Eating for pleasure or just wanting to eat? Reconsidering sensory hedonic responses as a driver of obesity

Appetite control: methodological aspects of the evaluation of foods

This report describes a set of scientific procedures used to assess the impact of foods and food ingredients on the expression of appetite (psychological and behavioural). An overarching priority has been to enable potential evaluators of health claims about foods to identify justified claims and to exclude claims that are not supported by scientific evidence for the effect cited. This priority follows precisely from the principles set down in the PASSCLAIM report. The report allows the evaluation of the strength of health claims, about the effects of foods on appetite, which can be sustained on the basis of the commonly used scientific designs and experimental procedures. The report includes different designs for assessing effects on satiation as opposed to satiety, detailed coverage of the extent to which a change in hunger can stand alone as a measure of appetite control and an extensive discussion of the statistical procedures appropriate for handling data in this field of research. Because research in this area is continually evolving, new improved methodologies may emerge over time and will need to be incorporated into the framework. One main objective of the report has been to produce guidance on good practice in carrying out appetite research, and not to set down a series of commandments that must be followed.

Bariatric surgery and taste: novel mechanisms of weight loss

PURPOSE OF REVIEW

The mechanisms by which obesity surgery and in particular gastric bypass cause weight loss are unclear. The review will focus on the concept of alterations in the sense of taste after obesity surgery.

RECENT FINDINGS

Patients after obesity surgery and gastric bypass in particular change their eating behaviour and adopt healthier food preferences by avoiding high-calorie and high-fat foods. Patients find sweet and fatty meals less pleasant not due to postingestive side effects but through changes in the sense of taste. The acuity for sweet taste increases after gastric bypass potentially leading to increased intensity of perception. Obese patients experience higher activation of their brain taste reward and addiction centres in response to high calorie and fat tasting. Gastric bypass may reverse these taste hedonics, perhaps through the influence on gustatory pathways caused by enhanced gut hormone responses after surgery.

SUMMARY

Elucidation of the metabolic mechanism behind the alterations in taste after obesity surgery could lead to the development of novel surgical and nonsurgical procedures for the treatment of obesity.

HPA axis response to stress predicts short-term snack intake in obese women

Prior research has linked heightened cortisol reactivity to stress with increased food consumption. This pilot study tested corollaries of the hypothesis that cortisol stress reactivity promotes obesity. Thirty-four lean and obese women completed an acute stress task and a non-stressful control task in counterbalanced order. Contrary to expectations, higher post-stress cortisol was associated with decreased post-stress snack intake in obese women but was unrelated to snack intake in lean women. Stress also blunted an expected rise in hunger only among obese women. Findings suggest that some obese women may be more sensitive to short-term anorectic effects of HPA axis activation.

How many calories are on our plate? Expected fullness, not liking, determines meal-size selection

The availability of highly palatable food is thought to stimulate the selection of larger meals (leading to weight gain and obesity). In this article, we explore aspects of this proposition. Specifically, we scrutinize two basic assumptions: (i) palatable energy-dense foods are more rewarding (desired), and (ii) these palatable foods are selected in relatively larger portions. In combination with palatability, we also consider the relative role for "expected satiation"-the extent to which a food is expected to deliver satiation. A total of 17 commonly consumed foods were assessed by 28 normal-weight participants at lunchtime. Critically, our measure of food reward and expected satiation involves comparisons between foods based on equicaloric portions. When assessed in this way, we find that food reward and ideal portion sizes (in kcal) are both closely associated with expected satiation, but not with "expected liking." Low expected satiation (not expected liking) predicts the selection of large portion sizes (in kcal) and foods with this characteristic tend to be more energy dense and are regarded as less (not more) rewarding (when compared calorie for calorie). Together, these findings challenge the role of palatability in meal-size selection and they highlight the importance of expected satiation, a "nonaffective" component of food reward.

