People with phenylketonuria cannot break down a substance in food called phenylalanine. Following a phenylketonuria diet is the main treatment for preventing serious symptoms.

People with phenylketonuria (PKU) cannot break down the amino acid phenylalanine. Amino acids are organic compounds in food that the body needs to function.

The body uses amino acids to carry out essential bodily functions, such as growing and repairing body tissues. The body breaks down and removes any leftover amino acids.

Other names for PKU include:

  • Folling disease
  • phenylalanine hydroxylase deficiency
  • phenylalanine hydroxylase deficiency disease

This article discusses which foods people with PKU should include or avoid in a phenylketonuria diet. It also discusses when to speak with a doctor about a PKU diet.

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The body needs amino acids to function. People with PKU cannot break down phenylalanine. Without treatment, it builds up in the blood and brain. The phenylalanine buildup leads to several symptoms, including brain damage.

Different foods contain different amounts of phenylalanine. The main treatment for PKU is following a specific low phenylalanine diet that is based on foods that are suitable and those that are not. This plan helps people with PKU maintain recommended phenylalanine levels in their body.

Usually, healthcare professionals also prescribe amino acid supplements.

Dietitians create phenylketonuria diets based on a person’s individual tolerance level for phenylalanine. This can vary with how severe a person’s PKU is. Their tolerance level also depends on their:

  • gender
  • age
  • phenylalanine blood level
  • phenylalanine hydroxylase activity, or the process where a person’s body converts phenylalanine into other substances
  • height growth
  • weight gain

Each eating plan provides a safe daily allowance of phenylalanine based on a person’s need. There are several methods a person with PKU can use to track their daily phenylalanine intake. Tracking involves a person documenting what they eat and how much of those foods they eat.

There are several ways of calculating how much phenylalanine is in each food, measured in grams per hundred grams of food. A healthcare professional or dietitian can help explain how to track and calculate these levels.

People with PKU need a healthcare professional to regularly monitor and adjust their phenylketonuria diet plan long term. There is no one standard phenylketonuria diet for every person with PKU.

Several low protein versions of common foods are available for people with PKU. Some foods are also medically formulated without phenylalanine, allowing people with PKU to safely consume them. However, these foods are typically very expensive.

Certain food types often naturally fit into a phenylketonuria diet.

It is essential for people with PKU to get enough energy or calories when following a very low protein eating plan. If they do not, it can lead to poor blood phenylalanine management.

With a phenylketonuria diet, people can maintain their energy levels by eating very low protein foods. People with PKU can eat these foods as part of their diet without measuring their intake of them. They include:

  • fats containing less than 1 gram of fat per 100 g, such as:
    • butter
    • margarine
    • ghee
    • vegetable oils
  • starches that contain less than half a gram of protein per 100 g, such as:
    • cassava flour
    • arrowroot
    • sago
    • tapioca
    • corn starch
  • vegan cheese with less than half a gram of protein per 100 g
  • sugars that contain less than half a gram of protein per 100 g, such as:
    • glucose
    • jam
    • honey
    • marmalade
    • golden syrup
    • maple syrup
    • fruit sorbets
    • sweets
  • herbs or spices
  • drinks that do not contain aspartame

A phenylketonuria diet can also include medically manufactured low protein foods such as bread and pasta.

A phenylketonuria diet can include artificial protein substitutes. These substitutes derive from phenylalanine-free amino acids supplements. They may contain added carbohydrates, fats, and vitamins.

They provide sufficient protein intake for people with PKU. They are also essential in promoting a person’s growth cycle. Available forms for consumption include:

  • amino acid powders
  • capsules
  • tablets
  • bars
  • liquids

Potatoes and cooked crisps made from vegetables such as beetroot or parsnips are concentrated in phenylalanine. They can negatively affect a person with PKU. It is advised that these foods are restricted in a person’s phenylketonuria diet.

Other fruits and vegetables that contain small amounts of phenylalanine usually do not affect a person with PKU. Healthcare professionals typically do not recommend restricting them in a phenylketonuria diet.

However, it is still important that people with PKU get professional dietary advice from a healthcare professional such as a dietitian for each specific fruit or vegetable.

Some examples of fruits and vegetables that are lower in phenylalanine include:

  • apples
  • bananas
  • blueberries
  • cherries
  • grapefruit
  • nectarine
  • avocado
  • peppers
  • eggplant
  • cabbage
  • carrots
  • tomatoes

All proteins contain phenylalanine. People following a phenylketonuria diet are typically advised to avoid foods high in protein, such as:

  • meat
  • fish
  • eggs
  • cheese
  • dairy
  • nuts and seeds
  • flour-based foods, such as:
    • bread
    • pasta
    • cakes
    • cookies
  • tofu, soya, and Quorn

Avoiding aspartame

People with PKU should also avoid the artificial sweetener aspartame. It contains high amounts of phenylalanine. Products that may include aspartame include:

  • diet sodas
  • some chewing gums
  • some medications
  • sugar substitutes
  • other low calorie products

The signs and symptoms of PKU can vary. They may be mild or severe. A person with mild PKU may not require any treatment.

Doctors typically detect that a person has PKU at birth. They work with dietitians who have PKU experience to create phenylketonuria diets.

It is important that people speak with a healthcare professional before making significant changes to their eating plan. The phenylalanine amount that’s safe to consume differs for everyone living with PKU.

It is recommended that a person with PKU work with a healthcare professional to develop a suitable eating plan specific to their needs. If a person is considering stopping their phenylketonuria diet, it is advised that they consult a healthcare professional for medical advice and follow-up visits.

People with PKU cannot break down an amino acid called phenylalanine that is found in food. If a person with PKU does not restrict the amount of phenylalanine they eat, it can lead to adverse health effects such as severe symptoms.

Following a phenylketonuria diet is the main treatment for PKU. Each phenylketonuria diet is different and tailored to an individual’s specific needs, including restricting intake of certain foods.

It is important that people get professional medical advice before starting the phenylketonuria diet.