Food flavor preferences produced by drinking glucose and oil in normal and diabetic rats: evidence for conditioning based on fuel oxidation

Obesity in C57BL/6J mice fed diets differing in carbohydrate and fat but not energy content

To investigate the contributions of carbohydrate and fat to obesity we measured the body weight, body composition and food intake of adult C57BL/6J mice fed ad libitum with various combinations of two semisynthetic diets that differed in carbohydrate and fat but not in protein, micronutrient or energy content. In Experiment 1, involving male mice, body weights were similar in groups fed diets comprised of (by energy) 20% protein, 75% carbohydrate and 5% fat (C75-F5) or 20% protein, 5% carbohydrate and 75% fat (C5-F75). However, mice fed a 50:50 composite mixture of the C75-F5 and C5-F75 diets (i.e., a C40-F40 diet) became substantially more obese. Mice that could choose between the C75-F5 and C5-F75 diets ate equal amounts of each diet and gained almost as much weight as did the group fed C40-F40 diet. Mice switched every day between the C75-F5 and C5-F75 diets gained no more weight than did those fed either diet exclusively. In Experiment 2, male and female mice were fed chow or one of 8 isocaloric diets that differed parametrically in carbohydrate and fat content. Groups fed diets in the middle of the range (i.e., C35-F45 or C45-F35) weighed significantly more and were significantly fatter than were those fed diets with more extreme proportions of carbohydrate and fat (e.g., C75-F5, C5-F75), an effect that was more pronounced in males than females. In Experiment 3 and 4, male mice fed versions of the C40-F40 formulation gained more weight than did those fed the C75-F5 or C5-F75 formulations irrespective of whether the carbohydrate was predominantly sucrose or predominantly starch, or whether the fat was vegetable shortening, corn oil, palm oil or canola oil; the type of carbohydrate or fat had little or no impact on body weight. In all four experiments, energy intakes differed among the diet groups but could not account for the differences in body weight. These results demonstrate that the proportion of carbohydrate and fat in the diet influences body weight independently of energy content, and that the type of carbohydrate or fat has little impact on body weight. Consuming carbohydrate and fat simultaneously or in close temporal proximity exacerbates obesity.

FGF21 and the Physiological Regulation of Macronutrient Preference

The ability to respond to variations in nutritional status depends on regulatory systems that monitor nutrient intake and adaptively alter metabolism and feeding behavior during nutrient restriction. There is ample evidence that the restriction of water, sodium, or energy intake triggers adaptive responses that conserve existing nutrient stores and promote the ingestion of the missing nutrient, and that these homeostatic responses are mediated, at least in part, by nutritionally regulated hormones acting within the brain. This review highlights recent research that suggests that the metabolic hormone fibroblast growth factor 21 (FGF21) acts on the brain to homeostatically alter macronutrient preference. Circulating FGF21 levels are robustly increased by diets that are high in carbohydrate but low in protein, and exogenous FGF21 treatment reduces the consumption of sweet foods and alcohol while alternatively increasing the consumption of protein. In addition, while control mice adaptively shift macronutrient preference and increase protein intake in response to dietary protein restriction, mice that lack either FGF21 or FGF21 signaling in the brain fail to exhibit this homeostatic response. FGF21 therefore mediates a unique physiological niche, coordinating adaptive shifts in macronutrient preference that serve to maintain protein intake in the face of dietary protein restriction.

Macronutrient selection by seven inbred mouse strains and three taste-related knockout strains

Many animals thrive when given a choice of separate sources of macronutrients. How they do this is unknown. Here, we report some studies comparing the spontaneous choices between carbohydrate- and fat-containing food sources of seven inbred mouse strains (B6, BTBR, CBA, JF1, NZW, PWD and PWK) and three mouse models with genetic ablation of taste transduction components (T1R3, ITPR3 and CALHM1). For 8days, each mouse could choose between sources of carbohydrate (CHO-P; sucrose-cornstarch) and fat (Fat-P; vegetable shortening) with each source also containing protein (casein). We found that the B6 and PWK strains markedly preferred the CHO-P diet to the Fat-P diet, the BTBR and JF1 strains markedly preferred the Fat-P diet to the CHO-P diet, and the CBA, NZW and PWD strains showed equal intakes of the two diets (by weight). Relative to their WT littermates, ITPR3 and CALHM1 KO mice had elevated Fat-P preferences but T1R3 KO mice did not. There were differences among strains in adaption to the diet choice and there were differences in response between males and females on some days. These results demonstrate the diverse responses to macronutrients of inbred mice and they point to the involvement of chemosensory detectors (but not sweetness) as contributors to macronutrient selection.

