Phenylketonuria (PKU) is a disorder in which a child lacks an enzyme necessary to process the essential amino acid known as phenylalanine. Essential amino acids can only be obtained from foods that are eaten since the body does not normally produce them. When levels of this amino acid (a building block of protein) become too high, it can cause brain damage that leads to progressive developmental delay and permanent mental retardation. About one in every 10,000 infants born in the United States has PKU, according to the National Institutes of Health.

PKU is the result of a defective recessive gene that is passed on from each parent to the child. If both parents are carriers of this gene, each of their children has a one in four chance of having PKU. Caucasians of northern European ancestry are at the highest risk for PKU. African-Americans and Asians are at much lower risk for PKU. Both male and females are equally affected.

Infants with PKU appear normal at birth. Symptoms commonly associated with untreated PKU include a distinctive musty or mousy odor on the child’s breath, skin, sweat and urine. Children with untreated PKU often have a lighter complexion than their siblings, because phenylalanine contributes to the production of melanin, which is the pigment responsible for hair and skin color. As children with untreated PKU get older they may have an abnormally small head (microcephaly), delayed mental and social skills, self-mutilation, stunted growth and seizures. 

PKU typically is diagnosed through blood tests.  A newborn screening test performed at the hospital after the baby’s birth is required in all states in the United States. If this screening test is positive, more specific tests are performed to diagnose the condition.

Children with PKU can live normal lives if they are treated early. The goal of treatment is to maintain blood levels of phenylalanine between 2 and 10 milligrams per deciliter (mg/dL). Treatment for PKU involves a special diet that is low in phenylalanine. This requires eliminating all high-protein foods, including meat, fish, milk, cheese, eggs, dried beans, peas and nuts. In addition, the artificial sweetener aspartame (commonly used in diet soft drinks) should be avoided because it contains phenylalanine. Children typically have to take a special liquid or supplement containing a nutritional substitute to help make up for the protein that is lacking in the diet. Phenylalanine levels in the blood should be checked regularly throughout childhood. PKU patients generally remain on restricted diets for life.