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Home > Vitafriendspku > Your pku journey > Understanding The Importance Of Staying On Diet

Understanding The Importance Of Staying On Diet

An Introduction To PKU

PKU is a genetic disorder that results in the build-up of an amino acid called phenylalanine, often abbreviated to “phe”.

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Amino acids are the building blocks of protein. When protein is eaten it is broken down by enzymes into amino acids (there are around 20 different amino acids).  Amino acids are needed for a wide variety of functions within the body such as muscle maintenance, growth, repairing cells and tissues, as well as making other body proteins and amino acids.  

Phe is found in all foods containing protein. In PKU, the build-up of phe is due to the deficiency of an enzyme called phenylalanine hydroxylase (PAH).   For those without PKU, PAH breaks down phe into another amino acid called tyrosine, and so, for those with PKU, a deficiency of PAH results in a build-up of phe and low levels of tyrosine.  Both amino acids have important roles in the body.  

If left untreated, chronic high levels of phe, together with very low levels of tyrosine, are harmful to the brain.   However, if detected early in life, and immediately managed, those with PKU can expect to lead a full and healthy life.   

PKU can be different from person to person. Some people have no or very little PAH enzyme activity– this is referred to as classical or severe PKU. Some individuals have more PAH enzyme activity - this is called moderate or mild PKU. In the case of mild PKU, the term ‘Hyperphenylalanaemia’ (Hyperphe for short), may be used.  

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PKU And The Teenage Years

The teenage years bridge the gap between childhood and adulthood. This will bring many changes to your body and mind, which may impact on your diet, especially when food has been such a big focus in your life for as long as you can remember. 

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You may recognise some of the following changes:

  • Unpredictable growth patterns – As a teenager, your growth does not always follow a steady, predictable pattern. You may go through periods of rapid growth followed by periods of slow growth. Some reach adult height early in the teenage years whereas others don’t reach it until much later. In PKU, this lack of predictability can sometimes make it more challenging to keep on top of your diet. 

  • Fitting in – When eating out with friends you will need to be organised, think ahead about your diet or even ring in advance to see if there will be food suitable for you.  Your friends may start to ask more questions and become more interested in your PKU and your protein substitutes. 

  • Taking control – Part of growing up and becoming independent is taking control of your low protein diet. This will include organising your prescriptions, and shopping and cooking for yourself. To help you achieve this, you may like to gradually take more ownership of your diet as you move through your teenage years. Your metabolic team will advise you on your target phe levels.  

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Staying On Diet

Despite these challenges, it is essential that you consume a healthy balanced low protein diet, which includes enough energy and nutrients to grow, be active and generally be the best version of you!

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There are significant benefits for you in following the diet;

  • Some teenagers report better focus and organisational skills when they are following their low protein diet and a feeling of ‘brain fog’ when they are not. You may feel a bit muddled and your friends may notice you’re not quite yourself. That’s not what you need when you have a big exam or you’re trying to have a night out with your friends. 

  • Most protein substitutes, as well as helping control your phe levels, contain vitamins and minerals to help you stay healthy. If you regularly skip taking your protein substitute or take less than you have been prescribed, this might affect your vitamins and minerals. Short-term you can feel tired and it could affect your appearance - you need good nutrition for healthy skin, nails and hair. Longer-term, this may lead to serious health issues such as muscle pain, osteoporosis and iron-deficiency anaemia. 


Despite becoming more independent in your teenage years it is important to remember you are not alone in this. As well as the support of your family, the specialist metabolic team are there to help you as much as they can. They’ll be more than happy to hear your thoughts and ideas on the diet (what you like and dislike, what works for you and what doesn’t) so that they can make the diet fit more around your lifestyle, instead of you having to fit around the diet!  During your teenage years you may also move over to an adult metabolic centre and a different metabolic team will take on your care.   

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Learn more about PKU

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