Nutrition in Children With Phenylketonuria

One of the first questions families ask after a baby is diagnosed with phenylketonuria is usually this:

“What will my child be able to eat from now on?”

Nutrition in phenylketonuria is not an ordinary “low-protein diet.” While enough energy, protein, vitamins and minerals are provided to support the child's growth and development, phenylalanine intake has to be regulated in a controlled way.

For this reason, nutritional treatment in children with phenylketonuria is one of the core parts of managing the condition, and it should be planned according to the child's age, growth, blood phenylalanine levels and individual phenylalanine tolerance.

What Is Phenylketonuria (PKU)?

Phenylketonuria, or PKU for short, is an inherited metabolic disease.

Phenylalanine is one of the basic amino acids found in the structure of proteins.

In phenylketonuria, the activity of the phenylalanine hydroxylase (PAH) enzyme, which converts phenylalanine to tyrosine, is usually insufficient.

As a result, phenylalanine rises in the blood.

When left untreated, high phenylalanine levels can be harmful, especially for the developing brain, and can affect neurological development negatively.

For this reason, early diagnosis and treatment are extremely important in PKU.

How Is Phenylketonuria Diagnosed?

Phenylketonuria is one of the conditions that can be detected early through the newborn screening program.

Finding a high phenylalanine level in the heel-prick blood sample does not mean that the diagnosis is directly confirmed. In babies with a suspicious screening result, the necessary confirmatory tests are performed and the diagnosis is established.

Once the diagnosis is confirmed, starting treatment as early as possible is important for neurodevelopment.

Why Is Nutritional Treatment Necessary in Phenylketonuria?

Phenylalanine is not a substance that is completely unnecessary for the body.

On the contrary, it is an amino acid needed for growth and protein synthesis.

The problem is that children with phenylketonuria cannot metabolize phenylalanine normally.

For this reason the aim is not to remove phenylalanine completely;

it is to provide the amount the child needs for growth while preventing excessive rises in the blood.

This balance is different for every child.

How Are Children With Phenylketonuria Fed?

Nutritional treatment in classic PKU rests on three elements:

Controlling natural protein and phenylalanine intake

Using phenylalanine-free or low-phenylalanine medical protein/amino acid products

Adding special low-protein foods to the daily diet when needed

Beyond these, the nutrition plan must also provide enough energy, vitamins, minerals and other nutrients.

Therefore, simply removing foods that contain protein does not create a correct PKU diet.

Is Protein Completely Forbidden in Phenylketonuria?

No.

This is an important distinction.

Protein is needed for a child's growth and for the renewal of body tissues.

However, because a significant portion of natural proteins contains phenylalanine, the amount of natural protein is given in a controlled way.

A child's tolerance for natural protein and phenylalanine can vary according to;

• The severity of PKU
• Their age
• Their rate of growth
• Their weight
• Daily energy intake
• The medical products used
• Blood phenylalanine levels

and other factors.

For this reason it is not right to apply another person's PKU diet amounts directly.

Which Foods Do Children With Phenylketonuria Eat in Limited Amounts?

Natural foods high in protein contain significant amounts of phenylalanine.

These include;

• Meat
• Chicken
• Fish
• Eggs
• Milk and dairy products
• Nuts
• Legumes
• A significant portion of regular flour and grain products

among others.

In classic PKU, some of these foods may be very strictly limited or eaten according to the daily natural protein allowance.

However, the approach of “the same list of forbidden foods for every person with PKU” is not correct.

Which food can be eaten and in what amount should be determined by the metabolic team according to the child's individual phenylalanine tolerance.

Are Vegetables and Fruit Unrestricted?

Not always.

Vegetables and fruit generally contain less phenylalanine than animal protein sources, but they are not all the same.

Some vegetables and fruit contain small amounts of phenylalanine and can be used more freely within the nutrition plan, while others may need to be included in the calculation.

For this reason it is not correct to set a general rule such as “all vegetables and fruit are unlimited.”

Amounts should be arranged according to the child's diet plan.

What Is Medical Food in Phenylketonuria?

A significant part of the protein needs of children with phenylketonuria can be met with special medical products.

These products are usually;

• Amino acid mixtures that contain no phenylalanine or very little phenylalanine
• Age-appropriate medical formulas
• Special products that support protein and micronutrient requirements

in form.

These are not a “supplement” in the usual sense.

They are one of the core components of PKU treatment.

Especially in childhood, using these products regularly is important for ensuring adequate intake of protein, tyrosine, vitamins and minerals.

Why Are Special Low-Protein Foods Used?

Regular bread, pasta, flour, rice and similar products contain a certain amount of natural protein and phenylalanine.

In the PKU diet, in order to make it easier to meet energy needs and to increase variety in eating;

• Low-protein bread
• Low-protein pasta
• Special flours
• Low-protein grain products

and other foods for special medical purposes can be used.

Why Does Aspartame Matter in Phenylketonuria?

The sweetener called aspartame releases phenylalanine when it is broken down.

For this reason, people with phenylketonuria need to be careful with products that contain aspartame.

Some;

• Sugar-free drinks
• Chewing gums
• Diet products
• Sugar-free desserts
• Medications or supplements

may contain aspartame.

Product labels should therefore be checked.

Products containing aspartame may carry a warning that they are a source of phenylalanine.

Why Is Reading Labels Important for Children With Phenylketonuria?

