Dr. Heena AroraPhenylketonuria (PKU) is an inherited condition in which the body cannot properly break down phenylalanine (Phe), an amino acid found in protein. Phe builds up in the blood and damages the developing brain. Untreated, PKU causes severe intellectual disability. Found at birth and treated early, it allows normal development. That makes PKU the original success story of newborn screening, and a good example of how a genetic diagnosis can change a child’s whole future.
Key points
- PKU is caused by changes in both copies of the PAH gene and is inherited in an autosomal recessive way (GeneReviews).
- It affects about 1 in 24,000 newborns worldwide, with rates as high as 1 in 4,000 to 1 in 10,000 in parts of Europe (GeneReviews).
- Treatment should start in the first week of life, ideally bringing blood Phe into range within two weeks (GeneReviews).
- Guidelines recommend keeping blood Phe at or below 360 µmol/L for life (ACMG guideline, 2025).
- Women with PKU need tight control before and during pregnancy: high Phe can harm a baby who does not have PKU (GeneReviews; ACMG guideline, 2025).
What happens in PKU?
The PAH gene makes phenylalanine hydroxylase, the liver enzyme that converts Phe into another amino acid, tyrosine. When the enzyme does not work, Phe from food builds up. High levels are toxic to the brain, especially in early childhood. Untreated or late-treated PKU causes permanent intellectual disability, behavioral and psychiatric problems, seizures and movement problems, and a small head size (GeneReviews). Children may also have lighter skin and hair, eczema and a musty body odor (MedlinePlus Genetics).
PKU is a spectrum. It is graded by how high blood Phe rises without treatment (GeneReviews):
| Type (untreated Phe) | What it usually means |
|---|---|
| Classic PKU (above 1,000 µmol/L) | Lifelong treatment needed |
| Mild or moderate PKU (360 to 1,000 µmol/L) | Lifelong treatment needed |
| Mild hyperphenylalaninemia (120 to 360 µmol/L) | Often no treatment, but monitoring, and control during pregnancy |
A small number of babies with high Phe have a different problem: a defect in making or recycling BH4, the enzyme’s helper molecule. These conditions need different treatment, so they are ruled out after a positive screen (GeneReviews).
How PKU is inherited
PKU is autosomal recessive. A child is affected only if they inherit a non-working PAH gene from both parents. People with one copy are healthy carriers. When both parents are carriers, each pregnancy has:
- a 1 in 4 chance of a child with PKU,
- a 1 in 2 chance of a healthy carrier child, and
- a 1 in 4 chance of a child who is not a carrier.
Most parents of a child with PKU have no family history and only learn they are carriers after the diagnosis. Brothers and sisters of a parent may also be carriers. Expanded carrier screening before pregnancy can identify couples at risk.
Newborn screening and diagnosis
Newborn screening uses a few drops of blood from the baby’s heel, usually taken in the first days of life. If Phe is high, the diagnosis is confirmed with (GeneReviews):
- A repeat blood Phe level and the ratio of Phe to tyrosine.
- Tests to rule out BH4 defects.
- Genetic testing of the PAH gene.
Guidelines now recommend genetic testing at diagnosis, because the exact changes help predict severity and response to medicine (ACMG guideline, 2025). A screening result is not a diagnosis: many babies with a high first result turn out to be unaffected.
Treatment
Treatment aims to keep blood Phe in a safe range, at or below 360 µmol/L, for life. The 2023 ACMG guideline found strong evidence that lifelong control gives better intellectual outcomes than stopping treatment in childhood (ACMG guideline, 2025).
- Low-Phe diet: limits high-protein foods such as meat, fish, eggs, dairy, nuts and pulses, and the sweetener aspartame. Most protein comes from a special Phe-free medical food (protein formula), plus measured amounts of natural protein (GeneReviews).
- Sapropterin: a form of BH4 that helps the enzyme work better in some people. About 25% to 50% respond, so a trial is offered to everyone except those with two completely non-working gene copies (GeneReviews).
