What is phenylketonuria or PKU?
Phenylketonuria is a hereditary metabolic disease that is diagnosed via the heel prick test after the birth of a child. This heel prick test determines whether the child’s blood contains too much of the substance phenylalanine. 

Long-term excess phenylalanine causes brain damage and adversely affects a child’s mental and physical development. To prevent this, a child must follow a diet.

If the dietary instructions are followed properly, the child can develop normally like any other child.

What kind of substance is phenylalanine?
Our daily diet consists of all kinds of products such as bread, milk (products), meat, vegetables, potatoes and so on. These products contain various nutrients: including proteins, fats, carbohydrates, water and vitamins. Our body uses these nutrients as building material, fuel and reserve.

Proteins are especially important as a building material. They are needed to build new cells and replace old cells. Cells form the building blocks of our body. Our body is made up of cells. We also speak of muscle cells, liver cells, skin cells and so on.

We can see proteins as long chains, which are made up of different beads. Each type of protein has its own amount of ‘beads’ in a specific order. We call these ‘beads’ amino acids.

Before the proteins from our food can be used as building material, they must first be digested in the stomach and intestines. The long chains are cut into pieces. The amino acids released are absorbed from the intestine into the blood. One of those amino acids is phenylalanine or phenylalanine (usually abbreviated as ‘Phe’).

Phe is found in all foods that contain proteins.

Because our daily diet contains more proteins than we need, we consume a surplus of amino acids. And therefore also to Phe.

What happens in patients with PKU?
In healthy people, excess Phe is converted into another amino acid: tyrosine. An enzyme is required for this. In patients with PKU, the enzyme that converts Phe into tyrosine is insufficient or not active at all. The body is therefore not/insufficiently able to break down Phe.

This creates an excess of Phe in the blood. The body tries to get rid of this in other ways. This produces so-called phenylketones, waste products of Phe that occur in the blood and urine of untreated PKU patients. This gives the urine a musty smell.

The diet
The diet is composed in such a way that the child receives sufficient amounts of all nutrients and exactly the right amount of Phe. 

This food should certainly not contain too much Phe, nor too little. Because although too much Phe harms development, too little Phe is not good for the child either! He needs it together with the other amino acids to build his cells.

This diet is called ‘a phenylalanine-restricted diet’. But because Phe is present in all proteins, in practice we do not use Phe, but protein. That is easier and more practical.

During the first few months, the child’s diet consists of a special diet food, combined with breastfeeding or regular infant formula.

Later, the child’s diet consists partly of normal foods, supplemented with special low-protein diet products. The child should not use products that contain a lot of protein (and therefore a lot of Phe), such as cheese, meat, chicken, fish and eggs. He may eat small or normal portions of other products. For example, think of potatoes, rice, cookies and chips. The quantities of these products must be weighed or measured.

The child may use products that contain little or no protein ‘unlimitedly’. Consider fruit, for example. In addition, there are special low-protein diet foods that can replace ‘normal’ products, such as low-protein bread, low-protein macaroni and low-protein biscuit. Nutricia has a number of these products. These products are available from facilities companies, among others. 

The result is a protein-restricted diet with just the right amount of Phe. However, this does result in a deficiency of other amino acids, vitamins and minerals. This deficiency is supplemented by food for medical use, namely amino acid preparations. These contain all amino acids (except phenylalanine of course) as well as vitamins and minerals. 

This food for medical use is recommended by a doctor or dietician and should be used under medical supervision.