Following a low-phenylalanine diet can prevent intellectual disability and other complications in people with phenylketonuria. The diet focuses on low protein foods and avoids high protein foods.

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Phenylketonuria (PKU) is a condition where a genetic mutation causes low levels or an absence of an enzyme called phenylalanine hydroxylase (PAH). PAH is normally involved in breaking down the amino acid phenylalanine (Phe).

In people with PKU, Phe accumulates in the body. Because Phe is toxic to brain tissue, children born with PKU can develop intellectual disability if PKU treatment isn’t started shortly after birth.

The main treatment for PKU is a special diet. This is aimed at reducing Phe intake while maintaining good overall nutrition. Keep reading to learn more about the PKU diet.

The PKU diet is a special diet that’s maintained throughout your lifetime. The goals of the PKU diet are:

  • Restricting natural intake of Phe: Amino acids are the building blocks of proteins, so Phe is found in protein sources. Due to this, people with PKU must follow a low protein diet.
  • Using a protein substitute: While the PKU diet aims to limit Phe intake, it’s still vital that people with PKU get other amino acids from their diet. This is typically achieved by using a protein substitute formula.
  • Maintaining a good nutrition status: It’s important that people with PKU have a balanced diet. To ensure this, you may need to take vitamins and mineral supplements or add them to the protein substitute formula.

PKU management generally aims to keep Phe levels in the blood within a range of 120–360 micromoles per liter (umol/L).

If you have PKU, your doctor will regularly monitor the levels of Phe in your blood. This involves collecting a small sample of blood from your finger and placing it on a special card. The card is then sent to a lab to be analyzed.

A dietitian will work with you to develop a PKU diet plan that meets your specific needs. They may adjust this plan periodically. For example, the Phe levels in your blood and your nutritional needs may change during growth spurts.

The importance of following a phenylketonuria diet

Researchers have found that following a low-Phe diet can reduce Phe levels and prevent intellectual disability and other neurological complications in people with PKU.

Stopping the PKU diet means that Phe can start to build up again in your body. This can lead to a variety of neurological changes, such as:

  • intellectual disability
  • difficulty at school or work
  • behavioral or mood changes
  • issues with memory, concentration, or problem-solving
  • trouble with visual-motor coordination
  • tremors
  • fatigue
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The types of foods that people with PKU can eat include:

Phe is an essential amino acid. Your body cannot produce essential amino acids naturally, and they need to be acquired through your diet. Due to this, you cannot completely eliminate Phe from your diet.

In order to receive some dietary Phe, people with PKU can eat carefully managed amounts of certain foods like potatoes or cereals.

Additionally, low protein substitutes specifically formulated for people with PKU are available for some foods and can be prescribed by your doctor. Some examples include:

  • flour mixes
  • breads
  • pastas
  • pizza bases
  • egg and milk substitutes

How to calculate the amount of phenylalanine in a food

If you have PKU, your diet plan will include a daily Phe allowance. This is the amount of Phe that you may have in 1 day. The daily Phe allowance can vary between individuals with PKU.

There are several ways to calculate the Phe content of food, and there’s little evidence to show one method is better than another. This online booklet provides helpful information about calculating protein exchanges and Phe content.

Your doctor can also provide you with a protein exchange card and recommend apps that can help you track how much Phe you’re getting.

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Because Phe is present in protein, people with PKU need to avoid high protein foods. These include:

Additionally, people with PKU should avoid the food sweetener aspartame. This is because the body converts aspartame into Phe.

The main way that PKU is managed is through a low-Phe diet. Sticking to the PKU diet can help keep Phe levels in the blood low and prevent PKU complications like intellectual disabilities and other neurological issues.

Because Phe is found in protein, this involves consuming a low protein diet. People with PKU can eat most fruits and vegetables as well as sugars, fats, and low protein starches while avoiding foods like meats, eggs, and dairy.

All dietary plans should be made with your doctor.

Phe is an essential amino acid, so it’s still important to get some Phe in your diet if you have PKU. Every individual with PKU is different and has a specific daily Phe allowance. Your doctor can teach you ways to keep track of your Phe intake each day.