Recently, green tea has become renowned for the health benefits of its amino acid components, such as L-theanine (subsequently called theanine) and γ-aminobutyric acid (GABA). Theanine, a popular functional food ingredient, has received attention for its multiple roles in the central nervous system (CNS) and autonomic nervous system (ANS). It has been found to affect dopamine and serotonin concentrations in the brain, thus producing an anxiolytic effect [1, 2]. Intake of theanine was also found to increase alpha brain wave activity in humans, which might lead to a relaxed but alert state [3, 4]. Kimura reported that theanine reduced heart rate and salivary IgA responses to an acute stress task (arithmetic task), suggesting that it could reduce stress by inhibiting cortical neuron excitation [5].

GABA has also recently been studied for its effects on the CNS and ANS [6–8]. Kanehira et al. demonstrated an anti-fatigue effect from GABA ingestion on salivary secretion levels of chromogranin A (CgA) and cortisol following an arithmetic task used as a fatigue load [9]. Nakamura et al. also reported the psychological stress-reducing effect of chocolate enriched with GABA (28 mg) on stress induced by an arithmetic task [10]. They found that the GABA-enriched chocolate improved recovery time from a stressful to a normal state, and that in those taking GABA chocolate, CgA values measured after the task were not increased compared to measures before ingestion.

These amino acids are thus likely to underlie the effects of consuming green tea on the brain, including aiding recovery from stress and resistance to depression, promoting a more relaxed state and better mood, and improving sustained attention for mental task performances. Since these psychological and physiological effects of theanine and GABA were observed in studies where the components were administered in a dose-dependent manner, it can be hypothesized that green tea containing more of the amino acids might attenuate stress responses in the ANS even more significantly. However, there is little scientific evidence comparing the anti-stress effects induced by different green tea samples containing different concentrations of amino acid components.

CgA is an acidic glycoprotein with 439 amino acids, occurring in the secretory granules of most neuroendocrine cell types. It is a major protein in adrenal chromaffin cells and adrenergic neurons. Responding to stress, CgA and catecholamines are co-released into the extra-cellular environment. Nakane found a prompt elevation in salivary CgA levels and a delayed increase in salivary cortisol levels when psychosomatic stress was induced by a test involving an oral presentation in front of an audience or a driving situation [11]. Nomura et al. also found that salivary CgA concentration depicted an increase during the mental stress tasks and decrease (recovering) during the intermissions, demonstrating the possible candidacy of CgA as a biomarker for a short-term mental workload [12]. These studies suggest that salivary CgA may be a sensitive and promising index for psychosomatic stress.

In this study, we assessed the anti-stress effects of two kinds of green tea - ordinary green tea (Sagara), and shaded white tea - on CNS activities in healthy people by measuring salivary CgA. We also evaluated subjects’ Profile of Mood States (POMS) scores and the Visual Analog Scale (VAS) scores as subjective ratings of mental state.

The results of CgA tests showed that U-K tests increased salivary CgA levels significantly only in the warm water condition. That is, the two tea treatments attenuated CgA increase induced by the stress of performing the U-K tests, demonstrating the anti-stress effects of the teas. Green tea contains many chemicals that are reported to affect the human cardiovascular system and the brain, including amino acids, theanine and GABA. Caffeine and catechins also have been reported to improve mood and reduce anxiety or self-rated stress [20, 21].

As expected, participants who ingested shaded white tea had significantly lower CgA levels after completing all U-K tests than those who ingested water. There was not, however, a significant difference found between Sagara and warm water. In other words, the anti-stress effect of consuming shaded white tea was considerably larger than that of the ordinary green tea, Sagara. Quinlan et al. reported that tea and coffee produced mild autonomic stimulation and an elevation in mood but there were no effects of caffeine dose [22]. They concluded that caffeine could exert dose-dependent effects on a number of acute autonomic responses, meanwhile caffeine level was not an important factor, and factors besides caffeine might contribute to those acute effects. Thus, the higher dose of caffeine in shaded white tea may not be the only component that caused the different results in CgA levels between the two tea samples.

