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Understanding The Importance Of Diet For Life
An Introduction To PKU

PKU is a genetic disorder that results in the build-up of an amino acid called phenylalanine or 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). This enzyme helps turn phe into another amino acid called tyrosine. This results in a build-up of phe and low levels of tyrosine. Both amino acids have important roles in the body.

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) is sometimes used.

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The Importance Of Diet For Life

As an adult with PKU you may not currently be following a low protein diet and it is possible, that you may have been told you no longer need to follow a low protein diet in your childhood or teenage years.

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Over the last 40 years, recommendations for the management of PKU have changed as experts in the field collect more evidence on the effect of phe levels on health.

In Europe, a group of experts produced PKU management guidelines in 2017 recommending ‘treatment for life’ for people with PKU. Although some adults say they don’t feel any different when they have high phe levels, others do report problems when their phe levels are high. There is also a lot of uncertainty over the long-term effects of high phe levels. The oldest individuals to be diagnosed through newborn screening are now in their 50s and the impact of high levels of phe on aging are not known. As there is currently no strong evidence that it is safe to discontinue dietary treatment in adults, the experts recommended treatment for life.

This means continuing a low protein diet lifelong, that restricts phe, alongside a protein substitute. When following a low protein diet, taking protein substitutes is essential to ensure that you keep your phe levels within the recommended range, maintain lean muscle mass and do not develop nutritional deficiencies.

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Being 'off diet'

Being “off diet” can mean different things to different people;

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  • Some relax their dietary protein restriction and reintroduce protein-rich foods such as meat, fish and beans but continue to take their protein substitute
  • Others may continue to follow a restricted protein diet but do not take protein substitutes
  • Some may completely abandon their diet and eat a “normal” diet containing foods that are naturally high in protein such as eggs and meat and no longer take a protein substitute.

The recommendation is diet for life and any changes to your diet should be discussed with your Specialist Metabolic team. If you are struggling to comply with your low protein diet, your metabolic team are a point of call to support you.

If you were to stop taking your protein substitute whilst remaining on a severely restricted protein intake, you will be putting yourself at risk of protein malnutrition and vitamin and mineral deficiencies. This will also lead to high phe levels.

The effects of being “off diet” can vary for each person but will result in high phe levels.

Research has shown that some adults who stop low protein diet can develop symptoms within only a few weeks after discontinuation. You will hear people referring to a feeling of brain fog when phe levels are high. This may affect how well you make decisions such as when you’re driving a car, operating machinery, or performing day to day tasks. Some people report not feeling any effects when they have high phe levels, although it is still unknown what the long-term effects of living with high phe levels may be.

Other possible risks associated with high phe levels may include:

  • depression, anxiety
  • tremors (the shakes)
  • stiff or weak legs
  • headaches
  • eczema
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Recommended Blood Phe Levels
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The current European guidelines recommend blood phe levels below 600 μmol/l, ideally in the range 120 – 600 μmol/l. If you are a female, you need to be aware your phe levels need to be under stricter control during both before you become pregnant (pre-conception) and pregnancy. Your specialist metabolic team will advise on this as these can differ from centre to centre and person to person. For more information on PKU and pregnancy, click here.

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Returning To Diet
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Many adults may not be currently following the ‘treatment/diet for life’ approach or even realise that they should be following this as they may have been told as a teenager they no longer needed to follow a diet and therefore have not been seen by a metabolic team for a while.

However, it is never too late to return to a low protein diet, research is showing that re-starting a low protein diet may improve associated risks.

Some people with PKU who have stopped their low-protein diet and then restarted it report when back on their low protein diet that they:

  • feel better
  • look better
  • can think more clearly
  • are less moody
  • find it easier to get along with others
  • feel less tired and have more energy
  • can concentrate
  • can work things out better, e.g. when trying to think strategically such as in team sports or at work.
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Starting A Low Protein Diet In Adulthood

Although returning to diet in adulthood may seem daunting, the diet for PKU has improved over the years:

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  • More choice, availability and better tasting protein substitutes and low protein foods
  • clinics specifically for adults
  • social media provides access to others with PKU
  • more accessible options for eating out, e.g. vegan options.
  • More options of low protein foods in supermarkets with the increase in coeliac and vegan food options.

When returning to diet, there are certain things to take into consideration such as:

  • Aiming to keep within your daily phe target you will be set by your metabolic team
  • Taking your recommended protein substitute regularly throughout the day
  • Monitoring your phe levels
  • Ensuring you have enough of your PKU supplies in stock

Your metabolic team will help you return to a low protein diet. If it has been a long time since you saw your metabolic team, ask your GP to refer you to your nearest metabolic centre to help you get started on a low protein diet. For more information see the Returning To Diet.

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Learn more about PKU

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