* Science-Based Nutrition * Dietary Supplement * Suitable for Vegetarians * Aids in the Relief of Minor Pain D-phenylalanine is a non-protein amino acid related to the essential amino acid L-phenylalanine. D- and L-phenylalanine are structural mirror images of each other. A small amount of D-phenylalanine may be converted to L-phenylalanine in the body. D-phenylalanine is reported to inhibit an enzyme that degrades enkephalins, which are naturally occurring substances in the body that belong to the class of compounds known as "endorphins."