Influence of palatability on motivation to operate for caloric and non-caloric food in non food-deprived and food-deprived rats

Palatability is the hedonic food component that is considered to override the homeostatic mechanisms that control food intake, and we compared how much effort non food-deprived and food-deprived rats were willing to spend in order to earn a palatable caloric (sucrose) or non-caloric (saccharin) snack. We first studied the dopaminergic response, in terms of dopamine levels and dopamine and cAMP-regulated phosphoprotein Mr 32,000 (DARPP-32) phosphorylation pattern, to two consecutive palatable caloric or non-caloric snacks in the nucleus accumbens shell (NAcS) of non food-deprived and fasted rats. We report that non food-deprived rats developed rapid habituation in the NAcS dopaminergic response to the second consumption of both caloric and non-caloric palatable food, while food-deprived rats developed rapid habituation only to saccharin. Next, we show that in self-administration experiments, non food-deprived rats spent a similar effort when operating for sucrose or saccharin. However, the same rats showed an increased response specifically for sucrose after 18-h fasting. After pre-feeding devaluation, rats reduced their response to sucrose but not for saccharin. These results strengthen the hypothesis that food intake is mainly controlled by palatability in non food-deprived rats and by caloric content in food-deprived rats. Moreover, they show that rapid habituation development was associated with a similar, basal working activity aimed at ingesting both caloric and non-caloric food, as observed in non food-deprived rats consuming sucrose or saccharin and in fasted rats consuming saccharin. Conversely, lack of habituation, as present in fasted rats consuming a caloric food, was associated with extra energy expenditure.

Neural and metabolic regulation of macronutrient intake and selection

There is considerable disagreement regarding what constitutes a healthy diet. Ever since the influential work of Cannon and Richter, it was debated whether the 'wisdom of the body' will automatically direct us to the foods we need for healthy lives or whether we must carefully learn to eat the right foods, particularly in an environment of plenty. Although it is clear that strong mechanisms have evolved to prevent consumption of foods that have previously made us sick, it is less clear whether reciprocal mechanisms exist that reinforce the consumption of healthy diets. Here, we review recent progress in providing behavioural evidence for the regulation of intake and selection of proteins, carbohydrates and fats. We examine new developments in sensory physiology enabling recognition of macronutrients both pre- and post-ingestively. Finally, we propose a general model for central neural processing of nutrient-specific appetites. We suggest that the same basic neural circuitry responsible for the homoeostatic regulation of total energy intake is also used to control consumption of specific macro- and micronutrients. Similar to salt appetite, specific appetites for other micro- and macronutrients may be encoded by unique molecular changes in the hypothalamus. Gratification of such specific appetites is then accomplished by engaging the brain motivational system to assign the highest reward prediction to exteroceptive cues previously associated with consuming the missing ingredient. A better understanding of these nutrient-specific neural processes could help design drugs and behavioural strategies that promote healthier eating.

Reduced sensitivity to cholecystokinin in male rats fed a high-fat diet is reversible

Adult rats chronically fed a high-fat (HF) diet maintain reduced sensitivity to cholecystokinin (CCK). We hypothesized that, similar to adult rats, pups fed a HF diet would also exhibit reduced sensitivity to CCK. To test this, male pups fed low-fat (LF) and HF isoenergetic (16.2 kJ/g) diets were administered CCK intraperitoneally (0.125-1 microg/kg) 1 wk following dietary adaptation. After receiving 0.5 microg/kg CCK, pups fed the HF diet suppressed food intake less (8.9 +/- 5.0%) than pups fed the LF diet (28.9 +/- 4.7%; P < 0.05) relative to intakes after saline administration. We then assessed the development and extinction of changes in CCK sensitivity by switching the diets between the groups. The HF-fed group, when switched to the LF diet, regained sensitivity by wk 4 and suppressed food intake following administration of 0.25 microg/kg CCK (33.1 +/- 5.7%; P < 0.05). The LF-fed group, when switched to the HF diet, lost sensitivity by wk 2 and did not suppress food intake after administrations of CCK compared with saline. Finally, we examined if HF-fed rats have an increased sensitivity to corn oil during brief access tests using a multibottle gustometer. At oil concentrations of 25, 75, and 100%, rats fed the HF diet sampled more oil than LF-fed rats (P < 0.05). These findings demonstrate that male rat pups fed a HF diet exhibit reduced sensitivity to CCK, the development of this reduced sensitivity is quicker than its extinction, and rats consuming a HF diet have increased oral sensitivity to oils.

Learned and cognitive controls of food intake

While much has been elucidated about the hypothalamic controls of energy balance, the epidemic of obesity continues to escalate. Recent work has suggested that extra-hypothalamic central nervous system structures may play a previously un-appreciated role in the control of ingestive behavior and body weight regulation. Because animals can and do learn about food and food-related stimuli, as well as the consequences of eating, we and others have sought to understand the cognitive process that underlies that learning. Additionally, we have begun to investigate the neuro-anatomical bases for complex learning about food and food cues. Here we review some evidence for learning about food as well as evidence that the hippocampus may play a critical role in the brain's ability to regulate body weight through such learning processes.

Behavioral controls of food intake

Recent conceptualizations of food intake have divided ingestive behavior into multiple distinct phases. Here, we present a temporally and operationally defined classification of ingestive behaviors. Importantly, various physiological signals including hypothalamic peptides are thought to impact these distinct behavioral phases of ingestion differently. In this review, we summarize a number of behavioral assays designed to delineate the effects of hormone and peptide signals that influence food intake on these ingestive mechanisms. Finally, we discuss two issues that we have encountered in our laboratory which may obstruct the interpretation of results from these types of studies. First, the influence of previous experience with foods used in these behavioral tests and second, the importance of the nutrient composition of the selected test foods. The important conclusion discussed here is that the behavioral analysis of ingestion is accompanied by theoretical constructs and artificial divisions of biological realities and the appreciation of this fact can only increase the opportunities of contemporary behavioral scientists to make significant and novel observations of ingestive behaviors.

