Aspartame is a dipeptide of two amino acids, aspartic acid and phenylalanine. These amino acids are abundant in the food supply and are found in all protein-containing foods, including eggs, meat, cheese, fish, cereals, fruit and milk. When aspartame is digested, it is broken down to aspartic acid, phenylalanine and a very small amount of methanol.
Phenylalanine is an essential amino acid, which means that the human body cannot make it itself (unlike non-essential amino acids), and that people must obtain it from their diet in order to remain healthy.
Phenylketonuria (PKU) is a very rare disease. About 1 person in 10,000 suffers from it. PKU can only be inherited. In North America, in Europe and in most countries around the world, babies are tested for PKU shortly after birth. Sufferers of PKU either cannot metabolize phenylalanine or have a restricted ability to do so. Such patients have to follow a special diet during their childhood and adolescence to grow and develop healthily and to reach adulthood. Women with PKU have to monitor their diet carefully during pregnancy. Because phenylalanine is an essential amino acid, diets for PKU patients contain a small amount of phenylalanine.
Although aspartame represents a minor source of phenylalanine in the food supply, products that are sweetened with aspartame are labeled as a source of phenylalanine. This requirement was introduced to inform PKU patients of the presence of aspartame in these products.
For people who do not suffer from phenylketonuria, phenylalanine is a normal and essential part of the diet.