Effective connectivity of a reward network in obese women

Exaggerated reactivity to food cues in obese women appears to be mediated in part by a hyperactive reward system that includes the nucleus accumbens, amygdala, and orbitofrontal cortex. The present study used functional magnetic resonance imaging (fMRI) to investigate whether differences between 12 obese and 12 normal-weight women in reward-related brain activation in response to food images can be explained by changes in the functional interactions between key reward network regions. A two-step path analysis/General Linear Model approach was used to test whether there were group differences in network connections between nucleus accumbens, amygdala, and orbitofrontal cortex in response to high- and low-calorie food images. There was abnormal connectivity in the obese group in response to both high- and low-calorie food cues compared to normal-weight controls. Compared to controls, the obese group had a relative deficiency in the amygdala's modulation of activation in both orbitofrontal cortex and nucleus accumbens, but excessive influence of orbitofrontal cortex's modulation of activation in nucleus accumbens. The deficient projections from the amygdala might relate to suboptimal modulation of the affective/emotional aspects of a food's reward value or an associated cue's motivational salience, whereas increased orbitofrontal cortex to nucleus accumbens connectivity might contribute to a heightened drive to eat in response to a food cue. Thus, it is possible that not only greater activation of the reward system, but also differences in the interaction of regions in this network may contribute to the relatively increased motivational value of foods in obese individuals.

Who likes it more? Restrained eaters' implicit attitudes towards food

Previous work has been unclear as to how restrained eating is related to implicit attitudes towards food. The present paper introduces a new variant of the affective priming paradigm to measure implicit attitudes towards food among restrained and unrestrained eaters, using food pictures as primes and emoticons as targets. Results of two studies show that while unrestrained eaters evaluate palatable foods more positively than neutral and unpalatable foods, restrained eaters do not. In addition, it is shown that restrained eaters have stronger evaluations of the negative aspects of palatable food, and this influences their food attitudes. Thus, the present findings suggest that restrained eaters' chronic goal of dieting impacts their implicit attitude towards food items via their perception of the negative aspects of palatable food. These results are discussed with regard to the role of attitudes in the regulation of eating behavior.

Children's liking and wanting of snack products: Influence of shape and flavour

Background

Children's food choices are guided by their preferences. However, these preferences may change due to repeated exposure.

Methods

This study investigated children's (n = 242, 7–12 yrs-old) liking and wanting for snacks over 3 weeks of daily consumption. The snacks differed in size (small vs large) or flavour (sweet vs sweet-sour). Two conditions were designed: 1) a monotonous group in which children continuously consumed the same snack across the 3 weeks, and 2) a free choice group in which children were allowed to freely choose amongst 3 different flavours of the snack each day during 3 weeks.

Results

Shape influenced long-term liking, i.e. small shaped snacks remained stable in liking over repeated consumption, whereas large shaped snacks with the same flavour decreased in liking. Mean wanting ratings for all snack products decreased over 3 weeks daily consumption. Flavour did not significantly influence liking and wanting over time. The ability to freely choose amongst different flavours tended to decrease children's liking (p < 0.1) and wanting (p < 0.001) for these products. Changes in liking rather than initial liking was the best predictor of snack choice during the intervention.

Conclusion

Wanting rather than liking was most affected by repeated daily consumption of snack foods over three weeks. In order to increase the likelihood that children will repeatedly eat a food product, smaller sized healthy snacks are preferred to larger sized snacks. Future research should focus on stabilizing wanting over repeated consumption.

Sip size of orangeade: effects on intake and sensory-specific satiation

Sensory-specific satiation (SSS) drives food selection and contributes to meal termination. We hypothesised that smaller sips would increase SSS due to increased oro-sensory exposure, irrespective of energy content. The objective was to determine the effects of sip size and energy content on ad libitum intake of orangeade and subjective SSS for orangeade. Orangeade intake and ratings of wanting and liking were measured before and after ad libitum orangeade consumption in a 2 × 2 cross-over design (n 53). Conditions differed in energy content (no-energy v. regular-energy orangeade) and in sip size (large, 20 g/sip v. small, 5 g/sip). The mean intake of both orangeades was lower when consumed with small sips than when consumed with large sips (regular-energy, 352 v. 493 g; no-energy, 338 v. 405 g; both P < 0·001). When consumed with large sips, the mean intake of no-energy orangeade was lower than that of regular-energy orangeade (P = 0·02). When consumed with small sips, subjective SSS (based on the desire to drink) was higher for no-energy orangeade than for regular-energy orangeade (P = 0·01), while mean intake was comparable. We concluded that smaller sip size, i.e. increased oro-sensory exposure per unit of consumption, can lower intake of sweet drinks. Only with low oro-sensory exposure (large sip size) was intake higher for an energy-containing sweet drink than for a no-energy sweet drink. This suggests that intake of sweet drinks is stimulated by (metabolic) reward value and inhibited by sensory satiation. This underpins the importance of SSS for meal termination.

