Conditioned flavour preferences reinforced by caffeine consumed after lunch.

Caffeine-consuming adult males and females were divided into two groups, those who regularly consumed a caffeinated drink after lunch ("users" n = 21) and those who did not ("nonusers" n = 23). After lunch on weekdays during a 2-week conditioning period, these subjects consumed a novel flavoured fruit juice drink paired with either a caffeine (100 mg) or a placebo capsule. Preferences for this "target" drink and six other novel-flavoured fruit juice drinks were assessed before and then after 5 and 10 conditioning trials. The users showed a significantly greater increase in preference for the caffeine-paired target drink than for the placebo-paired target drink, whereas the nonusers showed a slight trend in the opposite direction. These changes in preference did not generalise to the nontarget drink flavours. For habitual postlunch caffeine users, caffeine alleviated the postlunch dip in mood experienced by those in the placebo condition. Thus, the increase in preference for the caffeine-paired target drink was consistent with the improved mood state that resulted from caffeine consumption. It is unlikely, however, that the subjects were aware of this relationship. These results provide strong evidence for the existence of a reinforcing effect of caffeine, which probably plays a significant role in the acquisition of preferences for caffeine-containing drinks.