Encyclopedia / P / Phenylketonuria



Phenylketonuria (PKU) is a metabolic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase.

If untreated, the interrupted metabolism of dietary phenylalanine results in accumulation of the amino acid in body fluids with resultant, progressive, irreversible mental retardation.


PKU is a congenital (present at birth) disorder characterized by the presence of increased amounts of certain amino acids in the blood and urine.

The result is mental retardation in infants or young children if the disorder is untreated. It is not understood why high levels of phenylalanine cause severe mental retardation in children. The average IQ of untreated children is less than 50, and these patients have to be institutionalized as adults. Normal brain development may be disturbed by a high phenylalanine concentration.


The condition is caused by the inability to convert phenylalanine (an amino acid essential for optimal growth in infants and for maintenance of nitrogen balance in adults) into tyrosine (another amino acid) because of complete or near-complete deficiency of the enzyme alanine hydroxylase.

Excess phenylalanine is normally eliminated from the body by conversion to tyrosine. In PKU, this mechanism does not work, permitting an accumulation of phenylalanine.


Early diagnosis is essential because symptoms are not obvious in a newborn infant. Mental retardation may develop gradually and may not be evident for a few months. An early clue to the disease is light coloring of the skin, eczema (an itchy skin rash), and a musty odor.


Screening tests are now mandatory for all babies born in the U.S. and Canada.


The goal of treatment is to keep plasma phenylalanine levels within the normal range through diet. Limiting the child's intake of phenylalanine, however, must be done cautiously because it is an essential amino acid. A carefully maintained dietary regimen can prevent mental retardation and other complications.

Dietary treatment must be started soon after birth or some degree of mental retardation may be expected. The duration of diet therapy is controversial, but some restriction of dietary phenylalanine is necessary indefinitely. IQs may decline after a peak at the end of the controlled diet periods.

Other problems that may appear and become severe once the patient is off the diet include academic and behavioral problems, poor visual motor coordination, poor problem solving skills, and low developmental age.

Since phenylalanine occurs in almost all natural proteins, maintaining proper nutrition is difficult on a phenylalanine-low diet. Dietary use of special phenylalanine-free preparations is therefore essential. These include Lofenalac (for a low phenylalanine diet) and Phenyl-Free, both from Mead Johnson. Low protein foods such as fruits, vegetables, and some cereals are may be allowed.

Prospective mothers who have PKU should be started on a low-phenylalanine diet before conception, and every effort should be made to keep blood levels low throughout pregnancy.


Do the laboratory findings indicate high levels of phenylalanine?

Are there other abnormalities?

Is there a risk of mental retardation?

What modifications in diet are necessary?

Does dietary treatment have to be permanent?