Updated on December 16, 2025.
Phenylketonuria (PKU) is a rare genetic disorder that disrupts the body’s ability to process a substance called phenylalanine. PKU belongs to a group of diseases known as inborn errors of metabolism.
Phenylalanine is a substance found in proteins. As the body metabolizes protein, phenylalanine is broken down by an enzyme called phenylalanine hydroxylase (PAH). Due to inherited genetic mutations, when a person has PKU the body does not produce enough PAH or produces PAH that does not function correctly. As a result, people who have PKU cannot metabolize phenylalanine.
This can cause high amounts of phenylalanine to accumulate in the blood and lead to a dangerous imbalance of amino acids that can damage the brain. Left untreated or inadequately treated, PKU can lead to cognitive disability, development delays, seizures, and problems with mood and behavior. Skin conditions and changes to the skin can also occur.
Treatment can prevent these complications. In the United States, newborns are typically screened for PKU shortly after birth, enabling treatment to begin as early in life as possible.
Below is an overview of treating and managing PKU.
A low-phenylalanine diet is the main treatment
Limiting or avoiding phenylalanine consumption through diet is critical to the treatment of PKU. This means high protein foods must be significantly limited or avoided entirely. This includes meat, poultry, fish, eggs, dairy products, nuts, beans, breads, and any foods containing an artificial sweetener called aspartame. Certain vegetables and grains will also need to be limited.
Because protein is an essential nutrient needed for growth, development, and health, people with PKU will need to consume special nutritional supplements that provide protein without phenylalanine.
Diet and nutrition will be individualized
The definition of a "low phenylalanine" diet will vary from person to person. Some people will be able to safely tolerate more phenylalanine than others. This can be determined through regular blood testing to monitor the amount of phenylalanine in the blood. A healthcare provider will explain what these results mean and adjust a treatment plan accordingly.
Your healthcare providers will be your best source of information. Treatment for PKU should involve a multidisciplinary team, ideally at a medical clinic that specializes in the treatment of inborn errors of metabolism. A team typically includes metabolic physicians, registered dietitians specialized in PKU, and other healthcare providers.
Treatment may also include a medication
In addition to a low-phenylalanine diet, treatment may also include medications. Several medications are available. This includes an enzyme substitution therapy approved for adults with uncontrolled blood phenylalanine levels. It also includes oral medications that can help the body process phenylalanine and are approved for people 1 month and older. These medications are used along with a low-phenylalanine diet.
A new oral medication became available in 2025, and researchers continue to study new potential treatments, including gene therapies.
Social and psychological support are an essential part of treatment
Living with PKU is challenging. Needing to follow a lifelong, highly restrictive diet can be emotionally and mentally taxing, and it can affect many different aspects of a person's life, including social development, education, career, personal goals, and relationships. PKU is also challenging to caregivers of young people with the condition.
Social workers, counselors, mental health professionals, and support groups can all be valuable additions to a treatment plan for people of all ages living with PKU.
