Phenylalanine is an amino acid, one of the building blocks of proteins. It is a natural compound that is present in all proteins (vegetable or animal). The human body needs phenylalanine, as it is an integral part of all proteins in the body. Humans cannot make phenylalanine, so it is an essential component of our daily diet, without which the body cannot function.

There is a group of people who suffer from the rare hereditary disease phenylketonuria (PKU). These people lack an enzyme needed to digest phenylalanine, and surplus phenylalanine will be converted by the liver to a range of (toxic) compounds known as phenylketones, which are excreted in the urine. PKU patients who ingest phenylalanine will suffer from different toxicity syndromes, including mental retardation, especially in children and intellectual disturbances in adults.

Many sugar-free or light products contain aspartame as a sweetener. Aspartame is basically a small protein, that contains about 40% phenylalanine. Products with aspartame thus pose a risk for PKU patients. All products with aspartame thus have to be labelled with a sentence stating 'contains a source of phenylalanine'. As PKU patients generally know which products contain protein, the labelling is not necessary for these products. Only products such as chewing gum, that normally do not contain any protein but do contain aspartame are to be labelled.