As shown in Figure 2 and Table 1, levels of amino acids, including theanine, were substantially higher in shaded white tea than in Sagara. Detected amounts of GABA and theanine were 0.25 mg and 52 mg, respectively, in shaded white tea, while they were 0.15 mg and 17 mg in Sagara. Knowing that most human mood studies have used higher doses of theanine (for example, 200 mg in Juneja et al. [3] and Higashiyama et al. [23]; and 250 mg in Gomez-Ramirez et al. [24]), the low dose of 52 mg in shaded white tea in this study might have been insufficient. However, one previous study on the effect of theanine consumption on human brain alpha activity found significantly greater and increasing alpha activity after ingestion of 50 mg theanine compared with placebo [25]. Alpha activity is indicative of a state of wakeful relaxation, and is associated with decreased anxiety and improved performance under stress [26, 27]. At the same time, previous studies have indicated that theanine interacted with caffeine, and the combination promoted better effects than either alone [28]. Thus the mixture of theanine and caffeine in shaded white tea may have potentiated the anti-stress effect beyond that seen in studies using 50 mg theanine alone.

Besides the theanine and GABA, other amino acid components that showed much higher content levels in shaded white tea than in Sagara also deserve attention. These include an asparagine level forty-three times greater, a twice greater glutamic acid level, alanine (none in Sagara), an isoleucine seven times greater and an arginine level eight times greater. It is still unclear whether these components play important roles in improving mood or reducing stress. Further studies of these isolated amino acid components are necessary to confirm their physical and psychological effects. Nevertheless, green tea has been traditionally used as an herbal medicine since ancient times, and, in general, herbal medicines are complex mixtures of different compounds that often act in a synergistic fashion to exert their full beneficial effect. From this point of view, it seems plausible to consider that the high content of the amino acid compounds mixed with higher content level of caffeine contributed to the larger anti-stress effect of shaded white tea in this study.

It may be that the anti-stress effect promotes a more relaxed state that improves sustained attention, thus leading to the improved performance on the U-K tests. Participants tended to complete more calculations after ingesting shaded white tea than after ingesting warm water. Among the large body of literature on the performance effects of caffeine and catechins, amino acid components such as theanine have also been reported to have positive effects on brain activity such as memory and learning ability [3, 29]. Additionally, an in vitro study reported that glutamic acid in green tea increased electrical hippocampal activity in rats; such activity is commonly taken as representative for enhancement of spatial and time-dependent memory [30].

Finally, even though the results of the VAS indicated that shaded white tea elicited lower scores in terms of preference and familiar experience compared with the ordinary green tea Sagara, the intake of shaded white tea significantly decreased the TMD score measured immediately after ingestion compared with the baseline evaluation; meanwhile, ingestion of Sagara showed only a trend of this effect. Needless to say, there was no similar effect from warm water intake. We presume that this TMD score decrease came from the influence of the odor of the green tea. Our previous study showed that smelling shaded white tea increased the band power of alpha and beta in the frontal and occipital regions and suppressed the decrease of Vigor-Activity in POMS after mental tasks [31]. The mechanism might be that one of the main odor components of green tea stimulated the release of dopamine, which regulates brain functions, mood and attention in rats, and induced changes of event-related potential P-300 and perceived pleasure in human studies [32, 33].

The limitations of our study are that only young Japanese students participated in this study, and only the concentration of the salivary CgA was used as the index for ANS activity. Further study is needed to explore the anti-stress effects of consuming shaded white tea or other amino acid-rich green tea using participants across all age groups and from other geographical regions by measuring more ANS or/and CNS activities.

Salivary CgA concentration levels increased after mental stress load tasks, but ingestion of green tea inhibited this increase. This anti-stress effect was larger after the consumption of shaded white tea than after Sagara. Shaded white tea intake also lowered TMD score (POMS) and tended to improve performance on an arithmetic task compared to warm water, suggesting that shaded white tea might also improve mood during and after mental stress load.