What is phenylketonuria (PKU), and why is there a statement regarding PKU on products sweetened with aspartame?

Phenylketonuria (PKU) is a rare inherited disease that prevents the essential amino acid phenylalanine, one of the components of aspartame, from being properly handled by the body. An essential amino acid is required for normal growth, development, and body functioning and must be obtained from the diet, as the body cannot make it.

Because people with phenylketonuria cannot properly metabolize it, phenylalanine from the diet can accumulate in the body and cause health problems including mental retardation. In the U.S., and many other countries, routine screening for PKU is required for all newborns. In the U.S., about 1 in 15,000 babies is born with PKU. People with PKU are placed on a special diet with a severe restriction of phenylalanine from birth to adolescence or after so that they get just enough for proper growth and not too much as to cause adverse effects. In addition, women with PKU must remain on the special diet throughout pregnancy.

Since individuals with PKU must consider aspartame as an additional source of phenylalanine, aspartame-containing foods must carry a statement on the label "Phenylketonurics: Contains Phenylalanine" in the U.S.