L-Tyrosine

L-Tyrosine is an amino acid found in the diet that is metabolized to produce catecholamines such as dopamine (DA) and norepinephrine (NE).^[1] L-tyrosine is also used in the production of thyroid hormones.^[2] L-Tyrosine is often taken as a supplement to help increase catecholamine levels depleted during stressful or challenging conditions.^[1] It is taken as a free-form amino acid supplement, commonly alone or as part of weight loss or preworkout supplements.

The main benefits of L-tyrosine are related to its ability to replenish catecholamine levels in the brain, which can become depleted under stressful conditions.^[1] Multiple studies have demonstrated that L-tyrosine can help to prevent declined cognitive function under stressful, cognitively demanding conditions.^[3] Although it has not been shown to improve memory under resting conditions, L-tyrosine supplementation has been shown to alleviate reduced memory under acutely stressful conditions.^[4] Overall L-tyrosine appears to be safe and well-tolerated at moderate doses, with one 2-week human trial using a dose of 2500 mg, provided three times per day for two weeks,^[5] and another trial using single doses of up to 150 mg/kg of body weight (around 10,000 mg for a 68 kg/150 lb individual), both with no adverse effects.^[6]

Although L-tyrosine has shown beneficial effects in some studies, it has a mixed track record for efficacy, with some studies failing to observe positive effects on cognition.^[7][3] This may be partially explained by the fact that L-tyrosine increases catecholamine synthesis only when catecholamine levels are depleted, limiting efficacy to sufficiently stressful or challenging conditions. Efficacy may also vary in different individuals, as highlighted by one study that found that the effects of L-tyrosine on cognitive function become detrimental with older age.^[7] There are also possible contraindications for L-tyrosine supplementation in people with hyperthyroidism or anyone taking L-dopa or monoamine oxidase inhibitors. If this applies to you, consult with your physician before supplementing with L-tyrosine.

L-Tyrosine is used in the body to make the catecholamines dopamine (DA) and norepinephrine (NE), which can become depleted under stressful or cognitively challenging conditions. After supplementation, L-tyrosine levels in the blood peak after one to two hours, and stay elevated for up to eight hours.^[8] L-tyrosine then passes through the blood-brain barrier, where it is absorbed by brain cells and converted in a series of enzymatic reactions to DA, which can be converted to NE through the action of another enzyme.^[9] The ability of L-tyrosine to enhance catecholamine synthesis only in neurons that are actively firing may explain its ability to reverse the effects of neurotransmitter depletion during stress.^[10]