described as conditionally essential) in infancy. It’s also important for making glutathione, a key antioxidant, and for other growth-related processes, which is why infant formulas and neonatal parenteral nutrition routinely include cysteine.