Why are taurine and tyrosine considered essential in neonates but not in adults?

• A. Enzyme immaturity
• B. Low dietary intake
• C. Renal immaturity
• D. Liver immaturity

Answer: (B)

Neonates rely on their own metabolic pathways to make certain amino acids, but those pathways are not fully mature at birth. Taurine and tyrosine become essential in this period because the enzymes needed to produce them are underdeveloped, so the neonate cannot supply adequate amounts endogenously even if intake is reasonable.

Taurine depends on synthesis from cysteine, a process that is limited in newborns due to immature liver/kidney enzymes. Taurine also supports bile salt conjugation and CNS development, making external supply important for infants. Tyrosine is normally formed from phenylalanine by phenylalanine hydroxylase, an enzyme whose activity is reduced in the newborn liver, so tyrosine cannot be produced in sufficient quantities. In adults, these biosynthetic processes are fully mature, so both amino acids are typically nonessential.

Low dietary intake is not the fundamental reason they’re considered essential in neonates; even with typical milk/formula intake, the immature biosynthetic capacity requires providing these nutrients directly in early life.