Which two amino acids are essential in neonates but not in adults?

• A. Taurine and Tyrosine
• B. Cysteine and Methionine
• C. Phenylalanine and Valine
• D. Glycine and Leucine

Answer: (A)

Neonates rely on dietary intake for certain amino acids because their bodies can’t make enough of them yet. Taurine is essential in newborns because their capacity to synthesize it is limited, and it plays a crucial role in bile acid conjugation, as well as retinal and neural development. Tyrosine becomes essential in neonates because the enzyme that converts phenylalanine to tyrosine is not fully active early on, so they can’t rely on endogenous production and need dietary tyrosine.

In adults, these constraints aren’t present: phenylalanine can be efficiently converted to tyrosine, and taurine synthesis meets needs, so these aren’t required as essential dietary amino acids. That’s why taurine and tyrosine are the best answer.

Other pairs don’t fit the neonatal pattern as neatly; for example, some involve amino acids that are already essential in adults or include nonessential ones, so they don’t uniquely reflect neonatal essentiality.