Phenylketonuria (PKU) is an inherited genetic disease in which affected individuals are unable to process an amino acid called phenylalanine, leading to its toxic build up when protein-rich foods are consumed. Patients affected by PKU must therefore limit their protein intake in order to prevent serious health complications.

Our bodies require an adequate daily amount of protein in order to support growth, repair and make new cells. Proteins are digested in the stomach into small amino acids, which are the building blocks of biological organisms. There are only twenty amino acids in nature; nine of which are essential and must be obtained from the diet, while the rest can be synthesised by our bodies. When we eat too many amino acids, the liver breaks them down until they are finally excreted in the urine. Unfortunately, individuals with PKU are unable to process the amino acid phenylalanine, causing its levels to dangerously rise in the body. Since nearly every food that has protein contains phenylalanine, these individuals must carefully watch out and drastically cut their intake of many protein containing foods such as eggs, dairy, meat and nuts.

Figure 1. People with PKU must carefully watch out for protein containing foods and limit their intake. Foods such as meat and dairy are obvious sources of protein, but things like aspartame are less clear and can increase phenylalanine levels.

PKU is an autosomal recessive genetic disorder meaning that both parents must pass the faulty alleles in order for the offspring to develop the disease. A mutation in a gene located within chromosome 12 leads to the malfunction or complete absence of a liver and kidney enzyme known as phenylalanine hydroxylase. This enzyme is normally involved in the processing of phenylalanine into another amino acid called tyrosine. People suffering from PKU cannot convert phenylalanine into tyrosine, which causes a build up of the former in the blood. Since the body cannot process phenylalanine, it breaks it down through an alternate route which causes the production of compounds that are toxic to the nervous system. The build-up of these compounds adversely affects brain development and can lead to mental retardation if not treated from a very early age.

Figure 2. Individuals with PKU (right side) have a non-functioning and sometimes absent phenylalanine hydroxylase enzyme. This prevents the normal processing of phenylalanine into tyrosine.

Since consumption of mostly any protein food causes a dangerous build up of phenylalanine in people with PKU, they must refrain their intake to avoid damage to the nervous system. Unfortunately there is no cure for this disease and the primary treatment is to follow a restrictive protein diet that provides just the right amount of phenylalanine that is needed for health, but no more. Such a diet is very difficult to implement and must only be devised by a good nutritionist or dietician.

Complete avoidance of most protein foods means that people who follow this diet will have deficiency of amino acids which are needed for important bodily functions. Additionally, many protein foods have many vital nutrients that cannot be found elsewhere in high quantities: meat and dairy has vitamin B12 and iron; while oily fish has omega-3s and vitamin D. This means that individuals following this restrictive diet are likely to be deficient in many essential nutrients. Therefore, PKU individuals on a phenylalanine free diet are indicated to take a fortified powder formula on a daily basis which provides all the minerals and vitamins that would otherwise be missing. Additionally, the formula provides essential and non-essential amino acids, except for phenylalanine. All of this ensures that individuals with PKU can have an adequate intake of essential nutrients to carry a normal and healthy life.