Low protein diet
A low protein diet is the main type of treatment for phenylketonuria (PKU).
Your dietician will be able to draw up a detailed dietary plan for your child and they will also be able to help revise the plan over time as your child gets older and their needs change.
It is beyond the scope of this article to provide a complete overview of all the recommendations and information regarding low protein diets.
In general terms a 'traffic light' approach to diet is recommended where foods are categorised as follows:
- green foods - are those that are safe to eat because they contain very low levels of protein, although they should not be eaten to excess
- amber foods - contain protein and must be carefully weighed; they will be eaten in different quantities according to your blood phenylalanine level
- red foods - should never be eaten under any circumstances
Below are examples of green, amber and red 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
There are also a number of specially designed low protein versions of popular products, such as flour, rice and pasta that are specifically designed for people with PKU and related conditions. Many of these special foods are available on prescription so you will not have to pay the full price for them.
Examples of amber foods include:
- baked beans
- milk - a specially designed formula can be used as a milk substitute for babies and children
- normal rice
Examples of red foods include:
- all types of meat
- beer - lager and bitter
- any food that has been baked using flour, such as bread
As well as sticking to a low protein diet, people with PKU will also have to take regular dietary supplements. These contain all the essential nutrients that are absent from the low protein diet. The supplements contain a mixture of all the amino acids (apart from phenylalanine) that are essential for the body to grow and remain healthy. Supplements are available in various different 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. Aspartame is an artificial sweetener that can be found in:
- sugar substitutes, such as artificial sweeteners that are 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 state on the packet 'contains a source of phenylalanine'.
There are also a number of medications that contain aspartame, such as some cold and flu remedies for children. It is a legal requirement for any medication that contains aspartame to state it on the patient information leaflet that comes with the medication.
You should always make sure you read the patient information leaflet carefully before giving any medication to your child.
Tetrahydrobioterin is a new type of medication that has proved to be effective at reducing the level of phenylalanine in some children and adults with mild PKU. Tetrahydrobioterin is a synthetic form of a vitamin which helps the PAH enzyme to work better.
Other therapies under investigation
- Enzyme substitution therapy:
~ Phenylalanine ammonia lyase
- Large neutral amino acid
- Gene therapy
Phenylketonuria and pregnancy
Women with PKU have to take particular care during pregnancy. This is because high levels of phenylalanine can damage the unborn baby leading to heart defects, learning difficulties and other problems. Fortunately, providing that the mother’s phenylalanine levels are strictly controlled during pregnancy, all of these problems can be avoided and there is no reason why any woman with PKU should not be able to have a normal, healthy baby.
Due to the need to keep phenylalanine levels very low throughout pregnancy, it is recommended that all women with PKU plan their pregnancies carefully. Ideally, they should be following a strict preconception diet and sending in blood monitoring once a week before they become pregnant. Once the phenylalanine levels are within the target range for pregnancy, the couple can start to try to conceive.
During pregnancy, women are asked to send in blood samples twice weekly and will be in frequent contact with their dieticians. As soon as the baby is born, phenylalanine control can be relaxed and there is no reason why a mother with PKU cannot breast feed her baby.
If you do become pregnant when your phenylalanine levels are not adequately controlled it is important that you contact your PKU doctors and dieticians as soon as possible. Providing 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, it will be necessary to monitor the pregnancy very carefully