L-Phenylalanine — Frequently Asked Questions

It is one of the two amino acids from which aspartame is made. Aspartame is the methyl ester of a dipeptide formed from L-phenylalanine and L-aspartic acid, and that connection is the largest single industrial outlet for phenylalanine. It also explains why aspartame-containing products carry a phenylalanine declaration.

For people with phenylketonuria, an inherited disorder in which the enzyme that metabolises phenylalanine is absent or deficient. Phenylalanine accumulates to neurotoxic levels if intake is not controlled, and since digestion of aspartame liberates it, the declaration allows affected individuals to account for that source in their diet.

It is an essential amino acid, meaning the body cannot synthesise it and must obtain it from the diet, and it is used directly in protein synthesis. It is also the biosynthetic precursor to tyrosine and thence to the catecholamine neurotransmitters dopamine, noradrenaline and adrenaline, which underlies its use in nutritional supplements.

Because only the L-isomer is biologically active and it is the form required for both nutrition and aspartame synthesis. Optical rotation is the standard measurement confirming the material is the L-form and is not contaminated with the D-isomer, and it appears on the specification alongside assay and related substances.