Background: Food reinforcement, ie, motivation to obtain food, is associated with energy intake and obesity. Finding ways to decrease the reinforcing value of unhealthy foods may help with adherence to diets and maintenance of weight loss. Our previous study in nonobese adults showed that daily consumption of the same snack food (food consumed apart from meals) for 14 d significantly decreased its reinforcing value. Objectives: The aims of this study were to replicate and extend these findings to obese individuals and to examine the effects of different portion sizes of snack foods on food reinforcement. Design: Food reinforcement and liking were tested in 31 obese and 27 nonobese women at baseline and after 2 wk of daily consumption of 0, 100, or 300 kcal/d of the same snack food. Results: We found a significant interaction of phase, portion size, and body mass index on the pattern of operant responding for food. Obese women had a significant increase in food reinforcement after consuming the 300-kcal portion of food for 2 wk, whereas nonobese women had the opposite response. No significant differences were found on the reinforcing value with the 0-and 100-kcal portion-size conditions. Women in the 300-kcal group (obese and nonobese) reported a significant decrease in snack food liking from baseline to after daily intake. Conclusions: These findings suggest that obese and nonobese women respond differently to the daily intake of a snack food and that this may not be a viable mechanism for reducing food reinforcement in obese women. This trial was registered at www.clinicaltrials.gov as NCT00837694.Am J Clin Nutr 2009;90:304-13.

Caffeine use is increasing among children and adolescents, but the effects of caffeine use on behavior and physiology within this population remain understudied and poorly understood. The purpose of this study was to test the hypothesis that adolescents find caffeinated soda more reinforcing than noncaffeinated soda and that this would be related to the level of usual caffeine consumption and to sex. We measured operant responding for portions of caffeinated and noncaffeinated soda at baseline and after daily consumption of 32 oz of caffeinated and noncaffeinated soda for 1 week each in 12-17-year-old participants. Participants also completed a behavioral checklist, a beverage-liking questionnaire, and a 24-h dietary recall to assess the energy intake at baseline and again after each week of beverage consumption. There was no difference in reinforcing value of noncaffeinated or caffeinated soda as a function of usual caffeine consumption. However, males found the caffeinated soda significantly more reinforcing than did females after the exposure period. To our knowledge, this is the first study to show a sex difference in the reinforcing value of caffeinated soda. These data suggest that boys may be more susceptible to the reinforcing effects of caffeine.