CAFFEINE AND THEANINE
Caffeine is the most widely consumed psychoactive substance in the world, with coffee and tea representing our main dietary sources (1). The mechanism by which caffeine exerts its effects is now largely accepted to be through non-selective antagonism of adenosine A1 and A2A receptors (1), with A1 receptors being more closely related to neural activation and A2A receptor antagonism leading to vascular effects, including vasoconstriction and a reduction in cerebral blood flow (CBF) (1).
Caffeine has been shown to reduce cerebral blood velocity and CBF assessed using a number of different techniques (2-7). A linear dose relationship has been observed between reductions in CBF and caffeine following doses of 1,2.5 and 5mg/kg caffeine (8). Effects of caffeine on CBF are also dependent upon the level of caffeine habitually consumed. In a withdrawn state, habitual high consumers exhibit higher resting CBF as compared to low consumers, and caffeine use is significantly positively correlated with CBF following both placebo and caffeine (9,10). Caffeine has consistently been shown to improve reaction times (11-13) and alertness (14).
L-theanine is a naturally occurring amino acid found almost uniquely in tea (Camellia sinensis), where it co-exists with caffeine. Its chemical structure is similar to that of the neurotransmitter glutamic acid (15)
Anxiolytic effects have been reported, whereby 200mg L-theanine reduced acute stress responses (subjective perception, heart rate and salivary immunoglobulin A) induced by a mental arithmetic task (16), and a 250 mg dose of L-theanine slowed reaction time on a visual probe task, indicating reduced anxiety (17). EEG studies have also provided some support for these findings with increases in resting alpha activity observed following 50 mg (18) and 200mg L-theanine (18), which the authors interpret as being indicative of relaxation. A dose of250mg of L-theanine has also been shown to decrease tonic alpha activity during task performance (19).
In terms of cognitive function, L-theanine in isolation has been shown to engender decrements in performance (20,21). However, when administered together, L-theanine modulates or potentiates the effects of caffeine. For instance, Haskell et al. (22) reported that a number of effects were evident following a combination of L-theanine and caffeine that were not apparent when each treatment was administered alone; these included increased speed on several tasks, improved semantic memory and increased alertness. Other studies have employed lower doses of L-theanine (~100mg) and caffeine(~50mg) to explore these effects (23-25), providing support for the findings from studies of higher doses, in terms of improved accuracy (19) and speed (23) on tasks of attention, and improvements to measures of memory (20).
L-theanine has also been shown to antagonise the unwanted physiological effects of caffeine. Rogers et al. (17) demonstrated that although systolic and diastolic blood pressure were significantly higher following the administration of caffeine when compared to placebo or L-theanine alone, the combination of the two compounds led to an attenuation of this effect.
The combined effect of L-theanine and caffeine on cognitive performances clearly evidenced the beneﬁcial impression on recognition, visual information processing, attention and moods (23,24). Recently, Einother, Martens, Rycroft, and De Bruin (23) have reported that L-theanine and caffeine improve task switching.
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