PKU, or phenylketonuria, is an inherited metabolic disorder that occurs in one in 15,000 births (less commonly among African-Americans and people of Jewish descent). All babies in the U.S. are tested for the disease soon after birth. A child with PKU is missing a crucial enzyme that breaks down a protein called phenylalanine, which is found in many different foods. If PKU is left untreated, this protein can rise to high concentrations in the body, become toxic to the brain, and cause mental retardation.
Symptoms: PKU has no symptoms -- it's typically diagnosed through newborn testing. However, it is only detectable in a baby more than 72 hours old who has already ingested milk. Ideally, the testing should be done during the baby's fourth or fifth day of life.
Treatment: A child born with PKU can live a normal life if she's put on a strict diet. Usually started before the fourth week of life, this diet is low in foods that contain phenylalanine. Instead of breast milk or cow's milk, an affected child must be fed a special formula. As the baby gets older, she can eat certain vegetables, fruits, and grain products, but usually must avoid cheese, meat, fish, eggs, and foods made with some artificial sweeteners, such as aspartame. Regular blood tests of phenylalanine levels can help determine what an affected child can and can't eat.