The hepatic vagus mediates fat-induced inhibition of diabetic hyperphagia

Diabetic rats both overeat high-carbohydrate diet and have altered hypothalamic neuropeptide Y (NPY) and corticotropin-releasing factor (CRF). In contrast, a high-fat diet reduces caloric intake of diabetics to normal, reflected by normal hypothalamic NPY and CRF content. How the brain senses these changes in diet is unknown. To date, no hormonal changes explain these diet-induced changes in caloric intake. We tested whether the common branch of the hepatic vagus mediates the fat signal. We presented fat in two ways. First, diabetic and vehicle-treated rats were offered a cup of lard in addition to their normal high-carbohydrate diet. Second, we switched diabetic rats from high-carbohydrate diet to high-fat diet, without choice. In streptozotocin-treated rats, both methods resulted in fat-induced inhibition of caloric intake and normalization of hypothalamic neuropeptides to nondiabetic levels. Strikingly, common branch hepatic vagotomy (unlike gastroduodenal vagotomy) entirely blocked these fat-induced changes. Although a shift in hepatic energy status did not explain the lard-induced changes in diabetic rats, the data suggested that common hepatic branch vagotomy does not interfere with hepatic energy status. Furthermore, common branch hepatic vagotomy without diabetes induced indexes of obesity. Abnormal function of the hepatic vagus, as occurs in diabetic neuropathy, may contribute to diabetic obesity.

Different preferences for oils with similar fatty acid profiles

While preference for fat can be influenced by concentration and physical form, the influence of fatty acid composition on relative preference for oils has not been systematically investigated. Therefore, the purpose of the present investigation was to assess the relative preference for oils rich in oleic (Extra Light Olive Oil and Extra Virgin Olive Oil) and linoleic (Safflower Oil) acid. Male Fischer rats (n = 10) were used to determine preference in a two-choice testing procedure in which three pairs of oils were each tested twice. Preference testing occurred at dark onset at which time the rodent diet and water were removed and each rat was allowed 2-h access to his assigned pair of oils. There was a main effect of oil type (p<0.01), but no significant effect of oil pairing and no interaction between oil pairing and oil type. Rats preferred the Extra Light Olive Oil to the Extra Virgin Olive Oil (p<0.05). This is the first report of preference testing in which two oils with similar fatty acid profiles were included. The present data indicate that the fats with similar fatty acid profiles were not equally preferred, suggesting that a property other than the fatty acid composition of the oils accounts for the demonstrated preference.

Behavioral components of high-fat diet hyperphagia: meal size and postprandial satiety

Previously, rats fed a high-fat liquid diet (HF) ad libitum consumed more kilocalories and had greater weight gain than rats fed a liquid high-carbohydrate diet (HC) of equivalent energy density (Warwick, Z. S., and H. P. Weingarten. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 269: R30-R37, 1995). The present series of experiments sought to clarify the behavioral expression of HF hyperphagia by comparing HF and HC with regard to meal size and magnitude of postingestive satiety effect. Meal size of HF was greater than HC at 2.3 kcal/ml and also when diets were formulated at 1.15 kcal/ml. In a preload-test meal paradigm, an orally consumed HF preload was less satiating than a calorically equivalent HC preload across a range of preload volumes and intermeal intervals. Sensory-specific satiety was ruled out as an explanation of the relatively greater intake of test meal after an HF preload meal; an intragastrically delivered HF preload was less satiating than intragastric HC. Furthermore, a fat (corn oil emulsion) preload was less satiating than a carbohydrate (sucrose) preload when an evaporated milk test meal was used. These findings indicate that hyperphagia on an HF diet is expressed in increased meal size and decreased intermeal interval.

Flavor-cued modulation of intake in rats: role of familiarity and impact on 24-h intake

Following training with distinctively flavored nutritive solutions that differ in concentration and thus in caloric value, rats demonstrate flavor-postingestive consequence learning by preferentially consuming one of the flavors in two-bottle tests (both flavors in nutrient-identical solutions.) Experiment 1 investigated whether the relative familiarity of the flavor-nutrient combinations encountered in two-bottle tests contributes to the observed preference. One of the training concentrations (rather than the customary intermediate concentration) was used to present the flavors in testing; thus, one of the flavors was in a familiar context while the other was in an unfamiliar context. The results of two independent trials (rats trained with 1 and 5% sucrose; rats trained with 5 and 40% sucrose) confirmed that two-bottle test preference was not a preference for the familiar flavor-nutrient combination. Experiment 2 examined whether caloric expectancies based upon a previously learned flavor-postingestive consequence association would affect total daily intake. On alternating days, rats consumed 30 mL of dilute (5%) and concentrated (40%) sucrose, each distinctively flavored. When given 30 mL of 22.5% sucrose containing each of the flavors on separate test days, they ate less chow and thus fewer total calories over 24 h when given the flavor previously paired with concentrated sucrose. Experiment 3 replicated the design of Experiment 2 except that fat calories were used instead of sucrose; no significant adjustment of chow intake in extinction tests was noted, even when the number of fat calories used in training was increased (Experiment 4). Thus, rats did not exhibit flavor-cued modulation of chow intake when trained with fat, in contrast to responsivity to flavor cues when trained with sucrose. This differential responding to fat versus carbohydrate calories is consistent with previous observations, in a variety of paradigms, that modulation of caloric intake is less energetically appropriate when ingested foods are high in fat relative to high-carbohydrate foods.

