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Phenylketonuria – A Disorder That Affects Both Sexes Equally

PKU is the abbreviation for the metabolic disorder called phenylketonuria which occurs  in every 10.000-15.000 newborns and can affect both sexes equally. The disorder is caused by a mutated phenylalanine hydroxylase (PAH) gene which is normally responsable for creating PAH enzymes needed to break down amino acid called phenylalanine. High level of phenylalanine in patient’s blood can lead to serious health problems, including mental retardation.

The disease was first discovered in 1934. by a  Norwegian doctor and biochemist Dr. Ivar Asbjørn Følling, after testing urine of two mentally challenged siblings. Dr. Følling isolated phenylpyruvic acid from the tested urine and called this disease “Imbecilitas phenylpyruvate”.

Twenty years later doctor Horst Bickel published the results of a treatment on the PKU patients with the low protein diet, which showed a great improvement in patients’ neurological functions.

How does PKU occur?

PKU is, as we mentioned above, a metabolic genetic disorder that occurs at the moment of conception and as human chromosomes are carriers of hereditary characteristics, one of them can carry the PKU disorder, too.

It is known that one part of genetic material is inherited  from a mother and the other from a father. If both of the parents are PKU carriers, their child can still lead a healthy and PKU free life. It only means that they have just one of the PKU genes that has mutated, while the other functions normally. This is a main reason why a person doesn’t have any symptoms of a disease and ultimately may never find out the existence of the PKU gene in their bodies.

(Dr. Ivar Asbjørn Følling)

If two PKU carriers combine, the chance of getting PKU child is 1:4. In other words, even if you have a mutated PKU gene, it does not necessarily mean that your child will develop PKU.

In case that one of the parents has PKU disorder, the chance of having a PKU baby depends only on the other parent. If a baby has PKU, both of baby’s PKU genes have mutated, but if the parents with PKU genes are both healthy, the child will only be a carrier but will not have PKU. On the other hand, if one of the partners’ PKU genes has mutated, the chance for a healthy, non PKU child, would be 1:2. In the case the child doesn’t develop PKU, he/she will be the carrier of a  mutated gene.


The main problem for the PKU patients is the inability of the body to convert amino acid phenylalanine into tyrosine, other amino acid. The transformation of one amino acid into another happens by the help of enzyme called phenylalanine hydroxylase (PAH). This enzyme is either absent from PKU patients bodies, or not active enough to have ability to conduct the necessary reaction. Because of this reason, phenylalanine (Phe) accumulates in blood and can affect the brain functions and cause mental, as well as physical disorder. The only solution of this problem, for now, is the low protein diet that must be carried out throughout the whole life.

The detection of the disease in early life stage is crucial.  Nowadays this is quite possible, thanks to the newborns’ screening that is mandatory for every newborn baby. The sooner a baby is put on the low protein diet, the better chance they’ll have to lead a normal life without any side effects.

Want to learn more?


“The PKU Paradox: A Short History of a Genetic Disease” by Diane B. Paul and Jeffrey P. Brosco.

“Robert Guthrie–The Pku Story: Crusade Against Mental Retardation” by Jean Holt Koch.

“Low Protein Cookery for Phenylketonuria” by Virginia E. Schuett.

“Low Protein Food List for PKU” by Virginia E. Schuett.

“Low Protein Bread Machine Baking for PKU” by Virginia E. Schuett.

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