Influence of soda additives on motivation in adolescents
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Background: In a study done previously in our lab, we found that novel sugar sweetened caffeinated soda was more reinforcing than non-caffeinated soda in males but not females. It is unclear as to whether the same would be true for diet soda. Purpose: The purpose of this study is to test the hypotheses that caffeine increases the reinforcing value of soda relative to placebo and that the effect of caffeine on reinforcing value will be the same in diet and regular soda. We also predicted that the effect of caffeine on reinforcing value of soda will differ as a function of gender. Method: Male (n=-29) and female (n=25) participants between the age of 12- 17 were randomized in to either a diet or regular soda group. Each participant completed three study visits to the lab where they were asked to play a game to earn portions of either caffeinated or non-caffeinated soda labeled 1 and 2. Over the course of two weeks participants consumed beverages containing placebo and caffeine every day for one week each. At the end of two weeks, participants returned to the lab for a post-test and played the same computer game to earn portions of either caffeinated or non-caffeinated soda. Results: Caffeinated and non-caffeinated soda were both found to be less reinforcing over time with non-caffeinated soda significantly more reinforcing than the caffeinated soda in males only. The reinforcing value of non-caffeinated soda decreased more over time than did the reinforcing value of caffeinated soda. Diet and Regular soda were found to be equally reinforcing when caffeinated. Diet soda was rated higher for liking than regular soda over all with males liking it significantly more than females. For both soda types, liking of soda decreased over time. Discussion: Contrary to what was found in the previous study, this study found non-caffeinated soda to be more reinforcing than caffeinated soda. One potential explanation is that the caffeine was perceived as bitter and this gave caffeinated soda a lower reinforcing value. Since no interaction was found between soda type and caffeine condition, it is clear that while caffeine and diet soda are individually reinforcing in males, the combination of the two has no evident additive effect on reinforcing value.