Phenylketonuria: What Is It & How Is It Treated?

Wednesday, January 17, 2007 - 11:12am

By Erica Lesperance, RD, LD

If you’ve ever given birth in a hospital, you may remember your precious newborn being pricked in the heel until it bled, and the nurse blotting the blood in circles on a piece of paper.  If you expressed concern, the nurse may have told you not to worry because the chance of this test coming out positive was very slim.  Then you most likely never heard another word about it.  What was that test?  What if it would have been positive? 

Newborn screening

The test that your baby went through in the early days of life was a newborn screening test.  Newborn screening is the process of testing and screening newborns for certain dangerous conditions.  If these conditions are caught at birth, the child can be saved from potentially disastrous consequences.  While the conditions that are screened for vary from state to state, one that is tested for throughout the United States is called phenylketonuria (PKU).

What is PKU?

PKU is a condition in which an enzyme needed to break down protein in the body does not work properly.  More specifically, this enzyme cannot break down the amino acid phenylalanine (Phe), which is a component of all types of protein.  For those with PKU, eating a regular diet causes Phe to build up in the body, ultimately causing damage to the central nervous system.  This damage begins in the first few days of life and can result in mild to severe mental retardation, depending on the severity of the individual condition.  However, with early detection and treatment, these consequences can be avoided completely. 

How do you get PKU?

PKU is an autosomal recessive disorder.  To have a child with PKU, both parents must carry the PKU gene.  As a carrier of this gene, they are not affected with PKU themselves.  About one in every 50 people is a carrier, and while this may seem common, the chance of two carriers mating and passing on their PKU genes is rare.  In fact, PKU only affects one in every 10,000 babies born in the United States. 

Treatment: The good news

So why do we screen every newborn in the country for a disease that is so rare?  Because if a newborn is diagnosed with PKU and treatment is begun in the first week of life, she has a very good chance of escaping the disastrous effects of this disease.  Treatment consists almost exclusively of dietary modifications and restrictions.  Though the diet is strict and can be difficult, adhering to it will mean normal growth and development for a person with PKU. 

The diet & PKU

People with PKU need the same amount of protein as those without PKU, or perhaps even more.  However, they can’t consume their protein in the usual way because they need to restrict their Phe.  Their diet treatment consists of the following components: 

1.   Phe-free formula

In infancy, a special formula that does not contain Phe is used in combination with a regular protein source such as breast milk or regular infant formula.  The right balance allows the baby to grow normally.  During childhood and adulthood, the Phe-free formula is consumed alone, and the right amount of Phe is consumed through solid foods.  Phe-free formula must be consumed daily throughout life for effective treatment.

2.   Restricted foods for those with PKU

Protein-rich foods are restricted, including meat, seafood, poultry, nuts, eggs, legumes and dairy products.  Foods containing aspartame are also restricted because this sweetener contains a concentrated form of Phe.  Depending on the severity of the individual condition, regular breads, grains and some starchy vegetables are often restricted as well.

4.   Allowed foods for those with PKU

A low Phe diet is followed that includes fruits, some vegetables, a few grains and specially-made low protein products such as breads, pastas and imitation cheeses.  The variety of allowed foods has been vastly increased with the development of low-Phe products by Cambrooke Foods.

The PKU diet is for life

PKU treatment is most crucial in the early years of growth and development.  However, PKU is a condition that you are born with and will never grow out of, so the low-Phe diet should be followed throughout life in order to maintain normal physical and mental functioning. 

Conclusion: PKU is not common but it's worth being aware of it

While PKU only affects one in 10,000 US babies, there are still hundreds of babies born with PKU each year.  Perhaps someone in your child’s class has PKU, or you will have a friend whose child’s newborn screening test comes back positive for PKU.  Chances are good that you’ll never have a child that tests positive for PKU, but it never hurts to be armed with a little knowledge just in case.