Differential reinforcing and satiating effects of intragastric fat and carbohydrate infusions in rats

Food intake and preferences are modulated by the postingestive satiating and reinforcing actions of nutrients. This experiment compared the feeding effects of isocaloric intragastric (i.g.) carbohydrate (maltodextrin) and fat (corn oil) infusions in food-restricted rats fed low-fat (12% fat kcal) or high-fat (48% fat kcal) diets. In Experiment 1, the rats were given one flavored saccharin solution (CS+C) paired with i.g. carbohydrate infusions, a second flavor (CS+F) paired with i.g. fat infusions, and a third flavor (CS-) paired with i.g. water infusions during 30-min one-bottle training sessions. In subsequent two-bottle tests, the rats preferred both CS+s to the CS- (68-83%) and the CS+C to the CS+F (68-70%). In Experiment 2, the feeding inhibitory effects of the nutrient infusions on an ongoing meal (satiation test) or a subsequent meal (satiety test) were compared. The intake of a palatable Polycose+saccharin solution was suppressed by a concurrent carbohydrate infusion but not by a fat infusion. Also, i.g. carbohydrate preloads suppressed the intake of a subsequent (30-180 min) mixed carbohydrate+fat test meal more than did i.g. fat preloads. The satiety effects of the fat preloads were more pronounced in rats fed the low-fat diet than in rats fed the high-fat maintenance diet. Diet composition did not reliably influence the preference conditioning and satiation effects of the nutrient infusions. These results confirm prior reports that fat is less satiating than carbohydrate, and further demonstrate that i.g. carbohydrate infusions condition a stronger flavor preference than fat infusions.

Effect of a high-fat diet on food intake and hypothalamic neuropeptide gene expression in streptozotocin diabetes

Insulin-deficient diabetic rats are markedly hyperphagic when fed a high-carbohydrate (HC) diet, but normophagic when fed a high-fat (HF) diet. When maintained on a HC diet, diabetic rats also exhibit increased gene expression of the orexigenic peptide neuropeptide Y (NPY) in the hypothalamic arcuate nucleus, and reduced expression of the anorectic peptide corticotropin-releasing hormone (CRH) in the paraventricular nucleus, and these changes are hypothesized to contribute to diabetic hyperphagia. In this experiment we assessed whether the normophagia displayed by HF-fed diabetic rats is associated with the opposite profile of NPY and CRH expression. Our results show that relative to diabetic rats on the HC diet, the diabetic rats on the HF diet exhibited significantly reduced caloric intake (-40%), NPY expression in the arcuate nucleus (-27%), and elevated CRH expression in the paraventricular nucleus (+37%). Insulin and corticosterone, which are known to affect hypothalamic NPY and CRH expression, were not different between these two groups, making it unlikely that they can account for the differences in either feeding behavior or hypothalamic peptide expression. There was a small but significant increase in plasma leptin levels in the diabetic animals maintained on the HF, and large differences in parameters associated with elevated fat oxidation. These observations support the hypothesis that the normalization of food intake observed in diabetic rats consuming a HF diet may in part be mediated by reductions in NPY expression and elevations in CRH expression.

Learned suppression of intake based on anticipated calories: cross-nutrient comparisons

Following training with distinctively flavored solutions which differ in calories and thus in their postingestive effects, rats demonstrate flavor-postingestive consequence learning by preferentially consuming one of the flavors in two-bottle tests (both flavors presented in nutrient-identical solutions). The direction of the preference--for the flavor previously paired with more calories (F-hi) or for the flavor previously paired with fewer calories (F-lo)--depends critically upon the magnitude of postingestive effects experienced during training. The most frequent and more thoroughly investigated observation has been preferential consumption of F-hi (conditioned flavor preference). However, when relatively concentrated solutions (e.g., 5% and 30% sucrose) are used as the training nutrients, F-lo is preferentially consumed in two-bottle tests. This lesser intake of F-hi presumably reflects its previous association with the more potent satiating effect of the highly concentrated nutrient: conditioned satiety. The present series of experiments explored conditioned satiety across nutrient types. In each experiment, rats consumed 30 mL of distinctively flavored nutritive solution per day, alternating between a distinctively flavored high-calorie (1.6 kcal/mL) solution and a lower calorie (0.2 kcal/mL) solution containing a different flavor. Two-bottle testing evaluated whether conditioned satiety was evident. Experiment 1 found that rats trained with sucrose, fructose, glucose, maltodextrin, or saccharin-sweetened medium-chain triglyceride oil emulsion preferentially consumed F-lo in two-bottle tests. In contrast, rats trained with corn oil emulsions tended to preferentially consume F-hi. In Experiment 2, increasing the number of corn oil calories associated with F-hi produced a tendency toward preferential intake of F-lo in two-bottle tests. In Experiment 3, rats consumed a high-fat maintenance diet; sucrose-trained rats again consumed more F-lo than F-hi, whereas rats trained with corn oil emulsions showed a tendency (nonsignificant) to consume more F-lo in two-bottle tests. In Experiment 4, however, adding saccharin to corn oil emulsions did produce conditioned satiety. These findings demonstrate conditioned satiety as a robust phenomenon across various nutrient types; however, corn oil calories entrain conditioned satiety only under certain conditions.

