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Why do people with phenylketonuria need to restrict phenylalanine throughout life?

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BRCA2 tumor suppressor gene on chromosome 13 — human genetic disease Photomicrograph of the *BRCA2* tumor suppressor gene on chromosome 13 of the human genome. Inactivation of this growth-regulating gene is associated with a higher risk of developing breast cancer.

Individuals with phenylketonuria (PKU) must restrict phenylalanine intake throughout their lives because they lack the enzyme phenylalanine hydroxylase, which is necessary to convert the amino acid phenylalanine into tyrosine.1

Without this enzyme, phenylalanine accumulates to toxic levels in the blood and tissues. This accumulation, along with the buildup of abnormal metabolic byproducts, interferes with critical processes in the central nervous system, leading to the decreased production of essential neurotransmitters and the destruction of the myelin sheath surrounding nerve fibers. If left unmanaged, these metabolic disruptions cause severe cognitive dysfunction, intellectual disability, epileptic seizures, and other neurological damage.123

Maintaining a strict, low-phenylalanine diet is essential to prevent these toxic accumulations and allow for healthy neurological development and function. This dietary requirement is lifelong, as even in adulthood, high levels of phenylalanine continue to pose risks to cognitive health, and it is particularly critical for pregnant women to prevent severe damage to an unborn child.13