Post-ingestive Satiety Research Articles

A biphasic reduction in a measure of palatability following sucrose consumption in mice

Consumption of foods results in a transient reduction in hedonic value that influences the extent and nature of feeding behavior. The time course of this effect, however, is poorly specified. In an initial experiment, using an analysis of the microstructure of licking in mice we found that consumption of sucrose led to a rapid reduction in lick cluster size, a measure of palatability, which recovered after 10 min, but reemerged 60min after initial consumption. We then replicated the finding that lick cluster size is reduced after 60min, but not 10min, under conditions in which a number of potential behavioural confounds were removed. In Experiment 2 the effect was replicated using a between-subjects design that ruled out the possibility that the effect was a specific consequence of the within-subjects procedures used in the first experiment, in which mice may have come to expect sucrose at different time points within the feeding session. While Experiments 1 and 2 confounded the time between periods of access to sucrose with time since the start of the feeding session, this confound was removed in Experiment 3, and, similar to the previous experiments, it was found that a second reduction in palatability occurred after 60min. Therefore, the effect was dependent only on the time since the previous exposure to sucrose, demonstrating that sucrose consumption initiates a biphasic reduction in palatability. The reduction in lick cluster size after 60min was not typically accompanied by a reduction in consumption suggesting that the more slowly developing reduction in the palatability measure was not simply a consequence of post-ingestive satiety. The cause of the biphasic change is not yet clear, and may reflect independent processes or the consequence of a single process that initiates multiple changes in palatability over time.

Umami flavor enhances appetite but also increases satiety

Monosodium glutamate (MSG) has been shown to increase satiety when combined with protein. Inosine 5'-monophosphate acts synergistically with MSG when tasted, is present in high-protein sources, and may potentially further enhance satiety. We assessed effects of a combination of monosodium glutamate and inosine 5'-monophosphate (MSG/IMP) provided either alone or in a high-energy, high-carbohydrate and -protein soup on appetite during ingestion and postingestive satiety. Fixed portions (450 g) of a low-energy control and high-energy, high-carbohydrate and -protein soup preload with added monosodium glutamate and inosine 5'-monophosphate (MSG/IMP+) or without added monosodium glutamate and inosine 5'-monophosphate (MSG/IMP-) were consumed on 4 nonconsecutive days, and changes in appetite during soup intake and at a subsequent ad libitum lunch were assessed in 26 low-restraint volunteers by using a within-participant design. MSG/IMP+ conditions significantly reduced subsequent intake more than the MSG/IMP- condition did irrespective of energy. The high-carbohydrate and -protein condition also reduced intake independently of MSG/IMP. Energy compensation was greater in the MSG/IMP+ carbohydrate and protein conditions than MSG/IMP- condition. The addition of the MSG/IMP+ also increased the soup pleasantness and caused an immediate increase in appetite when the soup was first tasted. The addition of MSG/IMP to a low-energy preload had a biphasic effect on appetite by stimulating appetite during ingestion and enhancing postingestive satiety.

A randomized 3x3 crossover study to evaluate the effect of Hass avocado intake on post-ingestive satiety, glucose and insulin levels, and subsequent energy intake in overweight adults

