The effects of calories and taste on habituation of the human salivary response

Decoupling of desire and salivation over repeated chocolate consumption and the moderating role of food legalizing

This study aimed to investigate the responses in desire and salivation during repeated chocolate consumption, and examining how these responses are influenced by a relaxed relationship with food, or 'food legalizing'. Salivation is often used as a proxy for desire, though evidence for this correlation is mixed. We hypothesized that both desire and salivation would decrease with repeated chocolate intake. Additionally, research has suggested that eating styles may affect habituation rates. We proposed that individuals with the food legalizing trait would habituate more rapidly to chocolate, providing an alternative mechanism to reward sensitivity associated with restrained eating. Fifty healthy-weight individuals participated in the study, consuming five blocks of chocolate (each 4 g, 22 calories) over trials. After the trials, participants were allowed to eat as much chocolate as they desired. The results showed that salivation was not correlated with self-reported desire over repeated chocolate consumption. While desire decreased with repeated intake and predicted ad libitum consumption, salivation increased and did not predict ad libitum consumption. Furthermore, food legalizing moderated the rate of responding in terms of the desire to eat but did not affect salivary flow. These findings suggest that salivation is not a reliable physiological measure of desire when eating chocolate. Instead, salivation appears to reflect sensitization to the sensory characteristics of chocolate and is less predictive of subsequent ad libitum chocolate consumption than self-reported desire. Lastly, having a carefree relationship with chocolate may help regulate its consumption, highlighting the potential benefits of a relaxed attitude toward food.

Food reinforcement and habituation to food are processes related to initiation and cessation of eating

An individual bout of eating involves cues to start eating, as well as cues to terminate eating. One process that determines initiation of eating is food reinforcement. Foods with high reinforcing value are also likely to be consumed in greater quantities. Research suggests both cross-sectional and prospective relationships between food reinforcement and obesity, food reinforcement is positively related to energy intake, and energy intake mediates the relationship between food reinforcement and obesity. A process related to cessation of eating is habituation. Habituation is a general behavioral process that describes a reduction in physiological or affective response to a stimulus, or a reduction in the behavioral responding to obtain a stimulus. Repeated exposure to the same food during a meal can result in habituation to that food and a reduction in consumption. Habituation is also cross-sectionally and prospectively related to body weight, as people who habituate slower consume more in a meal and are more overweight. Research from our laboratory has shown that these two processes independently influence eating, as they can account for almost 60% of the variance in ad libitum intake. In addition, habituation phenotypes show reliable relationships with reinforcing value, such that people who habituate faster also find food less reinforcing. Developing a better understanding of cues to start and stop eating is fundamental to understanding how to modify eating behavior. An overview of research on food reinforcement, habituation and food intake for people with a range of weight status and without eating disorders is provided, and ideas about integrating these two processes that are related to initiation and termination of a bout of eating are discussed.

High reinforcing value of food is related to slow habituation to food

BACKGROUND

The rate of habituation to food and reinforcing value of food are processes that are related to food intake and body weight. Reinforcing value of food provides an index of the motivation to eat, while habituation provides an index on how repeated presentations of food relate to reduced behavioral and physiological responses to that food.

OBJECTIVE

To investigate the relationship between habituation phenotypes to salty, sweet and savory foods and reinforcing value of those foods.

DESIGN

A sample of 229 8-12-year-old children at risk for obesity completed reinforcing value and habituation tasks for foods that varied in savory, sweet or salty tastes. Multivariate Group Based Trajectory Modeling was used to create four unique patterns of habitation to foods based on detailed 10 s epochs of responding for food over a 24 min task. Differences in reinforcing value of food were assessed for these habituation phenotypes.

RESULTS

Results showed a graded relationship between the overall habituation phenotypes and reinforcing value of food, as those with the relatively flat habituation phenotype found food more reinforcing than those with phenotypes that showed rapidly decelerating responding across all taste categories.

CONCLUSIONS

Those who habituated slower found food more reinforcing than those with a rapid habituation phenotype. Implications of these phenotypes for understanding how habituation relates to food intake are discussed.