Diabetic rats prefer glucose-paired flavors over fructose-paired flavors

Prior studies indicate that glucose has a more potent postingestive reinforcing effect than fructose. The role of insulin in this effect was examined by comparing sugar-conditioned flavor preferences in normal and streptozotocin-diabetic rats. In Experiment 1, diabetic rats, like normal rats, preferred a cue flavor that had been mixed into 8% glucose solution over a flavor mixed with 8% fructose. Both taste and postingestive properties of glucose may have contributed to this preference. Experiment 2 evaluated postingestive reinforcement by pairing cue flavors with intragastric infusions of glucose and fructose. Both diabetics and normals acquired a preference for the flavor paired with intragastric 16% glucose infusions over the flavor paired with 16% fructose infusions although the preference was somewhat smaller in the diabetic rats. Taken together, the results indicate that a normal insulin secretory response to glucose is not required for glucose-conditioned flavor preference. The diabetic rats' reduced flavor preference in Experiment 2 suggests that insulin may play some role in glucose conditioning although this may be secondary to alterations in gastrointestinal motility characteristic of diabetic animals.

The composition of the maintenance diet alters flavor-preference conditioning by intragastric fat infusions in rats

Prior studies indicate that intragastric (IG) fat infusions condition only weak flavor preferences in chow-fed rats using brief daily training sessions. The present study attempted to facilitate fat conditioning by feeding rats a high-fat maintenance diet or by adding an oily flavor to the conditioning stimuli. In Experiment 1, rats were fed restricted rations of either a chow-corn oil mixture (48% energy as fat, HF group) or regular chow (12% fat, LF group). During 1-bottle training sessions, drinking a flavored (CS+, e.g., cherry) saccharin solution was paired with IG fat (7.1% corn oil emulsion). On other days, an alternate flavor (CS-, e.g., grape) was paired with IG water. In subsequent 2-bottle tests between the CS+ and CS- flavors, the HF rats displayed a stronger CS+ preference than the LF rats (90% vs. 62%). Experiment 2 tested the effect of a semisynthetic HF maintenance diet (48% fat energy), using a conditioning procedure similar to that of Experiment 1. The rats displayed only a moderate (72%) CS+ preference. When switched to the chow-oil maintenance diet and retrained with new CS flavors, they developed a 90% CS+ preference. In Experiment 3, chow-fed rats were trained and tested with oily CSs (i.e., 2% corn oil was added to flavored saccharin solutions). They failed to show a preference for the CS+ paired with IG oil. Thus, increasing the level of fat in the maintenance diet can greatly enhance preference conditioning with IG fat, but the amplitude of the effect is influenced by the composition of the high-fat food. In contrast, adding a fatty flavor to the conditioning stimuli did not improve fat conditioning.

Carbohydrate- and protein-conditioned flavor preferences: effects of nutrient preloads

Food-deprived rats were trained to associate one flavor (CSProt) with intragastric (IG) infusions of protein (PROT; 10% calcium caseinate), a second flavor (CSCHO) with IG infusions of carbohydrate (CHO; 10% protein), and a third flavor (CS-) with IG water infusions during 30 min/day training sessions. (The CS flavors were cherry, grape, and orange saccharin solutions.) In subsequent two-bottle tests the rats reliably preferred both the CSProt 2nd CSCHO to the CS- and equally preferred the CSProt and CSCHO. The preference for the two nutrient-paired flavors was not altered by IG preloads of PROT or CHO delivered as three loads 120, 40, and 5 min prior to testing. However, single oral + gastric preloads of CSCHO + IG CHO and CSProt + IG PROT 45 min prior to test selectively increased the preference for the CSProt and CSCHO, respectively. In subsequent gastric-only and oral-only tests single IG preloads of PROT and CHO, but not CSProt and CSCHO preloads, selectively altered the rats' preference for CSCHO vs. CSProt. In a second experiment with new rats, oral + gastric preloads again selectively altered the preference for the CSCHO vs. CSProt, but gastric-only preloads failed to have this effect. These results demonstrate that rats can learn to associate different flavors with the postingestive effects of different nutrients, and modify their flavor preferences after nutrient preloads. Oral + gastric preloads were most effective in altering flavor preferences, whereas gastric-only preloads had inconsistent effects and oral-only preloads were ineffective.

Probing the causes of high-fat diet hyperphagia: a mechanistic and behavioral dissection

High-fat diets promote hyperphagia in both rats and humans; however, understanding of the process by which dietary fat increases intake is incomplete. Since altering the fat content of a diet simultaneously changes both its sensory properties and postingestive effects, it is unclear whether high-fat diet hyperphagia is driven by oral influences, postingestive factors, or both. Previous findings from both animal and human studies indicate that relatively "less palatable" high-fat diets are overeaten relative to high-carbohydrate diets, indicating that the postingestive effects of high-fat foods are sufficient to promote hyperphagia. A program of research on rats is described, which isolates and assesses the independent effects of sensory and postingestive influences on intake of liquid high-fat and high-carbohydrate diets. An integrated series of experiments investigates both short-term (meal size, postprandial satiety) and long-term (ad lib intake over weeks) effects of diet composition on intake in order to "dissect" the causes of high-fat diet hyperphagia. Preliminary findings from this approach indicate that the postingestive effects of a high-fat diet promote longer meal size, less postprandial satiety per calorie, and greater daily calorie intake than a high-carbohydrate diet.