Looking only at the name of a food may not be enough.

Families need to learn to check;

• The amount of protein
• The ingredient list
• Whether it contains aspartame
• The special protein sources used

on the label.

Because the contents of packaged products in particular can change over time, the label of a product that has been used before should also be checked again.

How Is the Amount of Phenylalanine Calculated?

In the PKU diet, some foods may need to be included in the daily plan by calculating;

• Their protein amount
• Their phenylalanine content
• Their portion size

in advance.

The amount of phenylalanine a child can consume daily is not fixed, and a number given for another child cannot be used.

This amount is adjusted by the metabolic team according to blood phenylalanine levels.

Why Are Blood Phenylalanine Levels Monitored?

One of the main ways of assessing whether nutritional treatment is adequate is regular measurement of blood phenylalanine.

Depending on the result;

• The daily amount of natural protein
• Phenylalanine intake
• The amount of medical product
• Total energy and protein intake

can be readjusted.

Current guidelines also continue to emphasize the importance of lifelong metabolic control.

Why Can Phenylalanine Rise When a Child Is Ill?

During infection, high fever or insufficient energy intake, the body may start breaking down its own proteins.

This catabolic state can cause blood phenylalanine levels to rise.

For this reason, during periods of illness, it is important to;

• Maintain adequate energy intake
• Continue using the medical amino acid/protein product as much as possible
• Ensure fluid intake

for the child.

If the child is vomiting, unable to eat or the illness is prolonged, the metabolic team should be contacted.

Can a Child With Phenylketonuria Grow Normally?

Yes.

Children who are diagnosed early, start treatment in time and maintain good metabolic control can show normal growth and development.

For this, the diet must be adequate not only in terms of phenylalanine but also in terms of;

• Energy
• Protein
• Vitamins
• Minerals
• Fatty acids

and other nutrients.

For this reason PKU nutrition should not be seen only as “a diet made up of restrictions.”

Can Vitamin or Mineral Deficiencies Occur on the PKU Diet?

They can.

Significantly limiting natural protein sources can increase the risk of inadequate intake of some nutrients such as;

• Vitamin B12
• Iron
• Calcium
• Zinc
• Vitamin D
• Selenium

and others.

However, a significant portion of current medical amino acid products is enriched with various vitamins and minerals.

The risk of deficiency should be assessed according to the foods the child eats and the medical products they use.

For this reason, rather than using supplements casually, regular nutritional and laboratory follow-up should be carried out.

How Long Does the Phenylketonuria Diet Last?

The current approach is for treatment to be continued for life.

The current ACMG guideline also strongly recommends that phenylalanine control be maintained for life in PAH deficiency requiring treatment.

Therefore the idea that “the diet ends once the child grows up” is not correct.

Does the Diet Change as the Child Grows?

Yes.

The nutrition plan changes throughout;

• Infancy
• The transition to complementary foods
• The preschool period
• School age
• Adolescence

as the child develops.

Because a child's energy and protein needs change as they grow, natural phenylalanine tolerance and the amounts of medical food used may also be readjusted.

For this reason it is not correct to keep using the same diet list that was given years ago indefinitely.

How Is the Transition to Complementary Foods Handled in Babies With Phenylketonuria?

The transition to complementary foods is an important period for normal development in babies with PKU as well.

However, new foods need to be given with;

• Their protein content
• Their phenylalanine amount
• Their portion size

taken into account.

The aim is to let the baby meet different tastes and textures while maintaining metabolic control.

During this period it is important for the family to be taught how to measure foods, portion them and keep records.

How Should a Child With Phenylketonuria Eat at School?

For a child starting school, managing nutrition only at home is not enough.

It helps for the school to know;

• The child's PKU diagnosis
• Their special nutrition plan
• Why foods with normal protein content are limited
• The importance of the child's medical products

as part of daily care.

As they get older, the child should also start to understand their own condition and nutrition.

The aim is not for the child to take on all the responsibility at once, but to gain the ability to manage their own treatment in a way that suits their age.

Can a Child With Phenylketonuria Eat Out?

Yes, but it requires planning.

In restaurants;

• Ingredient information
• Portion size
• Protein sources such as milk, cheese, meat or egg added to a dish

may not always be clear.

Families can contact the restaurant in advance when needed, carry the child's suitable low-protein products with them and adjust the portion according to the daily plan.

It is important that PKU does not become a condition that completely restricts social life.

Are There Medication Treatments in Phenylketonuria?

Yes.

PKU treatment is no longer limited to diet alone.

In some patients, depending on the genetic and biochemical features of the condition, treatments such as sapropterin can increase phenylalanine tolerance.

Depending on age and clinical features, other pharmacological treatments may also come into consideration.

However, responding to a medication treatment does not always mean “the diet is no longer needed.”

The nutrition plan is readjusted according to the response to treatment.

Who Should Manage Nutrition Follow-Up in PKU?

Phenylketonuria requires multidisciplinary follow-up.

Follow-up may involve;

• A pediatric metabolic specialist
• A dietitian experienced in metabolic diseases
• A pediatrician
• A psychologist and other health professionals when needed

working together.

The child's;

• Height and weight
• Rate of growth
• Neurodevelopment
• Blood phenylalanine levels
• Food intake
• Use of medical products
• Vitamin and mineral status

should be monitored regularly.