Neural correlates of individual differences related to appetite

Using neuroimaging technologies to compare normal weight and obese individuals can reveal much about the pathophysiological state of obesity but such comparisons tell us little about what makes some normal weight individuals susceptible to obesity or about important individual differences amongst obese individuals. The current review therefore reviews neuroimaging research on individual difference measures that can illuminate these important topics. After introducing three neuropsychological models of the nature of motivation to approach rewarding stimuli, neuroimaging research on measures of impulsivity, craving, binge eating, restrained eating and disinhibited eating is reviewed. Although neuroimaging research on individual differences measures of brain activity related to appetite is in its infancy, existing studies suggest that such research could enrich the understanding, prevention and treatment of disordered eating and obesity.

'Liking' and 'wanting' food rewards: brain substrates and roles in eating disorders

What brain reward systems mediate motivational 'wanting' and hedonic 'liking' for food rewards? And what roles do those systems play in eating disorders? This article surveys recent findings regarding brain mechanisms of hedonic 'liking', such as the existence of cubic-millimeter hedonic hotspots in nucleus accumbens and ventral pallidum for opioid amplification of sensory pleasure. It also considers brain 'wanting' or incentive salience systems important to appetite, such as mesolimbic dopamine systems and opioid motivation circuits that extend beyond the hedonic hotspots. Finally, it considers some potential ways in which 'wanting' and 'liking' might relate to eating disorders.

Finding the critical cue: implementation intentions to change one's diet work best when tailored to personally relevant reasons for unhealthy eating

Implementation intentions promote acting on one's good intentions. But does specifying where and when to act also suffice when goals involve complex change that requires not merely initiating a behavior but rather substituting a habit with a new response? In a pilot study and two experiments, the authors investigated the efficacy of implementation intentions to replace unhealthy snacks with healthy snacks by linking different types of cues for unhealthy snacking (if-part) to healthy snacking (then-part). The pilot study identified cues for unhealthy snacking, differentiating between situational (where/when) and motivational (why) cues. Studies 1 and 2 tested the efficacy of implementation intentions that specified either situational or motivational cues in altering snacking habits. Results showed that implementation intentions specifying motivational cues decreased unhealthy snack consumption whereas the classic specification of where and when did not. Extending previous research, for complex behavior change "why" seems more important than "where and when."

Nonnutritive sweetener consumption in humans: effects on appetite and food intake and their putative mechanisms

Nonnutritive sweeteners (NNS) are ecologically novel chemosensory signaling compounds that influence ingestive processes and behavior. Only about 15% of the US population aged >2 y ingest NNS, but the incidence is increasing. These sweeteners have the potential to moderate sugar and energy intakes while maintaining diet palatability, but their use has increased in concert with BMI in the population. This association may be coincidental or causal, and either mode of directionality is plausible. A critical review of the literature suggests that the addition of NNS to non-energy-yielding products may heighten appetite, but this is not observed under the more common condition in which NNS is ingested in conjunction with other energy sources. Substitution of NNS for a nutritive sweetener generally elicits incomplete energy compensation, but evidence of long-term efficacy for weight management is not available. The addition of NNS to diets poses no benefit for weight loss or reduced weight gain without energy restriction. There are long-standing and recent concerns that inclusion of NNS in the diet promotes energy intake and contributes to obesity. Most of the purported mechanisms by which this occurs are not supported by the available evidence, although some warrant further consideration. Resolution of this important issue will require long-term randomized controlled trials.

