Caffeine is often described as the most widely consumed psychoactive drug in the world. Despite a substantial amount of research examining the effects of caffeine on mood and cognition, there remain a number of unresolved issues in this field, two of which formed the focus of this thesis. The first pertains to whether caffeine has any behavioural effects beyond a reversal of withdrawal effects purported to exist in habitual consumers following caffeine deprivation. A second relates to the biobehavioural effects of caffeine when consumed in combination with other potentially psychoactive components, as is usually the case in dietary forms of caffeine. This thesis, therefore, firstly compared the cognitive and mood effects of acute administration of caffeine to habitual consumers and habitual non-consumers of caffeine. The effects of combining caffeine with other naturally concomitant compounds were then explored, firstly by examining the impact of combining caffeine with L¬theanine (an inhibitory amino acid found in tea) and then by exploring the effects of guaranâ (a caffeine-containing whole extract). Finally, following on from these latter studies, an attempt was made to establish the lowest active dose of caffeine. Each experiment followed a placebo-controlled, double-blind, balanced cross-over design. In each study, treatment-related changes in cognitive performance were assessed with computerised assessment tools (the Cognitive Drug Research battery, a sentence verification task and serial subtractions), and mood was assessed using both Bond¬Lader and specifically tailored caffeine research visual analogue scales. Where appropriate, salivary caffeine levels and autonomic activity were monitored. Performance was similarly improved for habitual consumers and habitual non- consumers of caffeine following caffeine administration. The administration of caffeine in combination with L-theanine led to some modulation of the effects of caffeine. This was also demonstrated when examining the effects of guaraná. A direct comparison of caffeine and guaranâ with matched caffeine levels revealed differences in the effects of the two treatments. Exploration of the lowest active dose of caffeine revealed (largely impairing) effects of caffeine at doses lower than those found in decaffeinated beverages. These findings may have important implications for caffeine research. Firstly, they suggest that behavioural effects of caffeine cannot be attributed wholly to withdrawal reversal. Secondly, they demonstrate that other components commonly co- consumed with caffeine are likely to modulate its biobehavioural effects. Finally, they suggest that levels of caffeine hitherto thought to be inactive may have (negative) psychoactive properties.
|Publication status||Accepted/In press - 23 Jun 2008|