Phenylketonuria – causes, diagnosis, treatment. Diet in phenylketonuria

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Phenylketonuria is a rare hereditary disease that causes serious disturbances in metabolism and destabilizes the work of its systems. People suffering from phenylketonuria struggle with disturbances in the transformation process of phenylalanine – one of the important amino acids. How does phenylketonuria affect everyday life? What is the mechanism of this disease and its treatment?

Phenylketonuria – definition and characteristics of the disease

Phenylketonuria – in the professional literature also called phenylpyruvic oligrophrenia or simply PKU (from PhenylKetonUria), and in a mild form hyperphenylalaninemia – is an inherited metabolic disorder of genetic origin.

It is included in the diseases known as enzymopathies. Phenylketonuria is a disorder of the metabolism of the phenylalanine amino acid caused by a mutation of the gene responsible for the production of the enzyme – phenylalanine hydroxylase. This enzyme regulates the metabolism of phenylalanine – when it is not enough, excess of this amino acid accumulates in the patient’s blood, which leads to a number of abnormalities.

Let’s take a closer look at the mechanism of this disease and try to explain it in simpler language.

One of the ingredients of our food is protein: vegetable (its good sources are, for example, legumes) and animal (its sources include fish, meat and dairy products). These proteins are made of tiny molecules that break down into smaller elements – amino acids, during digestion. Our body can use them in various ways, for example to build muscle or create other important substances such as hormones.

Patients suffering from phenylketonuria are deficient in the enzyme responsible for the transformation of one of the essential essential amino acids: phenylalanine. This defect causes phenylalanine cannot be converted into another chemical e.g. into tyrosine necessary to build thyroid hormones or into melanin – a substance with the nature of a skin pigment.

As a consequence, phenylalanine and the products of its abnormal metabolism accumulate in the blood of a patient with phenylketonuria. The level of unconverted or incorrectly converted phenylalanine in the blood begins to rise, which in turn leads to disorders and malfunctioning of the body. People with phenylketonuria are primarily exposed to various types of neurological disorders and serious brain damage.

Find out more: Examples of diseases caused by monogenic disorders

Phenylketonuria – causes

Phenylketonuria is caused by a mutation in the gene responsible for coding the enzyme phenylalanine hydroxylase, which is located on the 12th chromosome. The disease is inherited recessively – so it can only occur if the phenylketonuria allele is present in both parents.

If both the father and mother have or are carriers of the condition, the child may:

1. be born healthy (25% probability);
2. be born sick (25% probability);
3. born as a carrier of the disease (50% probability).

If only one parent has phenylketonuria or a genetic burden towards it, there is certainty that the baby will be born healthy.

Phenylketonuria – symptoms and diagnosis

A child with phenylketonuria does not have any disturbing symptoms during the first weeks of life. The first symptoms usually appear around 3 months of age and may gradually worsen. Children with this disease usually have a typical phenotype: they have fair hair, a fair complexion and light blue eyes.

To typical early symptoms of phenylketonuria and include:

1. hyperactivity;
2. behavioral disorders;
3. a characteristic “mousy smell” emitted by a toddler;
4. psychomotor development disorders in a child;
5. decreased muscle tone;
6. vomiting and convulsions;
7. skin rashes of unknown origin.

It is worth knowing that in the case of phenylketonuria, early diagnosis and implementation of symptomatic treatment, including an appropriate diet, is extremely important. Therefore, in Poland tests for phenylketonuria are part of newborn screening tests carried out on the third day of life.

Such a set of tests allows for the quick detection of diseases and congenital defects in children – apart from phenylketonuria, there are, among others, tests for cystic fibrosis, congenital adrenal hyperplasia or congenital hypothyroidism. Taking a blood sample for tests is quick and painless for the child – the blood is collected from the toddler’s heel on a special tissue paper. The results of the screening tests are usually available around 3 weeks after the baby is born.

If the test for phenylketonuria is positive, it is also important to determine what the clinical form of the disease is. The clinical severity of phenylketonuria is classified based on the blood phenylalanine concentration related to the activity of phenylalanine hydroxylase (PAH):

1. classic phenylketonuria – blood phenylalanine level on a normal diet above 1200 μmol / l (PAH activity below 1%);
2. phenylketonuria mild – blood phenylalanine level on a normal diet 600-1200 μmol / l (PAH activity 1-3%);
3. mild hyperphenylalaninaemia – blood phenylalanine levels on a normal diet below 600 μmol / l (PAH activity 3-6%).

Also read: All newborns screened for rare metabolic defects

What can untreated phenylketonuria lead to?

In people who did not receive symptomatic treatment at an early stage of life, other disturbing symptoms, especially neurological ones, may appear as a result of irreversible changes in the nervous system. Untreated phenylketonuria can result in, for example:

1. behaviors that are close to autism, e.g. fear of others;
2. sleep problems;
3. problems with nervous tension;
4. gait disturbance;
5. impaired eye-hand coordination;
6. speech disorders;
7. aggression towards oneself and other people;
8. problems with concentration and attention;
9. epileptic seizures;
10. the development of intellectual disability;
11. psychosis.

Phenylketonuria – treatment and prognosis

Today we do not know of any effective cure for phenylketonuria – due to the genetic basis of this disease, there is no possibility of causal treatment. However, specialists emphasize that if the disease is detected at an early stage, it is possible to significantly inhibit its effects.

Hence the necessity of screening newborns – after confirming the diagnosis, symptomatic treatment can be implemented. Due to the specificity of the disease, which is phenylketonuria, it is crucial to follow a strictly defined, restrictive elimination diet.

Fenyloketonuria – diet

A child suffering from PKU is unable to process phenylalanine and stores it in excess. The diet of a toddler struggling with this disease should therefore be eliminative and limit the supply of phenylalanine. The goal of such therapy is to maintain blood phenylalanine levels within the range of 100 – 350 µmol / L in the first 5 years of life and <600 µmol / L throughout childhood.

In order to stop the negative impact of phenylketonuria on the child’s body as soon as possible, the diet should be introduced as soon as possible – in the first 2 weeks of a child’s life. Although breastfeeding a child is acceptable, it is considered that a much better solution is to reach for special protein mixtures with a reduced amount of this amino acid, dedicated to children with phenylketonuria. A good preparation of this type is e.g. casein hydrolyzate with a low content of phenylalanine and higher content of tyrosine.

As for older children, foods rich in phenylalanine, such as:

1. bread and confectionery;
2. eggs and dairy products: cheese, milk, dairy products;
3. meat, cold cuts, fish;
4. legumes;
5. nuts, chocolate, cocoa.

In a small amount – under the supervision of an attending physician and an experienced dietitian – are, however, fruit, vegetables, potato crisps and special low-protein products (with a reduced phenylalanine content) intended for patients with phenylketonuria – e.g. low-phenylalanine flour baked goods. However, you can consume without restrictions, among others: honey and sugar, vegetable oils or mineral waters.

Due to the restrictive nature of the diet and the large number of prohibited products, it is worth consulting a good dietitian on this matter. He will tell you how to compose meals so that they properly meet the needs of various nutrients. It will also help you find good and trusted producers of low-protein foods.

Although it is not easy, a restrictive elimination diet should be followed throughout childhood. It can be slightly liberalized and extended in adolescence (12-14 years), when the brain stops developing at such a high pace.

However, it should be remembered that the brain of a person with phenylketonuria is exposed to the neurotoxic effects of phenylalanine throughout his life. Adults suffering from this disease should also follow a low-phenylalanine diet to prevent the accumulation of this substance in the brain and possible damage to the nervous system.

Also check: Millions of Poles suffer from “rare” diseases

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