-tyrosine is an amino acid that can be synthesised in humans from the essential amino acid L-phenylalanine. Nutritional interest in L-tyrosine has expanded in recent decades because of military research showing that supplementation with L-tyrosine reduces battle stress and improves cognitive performance in soldiers. For example, research published in the Brain Research Bulletin1 showed that administration of a drink containing 2 grams of L-tyrosine five times per day was effective at improving memory and tracking tasks following a 6 day combat course, when compared to a carbohydrate drink. In addition, the L-tyrosine drink significantly decreased blood pressure in comparison to the carbohydrate drink. This research was important because it showed for the first time that the deteriorations in mental status and the blood pressure rises seen with combat stress could be ameliorated with administration of L-tyrosine.
Since this time a number of studies have investigated L-tyrosine in order to determine the mechanisms by which fatigue might be ameliorated with supplementation. Generally, it is now accepted that L-tyrosine passes into the brain using the large neutral amino acid transporter. From here tyrosine enters dopaminergic and adrenergic neurones where it can be converted to L-DOPA, dopamine, noradrenaline and adrenaline by the enzymes tyrosine hydroxylase, aromatic amino acid decarboxylase, dopamine hydroxylase, and phenylethanolamine N-methyltransferase, respectively. Under times of stress, adrenergic and dopaminergic neurones become depleted of their neurotransmitters through repeated release across the synapse and subsequent destruction via the monoamine oxidase enzyme system. L-tyrosine supplements can increase plasma concentrations of L-tyrosine which in turn increase brain levels, thus replenishing neurotransmitter levels to normal physiological levels.
Another mechanism by which L-tyrosine may alleviate fatigue is during sustained physical activity. Under such conditions, plasma fatty acids become elevated and this can result in the deposition of the amino acid L-tryptophan from its binding site on albumin in plasma. L-tryptophan is also able to cross the blood brain barrier using the large neutral amino acid transporter, and this raises levels of serotonin in certain areas of the brain, which may contribute to fatigue. This process is enhanced by a concomitant exercise induced fall in branched chain amino acid (BCAA) concentrations. The BCAAs are also able to cross the blood brain barrier and during exercise their inhibitory effects of the entry of tryptophan diminishes as they are oxidised in muscle tissue and plasma levels fall. Supplementation of tyrosine may antagonise entry of tryptophan into the brain and thus inhibit serotonin production and delay the onset of fatigue.
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