Caffeinated drinks are commonly drunk for their central nervous system stimulatory effects. Coffee is often used as a stimulant and this relates primarily to its caffeine content. Tea is considered by many to be similar to coffee as it also contains caffeine. However, studies show that tea may have a quite distinct effect on cognition and the function of the central nervous system compared to coffee. The different effects of tea, when compared to coffee, may relate largely to the presence of an non-proteinaceous amino acid that is present in the former. This amino acid, L-theanine, is present in both green and black tea and may have particular effects on the brain. Some of the health effects conferred on regular tea drinkers may stem from these cognitive effects. In particular, tea consumption appears to affect the alpha wave state of the brain, promoting a feeling of relaxation and calm. The cognitive effects of tea are therefore contrasted with the more stimulatory effects of coffee consumption.
The cognitive effects of caffeine and L-theanine have been investigated in studies. For example, one group of researchers1 investigated the effects of L-theanine and caffeine, both alone and in combination in healthy human subjects. This was achieved through observations of the effects of caffeine and L-theanine ingestion on behavioral and electrophysiological indices of visuospatial attention. Subjects performed 4 days of testing and during this time they were given either 100 mg L-theanine, 50 mg caffeine (the amount of L-theanine and caffeine in a cup or two of tea) or a placebo or combinations of these. The combination of caffeine and L-theanine was observed to increase in hit rate and target discriminability, two cognitive tests of visuospatial attention. However, caffeine alone only increased discriminability and L-theanine alone had no effect on either test. The combination of tests caused a lower overall tonic alpha wave power in the brain, something that has been shown previously at high doses of L-theanine in isolation.
These results therefore suggest that caffeine and L-theanine have synergistic cognitive effects. L-theanine is present almost exclusively in the tea plant, and as such tea is the only dietary source. Tea has been shown to be neuroprotective and this has been speculated to be due to the L-theanine content. In addition, tea has been shown to have mood stabilising effects, which have also been speculated to be due to L-theanine. The basis for this speculation rests on the observation that consumption of L-theanine increases the alpha frequency band (8-14 Hz) when tea is consumed at rest. One possibility regarding tea is that the caffeine provides a stimulatory effect on the central nervous system, but the L-theanine then balances this effect with an overall calming effect leading to an increased state of arousal that is more controlled and focussed that that of caffeine alone. The lack of effect of L-theanine alone in this study may have been due to the small dose administered as larger doses (250 mg) reduce tonic alpha wave power.
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