BackgroundThe behavioral outcome of food ingestion is a complex process that involves psychological and biological factors. Avocados are nutrient dense with properties that may favorably impact energy balance. This study sought to evaluate if incorporating approximately one half of a Hass avocado by addition or inclusion into a lunch meal will influence post-ingestive satiety, glucose and insulin response, and subsequent energy intake among overweight adults.MethodsThis was a randomized 3x3 single-blind crossover design study with 26 healthy overweight adults (mean ±SD age 40.8±11.0 years and BMI 28.1±2.4 kg/m2). Participants consumed a standardized breakfast followed by 1 of 3 lunch test meals [Control (C), avocado-free; Avocado Inclusive (AI); and, Avocado Added (AA)]. Participants rated five appetite sensations using a visual analog scale (VAS) before lunch and at specific intervals over 5 hours following the start of the test meal. Blood glucose and insulin were measured before lunch and at specific intervals over 3 hours following the start of the test meal. Mixed models were used to compare differences among the 3 test meals, and the area under the curve (AUC0-xh) was computed for the VAS and biological measures.ResultsThere were significant differences in the AUC(0-5h) for the self-reported feelings of satisfaction (P=0.04) and desire to eat (P=0.05) in the mixed model analysis. Compared to the C test meal, the AA test meal increased satisfaction by 23% (P=0.05) and decreased the desire to eat by 28% (P=0.04) for the AUC(0-5h). For the AUC(0-3h), the AA test meal increased satisfaction by 26% (P=0.02) and decreased the desire to eat by 40% (P=0.01) as compared to the C test meal. Compared to the AI meal, the AUC(0-3h) for blood insulin was higher in the C and AA meals (P=0.04 and P=0.05, respectively).ConclusionsThe addition of approximately one half of a Hass avocado at a lunch meal can influence post-ingestive satiety over a subsequent 3 and 5 hour period in overweight adults. A caveat to these findings is that the avocado contained an additional 112 kcal, which may have accounted for the observed increase in satisfaction and decreased desire to eat. Future trials are warranted to evaluate the effects of avocado intake on weight management in adults of varying BMIs and among insulin resistant individuals.

Does monosodium glutamate interact with macronutrient composition to influence subsequent appetite?

The influence of flavour enhancers such as monosodium glutamate (MSG) on satiation and satiety is unclear, and the present study aimed to explore this by examining the effects consumption of soups varying in MSG (1% MSG added or no MSG) and macronutrient content (added carbohydrate, protein or control) had on appetite. 24 non-obese, low-restraint male participants consumed a fixed portion of soup and rated their appetite before, immediately after intake and at 15minute intervals for 120min post-ingestion across six sessions. Added MSG significantly increased flavour pleasantness and tended to result in a smaller decrease in hunger immediately after soup ingestion. MSG also reduced rather than enhanced feelings of fullness immediately after ingestion of the high protein soup. As expected, hunger increased, and fullness decreased, over the subsequent 120min, but the increase in hunger was significantly lower in the MSG than no-MSG conditions with the protein soup between 30 and 60min post-ingestion. Overall these data suggest that MSG may have a bi-phasic effect on appetite, with reduced satiation mediated by effects on palatability, but potential for enhanced post-ingestive satiety particularly in the context of protein ingestion.

The emulsified lipid Fabuless (Olibra) does not decrease food intake but suppresses appetite when consumed with yoghurt but not alone or with solid foods: A food effect study

The lipid emulsion Fabuless (Olibra) has been shown in some studies to decrease short/medium term energy intake (EI) and prevent weight regain. The purported mechanism is the ileal brake. Whether Fabuless is efficacious under a range of dietary conditions is unknown since studies have administered the emulsion within a fermented, semi-liquid dairy yoghurt, and outcomes have been inconsistent. To determine whether Fabuless suppresses post-ingestive satiety and short-term food intake under a range of dietary conditions and forms we administered the emulsion co-presented with 185mL water, stirred into a semi-liquid dairy yoghurt, and co-presented with a solid food breakfast muffin. This was a cross-over study in 18 lean men randomised to 6 treatments: (i) lipid emulsion, LE (15g Fabuless, containing 4.2g lipid, 0.2MJ)+water, (ii) lipid control, LC (15g non-emulsified lipid/water, containing 4.2g lipid, 0.2MJ)+water, (iii) lipid emulsion+yoghurt, LE+Y (1.2MJ), (iv) lipid control+yoghurt, LC+Y (1.2MJ), (v) lipid emulsion+muffin, LE+M (1.2MJ), (vi) lipid control+muffin, LC+M (1.2MJ), each given as a test breakfast at 8.30am. Participants rated postprandial appetite sensations using visual analogue scales (VAS), and ad libitum energy intake was measured at a lunch meal 3.5h later. The lipid emulsion increased fullness compared with an energy-matched lipid control but only when administered within the semi-liquid fermented yoghurt (P<0.05). There were no effects on satiety ratings when co-presented with water or with the solid food muffin. Energy and macronutrient intake were not significantly decreased by any of the emulsion treatments. We conclude that effects are small, the format in which lipid emulsions are consumed influences postprandial satiety, and there is no evidence that this emulsion alters eating behaviour at the subsequent meal.

