Treating phenylketonuria  

Phenylketonuria (PKU) can be successfully treated with a low-protein diet and dietary supplements.

Low-protein diet

Your dietician will draw up a detailed dietary plan for your child that can be revised as your child grows and their needs change.

For more detailed information and advice, the National Society for Phenylketonuria (NSPKU) website has a dietary information booklet (PDF, 193kb) that's available to download. The NSPKU is the UK’s leading charity for people with PKU.

The NSPKU recommends a "traffic light" approach to diet, where foods are categorised as follows:

  • green foods  safe to eat because they contain low levels of protein, although they shouldn't be eaten to excess
  • amber foods  contain moderate levels of protein and must be carefully weighed; they'll be eaten in different quantities, according to your blood phenylalanine level
  • red foods  high-protein foods that should never be eaten under any circumstances

Green foods

Examples of green foods include:

  • fresh fruit  with the exception of bananas, figs and dates
  • most vegetables  with the exception of asparagus, bean sprouts, broccoli, Brussels sprouts and cauliflower
  • low-fat and low-protein butter and margarine 

Read more about fruit and vegetables.

There are also a number of specially designed low-protein versions of popular products – such as flour, rice and pasta – specifically designed for people with PKU and related conditions. Many of these special foods are available on prescription, so you won't have to pay full price for them.

Amber foods

Examples of amber foods include: 

  • potatoes, including chips
  • cereals, including oats and cornflakes
  • baked beans
  • milk  a specially designed formula can be used as a milk substitute for babies; a low-protein milk is also available for older people 
  • rice

Red foods

Examples of red foods include:

  • all types of meat
  • eggs
  • fish
  • cheese
  • nuts and seeds
  • flour-based foods – such as bread, cakes and biscuits
  • Quorn and tofu

There's some disagreement about whether a low-protein diet should be continued by adults. Some experts believe that the diet should be for life, while others feel that people should decide for themselves after a discussion with their doctor.

If you choose to return to a normal diet, it's important that you still attend regular follow-up appointments, so you can be monitored for any complications that might arise.

The appointments will also keep you informed about any relevant new evidence, and ensure that you're supported should you choose to go back on to a low-protein diet.

Dietary supplements

If you have PKU, as well as sticking to a low-protein diet, you will also have to take regular dietary supplements. These contain essential nutrients absent from the low-protein diet. 

In addition to vitamins and minerals, the supplements contain a mixture of chemicals called amino acids (not phenylalanine), which replace the protein that's missing from the diet and are essential for the body to grow and remain healthy.

Supplements are available in various forms – including powders, drinks and tablets – and need to be taken three or four times a day.


People with PKU also have to avoid food products that contain aspartame, because it's converted into phenylalanine in the body. Aspartame is a sweetener found in:

  • sugar substitutes – such as artificial sweeteners often used in tea and coffee
  • diet versions of fizzy drinks
  • chewing gum
  • squashes and cordials
  • some alcopops

All food products that contain aspartame or a related product should be clearly labelled.

There are also medications that contain aspartame, such as some children's cold and flu remedies. It's a legal requirement for any medication that contains aspartame to state it on the patient information leaflet that comes with the medication.

Phenylketonuria and pregnancy

Women with PKU have to take particular care during pregnancy, because high levels of phenylalanine can damage the unborn baby.

Providing that phenylalanine levels are strictly controlled during pregnancy, problems can be avoided and there's no reason why a woman with PKU shouldn't be able to have a normal, healthy baby.

It's recommended that all women with PKU plan their pregnancies carefully. You should aim to follow a strict diet and monitor your blood twice a week before becoming pregnant. It's best to try to conceive once phenylalanine levels are within the target range for pregnancy.

During pregnancy, you'll be asked to provide blood samples three times a week and will be in frequent contact with your dietician. As soon as your baby is born, phenylalanine control can be relaxed and there's no reason why, if you have PKU, you can't breastfeed your baby.

Contact your PKU doctors and dieticians as soon as possible if you become pregnant when your phenylalanine levels aren't adequately controlled. If your phenylalanine levels can be brought under control within the first few weeks of your pregnancy, the risk of damage to the baby should be small. However, your pregnancy will need to be monitored very carefully.

Video: the Expert Patient Programme

Many people who have a life-long condition such as phenylketonuria find that taking part in the Expert Patient Programme (EPP) useful. A GP explains how the EPP works

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Pregnancy and baby

All you need to know about pregnancy, birth and looking after a baby, including feeding and trying to get pregnant

Page last reviewed: 02/10/2014

Next review due: 02/10/2016