A Biochemical Birth Defect

PKU (phenylketonuria) is an inherited metabolic disorder that occurs in 1 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 that is found in many goods. If PKU is left untreated, this protein can rise to high concentrations in the body and cause mental retardation.

Children born with PKU can live a normal life if put on a strict diet. Usually started before the fourth week of life, this diet is low in foods that contain phenylalanine, including breast milk and cow's milk. Instead, an affected child must be fed a special formula.

As the baby gets older, however, she can eat certain vegetables, fruits, and grain products but usually must avoid cheese, meat, fish, and eggs. Regular blood tests of phenylalanine levels can help determine what an affected child can and can't eat.