Children with phenylketonuria (PKU) can live normal lives if they receive early treatment. It is estimated that children who are well treated will have intelligence quotient (IQ) levels that are within five to eight points of their siblings. Children who are left untreated – or who are not treated until after the age of three – will inevitably experience brain damage. However, very mild cases of PKU may not require treatment. 

The chief treatment for PKU is a special diet that is low in phenylalanine and contains just enough of this amino acid to promote normal growth and body processes. This begins in infancy, with a special formula that is low in this amino acid. Babies who are breastfed may consume a combination of special formula and some breastmilk. Blood levels of phenylalanine will be monitored throughout life. The goal of treatment is to maintain blood levels of phenylalanine between 2 and 10 milligrams per deciliter (mg/dL). The frequency of testing may vary by child, ranging from weekly to monthly.

As the children begin to eat solid foods, their diet will be restricted so that all high-protein foods are eliminated. Such foods include meat, milk, cheese, eggs and nuts. Children typically have to take a special liquid or supplement containing a nutritional substitute to help make up for the protein that is lacking in the diet.

Parents work with a physician and dietitian to create a proper diet for children with PKU. Children will have to remain on this diet through adolescence, and usually for the rest of their lives. Children who follow this diet closely can lead normal lives. Previously, it was believed that diets could be ended around age six, when brain growth was completed. However, more recent study has revealed that high blood levels of phenylalanine may lead to a decrease in a child's IQ (intelligence quotient) or to other developmental and behavioral problems.

It is important that children and others with PKU avoid consuming aspartame. This artificial sweetener contains phenylalanine and is found in many sweets, diet soft drinks and medications. Children with PKU will also have to consume limited amounts of certain other foods that contain phenylalanine, including pasta, rice, bread, cookies and some fruits and vegetables.

Many children and parents find it challenging to adhere to the strict diet necessary to prevent damage associated with PKU. Food diaries and computer programs are available to help parents and children to plan and create meals that are safe. In addition, specialty foods and ingredients are available that are low in phenylalanine. These allow children with PKU to indulge in favorites such as pizza, cookies and pasta.

Once a special diet has been established, patients will continue to undergo follow-up monitoring to ensure that their blood phenylalanine levels are in the proper range. Blood tests can determine whether or not these levels are too high or too low. Testing of babies may take place weekly, while testing is likely to be once or twice a month during childhood.