PKU (phenylketonuria): a metabolic disease

Phenylketonuria (PKU) is a hereditary condition involving a metabolic disorder. In the Netherlands, all babies receive a heel prick within 8 days after birth, which also examines the presence of this disease. PKU is incurable, but it can be treated. The child will have to follow a special low-protein diet for the rest of his life. If this is not done, it will lead to imbecility. What exactly is PKU and what is life like with this condition?

What PKU entails

Phenylketonia, abbreviated PKU, is an inherited metabolic disease. The liver cannot process one component of protein or cannot process it sufficiently. As a result, the amino acid (part of the protein) called phenylalanine will not be processed and it will accumulate in the blood. This causes brain development to be hampered. If left untreated, this will lead to dementia. This will then lead to a clear delay within a year and ultimately serious brain damage will occur. About 1 in 18,000 babies born in the Netherlands have PKU.

The heel prick

All babies born in our country are tested within 8 days using the heel prick. By means of a small prick in the heel, a small amount of blood is taken in which the phenylalanine value is determined. If the value of this amino acid is high, more tests will follow to determine whether PKU is present. It is a big shock for most parents, because they have sometimes never heard of it. Once the diagnosis has been made, treatment is started immediately. The baby is given a preparation to reduce the high level of phenylalanine. This prevents the brain from being damaged.


PKU is treated by preventing the accumulation of the amino acid phenylalanine in the blood. This amino acid is a natural component that occurs in almost all foods. Therefore, someone with PKU must follow a very strict diet that is poor in protein for the rest of their life. As a result, the patient will not receive enough nutrients. That is why he must take a special preparation that contains all the missing amino acids and minerals so that the body and brain can function and grow properly. Constant medical supervision and a dietician are also required. The blood should be examined regularly. The patient can prick himself at home and send the blood.


Fortunately, in our country PKU is already detected during the heel prick test. This was not the case in the past. If not treated, serious brain damage will occur. This is already clearly noticeable at the age of one year, because there is a delay in development. Other symptoms to tell if someone has PKU include:

  • Delay in development of psychological and social skills
  • Dementia
  • Spasticity in arms or legs
  • Eczema
  • Smaller skull
  • Epileptic
  • Hyperactive (ADHD)
  • Lighter skin color, blonde hair and blue eyes
  • Constantly shaking one or more body parts
  • Unusual position of the hands


Low protein diet

Foods that contain a lot of protein include milk (products), meat, chicken, fish, eggs, nuts, legumes and grain products. Proteins are needed in the body for growth, functioning of all kinds of processes and repair of tissue. Patients with PKU do need phenylalanine, but only in small amounts. Because the disease makes it difficult for the body to break down proteins, the patient is given a low-protein diet. Someone with PKU should not eat meat, fish, cheese, milk, eggs, bread or chocolate at all. For example, fruit and vegetables also contain some protein, but to a lesser extent. A PKU patient may eat this in limited quantities. Everything the patient eats must be weighed precisely. Nowadays there are diet products specifically for the PKU disease available. These products are basic products such as pasta, flour and rice. Whether it tastes good depends on how you prepare the meal.

Example of the diet

Below is an example of what a PKU patient’s diet might look like in a day.
Breakfast : 2 slices of low-protein bread, spread with diet margarine, 1 sandwich with sandwich spread (0.2 grams) and 1 sandwich with fruit sprinkles, 1 slice of gingerbread (0.6 grams) and 20 grams of an amino acid preparation dissolved in 50 ml of a double drink. Later in the
morning : Drink 1 cup such as water, tea, lemonade, soft drinks, broth or fruit juice. Possibly a low-protein cookie, low-protein candy or a piece of fruit.
Lunch : 2 slices of low-protein bread and 1 low-protein cracker, spread with diet margarine. Topped with sandwich spread (0.2 grams), fruit sprinkles and jam. Packet of Loprofin drink (= low-protein milk), 20 grams of amino acid preparation dissolved in 50 ml of double drink.
Later in the afternoon : Drink 1 cup of water, tea, lemonade, soft drinks, broth or fruit juice. Possibly a low-protein cookie, low-protein candy or a piece of fruit.
Dinner : 3 boiled potatoes (150 grams), a sauce spoon of gravy, 2 vegetable spoons of vegetables, a little apple sauce, 100 ml of custard (2 grams of protein per 100 ml), 25 grams of amino acid preparation dissolved in 50 ml of fruit juice.
Later in the evening : Drink 1 cup of water, tea, lemonade, soft drinks, broth or fruit juice. Possibly a low-protein cookie, low-protein candy or a piece of fruit.


If both parents are carriers of the PKU gene, the chance that their child will have it is 25 percent. A couple may decide to first find out whether they are carriers of the gene before starting to have children. To test for PKU, there are three important tests: the enzyme test, chorionic villus sampling or PKU screening. The enzyme test can be used to determine whether the parents are carriers of the gene. The chorionic villus sampling is done on a pregnant woman to see whether the fetus has the PKU gene. The PKU screening is the heel prick, which is mandatory in most countries.

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