Stimulation of fluid intake by maltodextrins and starch

Maltodextrin solutions are more potent stimulators of fluid intake than are starch suspensions, even though these two substances are both glucose polymers. When rats were offered a choice of two fluids, one containing maltodextrin and the other containing the same concentration of starch, rats preferred maltodextrin; this preference increased over successive days for rats given 7.4% or 10% carbohydrate but not for rats given 1% carbohydrate. Rats offered fluids containing artificial flavors mixed with maltodextrin and starch acquired preferences for flavors paired with 10% or 7.4% maltodextrin but not for flavors paired with 1% maltodextrin. In an attempt to determine whether this greater preference for maltodextrin is due to an innate flavor preference, rats were given a choice of maltodextrin containing sucrose octaacetate vs. plain starch; these rats initially preferred starch but subsequently ingested more maltodextrin flavored with sucrose octaacetate than plain starch. Direct evidence for a postingestive factor was obtained by giving rats intragastric infusions of either maltodextrin, starch, or vehicle whenever they drank a dilute saccharin solution. Both carbohydrates stimulated fluid intake, but maltodextrin did so more rapidly (within 1 day rather than the 2 days of training required with starch infusions). These results indicate that some difference in the postingestive effects of maltodextrin and starch contributes to the greater intake-stimulating effect of maltodextrin. The postingestive factor responsible for the difference between maltodextrin and starch has not been identified, but it is not the amount of utilizable energy because rats prefer maltodextrin over starch even when the starch suspension contains more calories than does the maltodextrin solution. It is proposed that the postingestive factor differentiating these substances is rate of digestion.

Taste and smell sensations enhance the satiating effect of both a high-carbohydrate and a high-fat meal in humans

The effects of meal sensory properties (tasty vs. bland) and nutrient composition [high-CHO (carbohydrate) vs. high-FAT] on hunger ratings, blood glucose and free fatty acids (FFA), taste perception, and subsequent food intake, were studied in human subjects. Aspartame and vanilla were used to augment meal palatability, yielding four isocaloric liquid meals: bland-FAT, tasty-FAT, bland-CHO, tasty-CHO. Normal-weight, nondieting young adults consumed each of the meals for breakfast on separate days. The main finding was that tasty versions of high-FAT and high-CHO meals were more satiating than nutritionally identical bland meals, as indicated by a greater decrease in hunger ratings following the tasty meals. Changes in blood glucose and FFA were related to meal nutrient composition, but not to meal sensory properties. High-CHO meals tended to be more satiating than high-FAT meals. Consumption of each of the meals produced a similar decrease in pleasantness ratings of food-related tastes. Intake of carbohydrates was significantly higher at a self-selected lunch 5.25 h following a tasty breakfast. These findings indicate that hunger is decreased to a greater extent by meals flavored with aspartame and vanilla relative to nutritionally identical, unflavored meals. The satiety-enhancing effect of oral stimulation was found for both high-FAT and high-CHO meals.

Experience with a macronutrient source influences subsequent macronutrient selection

We examined the influence of experience with a macronutrient on subsequent macronutrient selection. For 4 days, rats ate chow, or chow and a single macronutrient source. They then had simultaneous access to protein, carbohydrate (CHO) and fat sources, according to a standard macronutrient self-selection paradigm. When selecting among the macronutrient sources, rats pre-exposed to CHO ate more CHO, and those pre-exposed to fat ate more fat, relative to the other groups. Rats pre-exposed to protein ate more protein than did those pre-exposed to CHO or fat but not more than those that received no macronutrient pre-exposure. These selection patterns persisted for at least 12 days, when the test was ended because of the low protein intakes and poor growth of the rats pre-exposed to the CHO and fat sources. After 34 days of recovery with only chow to eat, the rats were again allowed to choose among the three macronutrients, and their patterns of selection were essentially unchanged. Similar results were found in a second experiment in which a 5-day interval was interposed between macronutrient pre-exposure and macronutrient selection. These findings show that experience with the macronutrients typically used in self-selection experiments can have a large, long-lasting, and sometimes detrimental effect on subsequent food selection by rats. Prior experience can be a more powerful influence than nutritional wisdom in determining the rat's food choice.

Role of dietary fat in calorie intake and weight gain

This paper reviews the literature on the role of dietary fat in calorie intake and body weight gain in humans and laboratory animals. An overview of 40 animal studies which compared growth on high-fat (HF) and high-carbohydrate (HC) solid/powdered diets indicated that the HF diet elicited greater weight gain in 33 out of 40 studies. Enhanced growth on the HF diet was often, but not exclusively, attributable to greater caloric intake. Additional evidence for the growth-enhancing effect of HF diets emerges from "diet option" and "supermarket" feeding studies in rats, and experimental and epidemiological studies in humans. Three principal factors that contribute to the different responses to HF and HC diets are (a) caloric density, (b) sensory properties and palatability, and (c) postabsorptive processing. It is concluded that both calorie intake and metabolic energy expenditure are biased towards weight gain when a HF diet is consumed, and that the high caloric density of high-fat diets plays a primary role in weight gain. Humans may be biologically predisposed to gain weight when a HF diet is consumed.

