Camellia sinensis, better known as tea, is a popular drink because of its energising and relaxing properties. Nutritionally tea is important because after water it is the most consumed beverage in the World. Much of the credit for the energising and rejuvenating properties of tea are historically attributed to the methylxanthines contained within the leaves. The methylxanthines in tea include caffeine, theobromine and theophylline, all of which have particular stimulatory properties in humans and other mammals. However, tea also contains other components that may provide some of its energising effects. L-theanine (γ-glutamylethylamide), a non-proteinogenic amino acid, is one such component that has been researched for its ability to improve cognition and mood. Green and black tea both contain high concentrations of L-theanine, which is also present on high concentrations in some mushrooms, such as Xercomus badium. A 250 mL cup of tea is estimated to contain around 40 mg of L-theanine, which equates to around half of the amino acids present.
Because if its possible positive effects on brain activity, researchers are interested in the absorption and metabolism of L-theanine. To these ends the pharmacokinetic activity of L-theanine has been investigated in humans. For example, in one study researchers1 administered a 100mg dose of L-theanine in the form of either green tea or a supplement to 12 healthy volunteers. Following administration, L-theanine levels increased significantly in plasma, erythrocytes and urine, to comparable levels in both groups. Peak levels were 24.3 μmol/L and 26.5 μmol/L in the capsule and the tea group, respectively. Administration of increasing doses of 50, 100 and 200 mg L-theanine caused a dose dependent rise of the area under the curve response in plasma. Urine levels of ethylamine and glutamic acid, known breakdown products of L-theanine, also increased in the plasma, and were detected in urine. Therefore L-theanine appears to be rapidly absorbed, metabolised and excreted in healthy human subjects, suggesting it is bioavailable.
These results support previous kinetics experiments involving L-theanine using animal models. For example, previously it has been shown that L-theanine is absorbed, metabolised and excreted in rats, with plasma, liver and brain tissue accumulating L-theanine. The cognitive effects of L-theanine are not fully understood, and while they might relate to the parent compounds, it is also possible that the breakdown products ethylamine and glutamic acid may contribute part or all of these effects. Studies using animal experiments have reported that L-theanine supplementation can increase brain levels of dopamine and γ-aminobutyric acid, which may explain the cognitive and anti-anxiety effects of tea, respectively. Experiments in humans have also shown that administration of L-theanine in doses of between 50 and 200 mg (1 to 4 cups of tea), increases the alpha brain waves associated with relaxation, with doses of 200 to 400 mg (4 to 8 cups of tea) being reported to decrease anxiety and stress significantly in both healthy and mentally ill patients.
The presence of both L-theanine and methylxanthines in tea invites speculation that they may act synergistically to enhance cognitive performance. Synergism may be expected because methylxanthines and L-theanine enhance cognition through different biochemical routes. Comparisons of tea with coffee are justified, because coffee offers similar motivational properties to tea, but tends to provide a sharper more defined alertness. Undoubtedly much of this comes from the high caffeine content of coffee, the predominate methylxanthine in the coffee bean. The presence of L-theanine in tea, as well as the lower caffeine and higher theobromine and theophylline content, may therefore explain the ability of tea to lower anxiety thresholds, compared to coffee. While synergism cannot be excluded, this calming effect of tea compared to coffee suggest that perhaps antagonism between L-theanine and methylxanthines may provide a more balanced enhancement of cognitive function when compared to the more euphoric coffee high.
