The effects of caffeine on olfactory function and mood: an exploratory study
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REVIEW
The effects of caffeine on olfactory function and mood: an exploratory study Lorenzo D. Stafford 1
&
Kaylee Orgill 1
Received: 29 June 2020 / Accepted: 23 October 2020 # The Author(s) 2020
Abstract Caffeine has been demonstrated to enhance olfactory function in rodents, but to date, the sparse research in humans has not shown any equivalent effects. However, due to the methodological nature of those human studies, a number of questions remain unanswered, which the present study aimed to investigate. Using a double-blind experimental design, participants (n = 40) completed baseline mood measures, standardised threshold and identification tests and were then randomly allocated to receive a capsule containing either 100 mg of caffeine or placebo, followed by the same olfactory tests and mood measures. Results revealed that despite a trend toward elevated arousal following caffeine for habitual caffeine consumers, there were no changes in odour function. In contrast, for non-caffeine consumers, caffeine acted to enhance odour (threshold) sensitivity but reduce odour identification. Overall, these findings demonstrate a complex profile of effects of caffeine on odour function and, given the evidence from the wider caffeine literature, it is proposed that the effects of caffeine might be limited to older populations. Keywords Smell . Adenosine . Coffee . Tea . Odour threshold . Odour identification
Introduction Caffeine is contained in a number of common beverages such as tea, coffee and energy drinks and has been consumed for over 2000 years (Barone and Roberts 1984). Caffeine is classified as a psychostimulant and has been extensively researched for its effects over the years, which include increases in arousal and enhanced performance in tasks requiring vigilance (see review Temple et al. 2017). However, there is still debate on the veracity of such effects in those who regularly consume caffeine, i.e. are the observed effects more about reversing the withdrawal symptoms of caffeine (James and Rogers 2005). At doses routinely consumed by humans, the main mechanisms of action are the antagonism of adenosine receptors (Patocka et al. 2019) and of particular interest here are its effects on the adenosine A2a receptors. Research has shown that the stimulatory effects of caffeine are largely achieved via the blockage of adenosine A 2a receptors (Svenningsson et al. 1999), and separately, we know that * Lorenzo D. Stafford [email protected] 1
Department of Psychology, University of Portsmouth, King Henry Building, King Henry I Street, Portsmouth PO1 2DY, UK
A2a receptors are found in the olfactory bulb (Kaelin-Lang et al. 1999). Evidence for the link between adenosine and odour function was also demonstrated in an elegant rodent experiment where both caffeine and separately an A2a receptor antagonist enhanced olfactory function (Prediger et al. 2005). Since no effects were found for an adenosine A1 receptor antagonist, that study suggested that the enhancements observed were due to caffeine’s action
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