What Is Phenylketonuria (PKU)?

What Is Phenylketonuria (PKU)?

Phenylketonuria, commonly referred to as PKU, is a genetic condition that causes elevated levels of the amino acid phenylalanine in the body. When PKU is not treated, the buildup of phenylalanine in the body can cause a variety of symptoms, including cognitive development challenges or intellectual disabilities. This rare disorder must be treated diligently through a specific diet that limits protein intake. The Special Olympics Arizona is breaking down phenylketonuria: what it is, the signs and symptoms, and how it will be treated.

What Is Phenylketonuria (PKU)?

Phenylketonuria is a rare inherited disorder that causes the amino acid, phenylalanine to build up in the body. Individuals with PKU will have a change in the phenylalanine hydroxylase gene, which is the gene that is responsible for creating the enzyme needed to break down phenylalanine. Without this enzyme, phenylalanine will continue to build up in the body, which can cause very dangerous side effects. This buildup occurs especially when individuals with PKU consume foods that contain protein or the substance aspartame, which is a common artificial sweetener. Individuals with PKU generally have to avoid consuming foods with protein or aspartame to limit the buildup of phenylalanine in the body and avoid the serious health effects that come with the buildup.

What Are the Signs & Symptoms of Phenylketonuria (PKU)?

Newborn babies generally will not have any symptoms of PKU, but they are screened very soon after birth so doctors can begin treatment as soon as possible. Signs and symptoms of PKU will usually begin within a few months of birth, and they can range from mild to severe. Common symptoms may include:

  • An odor in the breath, skin, or urine. The odor is often described as musty, and it will be caused by the excess phenylalanine in the system.
  • Neurological problems, including seizures.
  • Skin rashes, such as eczema.
  • Lighter in skin color, hair color, and eye color than other family members. This occurs because phenylalanine cannot transform into melanin, which is the pigment responsible for hair and skin color.
  • An unusually small head size.
  • Intellectual disabilities.
  • Hyperactivity.
  • Delayed development.
  • Behavioral, emotional, or social problems.
  • Mental health disorders.

The severity of the listed symptoms depends on the type of PKU, as well as the treatment and how early in the developmental years treatment begins. There are two types of PKU: classic PKU and other less severe forms of PKU. In classic PKU, individuals will generally experience symptoms that are more severe. The enzyme needed to break down phenylalanine is either missing completely or greatly reduced, resulting in high levels of the amino acid and potentially severe brain damage. In individuals with less severe forms of PKU, the enzyme still has some function, and phenylalanine levels are not as high, which results in a lower possibility of severe symptoms and brain damage.

Phenylketonuria Causes

PKU is caused by mutations in both copies of the phenylalanine hydroxylase (PAH) gene, which is responsible for creating the enzyme that breaks down phenylalanine. In an individual without PKU, this enzyme is formed and will work to break the amino acid phenylalanine down into components, or proteins, that the body can use. Without the enzyme needed to break down proteins, they will begin to build up in the blood and tissues of the body. This buildup will cause damage to the body, and due to the body’s sensitivity to phenylalanine, it will also cause damage to the brain when the buildup becomes severe.

PKU is a genetic condition that can be passed to children from their parents in an autosomal recessive pattern. This means that babies will receive one copy of the mutated gene that is responsible for PKU from their parents at conception. In the majority of cases, parents are carriers of the mutated gene without ever having symptoms of PKU themselves.

Phenylketonuria (PKU) Treatment

PKU treatment generally involves a very specific diet, among other things, to manage the phenylalanine levels in the body.


The main treatment for individuals with phenylketonuria will be a highly regulated diet, avoiding all foods that contain protein, among other foods. High protein foods, such as meat, fish, eggs, cheese, nuts, seeds, soy, quorn, tofu, and flour-based foods like bread, pasta, biscuits, and cakes, will need to be avoided since they are not processed properly in the body. There are other foods that will need to be avoided or closely regulated, such as potatoes and cereals. There are some meal substitutes that have been designed specifically for individuals with PKU to allow them to eat foods such as pasta and bread without having to consume the protein that may typically be incorporated into the meals. Many of these foods are available on prescription from a doctor.

In addition to the highly regulated diet, individuals with PKU must take an amino acid supplement to ensure they are getting all the necessary nutrients for growth. The phenylalanine levels will be closely monitored by a finger-prick blood collection, which will be tested in a lab. If high phenylalanine levels are confirmed in a baby, they will immediately be put on a low-protein diet to regulate PKU. Throughout their lives, individuals with PKU will work with dieticians to ensure they are consuming a low amount of protein while still consuming the necessary nutrients to lead a healthy life, and supplementing when needed. A proper diet consisting of low protein will help ensure individuals with PKU do not experience any brain damage from the buildup of phenylalanine.


In addition to avoiding protein, individuals with PKU must also avoid an artificial sweetener that is found in a number of foods, aspartame. Aspartame is converted to phenylalanine in the body after it is consumed, making it a risk for individuals with PKU to consume. Foods that commonly contain aspartame and should be avoided include sugar substitutes such as Splenda or Sweet and Low, diet versions of common sodas, chewing gum, squash and cordial, and some alcopops. There are also some children’s medicines that contain aspartame, such as liquid cold and flu medicines. All foods with aspartame should be clearly labeled, as should medicines.


Sapropterin is a medicine that can be taken by individuals with PKU to help regulate a mild form of PKU called hyperphenylalaninemia. Individuals who are under the age of 22, pregnant, or an adult that genetic tests have shown are likely to respond to treatment and is suffering from hyperphenylalaninemia may take sapropterin to regulate their system in addition to consuming a low-protein diet. While it is unclear how well sapropterin works in the long term, it is thought to help reduce the levels of phenylalanine in the body, allow individuals to eat a less restricted diet, and improve symptoms of PKU to help protect the brain in the short term.

Regular Blood Tests

A child with PKU will require regular blood tests to ensure the levels of phenylalanine are regulated in the body. Only a drop of blood will be needed for the blood test, which is generally taken through a finger prick. For children 1 year and younger, a weekly blood test is recommended. For children between the ages of 1 and 12, a blood test should be taken every other week. Individuals older than the age of 12 should have their blood tested once a month.

How Common Is Phenylketonuria (PKU)?

PKU is rare, affecting an estimated 1 in every 10,000 to 15,000 births in the United States each year.

What to Expect if Your Child Has Phenylketonuria (PKU)

PKU is a lifelong condition that requires a large amount of monitoring and a very regulated diet. As a parent of a child with PKU, you should expect to be very aware of your child’s diet and keep on top of regular blood tests. While it may be challenging to do so, a regulated diet and regulation of protein intake will help keep your child healthy and allow them to lead a very normal life. There are support groups and communities of other parents of children with PKU that can provide guidance and information on how to navigate this condition. But, with a well-regulated diet and regular blood tests to track phenylalanine, you should expect your child to lead a relatively normal and healthy life.

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