No evidence of differential effects of SFA, MUFA or PUFA on post-ingestive satiety and energy intake: a randomised trial of fatty acid saturation

BackgroundHigh fat diets have long been associated with weight gain and obesity, and the weak satiety response elicited in response to dietary lipids is likely to play a role. Suppression of appetite and food intake has consistently been shown to be diminished with high fat relative to either high protein or carbohydrate meals. There is however some evidence that the satiating capacity of lipids may be modulated when physicochemical properties are altered, but studies investigating the effect of lipid saturation on appetite have generated inconsistent findings. This study investigated the effects of changes in fatty acid saturation on post-ingestive satiety and energy intake.MethodsHigh-fat (HF) test breakfasts (2.0 MJ) containing 26 g lipid were given to 18 healthy, lean men in a 3 treatment randomised cross-over design, each treatment separated by a washout of at least 3 days. The breakfasts were high in saturated (SFA, 65% of total fat), polyunsaturated (PUFA, 76%) or monounsaturated (MUFA, 76%) fatty acids, and comprised 2 savoury muffins. Participants rated appetite sensations using visual analogue scales (VAS) to assess palatability immediately following the meals, and hunger and fullness prior to the HF breakfast and throughout the day. Energy intake was measured by covert weighing of a lunch meal which was served 3.5 h after the breakfast, and from which the participants ate ad libitum.ResultsThere was no difference in VAS ratings of pleasantness, visual appearance, smell, taste, aftertaste and overall palatability between the 3 high-fat test breakfasts. However, there was also no differential effect of the 3 treatments on ratings of hunger, fullness, satisfaction or prospective food consumption during the 3.5 h following the breakfast meal and over the full 6 h experiment. Energy and macronutrient intake at lunch also did not differ between treatments (mean, sem; SFA: 5275.9 ± 286.5 kJ; PUFA: 5227.7 ± 403.9 kJ; MUFA: 5215.6 ± 329.5 kJ; P > 0.05). The maximum difference in energy intake between treatments was less than 2%.ConclusionsThere was no evidence of a difference in post-ingestion satiety between high fat meals which differed in saturation profile in this group of lean, healthy men.Trial RegistrationACTRN12610000193077

Operant behavior in dwarf hamsters (Phodopus campbelli): Effects of rate of reinforcement and reinforcer flavor variety

We investigated operant behavior in a novel species, the dwarf hamster (Phodopus campbelli). In two experiments, hamsters were trained to lever-press for food reinforcement. In Experiment 1, rate of reinforcement was manipulated across conditions using four variable-interval schedules of reinforcement (delivering one to eight reinforcers per min). As predicted, within-session decreases in responding were steepest on the richest schedule. In Experiment 2, lever-pressing was reinforced by either a constant or a variety of flavored food pellets. Within-session decreases in responding were steeper when the reinforcer flavor remained constant than when it was varied within the session. In both experiments, subjects hoarded most reinforcers in their cheek pouches rather than consuming them in the operant chambers. These results are incompatible with post-ingestive satiety variables as explanations for within-session decreases in operant responding and suggest that habituation to repeatedly presented reinforcers best accounts for subjects' response patterns. Additionally, a mathematical model that describes behavior undergoing habituation also described the present results, thus strengthening the conclusion that habituation mediates the reinforcing efficacy of food.

Effects of exercise and restrained eating behaviour on appetite control

Obesity is a global epidemic; increased consumption of energy-dense food and reduced physical activity levels are likely to be the main drivers. Previous cross-sectional research has shown that sedentary males, unlike their active counterparts, are unable to compensate for previous energy intake (EI). Using a longitudinal design a 6-week exercise intervention was found to improve short-term appetite control, leading to a more 'sensitive' eating behaviour in response to previous EI, both acutely at a test meal and for the next 24 h. Although the mechanisms whereby acute and chronic exercise improves short-term appetite remain unknown, post-ingestive satiety peptides are likely to be involved. Acute exercise was found to increase postprandial levels of polypeptide YY, glucagon-like peptide-1 and pancreatic polypeptide but to have no impact on ghrelin, suggesting that exercise can trigger physiological changes in satiety hormone secretion that could help in appetite control and weight maintenance. In the context of an increased availability of highly-palatable food, dietary restraint may be increasingly important. Although restraint has been associated with abnormal eating behaviour, in the laboratory no counter-regulation was found in restrained eaters when presented with a buffet meal 60 min after a high-energy preload or when a pasta-meal was presented 3 h after preloading. Although restraint was not found to impact on polypeptide YY or TAG, lower postprandial glucose and insulin plasma levels were observed in restrained eaters, together with increased feelings of fullness. In conclusion, short-term appetite control seems to be favourably modified by exercise, while the impact of restraint on appetite seems to be more complex.

