Phenylketonuria (PKU) and Food

We often dig into the rarer forms of food allergy like FPIES and EoE, but today, we’re here to talk about a different food-related disorder that also has a funny acronym—PKU. Testing for this disorder is also the reason your baby’s heel is routinely pricked shortly after birth. 

What is PKU?

Phenylketonuria (PKU for short) is a rare genetic disorder that causes an amino acid called phenylalanine to build up in the body. The condition is caused by a defect in the gene that creates phenylalanine hydroxylase—an enzyme tasked with breaking down phenylalanine. The disorder requires lifelong management. 

In order for a child to be born with PKU, both parents must pass on a defective version of the gene. Interestingly, if only one parent passes on the mutated gene, the child won’t have any symptoms, but they’ll be a carrier of the altered gene. 

If PKU isn’t diagnosed at birth, and treatment isn’t started quickly, the disorder can cause irreversible brain damage and intellectual disabilities within a few months, along with behavioral problems and seizures in older children. 

How common is PKU?

Since the 1960s, doctors have generally tested babies for this disorder by taking a blood sample from the baby’s heel. Sometimes additional testing is performed to confirm (or rule out) a diagnosis. These tests are often performed within six weeks after birth. Fortunately, the disorder is fairly uncommon in the United States, affecting approximately 16,500 Americans (350 babies are born with it each year). 

How is PKU related to food? 

A hazardous buildup of phenylalanine can occur from eating high-protein foods like eggs, beans, fish, nuts, and certain meats. Aspartame, an artificial sweetener, can also cause a dangerous buildup. Most people with PKU require a special diet to prevent complications. 

The good news is that once a special diet and other treatments start, symptoms typically diminish. People who properly manage their diet usually don’t show any symptoms!

What does the PKU diet look like? Infants with PKU may take breast milk, but they usually also need to consume a special formula called Lofenalac. Formulas like Lofenalac contain all the amino acids that the body needs except for phenylalanine. Once the baby can tolerate solid foods, and throughout the course of their life, they have to avoid foods high in protein. Some specialty health stores also carry certain low-protein, PKU-friendly foods. 

What are the PKU symptoms? 

Similar to food allergies, PKU symptoms can range from mild to severe. Typically, symptoms begin to show up within the first few months of life. Symptoms can include:

  • A musty odor in the breath, skin, or urine, caused by too much phenylalanine in the body 

  • Skin rashes (eczema) 

  • Seizures, tremors, trembling, or shaking

  • Fair skin and blue eyes (phenylalanine can’t turn into melanin—the pigment that’s responsible for hair and skin tone)

  • Delayed development/growth 

  • Abnormally small head 

  • High levels of phenylalanine can disrupt neurotransmitters like dopamine and serotonin, which are important for memory, learning, mood, and motivation.  

  • The seriousness of the disorder depends on the type of PKU a person has:

Classic PKU is the most serious form. In these cases, the enzyme that’s necessary to convert phenylalanine is either missing or drastically lowered. This can lead to severe brain damage.

Variant PKU (or non-PKU hyperphenylalaninemia) cases that are mild or moderate means that the enzyme maintains some functioning, so phenylalanine levels aren’t as high. In this situation, the risk of significant brain damage is much lower. 

Are there medications to manage PKU? 

In 2007, the FDA approved sapropterin (Kuvan) for the treatment of PKU. It helps to lower phenylalanine levels. As with many drugs, it’s important to note that Kuvan doesn’t work for everyone—it’s most effective in children with mild cases of PKU.  

What are the best resources for managing PKU?

As with any special food diet, meal plans for PKU vary greatly from person to person. People managing PKU should work closely with their doctor and/or dietitian to ensure they’re getting the proper nutrients their body needs while limiting phenylalanine intake. 

If your family is managing PKU, certain states in the US provide some insurance coverage for formulas and foods necessary for treating PKU. Check with your insurance provider and state legislature to see if this coverage might be available for you!

– Meg & the Allergy Amulet Team 

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