Ingestion patterns of food, water, saccharin and sucrose in streptozotocin-induced diabetic rats

Changes in preference for sweet tasting solutions have previously been reported in diabetic rats. The present study was designed to use detailed analyses of drinking patterns to investigate changes in sweet taste perception in rats made diabetic with a streptozotocin (SZ) injection and tested with saccharin and a broad range of sucrose concentrations. Although meal patterns in diabetic rats have been studied extensively, drinking patterns and their relation to meal patterns have not been described. A second goal of this research was to use the detailed patterns of eating and drinking of the diabetic rat in an effort to understand the resulting hyperphagia. Rats were given IP injection of SZ or saline. Following the SZ injection, the rats showed marked increase in blood glucose levels, an increase in food and fluid intake and they failed to gain weight. The SZ-injected rats showed a loss of preference for saccharin and for high concentrations of sucrose. Patterns of sucrose ingestion that are correlated with taste perception were distorted in the diabetic rats. The detailed pattern analysis of eating showed that the diabetic-induced hyperphagia resulted from a marked increase in length of eating bouts. The increase in fluid intake in the diabetic rats resulted from both an increase in frequency and length of drinking bouts. Circadian patterns of both food and fluid ingestion were maintained in the diabetic rats.

Diet composition alters the acceptance of fat by rats

The acceptance of dietary fat by rats is influenced by changes in fat digestion and metabolism. In these experiments, rats were fed diets that differed in fat, carbohydrate and fiber content, and the acceptance of fat was measured. Rats fed a high fat (HF) diet ate more corn oil in 30-min or 6-h tests than did rats fed an isocaloric high carbohydrate (HC) diet. This effect was seen after the diets were switched and rats retested. Differences in dietary fiber between the isocaloric HF and HC diets did not account for this effect because rats fed HF diets, either high or low in fiber content, drank more oil than rats fed the HC diet. Rats fed the HF diet with added carbohydrate drank less oil than rats fed the HF diet, and the same amount of oil as rats fed the HC diet. Compared with rats fed the HC diet, rats fed the HF diet drank more oil in a two-bottle preference test with sucrose or when mixed with sucrose in a single-bottle test. Rats offered a variety of fats, sugars or other test foods, ate more nutritive liquid fats and some solid fats, but did not eat more sugar or other items if they were fed the HF diet rather than the HC diet. These studies taken together strongly suggest that rats fed a high fat diet show a greater acceptance of fat compared with rats fed a high carbohydrate diet.

Sham-feeding of corn oil by rats: sensory and postingestive factors

Previous research indicates that rats fed a high-fat (HF) diet increase their intake and preference for oil compared with rats fed a high-carbohydrate (HC) diet. To assess whether this increased intake was due to the sensory or postingestive properties of oil, rats were adapted to either the HF or HC diet and then allowed to sham-feed pure corn oil daily for 30 min. During the first 4 trials, rats fed the HF diet sham-fed more oil than did rats fed the HC diet; however, this difference diminished with repeated testing and was absent after 8 trials. In both diet groups, 4-5 calories (approximately 25%) of sham-fed oil could not be recovered and may have escaped to the intestine. These results suggest that, compared with rats fed a HC diet, rats fed a HF diet are initially attracted to the sensory properties of oil, but that the differential oil intakes of rats fed the HF or HC diet are maintained by postingestive, rather than sensory factors.

Flavor preferences and fructose: evidence that the liver detects the unconditioned stimulus for calorie-based learning

Previous work suggests that fuel oxidation in the liver provides an unconditioned stimulus for "calorie-based" flavor preference learning. To investigate this possibility in more detail, we manipulated liver metabolism by taking advantage of the greater specificity to the liver of fructose than glucose. Intact rats preferred flavored food ingested with a drink of 35% fructose solution to flavored food eaten with either no sweet drink (Experiment 1) or an equicaloric drink of glucose (Experiment 2). The effect on food preference did not depend on the taste of the sugar solutions: when given a choice between glucose and fructose solutions, the rats drank the same volume of each. Moreover, a preference for fructose-paired flavored food was obtained when fructose and glucose solutions were given by gavage (Experiment 3). Unlike intact rats, rats with hepatic vagotomy preferred equally flavored food paired with fructose solution and flavored food paired with no sugar solution (Experiment 1). They also avoided flavored food paired with gavage of fructose slightly, relative to flavored food paired with gavage of glucose (Experiment 3). These results suggest that the unconditioned stimulus for calorie-based conditioning is transduced in the liver, and that an intact hepatic vagus nerve is required for conditioning to occur.

Preference conditioning by concurrent diets with delayed proportional reinforcement

Mildly food-deprived rats were presented at the same time either high- and low-carbohydrate diets or protein-containing and nonnutritive diets differing in flavor in parallel with nutrient composition. After a few days of these concurrent 10-minute presentations, the rats preferred the flavor of the richer or nutritive diet. Such preference conditioning also occurred when there was a 10-minute delay between the presentation of two flavors in nonnutritive diet and intubation of a mixture of nutrients in proportion to the flavor intakes. Thus, rats are able to perceive the difference between the nutritional aftereffects of two distinctive diets presented concurrently and to learn to respond differentially to associated flavors even when there was a delay as long as that used in earlier studies with pyloric closure. Therefore, the reinforcement that has been suggested to arise from gastric chemoreception can be attributed instead to delayed postgastric effects.