Food reinforcement, energy intake, and macronutrient choice

BACKGROUND

Food is a powerful reinforcer that motivates people to eat. The relative reinforcing value of food (RRV(food)) is associated with obesity and energy intake and interacts with impulsivity to predict energy intake.

OBJECTIVE

How RRV(food) is related to macronutrient choice in ad libitum eating tasks in humans has not been studied; however, animal research suggests that sugar or simple carbohydrates may be a determinant of reward value in food. This study assessed which macronutrients are associated with food reinforcement.

DESIGN

Two hundred seventy-three adults with various body mass indexes were assessed for RRV(food), the relative reinforcing value of reading, food hedonics, energy intake in an ad libitum taste test, and usual energy intake derived from repeated 24-h dietary recalls. Multiple regression was used to assess the relation between predictors of total energy and energy associated with macronutrient intake after control for age, sex, income, education, minority status, and other macronutrient intakes.

RESULTS

The results showed that the relative proportion of responding for food compared with reading (RRV(prop)) was positively related to body mass index, laboratory-measured energy intake, and usual energy intake. In addition, RRV(prop) was a predictor of sugar intake but not of total carbohydrate, fat, or protein intake.

CONCLUSION

These results are consistent with basic animal research showing that sugar is related to food reward and with the hypothesis that food reward processes are more strongly related to eating than are food hedonics. This trial was registered at clinicaltrials.gov as NCT00962117.

Salivary habituation to food stimuli in successful weight loss maintainers, obese and normal-weight adults

OBJECTIVE

Research shows that slower habituation of salivary responses to food stimuli is related to greater energy intake and that obese (Ob) individuals habituate slower than those of normal weight (NW). No study has examined habituation rates in weight loss maintainers (WLMs) who have reduced from obese to normal weight, relative to those who are Ob or NW.

DESIGN

Salivation to two baseline water trials and 10 lemon-flavored lollipop trials were studied in 14 WLMs, 15 Ob and 18 NW individuals comparable in age, gender and ethnicity. Linear mixed models were used to compare WLMs with Ob and NW groups.

RESULTS

Salivation in the WLM and NW groups decreased significantly (for both P <0.005) across trials, indicative of habituation. Salivary responses in the Ob group did not habituate (P=0.46). When compared with Ob group, WLMs showed a quicker reduction in salivation (P<0.05). WLM and NW groups did not differ in habituation rate (P=0.49).

CONCLUSIONS

WLMs have habituation rates that are comparable to NW individuals without previous history of obesity, and show quicker habituation than those who are currently obese. These results suggest that physiological responses to food may 'normalize' with successful weight loss maintenance.

Habituation as a determinant of human food intake

Research has shown that animals and humans habituate on a variety of behavioral and physiological responses to repeated presentations of food cues, and habituation is related to amount of food consumed and cessation of eating. The purpose of this article is to provide an overview of experimental paradigms used to study habituation, integrate a theoretical approach to habituation to food based on memory and associative conditioning models, and review research on factors that influence habituation. Individual differences in habituation as they relate to obesity and eating disorders are reviewed, along with research on how individual differences in memory can influence habituation. Other associative conditioning approaches to ingestive behavior are reviewed, as well as how habituation provides novel approaches to preventing or treating obesity. Finally, new directions for habituation research are presented. Habituation provides a novel theoretical framework from which to understand factors that regulate ingestive behavior.

Habituation and within-session changes in motivated responding for food in children

Within-session decreases in instrumental responding to obtain food, consistent with habituation, have been reliably demonstrated in adults and children. This study tested the hypothesis that within-session decreases in instrumental responding for food are due to habituation rather than satiation. Thirty-eight 8-12-year-old children performed a computer-based operant task to earn points toward access to potato chips for 20 min, and for chocolate candies for the final 6 min of the session. Portion size of the food reinforcer (75 kcal vs 225 kcal) and food consumption (consumption/no consumption). There was no difference in the rate of response decrease between the two portion size conditions. Both the consumption and non-consumption groups showed response decelerations during the first 20 min, with responses in the consumption group decreasing at a faster rate. When the novel food was presented, participants in all conditions recovered responding. Although satiation may contribute to reductions in motivated responding for food when food is consumed, habituation provides a more complete explanation for the results observed in this study.

