PKU overview

Types of PKU

Causes & genetic inheritance of PKU

Symptoms of PKU

PKU dietary management and treatment

Living with PKU

PKU overview 

What is PKU?

Phenylketonuria (PKU) is a rare inherited metabolic disorder diagnosed through newborn screening. People with PKU cannot properly break down the amino acid phenylalanine (Phe) due to a deficiency or absence of the enzyme phenylalanine hydroxylase (PAH). When phenylalanine is not processed correctly by the liver, toxic by-products build up in the body. Without treatment, this can lead to developmental delay, neurological problems, and permanent cognitive impairment.

There is no cure for PKU, but it can be effectively managed with a low-protein diet, which helps control blood phenylalanine levels and supports normal growth and development.

Types of PKU

PKU manifests in different forms, with varying levels of severity. The most common form is classical PKU.

PKU varies in severity depending on how well the enzyme PAH works. The most severe and common type is classic PKU, where enzyme activity is very low or absent, leading to high phenylalanine levels. 

Milder forms, known as hyperphenylalaninaemia (HPA), occur when the enzyme still functions partially. People with HPA can tolerate more protein in their diet but still need regular monitoring and dietary control to prevent health problems.

Causes & genetic inheritance of PKU

PKU is an inherited genetic condition that is present from birth. Every cell in the body contains DNA, half inherited from the mother and half from the father. DNA carries genes that provide instructions for how the body functions. In PKU, both parents usually carry a changed (mutated) gene that affects the enzyme phenylalanine hydroxylase, which is needed to break down phenylalanine.

When both parents are carriers, there is a 1 in 4 (25%) chance with each pregnancy that their child will have PKU. Because this gene change is uncommon, the overall chance of a child being born with PKU in the general population is low.

Symptoms of PKU

Without dietary treatment, symptoms of PKU can be severe and varied. Common symptoms include

• intellectual disability
• growth delay
• behavioural problems
• seizures
• skin rashes
• a musty odour of the breath, skin, or urine. 

Early detection through newborn screening (heel-prick test) is crucial to prevent these complications.

PKU dietary management and treatment

Living with PKU requires ongoing care and commitment to a low-protein diet, along with regular medical check-ups and monitoring.

The role of Food for Special Medical Purposes and protein substitutes

The PKU diet focuses on limiting phenylalanine intake. This means avoiding high-protein foods such as meat, fish, eggs, dairy products, nuts, and soy, while including low-protein foods such as fruits, most vegetables, and specially formulated low-protein products.

People with PKU also use foods for special medical purposes (FSMPs) and protein substitutes, which provide essential nutrients without excess phenylalanine. These products should always be used under medical supervision and prescribed by a doctor or dietitian.

While dietary management remains the cornerstone of treatment, some medication-based therapies are also available for certain individuals. Research continues into gene therapy and enzyme replacement, which may offer further treatment options in the future.

Medical intervention

While dietary management remains the cornerstone of PKU treatment, some medication-based therapies are also available for certain individuals. In addition, research continues to explore new approaches, such as gene therapy and enzyme replacement, which may offer further treatment options in the future.

PKU diet and foods to avoid

The PKU diet focuses on limiting phe intake. This means eating mainly low-phe foods, such as fruits, most vegetables, and specially formulated low-protein products. It is important to carefully track phe intake to keep blood levels within the safe range.

Foods high in phenylalanine to avoid include:

• Meat
• Fish
• Dairy
• Nuts

Living with PKU

Living with PKU requires ongoing care and commitment to a low-protein diet, supported by regular medical check-ups. It’s important to have the guidance of a dietitian who specialises in metabolic disorders, to help manage dietary needs and ensure the right amount of protein substitute is prescribed.
PKU can present different challenges throughout life. The needs and experiences of a person with PKU will vary as they grow — from infancy through adulthood. For example, babies need a special low-phenylalanine infant formula alongside breast milk or standard formula; children learn to manage dietary restrictions; teenagers may face social pressures; and adults must balance dietary control with the demands of daily life.

Regular follow-ups with a doctor or dietitian are essential to keep treatment effective as the body’s needs change over time. By following a low-protein diet and adhering to medical guidance, people with PKU can lead healthy, active, and fulfilling lives.

For ideas and inspiration, visit our low-protein recipes page (insert link) to find a range of delicious sweet and savoury dishes.