The Alcohol Mixed with Energy Drink Debate: Masking the Facts! A Commentary on “Mixing an Energy Drink with an Alcoholic Beverage Increases Motivation for More Alcohol in College Students” by Marczinski and Colleagues (in press)

IN THEIR STUDY “Mixing an Energy Drink with an Alcoholic Beverage Increases Motivation for More Alcohol in College Students,” Marczinski and colleagues (in press) conclude that “An energy drink may elicit increased alcohol priming. This study provides laboratory evidence that AmED beverages may lead to greater motivation to drink versus the same amount of alcohol consume alone.” However, on inspection of the study, we believe that this conclusion is premature and that issues with the design, analysis, and interpretation of the data may merit different conclusions. The first, less serious issue relates to the nature of the vehicle, Squirt, which is described by the manufacturers as a caffeine-free citrus soda. If we assume that the participants had some knowledge of Red Bull, then the sensory characteristics of the vehicle and alcohol compared with energy drink and alcohol mixed with energy drink (AmED) may have produced different expectancies, potentially confounding the main outcomes of the study. The second, more substantive issue relates to the analyses performed. The study utilized a between-subjects design and compared the effects of a placebo (Squirt lemon drink), alcohol, Red Bull energy drink, and AmED. The study includes several subjective measures but focuses on “desire to drink” which was assessed at baseline then at 10, 20, 40, 60, and 80-minutes postdrink. These scores were appropriately subjected to a 2 (Alcohol) 9 2 (Energy Drink) 9 2 (Gender) 9 6 (Time) analysis of variance with time as a repeated measures factor. This revealed a significant interaction (p = 0.046). An appropriate analysis would then be to compare treatment groups at each time point which the authors then did using independent samples t-tests comparing alcohol and AmED groups. The authors state “No differences between alcohol and AmED for any of the time points were observed, although there was a nonsignificant trend for higher ratings for AmED versus alcohol alone at 40 minutes, t(38) = 1.47, p = 0.075.” The between-groups comparison, therefore, failed to reveal any significant effects. In addition to the above, the authors compared, within each group, each time point with their respective baseline scores using paired samples t-tests. Unfortunately, this analysis tells us nothing about the differences between groups (Nieuwenhuis et al., 2011), for example, whether there are any differences between the alcohol and the AmED groups. However, they have used the results of the within-groups analysis to make inferences about differences between groups, even though they have shown these to be nonsignificant. This same analytical approach was used by some of these authors in an earlier study which then also drew erroneous conclusions (Marczinski et al., 2011). Additionally, such analyses can be greatly influenced by chance baseline differences in the same measure and by regression to the mean. Indeed, inspection of baseline “desire for alcohol” ratings (table 1) reveals large variations in mean baseline scores (energy drink 13.00, vehicle 7.80, alcohol 6.65, and AmED 4.65). This reflects sampling variability associated with the study and provides a range estimate (8 units) with which to compare posttreatment differences. From the graph (fig. 1), the maximum differences between alcohol and AmED appear to be of a similar magnitude supporting the lack of significant differences found. From the Division of Pharmacology (JCV), Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Utrecht, The Netherlands; and Psychology Department (CA), Faculty of Health and Life Sciences, University of the West of England, Bristol, United Kingdom; and Centre for Human Psychopharmacology (AS), Swinburne University, Melbourne, Victoria, Australia. Received for publication July 11, 2012; accepted August 17, 2012. Reprint requests: Joris C. Verster, Division of Pharmacology, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Utrecht 3584CG, The Netherlands; Tel.: +31 3