Phenylketonuria, commonly referred to as PKU, is a genetic disorder that leads to the build up of the amino acid phenylalanine in the blood. Untreated, PKU can cause metal retardation and serious health effects. According to the March of Dimes, one baby out of every 25,000 born in the United States has PKU. If diagnosed early, treatments allow those with PKU to avoid the harmful effects of the disease.
Phenylalanine
Phenylalanine is an amino acid derived from protein. In persons without PKU, the body produces an enzyme, phenylalanine hydroxylase, or PAH, which breaks down phenylalanine into useful compounds. People with PKU have very low levels of PAH, or lack the enzyme entirely. Without PAH to break down phenylalanine, the substance builds up in the blood to toxic levels. This leads to brain damage, seizures and a lack of melanin in the skin.
Genetics
Genes regulate the production of PAH. Some people have one normal PAH gene and one abnormal PAH gene. These people do not have PKU, but they have the potential to pass on the disease. In order to have PKU, a child must inherit one abnormal PAH gene from each parent, so both parents must be carriers. Even if both parents are carriers, there is only a 25 percent risk that their baby will have PKU. About 50 percent of the time, the baby will be a carrier, with one abnormal PAH gene. Twenty-five percent of the time, the baby will have two normal PAH genes.
Diagnosis
Most states require doctors to test for PKU shortly after the birth of a baby. The test for PKU was the first routinely performed test to screen newborns for disease. Doctors began using the test in the 1960s. The doctor takes blood from the baby's heel and tests it for levels of phenylalanine. Abnormally high levels of phenylalanine indicate PKU. Further tests will be done to confirm the diagnosis.
Treatment
If a doctor diagnoses PKU in an infant, she'll prescribe a special formula for the baby that is low in phenylalanine. As the child matures, he'll switch to a diet that is low in protein and high in other nutrients, and may continue to take low-phenylalanine, high-protein supplements to provide the nutrition he needs. The child will need to avoid meat, cheese, fish, eggs, milk and other high-protein foods. Some people have to continue this diet for the rest of their lives, while others can gradually switch to a more normal diet as they age. People with phenylketonuria must avoid aspartame and artificial sweeteners containing aspartame, because it contains phenylalanine.
