What is Phenylketonuria?

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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 22 October 2016
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Phenylketonuria (PKU) is a treatable inherited disease that is caused by mutation of a single gene. The disease is inherited in an autosomal recessive pattern. This means kids with PKU must get a recessive gene from both parents in order to have the condition, but this doesn’t mean that either parent actually has phenylketonuria.

In this condition, the body is unable to process phenylalanine, an amino acid present in many foods. The main treatment for this condition is for people to observe a special diet free of phenylalanines, from infancy onward. Failure to avoid phenylalanine exposure leads to extremely serious complications, which can include mental retardation, seizures, mental instability, and retarded growth and development.

Fortunately, in many countries, it is mandatory to test newborns for phenylketonuria. Mandatory testing is performed in all US states, Canada, most of Europe including Eastern Europe and in some Asian countries. Babies of Caucasian or Asian descent more often inherit the condition than do people of African descent.

Testing is accomplished in the first few days of life via a blood sample, usually from the heel. Though it may seem difficult to allow a newborn to have a painful blood test, and some mothers cry as much as babies do when this occurs, it is absolutely necessary. Parents can be silent carriers, and the complications of this condition untreated may be devastating.


A diagnosis of phenylketonuria means eating in a different way for the rest of your life. This starts in infancy because breast milk and most formulas do contain phenylalanine. Special infant formula that is phenylalanine free is required to help promote healthy development. Kids and adults with PKU need education to find out which foods to avoid and there are even cookbooks that can help make the most of a PKU diet. Advice on diet should come from doctors and nutritionists or from cookbooks they recommend. Many food products like diet sodas do contain warning labels because they have phenylalanines.

It used to be common medical wisdom that people with phenylketonuria could take occasional breaks from PKU diets. Most doctors no longer recommend this. Also, if you have PKU and become pregnant it is very important to stick to a phenylalanine free diet. Presence of high levels of this amino acid can create significant problems for the developing fetus.

One of the challenges associated with PKU may be that it is more expensive to eat a PKU diet. Depending on where you live, some financial aid may be available to low-income families who need to purchase things like special formula. On the flip side, early diagnosis and adherence to a PKU diet means many people with this condition live very normal and healthy lives.


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Post 4

@KoiwiGal - It sounds like it would be an extremely strict diet though, if even breast milk contains the chemical. Back in the days before they knew what was happening a phenylketonuria prognosis must have been essentially fatal, or at least extremely debilitating.

And since it's a recessive gene that causes it, it would have seemed like it came out of nowhere, like a curse or something like that. I do wonder if the people who only have one gene have any kind of advantage though, like those with one sickle cell gene are immune to malaria. It seems strange that such a destructive genetic condition could have survived. But maybe the expression of the disease isn't common enough to influence genes.

Post 2

@Fa5t3r - I'm sure they can test for it, but what good would that do? If you're in a relationship with someone you aren't going to stop being in that relationship because there's a small chance you could have a child with a treatable illness.

Granted, it isn't pleasant having to live with any condition that requires a special diet, but it sounds like, as long as they stick to the diet, there isn't any other problem.

Post 1

I'd never heard of phenylketonuria before, but I guess that's the advantage of living in the modern world, where all babies can be tested for the condition before it makes a significant impact on their lives.

Although it's strange that they can test for the disease and not be able to test the parents for the recessive gene in the first place, to see what their chances of having a child with the condition might be.

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