Effects of temptation and weight on hedonics and motivation to eat in women

OBJECTIVE

The aim of the present study was to examine the extent to which food temptation influences liking, the hedonics of food, and wanting, the motivation to eat, and whether this effect differed between normal-weight and overweight women.

METHODS AND PROCEDURES

Ninety-seven normal-weight and overweight women participated in a randomized experiment, which used a two-by-two design with food temptation and body weight as independent variables. ANOVAs tested the effect of these factors on wanting and liking.

RESULTS

The most important finding of this study was that food temptation had a significant effect on wanting, but not on liking. Wanting was mainly influenced by temptation; however, this effect was moderated by weight. Interestingly, temptation caused a decrease in wanting, but only in normal-weight women. This effect of temptation could not be explained by a change in affect after manipulation or a difference in hunger before the start of the experiment.

DISCUSSION

A possible explanation for the finding that normal-weight women showed a decline in wanting after the confrontation with highly palatable food may be that normal-weight women are protected by a higher sensory-specific satiety. Moreover, it is possible that in these women goals regarding, for example, weight maintenance are more easily evoked, which may remind them of the positive consequences of not yielding into temptation.

Emotion and food. Do the emotions expressed on other people's faces affect the desire to eat liked and disliked food products?

The aim of the present study was to test if pleasure, neutrality and disgust expressed by other individuals on a photograph could affect the desire to eat liked or disliked food products. Forty-four men and women were presented with two series of photographs. The first series of photographs was composed of six food photographs: three liked and three disliked food products. The second series consisted of the same photographs presented with eaters expressing three different emotions: disgust, pleasure or neutrality. Results showed that the effect of the presence of an eater, and of emotions expressed by this eater, depended on the food category. For the liked foods, the desire to eat was higher when these foods were presented alone than with an eater expressing neutral emotion. When the eater expressed pleasure, the desire to eat these liked foods did not significantly increase. In contrast, when the eater expressed disgust, the desire to eat them significantly decreased. When the foods were disliked, the influence of the pleasant social context was stronger than for the liked foods. The desire to eat the disliked foods actually increased in the presence of an eater expressing pleasure. On the contrary, the disgust and neutral context had no effect on the desire for disliked foods.

Personality and eating behaviors: a case-control study of binge eating disorder

OBJECTIVE

Questions have been raised about the validity of binge eating disorder (BED) as psycho-pathologically distinct from other forms of overeating. Our purpose was to ascertain whether BED individuals differed in important ways from nonbinging obese adults.

METHOD

BED adults were recruited from the community as were weight-matched (obese) and normal-weight control (NWC) groups. All groups were equivalent for age and gender distribution, and were assessed on several personality traits and eating behaviors.

RESULTS

BED individuals and obese controls did not differ on the personality traits. Both were more reward sensitive, and had greater anxiousness, impulsivity, and addictive personality traits than NWC. However, BED individuals reported significantly greater hedonic eating compared with the obese, who had higher levels than NWC.

CONCLUSION

Our findings provided no evidence of a psychological identity unique to obese adults with BED although their eating behaviors are markedly hedonically driven-i.e., more responsive to factors external to physiological needs.

Reward sensitivity and the D2 dopamine receptor gene: A case-control study of binge eating disorder

OBJECTIVE

The sensitivity of dopamine reward pathways has been implicated in the risk for various psychiatric disorders including compulsive overeating. The evidence is divided, however, about the direction of causal association. One argument is that a Reward Deficiency Syndrome is the risk factor, while others contend that hyper-sensitivity to reward enhances the motivation for pleasurable activities like eating. Unfortunately, little human research has bridged the gap between psychological and neurobiological approaches to brain reward functioning and disorder. The present study addressed this issue by implementing psychological and biological markers of reward sensitivity in the assessment protocol.

METHODS

Adults with binge eating disorder (BED) were compared to samples of normal-weight and obese controls on two personality measures of reward sensitivity and were genotyped for six markers of the DRD2 dopamine receptor gene.