Oxytocin knockout (OT KO) mice overconsume palatable carbohydrate solutions, but not palatable lipid solutions.

OT KO mice display enhanced intake of nutritive and non-nutritive sweet solutions (i.e., sucrose and saccharin) compared to wild-type (WT) mice of the same C57BL/6 background strain. The present study further investigated the differential behavioral response of OT KO and WT mice to sucrose solutions, and also examined intake preferences of OT KO and WT mice for palatable but non-sweet isocaloric solutions of carbohydrate and fat. A progressive ratio operant licking procedure demonstrated that OT KO and WT mice display similar motivational drive to consume 10% sucrose. A series of two-bottle intake tests revealed that OT KO mice consume significantly larger amounts of sweet and non-sweet carbohydrate solutions (i.e., sucrose, polycose, and cornstarch) compared to WT cohorts. Intake pattern analyses revealed that OT KO mice overconsume carbohydrate solutions by initiating more drinking bouts compared to WT mice; bout sizes did not differ between the genotypes. In contrast, OT KO and WT mice did not differ in their daily or hourly evening intake of Intralipid, a palatable soybean oil emulsion, except on the first day of exposure. These findings indicate that the absence of OT in mice does not affect their appetitive drive to consume palatable sucrose solutions. Instead, the absence of OT may increase daily intake of palatable sweet and non-sweet solutions of carbohydrate (but not fat) by selectively blunting or masking processes that contribute to post-ingestive satiety. Research supported by National Institutes of Health Grants HD 44898 (JAA) and DK 31135 (AS).

Oxytocin knockout mice demonstrate enhanced intake of sweet and nonsweet carbohydrate solutions

Oxytocin knockout (OT KO) mice display enhanced intake of nutritive and nonnutritive sweet solutions (i.e., sucrose and saccharin) compared with wild-type (WT) mice of the same C57BL/6 background strain. The present study further investigated the differential behavioral response of OT KO and WT mice to sucrose solutions and also examined intake preferences of OT KO and WT mice for palatable but nonsweet isocaloric solutions of carbohydrate and fat. A progressive ratio operant licking procedure demonstrated that OT KO and WT mice display a similar motivational drive to consume 10% sucrose. A series of two-bottle intake tests revealed that OT KO mice consume significantly larger amounts of both sweet and nonsweet carbohydrate solutions (i.e., sucrose, Polycose, and cornstarch) compared with WT cohorts. Intake pattern analyses revealed that OT KO mice overconsume carbohydrate solutions by initiating more drinking bouts compared with WT mice; bout sizes did not differ between the genotypes. In contrast, OT KO and WT mice did not differ in their intake of Intralipid, a palatable soybean oil emulsion. These findings indicate that the absence of OT in mice does not affect their appetitive drive to consume palatable sucrose solutions. Instead, the absence of OT may increase daily intake of palatable sweet and nonsweet solutions of carbohydrate (but not fat) by selectively blunting or masking processes that contribute to postingestive satiety.

