**1. Introduction**

We have previously shown that consumption of GABA-enriched oolong tea is effective in reducing stress and improving cardiac rhythm in a university student cohort [1]. As constituents other than GABA are known to influence stress, we have quantified three other key chemicals in oolong tea known to influence stress: the methylxanthine caffeine, the flavonoid epigallocatechin gallate (EGCG) and the amino acid theanine in addition to GABA. The chemical structures of these constituents are shown in **Figure 1**. The aim of the current study was to quantify, using HPLC, the concentrations of these constituents in 200 mL cups of regular oolong and GABA-enriched oolong tea as prepared according to manufacturer's specifications and consumed by the students.

GABA is the major inhibitory neurotransmitter in the brain [2] and plays a pivotal role in stress as well as modulating autonomic and cardiovascular function centrally and peripherally. GABA-enriched foods and GABA in single oral administration have been shown to reduce stress as measured through heart rate variability, heart rate, and other stress-related biomarkers and psychological tests. Hence, GABA content in tea has been enhanced to produce an improved therapeutically useful beverage for consumption [3]. GABA-enriched tea is obtained by incubation of tea under anaerobic conditions, or cycling under aerobic and anaerobic conditions, which has been shown to accumulate GABA to produce a tea with concentrations of GABA >150 mg/100 g tea [4–9].

The possible roles for other oolong tea constituents of oolong tea, such as EGCG, theanine and caffeine, may then also relate to their interaction with GABA. There are several known mechanisms of action to explain the effects of caffeine in oolong tea in addition to its stimulant effect [10]. The most prominent is that it is a potent adenosine receptor antagonist, and it has also been shown to potentiate GABA release via its effect on A1 adenosine receptors [11].

EGCG may mediate some of its stress-reducing effects via interaction with the GABAergic system [12–15]. EGCG is readily incorporated into the brain following intragastric administration in mice [14], and has shown stress-reducing, anxiolytic and sedative properties in a number of animal models [12, 13, 15]. EGCG has also been found to be a biphasic modulator of GABAA receptors, at low doses enhancing diazepam action at recombinant GABAA receptors expressed in *Xenopus* oocytes [16]. *GABA-enriched Oolong Tea: Reducing Stress in a Student Cohort May Involve More than Just… DOI: http://dx.doi.org/10.5772/intechopen.106896*

Although the action of theanine is not well understood [17, 18], it has been proposed to contribute to the relaxation experienced following tea consumption. For example, an oral 200 mg dose of theanine was also shown to increase α-brain waves in the occipital and parietal cortices of human subjects, indicative of a relaxation effect [17]. While one study showed it to increase GABA release [19], theanine does not appear to impact behaviours mediated by the GABAergic system in animal models [19–21]. Instead, theanine may mediate its effects through modulation of other transmitter systems including dopamine [18, 22] and glycine [22].

The aim of the current study was to quantify, using HPLC, the concentrations of GABA, EGCG, caffeine and theanine in 200 mL cups of regular oolong and GABAenriched oolong tea as prepared according to manufacturer's specifications and consumed by the students.