RESULTS

Genotype x Group ANOVAs revealed significant main effects and an interaction on the personality measures for Taq1A. BED and obese subjects reported greater reward sensitivity than normal-weight controls, but only among those carrying the A1 allele. We also found that normal-weight controls with at least one copy of the T allele of the C957T marker had significantly lower reward sensitivity scores than any of the other groups who did not differ from each other.

CONCLUSIONS

Given evidence linking the A1 allele with reduced receptor density, an inverse relationship was expected between psychological measures of reward sensitivity and presence of the A1 allele. One explanation for our findings could be that the BED and obese participants possess another genetic variant that interacts with the A1 allele to produce higher dopamine activity. These findings have implications for future studies of the molecular genetics of BED and obesity, and for behavioural and pharmacologic therapies targeting these conditions.

Widespread reward-system activation in obese women in response to pictures of high-calorie foods

Behavioral studies have suggested that exaggerated reactivity to food cues, especially those associated with high-calorie foods, may be a factor underlying obesity. This increased motivational potency of foods in obese individuals appears to be mediated in part by a hyperactive reward system. We used a Philips 3T magnet and fMRI to investigate activation of reward-system and associated brain structures in response to pictures of high-calorie and low-calorie foods in 12 obese compared to 12 normal-weight women. A regions of interest (ROI) analysis revealed that pictures of high-calorie foods produced significantly greater activation in the obese group compared to controls in medial and lateral orbitofrontal cortex, amygdala, nucleus accumbens/ventral striatum, medial prefrontal cortex, insula, anterior cingulate cortex, ventral pallidum, caudate, putamen, and hippocampus. For the contrast of high-calorie vs. low-calorie foods, the obese group also exhibited a larger difference than the controls did in all of the same regions of interest except for the putamen. Within-group contrasts revealed that pictures of high-calorie foods uniformly stimulated more activation than low-calorie foods did in the obese group. By contrast, in the control group, greater activation by high-calorie foods was seen only in dorsal caudate, whereas low-calorie foods were more effective than high-calorie foods in the lateral orbitofrontal cortex, medial prefrontal cortex, and anterior cingulate cortex. In summary, compared to normal-weight controls, obese women exhibited greater activation in response to pictures of high-calorie foods in a large number of regions hypothesized to mediate motivational effects of food cues.

The role of implicit wanting in relation to explicit liking and wanting for food: Implications for appetite control

Eating is an action open to awareness by the individual; however, it cannot be claimed that processes that control the expression of eating habits are necessarily explicit. This distinction between implicit and explicit processes may enhance understanding of the expression of food reward (particularly the concepts of liking and wanting [Berridge, K. C., & Robinson, T. E. (2003). Parsing reward. Trends in Neurosciences, 26, 507-513] and its importance for human appetite control [Finlayson, G. S., King, N. A., & Blundell, J. E. (2007b). Liking vs. wanting food: Importance for human appetite control and weight regulation. Neuroscience and Biobehavioural Reviews, in press]. The present study investigated the effect of meal-induced satiation on implicit and explicit processes of liking (L) and wanting (W) by developing a computer-based procedure to measure L and W in hungry and satiated states. Explicit measures were derived from analogue ratings whilst an implicit W measure was derived from reaction time in a forced-choice procedure, which also identified food preferences. Seventy subjects (21.8+/-0.9 years, BMI: 22.2+/-0.5 kg/m2) completed the procedure before and immediately following consumption of a savoury test meal. Satiation caused explicit ratings of L and W to decrease in all food categories (p<0.01); but with a more marked decrease for savoury foods compared with sweet foods (p<0.01). Implicit W was increased for sweet categories (p<0.01), but not for savoury. Implicit and explicit measures of L and W independently correlated with preference for sweet foods. This study provides support that implicit and explicit processes of food reward can be simultaneously measured and dissociated using a test meal. Adjustments in hunger were linked to changes in explicit L and W in a manner consistent with sensory specific satiety, while a relationship between hunger and implicit W was absent. We suggest that implicit W is not systematically downregulated by the physiological consequences of food consumption in the same way as hunger and therefore may be largely independent of homoeostatic processes influencing intake.

