Phenylalanine !

Phenylalanine is an essential aromatic amino acid. The liver turns it into tyrosine; if it cannot, it builds up and damages the brain.

Updated 2026-11-01

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Key facts

Essential: it must come from the diet.
Phenylalanine hydroxylase turns it into tyrosine with BH4, O₂ and Fe²⁺.
If that route fails, it is shunted to phenylpyruvate, phenylacetate and phenyllactate, the urinary «phenylketones».
It is both glucogenic and ketogenic.

Phenylketonuria: high blood phenylalanine, intellectual disability if untreated and a musty or mousy odour.
The sweetener aspartame contains phenylalanine; foods with it carry a warning for phenylketonuria.
Poorly controlled maternal phenylketonuria harms the fetus (microcephaly, congenital heart disease).

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Which metabolites give phenylketonuria its name?

Phenylpyruvate and its products (phenylacetate, phenyllactate), which appear in urine when phenylalanine cannot become tyrosine.

Why do products with aspartame carry a warning?

Aspartame releases phenylalanine when digested, which is dangerous for people with phenylketonuria.

Is phenylalanine glucogenic, ketogenic or both?

Both: its breakdown ends in fumarate and acetoacetate.

Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: L-Phenylalanine (CID 6140) (PubChem, NCBI, 2026) · Public domain; Phenylalanine metabolism (Reactome, 2026); Phenylketonuria: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).

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