Television watching increases motivated responding for food and energy intake in children

BACKGROUND

Sedentary activities, such as watching television, may disrupt habituation to food cues, thereby increasing motivation to eat and energy intake.

OBJECTIVE

These experiments were designed to examine the effect of television watching on habituation of ingestive behavior in children.

DESIGN

In experiment 1, all children worked for access to cheeseburgers in trials 1-7 (habituating stimulus). In trials 8-10, children in the control group continued to work for cheeseburgers without any dishabituating stimuli, whereas children in the other groups received either a novel food (French fries) or television as dishabituating stimuli. Responding for food and amount of food eaten were measured. In experiment 2, all children had access to 1000 kcal of a preferred snack food. One group watched a continuous television show, and the control groups either watched no television or watched a repeated segment of a television show, which controls for the television stimulus but requires reduced allocation of attention.

RESULTS

In experiment 1, both the novel food and the television watching groups reinstated responding for food (P = 0.009) and increased the amount of energy earned (P = 0.018) above the level of the control subjects. In experiment 2, the continuous television group spent more time eating (P < 0.0001) and consumed more energy than the no television and the repeated segment groups (P = 0.007).

CONCLUSION

These experiments show that television watching can dishabituate eating or disrupt the development of habituation, which may provide a mechanism for increased energy intake associated with watching television.

Habituation of responding for food in humans

Habituation to repeated food stimuli has been demonstrated in various response systems across animals and humans. Patterns of responding to obtain food demonstrate many empirical characteristics of habituation, and the purpose of the present study was to determine whether motivated responding for food in humans follows an habituation pattern. Thirty-five nonobese men were randomized to groups in which they responded to gain access to repeated presentations of the same food or presentations of a variety of isocaloric food. Subjective ratings of food liking and hunger were assessed. Consistent with habituation theory, participants working for one type of food demonstrated a more rapid decrease in responding for food and in ratings of liking of the repeatedly presented food than participants working for varied foods. All participants showed similar reductions of hunger and resumed responding for a novel food stimulus. This study documents that motivated responding for food in human shares characteristics of an habituation process.

Dietary variety, energy regulation, and obesity

Increased variety in the food supply may contribute to the development and maintenance of obesity. Thirty-nine studies examining dietary variety, energy intake, and body composition are reviewed. Animal and human studies show that food consumption increases when there is more variety in a meal or diet and that greater dietary variety is associated with increased body weight and fat. A hypothesized mechanism for these findings is sensory-specific satiety, a phenomenon demonstrating greater reductions in hedonic ratings or intake of foods consumed compared with foods not consumed. Nineteen studies documenting change in preference, intake, and hedonic ratings of food after a food has been eaten to satiation in animals and humans are reviewed, and the theory of sensory-specific satiety is examined. The review concludes with the relevance of oral habituation theory as a unifying construct for the effects of variety and sensory-specific satiety, clinical implications of dietary variety and sensory-specific satiety on energy regulation, and suggestions for future research.

Differences in salivary habituation to palatable foods in bulimia nervosa patients and controls

OBJECTIVE

Bulimia nervosa (BN) patients have been shown to experience anomalous responses to food and food cues. We investigated the response to food over time by presenting repeated food cues and measuring changes in physiological (ie, salivation) and subjective responses.

METHOD

Subjects were 18 BN women and 18 matched control women. Two palatable food stimuli, regular or frozen yogurt, that varied in macronutrient composition but had similar sensory characteristics, were presented to subjects repeatedly during a laboratory session. After two baseline salivation measures, subjects were presented with eight trials of one of the two yogurts. On Trial 9 a lemon juice dishabituator was presented, with the yogurt stimulus presented again at Trial 10.

