What is the PKU Diet?

Vegetables are safe for a PKU diet.
Most vegetabes are allowed on a PKU diet. Debby Lewis-Harrison / Getty Images

Question: I work as an after-school daycare provider, and yesterday a parent asked me about caring for a child who's on a PKU diet. She said it had something to do with avoiding protein. What is the PKU diet, and how do I help a child follow it?

Answer: The PKU diet is necessary for people who have a genetic disorder called phenylketonuria (PKU). People with phenylketonuria are missing an enzyme that allows the body to convert phenylalanine (an amino acid) to tyrosine (another amino acid).

The phenylalanine builds up and causes brain damage; so everyone with PKU must avoid protein-rich foods.

Infants and babies with PKU need a special formula that doesn't contain phenylalanine. It's absolutely crucial that babies, toddlers, and children don't eat protein-rich foods because the brain is growing rapidly. A very rigid diet must be followed. All foods that are high in protein need to be avoided.

Foods high in protein include:

  • meat
  • fish
  • poultry
  • milk
  • yogurt
  • cheese
  • eggs
  • nuts
  • seeds
  • legumes
  • peanut and other nut butter

Dishes, snacks and meals that are made with these foods need to be avoided too -- so pizza, cake, cookies, hamburgers, hot dogs, ice cream, yogurt and many other foods. Even things like bread and some types of cereal can have too much protein. Aspartame also needs to be avoided since it's a combination of aspartic acid and phenylalanine.

Children and teens with PKU can eat fruits and vegetables and small amounts of cereals and crackers because those foods are low in protein.

Adults need to stick pretty closely to the diet as well.

But, the PKU diet is too low in protein for normal health, so children and adults with PKU need to consume special protein drinks formulated without phenylalanine. Also, this diet needs to be followed for life, and blood tests are regularly taken to keep track of phenylalanine levels in the body.


Is PKU Common?

According to the National PKU Alliance, about one in 15,000 babies has PKU. Babies in the United States and many other countries are screened shortly after birth, and as long as babies and children adhere to the diet, their brains will develop normally.

Adults with PKU may have a little more leeway with protein intake because the brain is fully developed, but the PKU doesn't go away and consuming too much phenylalanine can still cause mood problems and memory loss.

Learn More About Phenylketonuria


Maher AK. "Simplified Diet Menu." Eleventh Edition, Hoboken NJ, USA: Wiley-Blackwell Publishing, October 2011. Accessed April 2, 2016.

Merck Manual - Professional Version. "Amino Acid and Organic Acid Metabolism Disorders." Accessed April 2, 2016. http://www.merckmanuals.com/professional/pediatrics/inherited-disorders-of-metabolism/amino-acid-and-organic-acid-metabolism-disorders.

National Phenylketonuria Alliance. "Phenylketonuria Facts for School Faculty and Staff." Accessed April 2, 2016. http://www.npkua.org/Portals/0/PKU_schoolfacts.pdf.

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