Caffeine reduced flanker conflict difference scores on the Attention Network Test (p < 0.05), theanine increased difference scores (p < 0.05), and the combination did not differ from placebo.
Caffeine accentuated global processing of visual attention on the hierarchical shape task (p < 0.05), theanine accentuated local processing (p < 0.05), and the combination did not differ from placebo. Mood, salivary cortisol, and visual attention were evaluated. Emotional arousal was induced by highly arousing negative film clips and pictures. Using a double-blind, repeated-measures design, 36 participants received 4 treatments (200 mg caffeine + 0 mg theanine, 0 mg caffeine + 200 mg theanine, 200 mg caffeine + 200 mg theanine, 0 mg caffeine + 0 mg theanine) on separate days. We evaluated whether caffeine and theanine influenced cognition under emotional arousal. Consumed together, caffeine and theanine exert similar cognitive effects to that of caffeine alone, but exert opposite effects on arousal, in that caffeine accentuates and theanine mitigates physiological and felt stress responses. L-theanine in tea may account for the difference. It relaxes smooth muscle, stimulates the contraction of cardiac muscle, and enhances athletic performance. Theine stimulates the central nervous system (CNS), heightening alertness, and sometimes causing restlessness and agitation. Tea is perceived as more relaxing than coffee, even though both contain caffeine. Finally, caffeine acts as a mild diuretic.
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