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California Newborn Screening Program

Phenylketonuria (PKU)

What is PKU?

Phenylketonuria (PKU) is an inherited metabolic disorder that occurs when both parents pass a gene for PKU to their child. This specific disorder affects the breakdown of protein by the body.

Protein is found in most foods. When we eat foods with protein, our body breaks down the protein into smaller parts called amino acids. Amino acids are important because they help the body grow and function properly. Phenylalanine, also known as phe, is one of many amino acids found in protein. The body has enzymes that work to break down substances like amino acids in the body. An enzyme known as phenylalanine hydroxylase is necessary to change phenylalanine into another amino acid called tyrosine. When there is not enough of the enzyme phenylalanine hydroxylase in the body, phe builds up in the blood and other body tissues.

What are the symptoms?

The high amounts of phe in the body can lead to mental retardation, seizures or convulsions, a skin problem called eczema, and an unusual musty odor in the urine.

What is the treatment?

Typically, PKU is treated with a special diet that limits the amount of phe eaten.  Foods high in protein like milk, eggs, meats, fish, cheese, and other high-protein foods cannot be eaten if a person has PKU.  A person with PKU has to drink a special formula every day which has the necessary ingredients for the body's growth and development but contains very little or no phenylalanine. The special formula contains adequate amounts of essential amino acids other than phenylalanine. Phenylalanine can be found in all foods containing protein. By eliminating foods high in protein, aspartame (formerly known as NutraSweet®), and wheat products containing gluten (which is relatively high in protein), blood phe levels can be significantly reduced. Since phenylalanine is an essential amino acid, it should not be totally omitted from the diet: too low a phe level is not healthy, either. The recommended phe-level range is 120 - 360 m mol/L (2 to 6 mg/dl) for children.

Frequent monitoring of the blood phe level and adjusting of the diet is necessary to ensure both adequate nutrition and safe levels of phenylalanine. This strict diet should be followed indefinitely rather than discontinuing it at eight or nine years of age (which was the standard in the past). It has been demonstrated that those who discontinue dietary phe restriction can show a significant drop in intelligence quotient (IQ), a change in behavior, or even convulsions.

Parents' Guide to Phenylketonuria (PDF)



Last modified on: 8/24/2010 3:22 PM