Written by Louise Robertson, Sarah Boocock and Paula Hallam. This article was first published in ‘Dietetics Today’, the official magazine of the BDA – the Association of UK Dietitians. We have published it on our blog as people with PKU and health care professionals working with people with PKU may find this article useful. The below diet information is NOT for the general population, but for use in specialist prescribed low protein diets.


In the 1970’s and 1980’s many adults with Phenylketonuria (PKU) were advised to discontinue their low protein diet in adolescence1 as it was thought that the diet was no longer needed. However, ‘Diet for life’ has now been recommended since the 1990’s2 and many adults with PKU want to return to low protein diet to improve their symptoms of high phenylalanine (phe) concentrations. Examples of these symptoms include: poor concentration, extreme fatigue, problems with organisation skills and anxiety3.

Protein substitute must be taken regularly throughout the day.

It is nutritionally unsafe for someone with PKU to follow a low protein diet without taking protein substitute (PS)4. PS contains all the amino acids, apart from phe, and essential vitamins and minerals missing when following a low protein diet. Therefore, it is important to find a PS that is tolerated and can be taken 3-4 times a day, everyday to avoid nutrient deficiencies. There are a variety of PS available in liquid, powder, dessert and tablet form. Taste, volume, appearance and convenience of the PS can all influence which one is chosen to suit each person’s lifestyle.

When restarting the PKU diet a stepwise process should be used.

Restarting the PKU diet is very challenging and changes should be made one step at a time to avoid the diet becoming overwhelming and resulting in failure. After finding a suitable PS it is usually taken once a day initially, increasing to 3-4 times daily at a pace that suits the individual. Concurrently high protein foods should be cut out and low protein foods trailed and added into the diet. Finally exchanges (1 exchange = 1g of protein = 50mg of phenylalanine) should be used and this can help to achieve desired blood phe concentrations.

Low protein prescribable foods are a vital component of the PKU diet.

The PKU diet is very restrictive and low protein (LP) foods are an important way of reintroducing vital calories, carbohydrate and variety back into the diet once high protein foods are restricted. These factors all help to control blood phe concentrations5. There are a variety of LP foods available on prescription. These include flour, breads, pasta, rice, cereals, noodles, burger and sausage mix, cakes and biscuits. People with PKU can have LP foods delivered via a home delivery service and can purchase a pre-payment prescription certificate which allows them to obtain more LP food without having to pay for each individual item.

LP products

Low protein alternatives introduce variety into the PKU diet.

Aside from LP foods available on prescription there are naturally occurring LP foods such as fruits and vegetables (some vegetables are counted as exchanges). For example, mushrooms or aubergines are a good base for a LP meal. In addition, there are many suitable vegan products that can be purchased from the supermarket or online. These include rice and nut milks, frozen desserts and cheeses. These foods will help with the taste and variety of the PKU diet, factors that could help with adherence and reduce blood phe concentrations.

mush and beans 400

Organisation and planning is essential.

Following a LP diet is time consuming6. Most meals need to be cooked from scratch using LP products. Patients need to organise a continuous supply of LP foods via the GP and chemist / home delivery, plan meals, shopping lists and plan ahead if they are going out to eat. In patients with PKU, if their phe concentrations are consistently higher than the target range, or if they were consistently high as a child, then these patients can find organisation and planning difficult and will need a lot of help and support from their Dietitian, family and friends3,7.

Cooking skills are needed to ensure variety in a LP diet.

Patients may never have cooked their own LP meals before as their parents cooked for them as a child. Tips are needed on how to cook with LP flour, pasta, rice and other products. These can be given by the dietitian, dietetic assistants, company representatives, at LP cooking events and clinics and LP recipes books and websites. Having the right cooking equipment including 1g increment scales for measuring exchanges and a bread machine for making fresh LP bread can all help with adherence.

LP toms 400

Modern technology can help in adhering to a LP diet.

There are now Apps available for patients to monitor their own PKU – recording how many PS and exchanges they have had in the day and recording their own phe results. Reminders can also be set on phones to take PS. Peer support is important and there are various groups on Facebook and Twitter for patients to follow. Patient society group web pages and blogs of people with PKU from around the world can provide information and insight into how others cope with the same diet.

Regular feedback can help with motivation to continue the PKU diet.

Patients with PKU send in their own blood spot cards through the post to their regional metabolic laboratory for phe analysis. Their dietitian will report back to them with their phe concentrations in a timely manner so that dietary changes can be made if necessary and encouragement given. Ensure a blood phe is taken before restarting the low protein diet so that the change in phe concentrations can be seen.

More information on a low protein diet can be found on the NSPKU website.


1 Guest, J.F., Bai, J.J., Taylor, R.R., E., Lee, P.J., Lachmann, R. .(2013) Costs and outcomes over 36 years of patients with phenylketonuria who do and do not remain on a phenylalanine-restricted diet. J Intellect Disabil Res. 2013 Jun;57(6):567-79

2 Medical Research Council Working Party on Phenylketonuria. (1993) Recommendations on the dietary management of phenylketonuria. Arch Dis Child, 68: 426-427.

3 Gentile, J.K., TenHoedt, A.E., Bosch, A.M. (2010) Psychosocial aspects of PKU: hidden disabilities-a review. Mol Genet Metab. 99 Suppl 1:S64-7

4 Das, A.M., Goedecke, K., Meyer, U., Kanzelmeyer, N., Koch, S., Illsinger, S., Lucke, T., Hartmann, H., Lange, K., Lanfermann, H., Hoy, L., Ding, X.Q. (2013) Dietary habits and metabolic control in adolescents and young adults with Phenylketonuria: Self-imposed protein restriction may be harmful. JIMD Rep (Epub ahead of print)

5 Cochrane, B., Schwahn, B., Galloway, P., Robinson, P., Gerasimidis, K. (2014) A questionnaire survey on the usage of low protein staple foods by people with phenylketonuria in Scotland. J Hum Nutr Diet. (Epub ahead of print)

6 Eijgelshoven, I., Demirdas, S., Smith, T.A., van Loon, J.M., Latour, S., Bosch, A.M. (2013).The time consuming nature of phenylketonuria: a cross-sectional study investigating time burden and costs of phenylketonuria in the Netherlands. Mol Genet Metab. 109 (3): 237-42.

7 MacDonald, A., van Rijn, M., Feillet, F., Bernstein, L., Bosch, A.M., Gizewska, M., van Spronsen, F.J. (2012) Adherance issues in inherited metabolic disorders treated by a low natural protein diets. Ann Nutr Metab. 61 (4): 289-95.

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