The primary bioactive constituents of tea are polyphenols, amino acids, and caffeine [62]. Between 30 and 42 % of the dry weight of green tea is formed from catechins; caffeine and L-theanine (amino acid) respectively account for around 2 to 5 % and 3 %. Its chief catechin polyphenol, epigallocatechin gallate (EGCG), contributes 50 to 80 % of total catechins [63], and has neuroprotective effects due to its antioxidant action, suppressing inflammatory processes and inhibiting cell proliferation [64]. Other prominent catechins include epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC), although these are present in smaller quantities. L-theanine is a non-protein amino acid analogue of γ-N-ethyl-L-glutamine, and has been shown to bind to AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid), N-Methyl-D-aspartate (NMDA), group I metabotropic glutamate, and kainate receptors, inhibiting glutamate reuptake and potentiating GABA, dopamine, and serotonin [65–67]. L-theanine is thought to have anxiolytic properties due to potentiation of GABA [68], the primary inhibitory neurotransmitter in the brain. Caffeine inhibits adenosine (A1 and A2a) receptors, increasing dopamine transmission and ACh, resulting in increased arousal and enhanced cognition [69]. These three constituents are thought to interact, resulting in the overall cognitive-enhancing and anxiolytic effects of tea, with some evidence that L-Theanine inhibits caffeine’s stimulatory effects [70].

Animal studies have demonstrated effects of green tea extract and its individual constituents on cognition and anxiety. Work on green tea extract has demonstrated effects after both chronic and acute administration. Improvements in motor coordination (Morris water maze, rota rod test, locomotor activity measured with actophotometer, and negative geotaxis measured with mid-air righting) and reduced anxiety (elevated plus maze, open field test) in valproic acid induced oxidative stress model mice were seen after 300 mg/kg of green tea extract [71]. In addition, enhanced learning and memory in aged rats were observed after 8 weeks of 0.5 % green tea extract, measured by elevated plus maze and passive avoidance tests [72]. Other acute research on individual constituents showed that standardised EGCG extract (10 mg/kg) reduced cognitive dysfunction and improved positive mood in Parkinson’s disease model rats [73], and that 100 μml/20 g of EGCG resulted in amnestic and anxiolytic effects on behaviour in mice in elevated plus maze test [74]. However, Stringer, Abeysekera, Dria, Roper, and Goodlett [75] did not show cognitive improvement in Down syndrome model mice following chronic administration of EGCG (~20 mg/kg/day) for 3 or 7 weeks. Heese et al. [68] found that acute 10 mg/kg L-theanine alone did not exert anxiolytic effects; however, when combined with midazolam, a synergistic/additive effect occurred, resulting in decreased anxiety and motor movements on the elevated plus maze. L-theanine has also been shown to reduce opioid withdrawal signs in rhesus monkeys (e.g., fighting), and produce anxiolytic effects in mice without affecting motor behaviour [70].

Work examining the combined and separate effects of acute L-theanine and caffeine on human cognition and mood has demonstrated that L-theanine suppresses the stimulatory effect of caffeine. For example, a study on the acute effects of caffeine and L-theanine in habitual (n = 12) and non-habitual (n = 12) caffeine drinkers demonstrated that 75 mg caffeine alone improved attention and positive mood (VAS), reduced oxygenated haemoglobin, and increased deoxygenated haemoglobin [76]. When 75 mg caffeine and 50 mg L-theanine were combined, increased deoxygenated haemoglobin was still apparent; however, this was somewhat attenuated compared to caffeine alone. Those authors concluded that caffeine and L-theanine interact, and the presence of L-theanine results in the attenuation of the vasoconstrictive and behavioural effects of caffeine. Further research is required to identify which constituent contributes to the positive effects on mood. Another study (n = 48) showed that acute dosing of caffeine (250 mg) increased alertness, jitteriness, and blood pressure, but when combined with 200 mg L-theanine, caffeine’s effect on blood pressure was attenuated; L-theanine alone slowed reaction times on a visual probe task [77]. Importantly, Camfield et al. [78] conducted a meta-analysis on 11 randomised placebo-controlled trials to determine the acute effects of EGCG, L-Theanine, and caffeine. Combined caffeine and L-theanine was found to improve cognition (alertness and attentional switching accuracy); however, a subsequent analysis showed a trend towards an enhanced effect for caffeine compared to L-theanine.

