Phenylketonuria (PKU)

What is PKU?

Phenylketonuria, abbreviated as PKU, is a recessive autosomal inherited metabolic disorder. Phenylalanine hydroxylase (PAH) is absent or non-functional in individuals with this condition.
Phenylalanine hydroxylase is the enzyme that breaks down phenylalanine (Phe) into tyrosine.
Phenylalanine is an essential amino acid for our body. We cannot synthesize it ourselves and must obtain it through our diet.

Due to the defective or absent breakdown, this amino acid accumulates in the blood and tissues. As a result, nerve cells are damaged, which can lead to brain damage, progressive intellectual disability, behavioral problems and epilepsy.

How is it diagnosed?

In Belgium, the heel prick test (Guthrie test) is systematically performed within the first days after birth. PKU is one of the conditions screened for. About 1 in 18.000 Belgian children has PKU. Only if both parents are carriers of the PKU gene can they have a child with PKU. If neither parent has PKU, but both are carriers, the chance of each child having PKU is 25%.

Dietary treatment

Protein-rich foods are natural sources of phenylalanine. Therefore, it is important that someone with PKU avoids these proteins.

PKU is well managed with a lifelong low-protein diet. In addition, PKU patients must take specifically developed amino acid mixtures daily and for life because a low-protein diet causes a deficiency in other essential amino acids.
The low-protein diet consists of a complete diet with a limited amount of phenylalanine.

• The degree of restriction of the amino acid phenylalanine depends entirely on the phenylalanine concentration in the serum. The amount of phenylalanine per kilogram of body weight varies individually and is partly determined in part by physiological processes such as growth. It is recommended to distribute the allowed amount of phenylalanine evenly throughout the day. The use of low-protein diet products is advisable.
• Supplementation of daily requirements of essential and non-essential amino acids via synthetic L-amino acid preparations.
• Ensure sufficient energy intake from fats and carbohydrates (to prevent the body from using protein for energy, which releases phenylalanine). Increased energy intake can be achieved by mixing dextrin-maltose, glucose, corn oil, butter, margarine, cream,… into daily food. Additional use of low-protein diet products can also contribute significantly.
• Provide sufficient vitamins, minerals and trace elements.
• Ensure enough linoleic acid in the diet.

Additionally, the sweetener aspartame must be removed from their diet. Aspartame is a source of phenylalanine and is often found in “light” products.

PKU patients are strictly monitored by the treating physician and dietician. Special low-protein foods developed for this condition can only be obtained with a prescription at the pharmacy. PKU patients are entitled to reimbursement for these products through health insurance.