Phenylketonuria (PKU) is a rare metabolic disorder in which a child lacks an enzyme necessary to process an amino acid known as phenylalanine. This amino acid (a building block of protein) is essential for normal growth in infants and children and to help the body process protein. However, if the body allows too much phenylalanine to build up, brain damage can result.

About one in every 10,000 newborns in the United States has PKU, according to the National Institutes of Health. PKU rates vary throughout the rest of the world, depending on a child’s ethnic background and country of origin. In the United States, Caucasians of northern European ancestry are at the highest risk for PKU. African-Americans and Asians are at much lower risk for PKU. Worldwide, the highest incidence of PKU appears to occur in Turkey.

Unless treated, children with PKU usually will experience brain damage by the end of the first year of life. However, maintaining a strict low-protein diet can prevent or minimize the damage caused by PKU. Most experts recommend that the diet be continued throughout life. Going off the diet later in life has been associated with a drop in cognitive function and other complications, such as movement disorders and attention-deficit hyperactivity disorder.