It’s been ages since I wrote a post but I’m not going to come up with a load of excuses….we are all busy and I just need to make time to write. So here goes…

A few people have asked me about the work that I do with children and adults with a condition called Phenylketonuria…or PKU for short, as the full word is much too long and cumbersome to keep saying/writing!

What the PKU is that??

PKU is a rare genetic condition, meaning it is “passed down” from your parents. Both parents need to be carriers of the “mistake” in their genes, in order to pass it on to their child.

baby heel

All newborns in the UK are screened for PKU (and other disorders) with the ‘heel prick’ blood test taken on day 5 of life. The blood test is to check if one of the building blocks of protein (called amino acids) is raised or not. In PKU the building block that is raised has another long name called phenylalanine.

Although PKU is a rare disorder (it occurs in about 1 in 10 000 babies in the UK), this still means that 60 – 70 babies per year are born with PKU in the UK.

So why is a dietitian involved in looking after children and adults with PKU, you might ask?

Well, the answer is a simple one…the treatment for PKU involves a special diet, restricting the amount of protein in the diet.

Remember that long word I mentioned earlier…phenylalanine? Well that is present in all high protein foods that we eat, such as all types of meat, fish, chicken, eggs, cheese, milk, yoghurt, nuts, seeds, soya, quorn and tofu. As well as in foods that you may not think are high in protein, such as bread, pasta, cakes and biscuits (made with wheat flour). People with PKU need to avoid all these high protein foods.

What happens when we eat foods containing protein?

When we eat foods containing protein, the body breaks down the protein in to smaller building blocks or amino acids with special chemical scissors called ‘enzymes’. The amino acids are then broken down further into other chemicals that can be used by the body to perform important jobs.

People with PKU cannot break down phenylalanine, as they do not have the ‘enzyme’ (or chemical scissors) to break it down. This means that phenylalanine builds up in the blood and (only) IF UNTREATED can lead to problems with the brain and a child’s development can be affected.

When PKU is treated and properly managed, people with PKU grow up to be healthy and have every chance of attending university, building a successful career and enjoying life, just as everyone else!

This is why all newborns in the UK are screened for PKU so that it can be diagnosed and treated at a very early age (typically treatment is started by 2 – 3 weeks of age). 

But how do children with PKU grow without high protein foods?

This is a very important question and I must explain this in more detail so that you understand what the PKU diet is all about.

Although people with PKU cannot break down phenylalanine, they do still need very small amounts of this amino acid to grow, as it is an ESSENTIAL amino acid, meaning that your body cannot make phenylalanine and it needs to be taken in from the diet. This is provided in small, measured quantities of foods such as potatoes, peas, sweetcorn, spinach, kale, breakfast cereals, rice and broad beans, as a few examples. These are called ‘exchanges’.

What is a protein substitute?

In order to make the diet safe and nutritionally adequate for children (and adults) with PKU, they need something called a ‘protein substitute’ to provide them with all the building blocks of protein that they need to grow.

There are 20 different types of building blocks (or amino acids) needed to make proteins in our bodies.

The protein substitute contains all the other amino acids, except phenylalanine, to provide “safe” protein as well as vitamins and minerals. This is because when you remove high protein foods from the diet, you also remove lots of other important nutrients such as calcium, zinc, iron and many vitamins. These all need to be replaced by a special protein substitute taken 3 times /day.

So what CAN people with PKU eat?

All fruits and some vegetables can be eaten in normal portion sizes without measuring / weighing out. There are also special low protein prescribable foods that are important to provide enough energy (calories) and variety in the diet. There are low protein versions of bread, pasta, rice, flour, cakes and biscuits.

Example of a meal plan for a child with PKU on 6 protein exchanges/day:

Breakfast                                                                              Protein Exchanges

Breakfast cereal eg. Cornflakes 20g                                                1

Low protein milk                                                                                     0

Fresh strawberries                                                                                  0

Protein Substitute


Low protein bread (2 slices)                                                              0

Cheese spread 30g                                                                                2

Carrots, cucumber, baby tomatoes                                               0

Banana with low protein custard                                                    0

Protein Substitute

Evening meal

Low protein pasta with tomato-based sauce                            0

Peas 25g                                                                                                    1

Sweetcorn 35g                                                                                        1

Coconut yoghurt 45g                                                                           1

Fresh mango                                                                                           0

Protein Substitute

TOTAL                                                                                                    6

I hope you have enjoyed this post and that it has given you an insight into what the PKU diet is all about and an appreciation for what it is like to follow a special diet…please see below for some further reading options.

Further reading:

Paula x