People with PKU can't process one of the amino acids found in many foods. The amino acid, called phenylalanine or "Phe" for short, builds up in the body.1
Too much Phe is toxic to the brain and can cause many problems. In infants and children, if PKU is not treated, the resulting high Phe can cause severe mental retardation.1 Even if PKU is treated problems like brain changes, lower IQ, and behavior problems may still occur.
There are ways to treat or inhibit this from causing significant trauma to an individual. Most who have this disorder take supplements, know what foods to avoid, and are able to function normally.