Phenylketonurie (PKU)

The hereditary metabolic disease phenylketonuria (PKU) occurs only rarely, but if a child develops the disease, it requires consistent nutrition from the very first minute in order to prevent damage to the development of the brain and the resulting complications.

Phenylketonurie (PKU)

What is phenylketonuria?

Phenylketonuria is an inherited metabolic disease in which a certain protein component accumulates in the body, which particularly restricts brain development in children. See electronicsencyclopedia for Slang Peutz-Jeghers Syndrome.

Statistically, only one child in every 10,000 born in the Federal Republic of Germany is affected. If the disease is diagnosed early in this child, phenylketonuria can be treated and the child allowed for fully normal development.


Phenylketonuria is caused by a genetic defect. This means that the protein component phenylalanine, which is found in all protein-containing foods and is therefore ingested with food, can no longer be broken down. In healthy people, an enzyme is active for this, which in people with phenylketonuria, depending on the severity, only works partially or has completely stopped working.

Symptoms, Ailments & Signs

If the disease is not recognized and treated immediately after birth, the first signs appear around the age of three months. Brain maturation is disturbed, the brain does not grow adequately, which becomes visible in older children with a significantly smaller head. Mental development lags behind. When they reach puberty, there are severe mental disabilities.

Due to the damage to the brain cells, the children suffer from hyperexcitability, which often causes epileptic seizures. Muscle cells are also damaged by the disease. The musculature tenses spasmodically (spasticity), which makes normal movements more difficult. Furthermore, behavioral disorders in the form of hyperactivity and aggressiveness occur. Children tend to have uncontrolled outbursts of anger.

A typical sign of PKU is the smell of acetone that sufferers give off. It is caused by the formation of phenyl acetate in the body. This substance is excreted through the sweat glands and is also found in the urine. Characteristic of the appearance of those suffering from PKU.

Since their organism cannot produce enough of the pigment melanin, they often have white-blonde hair, very light sensitive skin and light blue, transparent shimmering eyes. The pigment disorder can cause eczema-like skin rashes. The severity of the symptoms depends on how severely the metabolism is restricted and deviates from the norm.

Diagnosis & History

Phenylketonuria can be diagnosed via a blood test, which is usually carried out during newborn screening as part of the so-called U2 check-up. During the blood test carried out here, the phenylalanine level of the infant is determined, among other things.

Since the metabolism is disturbed in the case of a disease with phenylketonuria and the phenylalanine can no longer be broken down, it is increasingly deposited in the blood and can be detected here. An elevated phenylalanine level of more than 1 to 2 milligrams per deciliter in the blood indicates a disease with phenylketonuria.

If phenylketonuria is suspected during pregnancy, a DNA test of the infant can be carried out prenatally using an amniotic fluid test and a diagnosis can be made. If phenylketonuria has been diagnosed, further investigations are then carried out into the severity of the disease, on which the treatment is geared.

If phenylketonuria is not treated, the first symptoms of the metabolic disease appear after about three months, which quickly lead to serious complications, including delayed mental development up to severe mental disabilities, disturbed muscle tension up to epileptic seizures.


In any case, phenylketonuria must be treated immediately after the birth of the child. If this treatment is not carried out, it can lead to serious problems with the growth and development of the child, which can no longer be compensated for later. Those affected usually suffer from a metabolic disorder due to phenylketonuria.

This also leads to increased irritability, so that the children appear aggressive. A delay in motor and mental development can also become apparent and significantly reduce the quality of life of those affected. In many cases, the parents also suffer from depression or other mental disorders. Those affected also show an unpleasant body odor, so that affected children and young people can become victims of teasing or bullying.

Epileptic seizures also occur, which in the worst case can lead to death. Pigment disorders can also occur in children. The treatment of phenylketonuria is not associated with complications. Proper nutrition can prevent these undesirable developments, so that there are no more complications in adulthood.

When should you go to the doctor?

Since phenylketonuria can cause severe damage to the child’s brain, this disease must be treated immediately in any case. Early diagnosis is very important to prevent further complications or impaired development of the child. As a rule, phenylketonuria becomes noticeable through a slower development of the child, mental disabilities and epileptic seizures also play a role.

In the event of an epileptic seizure, call an ambulance or go to the hospital immediately, as this is the only way to prevent further damage. At a young age, phenylketonuria often leads to outbursts of anger or severe behavioral disorders. If these symptoms also occur, a visit to a doctor is necessary. Pigment disorders or spots on the skin can also indicate this disease.

The treatment of phenylketonuria depends on the exact symptoms and is carried out by different specialists. Psychological treatment, in which parents or relatives can also take part, is often useful. With early diagnosis and treatment, the life expectancy of the patient is usually not negatively affected.

Treatment & Therapy

If phenylketonuria has been diagnosed, it can usually be treated quite well, so that children do not have to suffer from maldevelopment of the brain and instead develop completely normally mentally.