RESULTS

We found that control subjects had decreased salivation after repeated food presentations. In comparison, BN subjects failed to show a decrease in salivation. The desire to binge increased over trials for the BN subjects, but remained stable for normals.

CONCLUSIONS

These data suggest that salivary habituation may be abnormal in BN patients.

The effect of dietary fat on salivary habituation and satiation

This study was designed to explore the effect of dietary fat and carbohydrate on oral habituation. Forty women (18-21 years old) were randomized to 1 of 4 yogurt conditions that varied levels of dietary fat and carbohydrate. Subjects consumed 0.4 g of yogurt per pound per trial until they indicated fullness in up to 15 trials, with salivation, number of trials to fullness, and ratings of hedonics, appetite, and fullness measured. Subjects in the high-fat conditions demonstrated a significantly faster rate of salivary habituation than subjects in the low-fat conditions, and consumed less volume of yogurt but consumed more calories. The rate of habituation was not influenced by carbohydrate content of the yogurt. These findings suggest that oral habituation in humans is sensitive to macronutrient content of food.

Allocation of attentional resources during habituation to food cues

Two experiments were designed to test the hypothesis that habituation to repeated food cues can be inhibited by allocating processing resources to nonfood cues. In two experiments, the salivary response to 10 presentations of lemon yogurt was assessed while subjects engaged in a controlled cognitive search task (demanding attentional resources), an automatic search task (needing fewer attentional resources), or no task. In Experiment 1, the controlled and automatic search tasks differed in the number of memory set items. In Experiment 2, the size of the memory sets was held constant, and individuals were provided practice to stabilize the different search strategies in the task. The automatic search and no task groups habituated to the repeated presentation of food cues in both experiments, but the controlled search group did not. These results support the hypothesis that allocation of attentional resources to external cues can influence the processing of food cues.

Changes in taste and satiety in dietary-restrained women following stress

Stress has been shown to increase food consumption in women with high levels of dietary restraint (restrainers) but decrease consumption in nonrestrainers. The present study was designed to replicate the differential eating pattern following stress and measure physiological and subjective responses to food over repeated taste presentations. Restrained (N = 16) and nonrestrained (N = 16) women were given eight taste presentations of a food followed by an ad lib taste test. Between taste trials, half of the restrainers and nonrestrainers performed a variation of the Stroop stressor while remaining subjects sat quietly. Salivation to the food cue was measured at each trial as well as ratings of food liking, hunger, fullness, and arousal. Results showed significant effects of restraint on food liking and stress condition on hunger. Restrainers increased liking ratings over taste trials whereas ratings for nonrestrainers increased and then decreased to baseline levels by the last trial (p = 0.05). Nonstressed subjects showed an increase in hunger ratings, whereas ratings for stressed subjects did not show any stable directional pattern (p = 0.04). Salivary responses decreased for all groups (p = 0.01). A significant interaction of restraint by stress for intake was found (p = 0.03); restrainers increased consumption following stress whereas nonrestrainers decreased consumption as compared with controls. The data suggest that the Stroop task can influence intake in restrainers, but the changes in intake did not directly correspond to the changes observed during the presentations of the taste cues and stressor.

Differences in salivation to repeated food cues in obese and nonobese women

In a series of studies we have shown that salivation, a cephalic phase preingestive response, habituates to repeated presentations of olfactory or gustatory cues in nonobese subjects. Previous research has studied the differences in anticipatory response to food cues in obese vs. nonobese subjects. This study was designed to assess if obese and nonobese females differed in their patterns of salivary response to repeated presentation of palatable food cues. The salivary response to 10 gustatory presentations of lemon yogurt was studied in 10 obese and 10 nonobese nonrestrained women. Results showed significant differences in the pattern of salivary responding, with obese subjects showing a significantly slower decline in salivation than nonobese subjects. These results are consistent with the hypothesis that obese women differ from nonobese women in their pattern of response to repeated food cues. The results are discussed in relationship to models of intake that focus on differences in satiety or differences in the reinforcing value of food between obese and nonobese subjects.