L-phenylalanine is an essential amino acid required for the synthesis of proteins. In addition, L-phenylalanine is the substrate for the formation of the catecholamine neurotransmitters and hormones. The catecholamines are adrenaline, noradrenaline and dopamine, and in the brain they play an important role in motivational behaviour. Low levels of adrenaline, dopamine and noradrenaline are associated with anxiety and depression. Stress is able to deplete the catecholamine neurotransmitters because they are required for the adaptation stage of the stress response. Supplemental L-phenylalanine has been shown to increase brain levels of phenylalanine, and this in turn increases brain levels of the catecholamines. L-tyrosine, a metabolite amino acid of L-phenylalanine has been shown to reduce the stress effects of battle and has been more heavily researched that L-phenylalanine, but both have similar effects in the central nervous system. The normal dose is 500 mg three times per day on an empty stomach.
Amino acids can be found in both L- and D- forms, which are stereoisomer. Usually, only the L-forms of amino acids are used by humans. L-phenylalanine is the form of the amino acid that is required for protein synthesis and other amino acids must also be in their L-form (e.g. L-tyrosine, L-glutamine, L-serine, L-methionine). However, D-phenylalanine is an exception as it appears to provide some physiological benefits with regards to pain. D-phenylalanine is absorbed much like L-phenylalanine, where it passess in to the brain. Here it inhibits the breakdown of the natural opioids in the brain, the enkephalins. Thus D-phenylalanine has a pronounced pain relieving effect. D-phenylalanine in not available as a supplement, but DL-phenylalanine, a racemic mixture of the two stereoisomers, is available. Taking DL-phenylalanine provides the D-phenylalanine necessary to have the pain relieving effects. As with L-phenylalanine, doses for pain relief are around 500 mg three times per day, taken on an empty stomach.