However, this is only possible with a consistently adhered to low-phenylalanine diet. There is no other treatment option, for example medication, to treat phenylketonuria and avoid damage to the brain. It is particularly important to almost completely abstain from foods containing phenylalanine during the period during which the brain is developing – i.e. in the period from birth to puberty.

The earlier the diet is started, the better the course of phenylketonuria. For this reason, the diet is usually started as soon as a newborn is born, and infants are given a special infant formula that contains very little phenylalanine in place of breast milk. Even if a low-phenylalanine diet is particularly important up to puberty, doctors recommend a lifelong diet low in phenylalanine in order not to be impaired by the disease into old age.

However, it is important in the diet for the therapy of phenylketonuria that the diet contains little phenylalanine but does not completely prevent the absorption of phenylalanine. Because phenylalanine is a vital amino acid that is particularly necessary for the growth of children suffering from phenylketonuria. However, a certain value in the blood should not be exceeded. That is why people with phenylketonuria need regular blood tests.

Outlook & Forecast

The prognosis for classic phenylketonuria is extremely good if the diagnosis is made early, preferably in the newborn child. If a low-phenylalanine diet is administered in early infancy, no physical or mental impairments are to be expected. Those affected lead normal lives and have an average life expectancy.

An important prerequisite for a positive prognosis is strict adherence to the diet, especially in the first six years of life, when the brain develops particularly well. Women with phenylketonuria can also become pregnant and have children of their own. Although it is a hereditary metabolic disease, no harm is to be expected for the child. However, the diet must be strictly adhered to during pregnancy.

Without the low-phenylalanine diet, the disease already leads to disorders of brain development in early childhood. Neurological abnormalities can already be seen from the 4th month of life. Since the brain damage is irreversible, there is no prospect of a cure. The intelligence quotient will usually be permanently below the norm. Seizures, behavioral disorders and motor disabilities can also occur.

A special case is the atypical form of phenylketonuria, which is characterized by a lack of coenzyme BH4 (tetrahydrobiopterin). The prognosis of mental development in these patients cannot be assessed consistently. Despite an early diet, damage to the nervous system can occur.


Since phenylketonuria is an inherited metabolic disease, there is no way to prevent phenylketonuria. However, people with phenylketonuria can prevent the symptoms and complications that usually occur with phenylketonuria by eating a diet containing foods low in phenylalanine.

In order to protect their own children from the consequences of a disease with phenylketonuria, affected persons who are already pregnant, whose partner is pregnant or who are planning a pregnancy, should strictly adhere to a low-phenylalanine diet. If the child falls ill, care should also be taken to ensure that the child eats a diet low in phenylalanine.


Since phenylkentonuria is one of the hereditary metabolic disorders and cannot be cured causally, but only treated, aftercare and treatment are largely identical. This primarily includes a low-phenylalanine diet and regular monitoring of the PA level in the blood. These measures must be strictly adhered to up to the age of six, but they are necessary for life.

In the case of atypical phenylketonuria, the coenzyme BH4 must also be supplemented. If the treatment was started too late, the aftercare can also extend to existing damage. These are usually in the area of ​​brain development and can vary in severity.

Here, the early start of support measures (e.g. ergotherapy) can be useful to counteract behavioral disorders. These include sleep problems, compulsive control, self-harm or antisocial behavior, the severity of which can often be successfully mitigated. In the case of motor disorders, targeted physiotherapy can improve the symptoms.

If the prescribed measures are observed, many patients are now able to lead a largely normal life. When you reach puberty, the most sensitive phase is over. Larger, neurological defects are then usually no longer to be expected, even if the brain chemistry can be temporarily impaired by fluctuations in the PA level. The supply of dopamine preparations helps against this.

You can do that yourself

In classic phenylketonuria, adherence to a lifelong vegan diet without the source of phenylalanine aspartame is paramount. Newborns are fed only phenylalanine-free formula. Fruit and vegetable jars are suitable as supplementary food. Parents of children suffering from PKU must calculate the phenylalanine content of all foods fed and strictly adhere to limit values.

Later, the baby food is replaced by a protein powder specially developed for this disease. The amino acid blend for PKU patients must be added to each meal. It forms the lifelong main component of the necessary low-phenylalanine diet. It is therefore advisable to get the toddler used to the smell and taste of the powder in a playful way.

There is now a wide range of training and cooking courses for affected children of all ages. These pedagogical measures promote personal responsibility step by step. At the same time, they support a positive approach to nutritional restrictions. Finished products low in phenylalanine enable the little patients to take part in excursions and school trips. Parents of sick children are exposed to strong organizational and psychological stress. It is helpful for them to exchange ideas with fellow sufferers in self-help groups.

Adult phenylketonuria patients who wish to have children must exercise particular care when planning a family: Only compliance with a strict diet ensures normal phenylalanine levels during pregnancy. Normal values ​​are absolutely necessary at the time of conception in order to avoid serious damage to the unborn child.