- Sepiapterin: a newer oral medicine that raises BH4 inside cells. It was approved in the US in July 2025 for children and adults, and in its main trial most people were able to eat more natural protein while keeping Phe controlled.
- Pegvaliase: an injected enzyme that breaks down Phe, approved for adults whose Phe stays above 600 µmol/L despite other treatment. It needs a careful start because of the risk of severe allergic reactions (GeneReviews).
Regular blood Phe checks, usually from a dried blood spot sent from home, guide the diet. Teenagers and adults often find the diet hardest to keep up, which is why the newer medicines matter.
Maternal PKU: why pregnancy needs planning
This is the part of PKU that is most often missed. If a woman with PKU has high Phe during pregnancy, Phe crosses the placenta and can harm the baby, even though the baby usually does not have PKU (GeneReviews):
- Intellectual disability in more than 90% of children when the mother’s Phe stays above 360 µmol/L
- Small head size (microcephaly)
- Heart defects (about 8% to 12% when Phe is above 600 µmol/L)
- Other birth defects and poor growth
The fix is control before conception. Guidelines recommend keeping Phe at or below 360 µmol/L for at least three months before pregnancy and 120 to 360 µmol/L throughout pregnancy (GeneReviews; ACMG guideline, 2025). Any woman with PKU, including one who stopped her diet years ago, should talk to her metabolic team before trying to conceive and use reliable contraception until her levels are under control.
PKU in India
In India, newborn screening is still not universal. Despite recommendations in national child health policy, few States run statewide programs, and screening often depends on the hospital where a baby is born (Kuriakose et al., 2026). Parents can ask whether their baby’s newborn screen includes PKU. Without screening, PKU is often diagnosed only after developmental delay has appeared, when the damage cannot be reversed.
A geneticist’s view: PKU shows what newborn screening can do: a heel-prick test and a special diet turn a cause of severe disability into a condition people live full lives with. Two things worry me most: families in places without routine screening, who never get the chance to start treatment early, and young women who stopped their diet as teenagers and do not know their Phe level before pregnancy. If that is you, see your metabolic team before you plan a pregnancy. – Dr. Sumeet Kumar
Frequently asked questions
Is PKU curable?
No, but it is very treatable. With early, lifelong treatment, people with PKU can expect normal development, school and work.
Can adults with PKU stop the diet?
Current guidelines recommend lifelong treatment. Adults who stop can develop problems with attention, mood and thinking, which can improve when control is restored. Talk to your metabolic team before making changes.
If I have PKU, will my children have it?
Only if your partner is also a carrier or has PKU. If your partner is not a carrier, all your children will be carriers but will not have PKU. For women with PKU, though, Phe control during pregnancy is essential either way.
Why do diet drinks say “contains phenylalanine”?
Aspartame, a common sweetener, is broken down into phenylalanine. The warning is for people with PKU, who need to avoid it.
We are both carriers. What are our options?
Each pregnancy has a 1 in 4 chance of PKU. Options include preimplantation genetic testing with IVF, prenatal testing, or a pregnancy without testing with a plan for treatment from birth. A genetic counselor can go through them with you.
References
- Arnold G, Vockley J. Phenylalanine hydroxylase deficiency. GeneReviews. Updated 20 November 2025.
- Smith WE, et al. Phenylalanine hydroxylase deficiency diagnosis and management: a 2023 evidence-based clinical guideline of the American College of Medical Genetics and Genomics (ACMG). Genet Med. 2025;27:101289.
- Kuriakose LM, Mohan D, Choudhury MC. Newborn screening in Karnataka: a scoping review of the landscape. PLOS Glob Public Health. 2026;6:e0007183.
- MedlinePlus Genetics. Phenylketonuria.
Dr. Sumeet Kumar is a human geneticist and the founder of GenesWellness. He earned his PhD in human genetics from the University of Delhi in 2021, with research on the genetics of Parkinson’s disease, and his work is published in peer-reviewed journals. He has been quoted as a genetics expert by Medical News Today and Salon. At GenesWellness he builds the site’s genetics calculators and explains, in plain language, how genes shape health and family traits.