Study (author, year, location)

Participants (gender, health, age)

Sample size (n)

Study design

Time period

Tea intervention

Dosage

Health outcome(s)

Main findings

Kahathuduwa, et al. [28] USA

Healthy males

n = 9

4-way crossover design

60 minutes post intervention

L-theanine, 160 mg of caffeine, their combination, or the vehicle (distilled water; placebo)

Solutions of 200 mg of L-theanine, 160 mg of caffeine, their combination, or the vehicle (distilled water; placebo)

Mind wandering

L-theanine and caffeine appear to have a synergistic action in decreasing mind wandering.

Kahathuduwa, et al. [35] USA

Healthy male volunteers, mean 21.9 yrs.

n = 20

Placebo-controlled, five-way crossover trial

4-hours

Black tea. L-theanine, caffeine, their combination, and a placebo (distilled water)

Black tea (150 ml) providing L-theanine (200 mg), caffeine (160 mg).

Cognitive and recognition visual reaction time and neurophysiological measures of attention

A dose of theanine equivalent to around eight cups of back tea improves cognitive and neurophysiological measures of selective attention, to a degree that is comparable with that of caffeine. Theanine and caffeine seem to have additive effects on attention.

Einöther, et al. [3] Netherlands

Dutch regular tea drinkers, mean 33.5 yrs.

n = 153 (n = 30 M)

Parallel study, 3 groups

75 minutes post intervention

Tea, placebo tea, or a glass of water

200 ml black tea, placebo or water

Mood

The effects of a single cup of tea may be limited to an immediate increase in pleasure and decrease in arousal, which can increase interest in activities.

Ide, et al. [36] Japan

Nursing home residents with cognitive dysfunction, mean 84.8 yrs.

n = 33 (n = 4 M, n = 29 F)

Double-blind, randomized controlled study

12 months

Green tea or placebo

2 g/day of green tea powder (containing 220.2 mg of catechins) or placebo powder (containing 0.0  mg of catechins).

Cognitive function

12 months green tea consumption may not significantly affect cognitive function but did prevent an increase of oxidative stress.

Dodd, et al. [37] UK

Twelve habitual consumers and 12 non-habitual caffeine consumers, mean 21.8 yrs

n = 24

Placebo-controlled, double-blind, counterbalanced, crossover study

30 minutes post intervention

L-theanine, L-theanine + caffeine or a placebo

50 mg L-theanine, 75 mg caffeine plus 50 mg L-theanine, and placebo (equivalent to 1-2 cups of tea)

Cerebral blood flow, cognition, mood

Combining L-theanine with caffeine, at levels and ratios equivalent to 1-2 cups of tea, eliminated vasoconstrictive effects and behavioural effects of caffeine.

Ota, et al. [38] Japan

Healthy adults, mean 31 yrs

n = 14 (n = 7 M, n = 7 F)

Randomized controlled trial

120 minutes post intervention

L-theanine or placebo

L-theanine given at 0, 200, 400, and 600 mg

Sensorimotor gating

The observed effect with 200-400 mg of L-theanine on PPI suggested that L-theanine at a particular dose range increases sensorimotor gating in humans.

Yoto, et al. [39] Japan

Healthy volunteers, mean 23.4 yrs.

 n =18 (n = 9 M, n = 9 F)

Single-blind randomized crossover (3 separate trials)

60 minutes post intervention

Green tea, white tea and warm water

250 ml tea sample

Mental stress, mood disturbance/states

Green tea had anti-stress effects - lowering levels of salivary CgA.

Shiah & Radin [40] Taiwan

Taiwanese adults

n = 189

Randomized trial

1-week

Oolong tea

600 mL in the morning and again in the afternoon

Mood

Tea blessed by a monk improved mood more than ordinary tea derived from the same source. Belief that one was drinking treated tea produced a large improvement in mood, but only if one was actually drinking the treated tea, indicating that belief and intentional enhancement interact.

Foxe, et al. [41] USA

Medically healthy participants, mean 26 yrs

n = 27

Double-blind, randomized, cross-over study

4-days

Theanine, caffeine, a combination, or placebo

Theanine (100 mg), Caffeine (50 mg),

Vigilance and sustained attention tasks

When treated with placebo, participants showed a rise in error rates, a pattern that is commonly observed with increasing time-on-task, whereas after caffeine and theanine ingestion, error rates were significantly reduced.

De Bruin, et al. [42] Netherlands

Healthy participants

n = 26

Two double-blind, randomized, placebo-controlled, crossover studies.

Short-term approx. 30 minutes post intervention

Black tea or placebo (two 200 ml servings)

About 23 mg theanine and 50 mg caffeine per cup

Attention, self-reported alertness

Auditory (p < 0.001) and visual (p = 0.030) intersensory attention were enhanced after black tea compared to placebo ingestion.

Giesbrecht, et al. [43] Netherlands

Young adults, mean 21.2 yrs

n = 44

Randomized, placebocontrolled, double-blind, within-subjects design

Approx. 70 minutes post intervention (self-paced)

Combination of L-theanine and caffeine vs placebo treatment

97 mg L-theanine and 40 mg caffeine

Cognitive performance, self-reported mood

During a challenging cognitive task 97 mg of L-theanine in combination with 40 mg of caffeine helped to focus attention.