Sensory-specific satiety, its crossovers, and subsequent choice of potato chip flavors

We investigated the influence of liking and flavor intensity on the development of sensory-specific satiety (SSS) to different potato chip flavors, and the influence of these measures, as well as measures of want-to-eat and similarity, on the subsequent choice of a potato chip flavor. In the first study, 35 subjects participated first in a taste test to measure flavor intensity, liking and similarities among six different flavors of potato chips. They then completed six SSS sessions, ending each session by choosing one of the six flavors for additional consumption. SSS varied among the six chip flavors, but was poorly related to either liking or flavor intensity. Subjects chose better-liked flavors, flavors dissimilar to recently consumed flavors, flavors differing in intensity from the recently consumed flavor, flavors that produced less SSS and flavors that produced less change in wanting-to-eat them. In the second study, we used data from a consumption diary panel, and replicated the key finding that when people switch flavors, the similarity to the flavor consumed on the previous occasion decreases the probably of that chip being chosen. Thus switching among flavor choices was driven by liking, the desire for variety and the desire for a product that produced less SSS.

Flavour-flavour learning occurs automatically and only in hungry participants

A novel flavour may become liked if it is presented repeatedly and in combination with a second flavour that is already liked. Conceptually, this 'flavour-flavour learning' is important, because it can account for many of our everyday food and flavour preferences. However, relatively little is known about the underlying process because learning paradigms have lacked reliability. Based on previous research we explored whether learning is determined by three variables; i) hunger state, ii) demand and contingency awareness, and iii) dietary restraint. Participants (male n=15/female n=15) consumed three different and novel-tasting fruit teas. One of the teas had a non-caloric sweetener added (CS+) and two were unsweetened (CS-). Before and after this training the participants ranked their preference for unsweetened versions of the three teas. We found that the training increased preference for the CS+ relative to the CS- teas. However, this effect was only found in hungry participants. We also found little evidence that learning was related to whether the participants could identify (recognition test) the specific tea that had been sweetened during training, suggesting that the underlying process is automatic and it operates outside conscious awareness. Learning was not predicted by dietary restraint (measured using the DEBQ-R scale). Together, these findings provide further evidence for a linkage between flavour-flavour learning and flavour-nutrient learning.

Liking vs. wanting food: importance for human appetite control and weight regulation

Current train of thought in appetite research is favouring an interest in non-homeostatic or hedonic (reward) mechanisms in relation to overconsumption and energy balance. This tendency is supported by advances in neurobiology that precede the emergence of a new conceptual approach to reward where affect and motivation (liking and wanting) can be seen as the major force in guiding human eating behaviour. In this review, current progress in applying processes of liking and wanting to the study of human appetite are examined by discussing the following issues: How can these concepts be operationalised for use in human research to reflect the neural mechanisms by which they may be influenced? Do liking and wanting operate independently to produce functionally significant changes in behaviour? Can liking and wanting be truly experimentally separated or will an expression of one inevitably contain elements of the other? The review contains a re-examination of selected human appetite research before exploring more recent methodological approaches to the study of liking and wanting in appetite control. In addition, some theoretical developments are described in four diverse models that may enhance current understanding of the role of these processes in guiding ingestive behaviour. Finally, the implications of a dual process modulation of food reward for weight gain and obesity are discussed. The review concludes that processes of liking and wanting are likely to have independent roles in characterising susceptibility to weight gain. Further research into the dissociation of liking and wanting through implicit and explicit levels of processing would help to disclose the relative importance of these components of reward for appetite control and weight regulation.

Evolved to satisfy our immediate needs: Self-control and the rewarding properties of food

Evolutionary explanations of overeating in modern society emphasize that humans have evolved to eat to their physiological limits when food is available. The present paper challenges the idea that eating is driven by the availability of food only and proposes that it is regulated by strategic anticipatory behaviour in service of the most profitable long-term scenario as well. Our alternative explanation emphasizes the interaction between the reward system that regulates the liking and wanting of food and the role of self-control, which is involved in maintaining the best outcome in the long run.