Other research utilising L-theanine alone has demonstrated anxiolytic effects. A randomised controlled trial (n = 60) on 8 weeks of 400 mg/day L-theanine administration significantly reduced anxiety and positive psychopathology scores (on the Positive and Negative Affect Schedule [PANAS]) in participants with a DSM-IV diagnosis of schizophrenia or schizoaffective disorder [79]. In relation to acute studies, Lu et al. [65] demonstrated that 200 mg L-theanine exerts calming effects under resting conditions (measured on visual analogue mood scale [VAMS]), however does not demonstrate anxiolytic effects under increased anxiety conditions (n = 16). Further, a recent study (n = 34) on the effects of an L-theanine-based nutrient drink (197 mg L-theanine) on responses to a cognitive stressor demonstrated a reduced salivary cortisol response and increased magnetoencephalogram (MEG) alpha activity (indicating decreased cortical arousal) following treatment, cf. placebo, for individuals higher in trait anxiety (STAI) [80]. Other work has shown that 200 mg L-theanine significantly attenuates task-related blood-pressure increases in labile individuals (n = 14), and reduced tension-anxiety scores measured on the Profile of Mood States (POMS) [81]. In the meta-analysis by Camfield et al. [78], no overall acute effects on anxiety were found in the first two hours post-dose, as measured by the STAI. However, isolated studies [77, 82] that were included in the review did report anxiolytic effects for 200 mg L-theanine in comparison to placebo (with standardised mean differences [SMDs] from 0.42 to 0.84).

The flavonoid EGCG has received less attention in human trials. A double-blind, placebo-controlled, cross-over design study (n = 31) reported that acute administration of 300 mg EGCG increased electroencephalogram (EEG) alpha, beta, and theta activity, increased self-reported calmness, and reduced self-reported stress [83]. Wightman, Haskell, Forster, Veasey, and Kennedy [84] found that 135 mg acute administration of EGCG reduced heart rate, oxygenated and total haemoglobin in the frontal cortex, but did not affect cognitive performance or mood (n = 27). An fMRI study (n = 12) found an acute dose-dependent effect of green tea extract in a milk whey based soft drink (0.05 % extract/250 ml or 500 ml) on increased activation in the dorso-lateral prefrontal cortex cf. controls, in a working memory task [85].

Commonly referred to as Sage, the Salvia genus (e.g., S. officinalis, S. lavandulaefolia, and S. elegans) belongs to the mint (Lamiaceae) family and includes almost 1000 species from across the world. Common uses include cooking or traditional medicinal herbs, ornamental plants, and occasionally religious ceremonies. More recent clinical applications of sage typically take the form of a standardised essential oil extract in combination with an inactive agent (e.g., sunflower oil) or dried plant extract in an alcohol solution, for oral administration [42, 86].

Preliminary findings suggest promising effects after oral administration of sage (particularly S. officinalis or S. lavandulaefolia) as a cognitive enhancer in individuals with AD and an anxiolytic in healthy, non-clinical individuals. However, to date no studies have investigated the effects of sage as a cognitive anxiolytic in individuals with a clinical anxiety disorder. Nor have studies investigated the long-term effects of sage on cognitive function. Further research is needed.

Commonly referred to as Rosemary, Rosmarinus officinalis is also a member of the mint (Lamiaceae) family. Recent clinical applications of rosemary involve aromatherapy of the essential oil via the skin (massage) or olfactory system (inhalation).