Resistance and susceptibility to weight gain: Individual variability in response to a high-fat diet

An obesigenic environment is a potent force for promoting weight gain. However, not all people exposed to such an environment become obese; some remain lean. This means that some people are susceptible to weight gain (in a weight-promoting environment) and others are resistant. Identifying the characteristics of appetite control and food motivation in these two groups could throw light on the causes of weight gain and how this can be either treated or prevented. We have investigated the issue experimentally by identifying people who habitually consume a high-fat diet (greater than 43% fat energy). These individuals have been termed high-fat phenotypes. We have compared individuals, of the same age (mean = 37 years old) and gender (male), who have gained weight (BMI = 34) or who have remained lean (BMI = 22). The susceptible individuals are characterised by a cluster of characteristics including a weak satiety response to fatty meals, a maintained preference for high-fat over low-energy foods in the post-ingestive satiety period, a strong hedonic attraction to palatable foods and to eating, and high scores on the TFEQ factors of Disinhibition and Hunger. The analysis of large databases suggests that this profile of factors contributes to an average daily positive energy balance from food of approximately 0.5 MJ. This profile of characteristics helps to define the symptomatology of a thrifty phenotype.

The degree of saturation of fatty acids influences post-ingestive satiety

Two studies were designed to compare the effects on post-ingestive satiety of manipulating the degree of saturation of fatty acids, at a fixed chain length (18 C atoms), in a fixed energy (5.68 MJ for males; 3.97 MJ for females), high-fat (55% energy) lunch meal. Two different groups of twenty subjects (ten males and ten females) took part in each study. All forty subjects were of normal weight and aged between 18 and 36 years. Study 1 compared the effects of fat A (oleic blend, high in monounsaturated fatty acids (MUFA)) with those of fat B (linoleic blend, high in polyunsaturated fatty acids (PUFA)) and fat C (stearic-oleic blend, high in saturated fatty acids (SFA)). Study 2, which was designed to confirm and extend the findings of Study 1, compared the effects of fats A, B and C with those of fat D (a linoleic-oleic blend). Energy and nutrient intakes were monitored for the rest of the day and for the following day. Profiles of hunger, fullness and other sensations were monitored by continuous tracking and end-of-day questionnaires. In each meal the fat content was exclusively composed of one particular type (A, B, C or D) and was divided equally between the main course and dessert. Study 1 revealed a significant effect of fat type (degree of saturation) on intake of nutrients at the following (dinner) meal (smallest F[2,36] 3.38, P < 0.05), on post-ingestive ratings of motivation to eat (smallest F[2,36] 4.18, P = 0.02) and on energy intake over the whole test day (F[2,36] 3.39, P < 0.01). Subjects consumed significantly more energy after consumption of the lunch containing fat A than after the lunches containing fats B or C and there was a trend for these effects to continue into the second day. In Study 2, fat C produced more similar effects on appetite to fat A and there was a tendency for subjects to consume more over the whole test day when they had consumed the lunch containing fat A than when they had consumed the lunch containing fat B. Furthermore, when the data from fat conditions A and B in both studies were combined (n 40) the results of Study 1 were confirmed. Overall, the results of these short-term studies indicate that PUFA may exert a relatively stronger control over appetite than MUFA and SFA.

Filling the gut activates paradoxical sleep in suckling rats

Recent studies with rat pups suggest that suckling and sleeping are coordinated through milk-related events in the gut. Our experiments revealed that suckling rats respond to milk in the upper gastrointestinal tract by displaying more paradoxical sleep (PS) as the volume increases to 4% of the pup's body weight. Conversely, gastric loads larger than 4% reduced PS as a function of the volume. We also discovered that filling the stomach with warm non-nutritive paraffin is as effective as an equivalent volume of warm milk for enhancing PS. Although the temperature of the gut load did not appear to play a major role in the amount of PS displayed, increasing ambient temperature from 22 degrees C to 32 degrees C increased PS significantly. Moreover, a gut load of milk (4% body weight) was more effective than the same volume of water or no load for enhancing PS. Gut loads that stay in the stomach and warm ambient temperature appear to work in an additive manner to enhance PS. The electrophysiological data together with the stomach volume data and behavioral observations of nipple attachment revealed that milk-related stimuli along the gastrointestinal tract, especially gastric distension, alter sleep patterns in predictable ways that permit us to distinguish postingestive satiety from a deprivation state and nimiety in suckling rats.

