Measuring the Reward Value of Aspartame-Sweetened Yogurt Shake via Continuous and Progressive Ratio Schedules in Humans

The purpose of this study was to develop a methodology for measuring the reward value of food. Potentially, this methodology may help determine why people tend to regain weight after stopping a diet program. Human subjects were given aspartame-sweetened yogurt shake and non-sweet yogurt shake on continuous, or uninhibited, and progressive ratio, or inhibited for increasing intervals, schedules. It was expected that subjects would consume more of the sweet than non-sweet shake on each schedule, and that the progressive ratio schedule would amplify the difference between intake of sweet and non-sweet shake. For males and females together, intake measured reward value only on the continuous schedule. Also, males consumed significantly higher quantities of both sweet and non-sweet shake than females on the continuous schedule, indicating that males may consume markedly more than females only when the food is easily obtained. A second variable, post-meal palatability rating on a nine-point scale, measured reward value for both sexes together on the continuous schedule, and for females, also on the progressive ratio schedule. This variable predicted intake in females, suggesting that females place more emphasis on taste in determining intake than do males. On the continuous schedule, a one-point increase on the nine-point palatability scale corresponded to a 100 gram intake increase. Ultimately, the study showed that when there is no obstacle to obtaining the reinforcer, intake and post-meal palatability rating on a nine-point scale predict reward value, and that males and females stress different factors in deciding how much to consume.