The lifecycle of dietary Phe

What happens when you eat protein.

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The gut

Protein breaks into amino acids.

Three matter here: phenylalanine, tyrosine and tryptophan. All three are absorbed into the blood.

The liver

PAH converts Phe to Tyr.

The reaction takes place mainly in the liver and requires the cofactor BH4. Tyr from this step supplies the downstream pathways.

The blood

Unconverted Phe stays in the blood.

The blood–brain barrier

Phe, Tyr and Trp share one transporter into the brain.

The LAT1 transporter carries Phe, Tyr, Trp and the other large neutral amino acids across the barrier, and has the highest affinity for Phe. Transport of each depends on its concentration relative to the others. The plasma Phe:Tyr ratio is a proxy for that competition. The ratio of Phe to the full large neutral amino acid pool is the more complete measure.

The brain

Neurotransmitters are made from the Tyr and Trp that cross.

Tyr is converted to dopamine and norepinephrine. Trp is converted to serotonin and then melatonin. Lower brain Tyr and Trp mean lower synthesis of each.

The systems map

Five organ systems depend on Phe converting to Tyr.

Thyroid. Brain and heart. Skin and hair. Muscle and every cell. Mood and sleep depend on Trp crossing beside it. Unconverted Phe is cleared through the kidneys.

Two hours after the meal

Carriers after a Phe load.

Carriers are classified as unaffected. In 2026 a University of Guelph study measured carriers after an oral Phe load. The Phe:Tyr rise was larger than in non-carriers, and anxiety scores correlated with postprandial Phe and Phe:Tyr.

Diet, supplementation and drugs.

The end

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