Phenylketonuria (PKU)

What is PKU?

PKU is a genetic disorder of phenylalanine metabolism.  Phenylalanine is an amino acid.  Amino acids are the building blocks for proteins.  PKU is usually caused by a deficiency of the liver enzyme phenylalanine hydroxylase which converts phenylalanine into another amino acid, tyrosine.  Phenylalanine levels increase to toxic levels which can be damaging to the brain if left untreated.  PKU is treated by a low protein, low phenylalanine diet.

How common is PKU?

PKU affects about 1 in 10,000 births.  Approximately 1 person in 50 is a carrier for PKU in the UK and if two PKU carriers have a baby they have a 25% chance of having a baby with PKU.  Approximately 70 babies with PKU are born per year in the UK.  All new born babies in the UK are screened for PKU using the heel prick test or Guthrie test.  Babies who have a raised level of phenylalanine are referred immediately to a specialist team so that dietary treatment can start. 

What is the treatment for PKU?

PKU can be successfully treated by diet.  The aim of the dietary treatment for PKU is to prevent excessive amounts of phenylalanine building up in the blood. Treatment is by strict dietary restriction of phenylalanine using a low protein diet.  High protein foods like meat, poultry, fish and eggs and some dairy products are avoided as they are too high in protein.  Other foods like bread, biscuits, cakes and other baked goods as well as pasta, rice and cereal contribute to protein intake and their intake must be carefully monitored.  This is done by using an exchange system of 50mg phenylalanine exchanges (i.e. the weight of food which provides 50mg of phenylalanine) which can be swapped to vary the diet.  There are many low protein food alternatives available such as bread, biscuits, all purpose flour for baking, pizza bases and pasta.  These products provide the diet with variety and a useful source of energy.

A very important part of the treatment is the inclusion of a phenylalanine-free protein substitute.  Adequate protein is essential for growth and without the supplement the diet would be deficient in protein.  The protein substitute can be given as a drink, taken as a paste or as a capsule depending on age and dietary prescription.  Achieving normal growth and nutrition is vital while keeping the diet as flexible and as palatable as possible. Phenylalanine levels are regularly measured and the diet adjusted in consultation with the dietitian.  Keeping phenylalanine at the recommendedlevel is important so that the brain develops normally.  Those who keep their phenylalanine levels well controlled are more likely to do well in education, have improved concentration and less behavioural problems. 

Is there anything else that needs to be avoided?

Aspartame is an artificial sweetener which contains phenylalanine and must be avoided.  It is commonly used to sweeten fizzy drinks, squashes and cordials, desserts and chewing gum.

How long should the PKU diet continue for?

It used to be thought that the restrictive diet could be stopped late in childhood but more recent studies show that some form of protein restriction should continue for life.  It is particularly important for PKU women who are contemplating getting pregnant to keep strict control on their phenylalanine levels as high levels can be dangerous for the baby.

The Juvela Low Protein Team are also doing our best to develop low protein recipes to make your diet as varied as possible, so that you are not missing out at mealtimes and snack time!

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