What is PKU?

Phenylketonuria (PKU) is lifelong metabolic disorder characterized by the body’s inability to break down the amino acid phenylalanine (phe). Individuals who do not have PKU are able to break down phe and converted it into tyrosine using an enzyme called phenylalanine hydroxylase (PAH). However, people born with PKU either have an insufficient amount of PAH or lack it completely. What does this mean right? Basically, it means people born with PKU cannot break down protein properly.

How is PKU diagnosed?

PKU is diagnosed through the Newborn Screening. This is a blood test performed on all infants born in the United States that screens for several treatable disorders. This disorder affects approximately one out of every 10,000 babies born in the United States, and it is inherited in a genetic fashion. For a child to develop PKU, an infant must receive two ‘PKU genes’ (one from each parent). When an individual only has one of these genes they are referred to as a carrier.

What are the symptoms of PKU?

  • Eczema
  • Fatigue
  • Tremors
  • Headaches
  • Irritability
  • Poor concentration
  • Depression
  • Anxiety
  • Decrease in intelligence (IQ)
  • Seizures in infants left untreated for a prolonged amount of time
  • Cognitive impairment in infants left untreated for a prolonged amount of time

All of these symptoms can be avoided though when proper treatment is put into place and continued throughout life!

Treatment for PKU

The treatment for PKU includes a diet that is low in protein and a metabolic formula, both of which need to be continued for life. The diet excludes many foods such as meat, dairy, beans, nuts, and most grain products, and it consists mainly of fruits, vegetables, and medically modified foods (breads, pastas, etc. that are medically made to contain little protein). The metabolic formula often comes in the form of a drink and provides all the necessary nutrients that individuals with PKU lack due to their restrictive diet. Ideally, the goal of this treatment is to keep blood phenylalanine levels between 2-6 mg of phe/dL of blood (120-360 ummol/L). Frequent blood tests are performed to monitor the amount of phenylalanine in the blood. In some cases a medication called Kuvan can assist in lowering blood phe levels (go to http://www.kuvan.com for more information). Currently, there is no cure for PKU. With proper lifelong treatment individuals born with this disorder are able to lead happy, healthy lives!!

Maternal PKU

It is especially important for women with PKU to monitor their diet closely during pregnancy as high levels of phenylalanine can harm the baby. Issues that can occur in babies born to PKU women due to lack of treatment include:

  • Low birth weight
  • Spontaneous miscarriage
  • Microcephally (small head)
  • Heart defects
  • Mental retardation
  • Learning or behavior problems later in life

Women with PKU can have healthy children though if the diet is monitored closely during pregnancy and ideally prior to pregnancy as well.



Amino acids- These are the ‘building blocks’ of protein or what protein is made of.

Atypical PKU- This is also called mild PKU. When off diet blood phe levels will be between 10-20 mg/dl (600-1200 ummol/L).

Carrier- Someone who carries one affected gene and one normal gene. In the case of PKU, carriers are not affected by the one mutated gene.

Classical Phenylketonuria- Classical PKU is the most severe type of PKU. When off diet blood phe levels will be above 20 mg/dl (1200 ummol/L).

Eczema- Eczema is a skin rash that causes the skin to become red, itchy, and scaly.

Enzyme- Enzymes control the rate at which the body breaks down foods. They break down amino acids and convert them into new amino acids that the body can use.

Hyperphenylalaninemia- Commonly called hyperphe, this is the least severe form of PKU. When off diet blood phe levels will be between 4-10 mg/dl (240-600 ummol/L), and in some cases a low protein diet is not required.

Metabolic- Metabolic refers to the way the body breaks down foods. If someone has a metabolic disorder, then their body does not break down nutrients properly.

Newborn Screening- A blood test that detects inherited, treatable disorders. It is performed by pricking a newborns heel and taking a few drops of blood on filter paper which is sent to a lab for analysis.

Phenylalanine (phe)- Phe is one of the twenty amino acids. Most foods contain phe. It is often found in foods that are high in protein, and foods that contain aspartame.

Phenylalanine Hydroxylase (PAH)- The enzyme that breaks down phe and turns it into tyrosine.

Phenylketonuria (PKU)- A genetic disorder characterized by the inability to process phenylalanine. With treatment there are no outward signs of PKU.

Tyrosine- The amino acid that phe is converted into in individuals without PKU. In individuals who do have PKU tyrosine is consumed in the metabolic formula.


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