Phenylketonuria (PKU)

Phenylketonuria (fennel-key-ton-uria) is often called PKU for short. PKU is a condition some children have that affects their ability to properly use protein from their food. The problem concerns a specific part of protein, the amino acid called phenylalanine ("phe" for short). When a person eats foods containing protein, enzymes break down the protein into separate amino acids as building blocks for body growth and repair. In PKU, one of the enzymes does not function properly, the one needed to convert "phe" into another amino acid, tyrosine. As a result, "phe" accumulates in the blood and other parts of the body. The excess "phe" prevents the brain from growing and developing normally which results in severe intellectual disabilities. It also causes other problems such as skin rash, excessive restlessness, irritable behavior and a musty body odor.

How is it Treated?

The main treatment is a diet low in "phe". If the diet is started early enough and closely followed, the child's development will be normal in almost all cases. Some "phe" is essential for growth. Too much "phe" is harmful. The diet must be carefully planned to allow enough "phe" for the child to grow normally, yet not enough to produce the harmful effects of excessive "phe". This balance between too much and too little "phe" is different for each child. A child's needs depend on the severity of the enzyme deficiency and the child's age, growth rate, and current state of health. The right amount of "phe" for the child is determined through blood tests that measure the amount of "phe" in the child's blood. The diet prescription is adjusted accordingly by your physician and nutritionist. Several special formula-milk substitute products are available. These special formulas make it possible to plan a diet that is low in "phe" but adequate in protein, calories, and essential vitamins and minerals.  In addition to special formulas, the child with PKU can have foods that are low in protein and "phe" in measured amounts.  This includes most fruits and vegetables, some cereals and candies and special breads, cookies and pastas. All high-protein foods such as milk, meat, eggs and cheese, which contain large amounts of "phe", must be eliminated from the diet. A diet record, with the exact kinds and amounts of food eaten, will need to be kept just before each monitoring blood test. The nutritionist will use this record to decide what changes, if any, need to be made in the diet prescription.

How Does One Get PKU?

The child gets PKU by inheriting it from their parents. Inherited characteristics such as eye color, hair color and PKU are carried on special parts of the cell called genes. PKU occurs in a child who has two genes for PKU, one inherited from their father and the other from their mother. A child with only one gene from either the mother or the father is a carrier of PKU. Carriers do not have PKU and do not need to remove “phe” from their diet. It is estimated that one in every 70 persons is a carrier for PKU and that the disorder affects one in every 15,000 to 20,000 infants born in the United States.


Severe intellectual disabilities and other problems resulting from PKU can be prevented by early diagnosis and proper diet control. The success of diet control requires the cooperation of everyone who comes in contact with the child. These efforts are without question, well rewarded.

Points to Remember

  1. PKU is an inherited condition in which the body cannot break down and use phenylalanine, one of the amino acids in protein.
  2. PKU cannot be cured but can be treated if a special diet is started early in the newborn period. Current research indicates that the special diet should be continued at least through adolescence and possibly throughout life. A young woman with PKU will need strictly follow the diet restrictions both prior to and during pregnancy.
  3. The special PKU diet provides all protein, calories, vitamins and minerals needed for normal growth and development.
  4. It is very important that the special PKU diet be carefully followed at all times.
  5. The child with PKU requires diet control and regular blood tests but is otherwise a normal child and should be treated as such.
  6. Successful management of PKU involves not only the parents and physician but requires the cooperation of everyone who comes in contact with the child, including relatives, neighbors, friends, babysitters, teachers, and physicians.

The Texas Department of State Health Services provides, without charge to residents of Texas who are diagnosed with PKU, blood specimen analysis through the Texas Department of State Health Services Laboratory in Austin. 

Adapted from "What is Phenylketonuria" Metabolism Clinic, James Whitcomb Riley Hospital for Children, Indianapolis, Indiana