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Phenylketonuria (PKU) and Your Baby

Phenylketonuria and Your Baby - Grown Ups Magazine - Understanding the signs and symptoms of phenylketonuria in infants.
Understanding the signs and symptoms of phenylketonuria in infants.

Phenylketonuria and Your Baby - Grown Ups Magazine - Understanding the signs and symptoms of phenylketonuria in infants.

Phenylketonuria (PKU) is a serious condition caused by a genetic mutation in which individuals are unable to break down an amino acid called phenylalanine, allowing the amino acid to accumulate in the body. Many foods and beverages that contain phenylalanine (such as Diet Coke) are labeled as such so patients with PKU can help monitor their intake of the amino acid.

What Causes PKU?

Children with PKU have inherited the genes associated with it from their parents. Both parents must have the gene in order for the child to inherit the condition, yet the parents may not necessarily have the condition itself. In most cases, the parents are not aware that they are carriers.

Because the gene doesn’t work properly, patients with PKU produce little to none of the enzyme needed to break down phenylalanine. Eating foods high in protein, such as milk, nuts, cheese, or meat can cause dangerous levels to build up within the body, leading to serious health issues.

PKU is usually diagnosed in babies of European descent. The incidence of PKU among people of Asian, Latino, and African descent is very uncommon. Because the affected individual’s body can’t transform phenylalanine into melanin, people with PKU generally have fair skin and blue eyes. Pregnant women with PKU who do not manage their condition during pregnancy do not necessarily pass the condition on to their child; however, high levels of phenylalanine in the mother’s blood may adversely affect the baby.


Newborns do not exhibit signs of PKU unless the condition is left untreated during the first few months of life. Symptoms can be mild or severe and include:

  • Mental retardation
  • Behavioral problems
  • Social problems
  • Seizures
  • Tremors
  • Jerking of the arms and legs
  • Hyperactivity
  • Stunted growth
  • Skin rashes
  • Microcephaly (small head)
  • Musty odor of the breath, urine, or skin (caused by phenylacetate accumulation in the body)

Classic PKU is the most severe form of the condition and results in permanent mental retardation. Mild or moderate PKU is less severe and carries less risk of brain damage.


Babies can be screened for PKU shortly after birth. Blood collected from the baby’s heel one to two days after birth can be analyzed for metabolic disorders, including PKU. If PKU is suspected, additional blood and urine tests may be combined with genetic testing to confirm the diagnosis. Babies born with PKU must go on a special diet immediately to prevent build-up of phenylalanine within the body that leads to mental retardation.


A strict diet that limits the amount of phenylalanine a person ingests is the primary treatment for PKU. A protein-substitute formula is prescribed for infants and continued throughout life to ensure intake of essential amino acids necessary for health and growth. Some patients with PKU can have a limited amount of phenylalanine in their diets, but the amount that is considered safe varies by person.

The PKU diet must be followed for life to prevent dangerous levels from building up within the body. The amount of PKU ingested at each snack and meal everyday must be monitored closely to make sure the amount of protein is limited to safe levels. Blood tests monitor the levels and are especially important during growth spurts and pregnancy. Foods that should be strictly limited or avoided because of their high-protein levels include:

  • Milk
  • Nuts
  • Eggs
  • Cheese
  • Soybeans
  • Beans
  • Pork
  • Fish
  • Chicken
  • Beef
  • Peas
  • Chocolate candy
  • Beer

Furthermore, artificial sweeteners like NutraSweet contain aspartame, which releases phenylalanine when digested. Sugar-free foods and diet sodas containing aspartame carry a warning label to people with PKU and should be avoided.

One drug, Kuvan (sapropterin), has been approved by the FDA for treatment of PKU, and works by increasing a body’s tolerance to phenylalanine. The drug is used in conjunction with a PKU diet, but is not effective for everyone with the condition.Following the PKU diet and adhering to treatment plans and schedules can and do help people with PKU lead normal lives.

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