Weight, Protein, Fat, and Timing of Preloads Affect Food Intake

Porrini, M., A. Santangelo, R. Crovetti, P. Riso, G. Testolin and J. E. Blundell. Weight, protein, fat, and timing of preloads affect food intake. Physiol Behav 62(3) 563–570, 1997.—Two foods, one rich in protein (HP) and one rich in fat (HF), were employed to evaluate the effect of macronutrients on food intake and to underline the differences that occurred when the foods were served as uniform meal, as first course of a varied meal, and as a snack 2 h before a varied meal. Our results showed that HP food always exerted a higher effect on both intrameal satiation and postingestive satiety than HF food. When a uniform meal was consumed, satiation for the specific food was reached before fullness; in this condition, sensory characteristics of foods played an important role in controlling food intake and made the uniform meal more satiating than the varied one. The consumption of a snack far from a meal did not contribute to satiety; consequently, gastric filling seems to be an important factor determining the amount consumed in a varied meal.

Independent Effects of Palatability and Within-meal Pauses on Intake and Appetite Ratings in Human Volunteers

The effects of the introduction of timed pauses within meals and palatability on food intake and changes in rated appetite during a meal were assessed in three experiments in which volunteers ate a lunch of pasta with a tomato sauce. Eating was monitored using a disguised electronic balance attached to a micro-computer, which also allowed the introduction of timed pauses within meals. In the first experiment, 16 subjects were tested with both a bland and palatable food (with 0·27% oregano), with eating uninterrupted or with pauses after every 50 g consumed during which appetite ratings were completed. Both the addition of oregano and the introduction of regular within-meal pauses enhanced overall intake. Rated hunger increased in the early stages of eating the palatable food in the interrupted condition, and then declined, whereas hunger declined throughout with the bland food. Similarly, the linear function relating intake to time in the uninterrupted condition was greater with the palatable food. In the second experiment, nine subjects ate the palatable food with no pauses within meals, with 30-second pauses with appetite ratings or with 30-second pauses in a non-appetite task. Intake was greater in both pause conditions than when eating was uninterrupted. In Experiment 3, the effect of pause duration was investigated in a further 16 subjects, with either no pause or a pause of 5, 30 or 60 seconds. Subjects ate more in all pause conditions than with no pauses, while ratings of hunger and fullness suggested that subjects were less satisfied at the end of the meal with longer pauses. These data confirm previous work which suggests that palatability exerts its effect by stimulating appetite and eating rate, but also suggest that the introduction of pauses within meals enhances intake as well, contradicting the idea that pausing within meals should reduce intake by allowing more time for post-ingestive satiety to develop.

Effects of Amount and type of Dietary Fibre (Soluble and Insoluble) on Short-Term Control of Appetite

Fibres which are soluble or insoluble in water have different physiochemical properties and may therefore be expected to exert different effects on post-ingestive satiety signals. This study compared the effects on short-term (24 h) appetite of two equienergetic high (22 g) fibre breakfasts, an equienergetic low fibre breakfast and a low energy, ‘light’ breakfast. Psyllium gum (the soluble fibre) and wheat bran (the insoluble fibre) were incorporated into breakfast cereals and consumed at breakfast by sixteen healthy, normal weight males after an overnight fast using a repeated measures, counterbalanced design. Ad libitum energy intake was assessed at a test snack 1.5 h after breakfast, later in the day using food boxes and the following day using food diaries. Motivation to eat and gastrointestinal sensations were tracked for the next 24 h. Hunger ratings showed a trend towards the subjects being less hungry and they consumed significantly less energy at snack time after the high insoluble than after the high soluble fibre breakfast cereal. The soluble fibre breakfast produced a greater suppression of snack intake than the light breakfast, but smaller suppression than the other breakfasts. Interestingly there was a trend toward reduced hunger and voluntary energy consumption following the soluble fibre compared with the insoluble fibre much later in the day (9.5-13.5 h after breakfast) although this was not significant. There was no significant effect of breakfast type on total day energy intake. The results suggest that different types of fibre modulate the timecourse of appetite control and may produce alterations in the experience of motivation and patterns of eating without necessarily effecting total energy intake.

Control of human appetite: implications for the intake of dietary fat.

