Green tea has a number of health effects and these effects relate to the phytochemicals within the Camellia sinensis (tea) plant. One important constituent of green tea is the amino acid L-theanine. In animals, L-theanine is absorbed across the gut into the bloodstream and is then transferred to the major organs including the brain within about 30 min. L-theanine can increase levels of 5-hydroxytryptamine and decrease levels of corticotropin hormone and cortisol. L-theanine can also decrease levels of adrenaline and noradrenaline in the prefrontal cortex and hippocampus. L-theanine may have immune stimulatory effects because of its ability to reduce cortisol levels. L-theanine also appears to have neuroprotective efforts, perhaps because of its ability to reduce cortisol levels. The overall effects of L-theanine in the brain are to cause a reduction in excitability and a calming effect through the stimulation of an alpha wave state. This likely explains the calming effects of green and black tea.
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