Phenylalanine -- an amino acid and one of the building blocks of protein -- has no side effects as a component of food in healthy individuals. However, in those with a disease called phenylketonuria, or PKU, phenylalanine causes a variety of side effects. This is because individuals with this disease can't properly metabolize the amino acid.
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Twenty common amino acids make up the proteins in your body and in the foods you eat. Phenylalanine is one of these; in fact, it's one of the essential amino acids -- there are eight of these -- that you must obtain from foods, because you cannot make them from other molecules. Many protein-containing foods are rich sources of phenylalanine, including meats, dairy, eggs and soy.
Phenylketonurics -- those with PKU -- have a genetic disorder that prevents them from producing an enzyme called phenylalanine hydroxylase. Your cells use this enzyme to convert phenylalanine into another amino acid called tyrosine, explain Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry." This not only makes tyrosine essential in patients with PKU -- those without the disease can make their tyrosine, while phenylketonurics must consume it -- it also leads to a build-up of phenylalanine.
Effects of PKU
If you have PKU and consume foods high in phenylalanine, the amino acid builds up in your body. You then metabolize it by an alternate pathway -- not the one used to make tyrosine -- into compounds called phenylketones. These can then lead to a variety of health effects. PubMed Health explains that the effects of PKU range from mental retardation -- perhaps the most commonly recognized effect -- to delays in physical development, rashes and seizures.
Prevention of Effects
No treatment is available for PKU. It is a genetic disorder, and you can't supplement those with the disease with the missing enzyme, since there's no way to get the enzyme from the digestive tract into the cells. However, babies diagnosed with PKU are fed a low-phenylalanine diet well into their teen years. During adulthood, it becomes less necessary to follow an extremely strict diet. This helps prevent the worst of the PKU-associated symptoms and side effects.