Flavor preferences conditioned by intragastric fat infusions in rats

The present series of experiments determined if rats would learn to prefer cue flavors that were paired with intragastric (IG) infusions of fat. Adult female rats were fitted with gastric catheters and were trained to drink a flavored (CS+) solution, e.g., cherry-water, associated with IG infusions of a corn oil emulsion. On other days, an alternately flavored (CS-) solution, e.g., grape-water, was paired with IG infusions of water. The preference between the CS solutions was then assessed with 2-bottle choice tests. In Experiment 1, food-restricted rats were given saccharin-sweetened CS solutions for 10 min/day followed by IG infusions of a 7.1% oil emulsion or water. In subsequent choice tests they displayed a small (60%) but significant preference for the CS+ solution. In Experiments 2-5, rats had access to the CS solutions for 20 or 23 hr/day; drinking the CS solutions automatically triggered IG infusions of oil or water (electronic esophagus preparation). Infusions of 7.1% oil failed to produce a significant preference for the CS+ solution in rats given ad lib access to chow, and offered unsweetened CS flavors. Increasing the oil concentration (14.5% and 29%) did not facilitate the formation of a preference, nor did infusing the rats with a preingested oil emulsion. However, rats restricted to 2 hr/day access to chow, and 20 hr/day access to the CS solutions acquired a significant (85%) preference for the CS+ flavor; subsequent training and testing with saccharin-sweetened CS solutions increased the preference for the CS+ flavor to 95%. Ad lib fed rats trained with saccharin-sweetened CS solutions also developed a reliable preference (76%) for the CS+ flavor paired with IG oil; this preference persisted during four extinction tests when both CS+ and CS- flavors were paired with IG water. These results demonstrate that, as with other macronutrients (carbohydrate, protein), the postingestive effects of fats can condition flavor preferences, and suggest that the postingestive actions of nutrients play an important role in food selection and reward.

Diabetes and a high-fat/low-carbohydrate diet enhance the acceptability of oil emulsions to rats

Acceptability of corn oil and coconut oil emulsions was examined in streptozotocin-diabetic (55 mg/kg, IP) and normal rats fed either a high-fat/low-carbohydrate (HF/LC) or low-fat/high-carbohydrate (LF/HC) diet. Intake of five concentrations of the emulsions (2.5, 5, 10, 20 and 40%) was measured in 30-min, one bottle intake tests. Diabetic rats consumed more of the oil emulsions than did normal rats. Emulsion intake by diabetic rats increased, then decreased across concentrations, whereas emulsion intake by normal rats showed little change across concentrations. No differences in the intake of corn and coconut oil emulsions were observed. Total oil consumption was higher in diabetic than in normal rats at the three highest emulsion concentrations. Total oil consumption was also higher in rats fed the HF/LC diet as compared to those fed the LF/HC diet. These findings suggest that the presence of diabetes or feeding a HF/LC diet can enhance oil emulsion intake of rats in short-term tests.

Taste-to-postingestive consequence conditioning: is the rise in sham feeding with repeated experience a learning phenomenon?

Although the progressive increase in intake with repeated sham feeding experience is assumed to reflect the extinction of learned satiety, the involvement of associative learning in this phenomenon has never been directly demonstrated. We show that: a) animals attenuate sham feeding on initial exposure only with foods tasting like those they have fed normally before, and b) latent inhibition, which retards the formation of CS-US associations in classical conditioning preparations, prevents the association of a taste with its postingestive consequences. These data suggest both that learning plays a role in the development of sham feeding and that associative linking of a food's taste with its postingestive consequences occurs during ingestion. The present results identify properties of taste-to-postingestive consequence conditioning and indicate how the sham feeding preparation can be used to identify the physiological events mediating this learning.

Drinking saccharin increases food intake and preference--IV. Cephalic phase and metabolic factors

Rats that drink saccharin solution increase their short-term food intake and develop a preference for flavored food eaten when saccharin is ingested. In this paper, we examined whether these effects are mediated by cephalic-phase metabolic reflexes. The results show that the cephalic-phase insulin response could be dissociated from food intake in three ways. (1) Drinking saccharin increased the food intake and food preference of rats with sham surgery (SHM) or celiac vagotomy (CV), but not hepatic vagotomy (HV); it produced a short-lived increase in plasma insulin levels in all three groups, but the insulin response of both the CV and HV group was attenuated relative to the SHM group. (2) Rats increased food intake even when a 90 min interval was imposed between drinking saccharin and eating food, although insulin and glucose levels returned to normal within 30 min of drinking saccharin. (3) Streptozotocin-induced diabetes did not affect the increased feeding response to saccharin. The failure of rats given hepatic vagotomy to increase food intake and food preference when drinking saccharin suggests that a hepatic mechanism is involved. We propose that drinking saccharin increases food intake by temporarily altering the disposition of metabolic fuels towards storage and away from oxidation. Flavored food eaten after drinking saccharin becomes preferred because it provides fuel to counteract this reduction in oxidation.

The psychoneuroendocrinology of diabetes mellitus in rodents

The metabolic-endocrine state of diabetes mellitus affects the brain and behavior of diabetic animals. Feeding, paradoxical sleep, analgesia, submissive behavior, and avoidance behavior, are generally increased in diabetic compared with nondiabetic rodents. In contrast, sexual behavior, aggressive behavior and sensitivity to the behavioral effects of amphetamine are decreased in diabetic rodents. This review examines behavioral changes in diabetes mellitus within the context of known disease-linked alterations in hypothalamo-pituitary relationships and brain monoamine metabolism.