Caffeine is anxiogenic and theanine is claimed to induce relaxation. One functional magnetic resonance imaging (fMRI) study and two behavioural studies were designed to better understand the effects of these two components of tea, the world's second most popular drink, on anxiety. Single doses (250 mg) of caffeine and/or theanine were administered to healthy adults. Given previous findings, the measures also included self-ratings of other mental/physical states, blood pressure, heart rate and cortisol. Initially, an anxiety-induction manipulation, comprising an acute stressor task, was refined in three pilot studies. This was applied in the first behavioural study (n = 151) that examined the reputed anxiolytic effect oftheanine alone and in combination with caffeine. As expected, caffeine (vs. placebo) increased anxiety and blood pressure. Theanine did not reduce caffeine- or task-induced anxiety, whereas it did reduce the caffeine-induced rise in blood pressure. Another noteworthy result is that caffeine failed to enhance mental alertness in non/infrequent caffeine consumers. Using an emotional face processing task, fMRI and a region of interest approach to examine the neural correlates of caffeine-induced anxiety, Study 2 found that caffeine (vs. placebo) selectively induced threat-related (angry/fearful faces> happy faces) midbrain- periaqueductal gray activation and abolished threat-related medial prefrontal cortex wall activation. Furthermore, effects of caffeine on extent of threat-related amygdala activation correlated negatively with level of dietary caffeine intake. Returning to theanine, Study 3 examined the potential oftheanine to reduce anxiety using a stronger anxiety-induction manipulation, provided by the 7.5% carbon dioxide (C02) inhalation model of anxiety. Theanine (vs. placebo) did not reduce anxiety increased in this context, whereas there was some indication that it reduced the CO2-induced rise in blood pressure. Together, the findings of these studies enhance understanding of caffeine-induced anxiety, and challenge the claim that theanine is anxiolytic. Instead, they suggest theanine may be useful for reducing raised blood pressure. More broadly, this research provides insight into the role of caffeine and theanine in health and wellbeing, and informs claims for products containing these compounds.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:559480 |
| Date | January 2011 |
| Creators | Smith, Jessica E. |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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