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What Is Phenylketonurics Phenylalanine?

By Ruth Coleman

According to the U.S. National Library of Medicine, one infant in every 10,000 to 15,000 is born with phenylketonuria. It is a genetic disease, referred to as autosomal recessive because a child born with this disease has inherited a mutated gene from each parent. The mutated gene is called the PAH gene.


In phenylketonuria the amino acid phenylalanine accumulates in the body and some of the amino acid is changed to a substance called phenylketones, which is then excreted in the urine. Normally, phenylalanine becomes part of protein or is changed to tyrosine, explains Gregory Barsh, M.D., Ph.D., professor of pediatrics and genetics at Stanford University School of Medicine in “Pathophysiology of Disease.” However, people with this disease do not have the enzyme that is needed to change phenylalanine to tyrosine, so the amino acid accumulates. Enzymes are proteins that speed up the time a reaction will take.


Infants born with phenylketonuria do not initially show any symptoms of this disease. They will start to have symptoms once the phenylalanine builds up, which may take several months, according to Chin-To Fong, M.D., Chief of the Division of Pediatric Genetics at the University of Rochester School of Medicine and Dentistry in “The Merck Manual for Healthcare Professionals.” Affected children may have a rash, body odor, light skin color and light eye and hair color. If they are not treated, they will grow more slowly than normal, and become moderately or severely mentally retarded.

Pregnant Women and Phenylketonuria

According to the Centers for Disease Control and Prevention, if pregnant women with this disease do not adhere to a special diet, even before becoming pregnant, their infants are at risk of becoming mentally retarded. In fact, people with phenylketonuria need to stick to this diet for life, but many stop following it once they reach adolescence. The special diet involves eliminating all foods that contain high amounts of protein, such as dairy products, fish, meat, eggs and poultry, because proteins are broken down to amino acids and they cannot process the phenylalanine amino acid.

What Does “Phenylketonurics: Contains Phenylalanine” Mean?

Some artificial sweeteners have the notice “phenylketonurics: contains phenylalanine” on the box and sugar packet. This notice is simply telling everyone who has phenylketonuria, both men and women, that the product contains phenylalanine and is, therefore, a product that people with phenylketonuria should not use. The warning does not apply to those who do not have this disease, just as a warning that a product is made using equipment that processes peanuts would not apply to people who are not allergic to peanuts.

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