The human appetite system contains central and peripheral mechanisms that interact with environmental features, especially with the physical and nutrient composition of the food supply. Foods varying in nutrient composition exert different physiologic effects, some of which function as satiety signals. High-fat diets (low food quotient) lead to high levels of energy intake. This effect is termed passive overconsumption and overcomes fat-induced physiological satiety signals. High-fat foods exert a weak effect on satiation (intra-meal satiety), and fat has a weaker effect, joule for joule, on postingestive satiety than do other macronutrients. The frequency of obesity is greater among high-fat than low-fat consumers. However, the development of obesity on a high-fat diet is not a biological inevitability. The investigation of people who resist the weight-inducing properties of high-fat diets is a key research strategy. Understanding the appetite control system suggests behavioral, nutritional, and pharmacologic strategies for modifying dietary fat intake.

Nutrition and appetite control: implications for the regulation of body weight

There is an important relationship between the expression of human appetite and the macro nutrient composition of the prevailing diet. Evidence indicates a correlation between the amount of fat consumed and body weight gain or the degree of adiposity. The pattern of eating behaviour is represented by the size and number of eating episodes (meals and snacks). The size of eating episodes is influenced by the process of satiation and the intervals between episodes by post-ingestive satiety. Protein appears to exert the most potent control over appetite. Carbohydrate also generates strong post-ingestive satiety with the intensity and duration depending upon the particular structure of the carbohydrate. High fat foods exert weak control over satiation and generate a weak satiety response relative to the proportion of energy ingested. High fat foods therefore have the potential to generate a form of passive overconsumption. The high energy density and potent oro-sensory qualities of high fat foods allows the rate of ingestion to overcome fat-induced satiety signals. This situation suggests behavioural, nutritional and pharmacological strategies to prevent fat-induced passive overconsumption and to improve the effect of fat on appetite control.

Learned preferences for real-fed and sham-fed polycose in rats: Interaction of taste, postingestive reinforcement, and satiety

Nutrient-conditioned flavor preferences have been obtained with a variety of procedures, but one study indicates that rats do not readily learn to prefer the flavor of real-fed food to the flavor of sham-fed food. This issue was reexamined in food-deprived rats trained to real feed (gastric fistula closed) and sham feed (gastric fistula open) different flavored Polycose solutions (cherry or grape). Sham-feeding intakes in the one-bottle training sessions (30 min) were limited to the amount consumed in real-feeding sessions; intakes were not limited during two-bottle preference tests (10 min). In Experiment 1, when tested with 32% Polycose solutions, the animals tended to prefer the sham-fed Polycose to the real-fed Polycose. When subsequently trained and tested with 8% Polycose solutions, the same animals displayed a strong preference (≈90%) for the real-fed Polycose. These findings were confirmed in Experiment 2 using separate groups. Rats trained with 8% Polycose readily learned to prefer the real-fed solution, and rats trained with 32% Polycose initially preferred the sham-fed Polycose solution. With additional training the 32% Polycose group developed a preference for the real-fed solution. In Experiment 3, rats trained to real feed flavored 8% and 32% Polycose solutions developed strong preferences for the 32% solution. This finding suggests that the tendency to prefer sham-fed to real-fed 32% Polycose is not due to aversive postingestive effects of the real-fed solution. Taken together, the results indicate that Polycose has a postingestive positive reinforcing effect that can be revealed using the sham-feeding preparation. The lack of an immediate preference for real-fed over sham-fed 32% Polycose may occur because 1) postingestive reinforcement only slightly improves the already highly preferred taste of the real-fed solution and 2) postingestive satiety reduces the attractiveness of the real-fed solution.

Persistence of Sham feeding after real-fed gastric loads in rats

Esophagostomized rats were permitted to “real-feed” a glucose solution (Experiment 1) or milk (Experiment 2), by receiving intragastric (IG) infusions concurrent with sham feeding. This was done for the first part of the feeding session; then the IG infusions were discontinued so that subsequent ingestion was sham. When such infusions stopped after 80% of real-meal volume had been delivered, they had no measurable effect on subsequent oral ingestion: sham feeding persisted as if nothing had been delivered IG. Therefore, the onset of postingestive satiety, at least in experienced sham-feeding rats, is all-or-none or close to it: if the IG loads are even slightly short of what is required to abolish sham feeding, then they do not affect sham feeding at all. This is so even if the postingestive events occur within their normal behavioral context.