A new study suggests that while caffeine does not affect odor function among regular users, it does enhance odor sensitivity among those who do not typically consume caffeine. The findings were published in Psychopharmacology.
Animal studies among rodents have shown that caffeine can enhance odor function, possibly because caffeine antagonizes certain receptors in the olfactory bulb, called adenosine A2A receptors. As of yet, it is unclear whether humans experience similar effects with caffeine.
Since very few studies have explored the possible odor enhancing effects of caffeine, study authors Lorenzo D. Stafford and Kaylee Orgill opted to conduct a study of their own. While previous studies used coffee to test these effects, Stafford and Orgill decided instead to use a double-blind experimental design where subjects were randomly given either a caffeine pill or a placebo pill. Neither the experimenters nor the participants knew who received which pill.
A total of 40 subjects were recruited for the study and asked not to consume any food or drinks containing alcohol, caffeine, taurine, glucose, or aspartame in the 12 hours before their lab session. Once at the lab, participants either received a 100 mg caffeine pill or a placebo pill. Thirty minutes after taking the pill, participants completed a test of odor sensitivity. They were then presented with fifteen different odors, one at a time, and asked to identify each one from a list of four options (e.g., lavender, vanilla).
The participants were asked questions concerning the amount of tea, coffee, and soft drinks they typically drink. The researchers noted that eight participants were not regular consumers of caffeine — four of these participants were assigned to the placebo group, and four were assigned to the caffeine group.
When looking at the full sample, there were no significant differences in odor identification or sensitivity between those who took the caffeine versus the placebo. However, when the researchers compared the regular caffeine consumers to the non-consumers, differences emerged.
First, non-consumers of caffeine who took the caffeine pills performed better on the odor sensitivity test compared to regular users of caffeine. Moreover, among the non-consumers, those who took the caffeine pills performed worse on the odor identification test compared to those who took the placebo pill. Oddly, this suggests that, among those who did not regularly drink caffeinated drinks, caffeine improved their sensitivity to smell but reduced their ability to identify odors. The authors say that caffeine’s stimulant effects may have had a more pronounced impact on performance during the odor sensitivity test given that it was a longer, more tedious task than the identification test.
When discussing why caffeine consumers did not experience any changes with the caffeine pill, Stafford and Orgill suggest that this group is likely more resistant to the effects of caffeine. After being asked to refrain from drinking coffee for 12 hours before the experiment, these participants may have been experiencing caffeine withdrawal. The caffeine pills may have simply canceled out these withdrawal effects, without affecting sense of smell.
The authors note that previous studies that pointed to the odor-enhancing benefits of caffeine were conducted among older samples. It could be that such effects are limited to older individuals. Again, the researchers consider the role of adenosine receptors, noting that “age-related increase in adenosine A2A receptors may play a role in declining odour function, which can be temporarily reversed by caffeine’s antagonism of those adenosine A2A receptors.”
The study, “The effects of caffeine on olfactory function and mood: an exploratory study”, was authored by Lorenzo D. Stafford and Kaylee Orgill.