The opioid-palatability hypothesis The evidence discussed to date suggests that opioid peptides play a specific role in some aspect of the neural system underlying the expression of palatability, the opioidpalatability hypothesis. In order to evaluate fully this idea in humans, an alternative measure of orosensory reward is needed in order to get around the potential problems of interpreting hedonic rating data. One approach has been to examine in detail changes both in eating rate and the pattern of changes in subjective appetite during meals following manipulations of palatability alone [79]. Increasing the rated palatability of a food increased intake and also resulted in increases in rated hunger during the early stages of the test meal [80]. One approach to examining these complex data was to model the changes in rated appetite as a function of cumulative intake within the meal. The resulting relationship was best described by a quadratic function with the three components of the function appearing to map onto separate aspects of short-term feeding motivation. Flavour modification only altered the linear component of this function [79], suggesting that this component is sensitive to palatability manipulations. This approach to studying human eating is similar to that developed in animal research [81], and makes a useful model for dissociation of different motivational influences on eating [79]. The opioidpalatability hypothesis was tested using this model by examining the effects of the opioid antagonist naltrexone on intake, eating rate and changes in subjective appetite within a test meal on a baseline (pre-test) day, and after both a placebo and a naltrexone treatment, administered doubleblind [30]. The opioid-palatability hypothesis predicted that opioid blockade should reduce intake and attenuate the short-term stimulation of appetite through palatability indicated by the linear component of the best-fit quadratic function. The results of this study were consistent with this prediction since subjects not only ate less after naltrexone (Fig. 1a), they also no longer showed any increase in appetite during the early stages of eating (Fig. 1b). Additionally, the study tested two different foods (pasta with a less palatable cheese sauce and more palatable tomato sauce: Fig. 1), and the effects of naltrexone were more evident with the more palatable version. These data clearly suggest that opioid release underlies the ability of palatability to stimulate appetite in humans.
Publication bibliography
Yeomans, Martin R.; Gray, Richard W. (2002): Opioid peptides and the control of human ingestive behaviour. In Neuroscience & Biobehavioral Reviews 26 (6), pp. 713–728. DOI: 10.1016/S0149-7634(02)00041-6