The health effects of tea are well reported. Studies show that tea has a number of beneficial effects, including anti-cancer, cardiovascular and neuroprotective effects. The ability of tea to protect mental function, and perhaps even enhance it, is well reported, but he mechanisms of this effect are not fully understood. One possible reason for the neuroprotective effects of tea are the presence of the amino acid L-theanine (N-ethyl-L-glutamine). L-theanine is a non proteinaceous free amino acid found almost exclusively in tea (Camellia sinensis) leaves, where it account for almost 50 % of the amino acids and 1-2 % of the total dry weight of tea leaves (L-theanine is also found in the mushroom Xerocomus badius). L-theanine is likely transported into the brain using the leucine transporter located on the blood brain barrier, with uptake to the brain occurring within 30 minutes of ingestion. When in the brain, L-theanine may affect a number of neurotransmitter systems including the GABA and glutamate systems.
The ability of L-theanine to affect the GABA system would explain its calming and relaxing properties. Tea is known to increase the alpha wave state of the brain, a condition also conferred by the ingestion of GABA. Some evidence suggests that GABA concentrations increase in the brains of animals following administration of L-theanine. Also it has been shown that neuroprotection from L-theanine may be conferred through activation of the GABAA receptor in mice. This effect is blocked by administration of GABAA antagonising drugs, providing further evidence that these receptors may be activated by L-theanine. However, L-theanine may also antagonise glutamate receptors. Activation of the glutamate receptor increases excitability of neurones in the brain. By blocking this receptor, L-theanine may decrease electrical excitability and cause a general inhibitory effect in the brain. Blocking the glutamate receptor is therefore similar to activating the GABAA receptor, and evidence suggests that L-theanine may do both.
The effects of L-theanine on neurotransmitter systems is complex and not fully understood. L-theanine may change levels of the monoamines noradrenaline, dopamine and serotonin. However, the exact effects are not clear, and different parts of the brain may experience different changes to the levels of monoamines following L-theanine consumption. The overall effect of L-theanine on brain physiology is more well reported, with significant relaxation and calming effects, probably brought about as a result of increases in alpha waves and decreases in beta waves in the brain. However, these changes to brain waves may not be as great as for GABA supplements. The safety of L-theanine has been demonstrated and side effects are not reported from ingestion. L-theanine also has been shown to produce no drowsiness in humans, and so the calmness and relaxation is conferred while maintaining alertness, rather than through a sedative effect.
Tea plants synthesise L-theanine in their roots from the amino acid L-glutamic acid and ethylamine. The L-theanine is then transported to the leaves where it accumulates. L-theanine is structurally very similar to L-glutamic acid. As L-glutamic acid is an excitatory neurotransmitter in the brain and central nervous system, this may explain the ability of L-theanine to favourably modify brain activity from excitatory to inhibitory. In mammals, L-theanine is absorbed from the gastrointestinal tract and following absorption can be metabolised through a hydrolysis back to L-glutamic acid and ethylamine in the kidney. However, L-theanine can cross the blood brain barrier and enter the brain. Blood levels of L-theanine are elevated for about 2 hours post consumption, with peak blood levels occurring about 1 hour after ingestion. However peak levels in the central nervous system may occur up to 5 hours after ingestion. Drinking tea throughout the day may therefore alter neurochemistry for some time following consumption.
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