Why do products which contain aspartame have the label 'Contains a source of phenylalanine' on the packaging?

Phenylalanine is an essential amino acid. That means that it is, an amino acid which our bodies cannot make and which we must obtain it from our diet. It is also one of the amino acids which is used to make aspartame. Phenylalanine is found in all protein-containing foods including, cheese, eggs, meat, fish and mothers’ milk.

Products which contain aspartame have a label which says 'Contains a source of phenylalanine'. This label is there to help people with the rare inherited genetic disorder phenylketonuria (PKU). These people cannot metabolize phenylalanine from any source and need to follow a very strict diet to avoid having more than a small amount of this amino acid. (Children who are diagnosed as suffering from PKU do, in fact, need a very small amount of phenylalanine to grow and develop properly.)