Transient overconsumption of novel foods by deafferentated rats: effects of novel diet composition

We recently demonstrated that capsaicin-treated rats consume more of an unfamiliar high-fat diet than vehicle-treated controls, but only on initial exposure (Chavez et al, 1997). We hypothesized that negative feedback signals carried by capsaicin-sensitive visceral afferents are critical for the regulation of intake of novel foods, but redundant pathways take over during subsequent exposures. To examine the role of nutrient content of the novel diet, rats were systemically treated with capsaicin (n = 15) or vehicle (n = 10), and exposed to 1) a fat/olestra diet that was isocaloric with chow; 2) a readily accepted fat-free cake; and 3) pure corn oil. Each 3-h feeding trial was preceded by 24-h food deprivation. Treated rats did not overconsume familiar chow, but did consume 50% more than controls of both the fat/olestra diet and the corn oil on first exposure; this suggests that capsaicin eliminated visceral afferents that normally carry satiety signals. However, the effect with the fat/olestra mixture was due primarily to depressed intake by controls, unlike the pure fat diets; this apparent neophobic response was blunted in treated rats. Because treated rats failed to overconsume the fat-free cakes, the neural system damaged by capsaicin appears to be linked to energy or fat sensory mechanisms, and possibly to hedonic responsiveness.     

High-fat hyperphagia in neurotrophin-4 deficient mice reveals potential role of vagal intestinal sensory innervation in long-term controls of food intake

Neurotrophin-4 (NT-4) deficient mice exhibit substantial loss of intestinal vagal afferent innervation and short-term deficits in feeding behavior, suggesting reduced satiation. However, they do not show long-term changes in feeding or body weight because of compensatory behaviors. The present study examined whether high-fat hyperphagia induction would overcome compensation and reveal long-term effects associated with the reduced vagal sensory innervation of NT-4 mutants. First, modifications of a feeding schedule previously developed in rats were examined in wild-type mice to identify the regimen most effective at producing hyperphagia. The most successful schedule, which was run for 26 days, included access to a 43%-fat diet and pelleted chow every other day and access to only powdered chow on the alternate days. On high-fat access days mice consumed 25% more calories than mice with continuous daily access to the same high-fat diet and pelleted chow. This feeding regimen also induced hyperphagia in NT-4 deficient mice and their wild-type controls: on high-fat exposure days mutants consumed 35% more calories relative to continuous-access mutants, and wild types ate 25% more than continuous-access wild types. Moreover, on high-fat access days the alternating NT-4 mutants significantly increased caloric intake by 9% compared to alternating wild types. Thus, high-fat hyperphagia appeared to override compensation, permitting short-term changes in meal consumption by mutants that accrued into long-term changes in total daily food intake. This raises the possibility that intestinal vagal sensory innervation contributes to long-term, as well as to short-term regulation of food intake.     

Influence of early dietary experience on energy regulation in rats

The environmental mechanisms that have contributed to the rapid increases in overweight and obesity in the US over the past 25-30 years have yet to be fully specified. One hypothesis that has been forwarded is that increased consumption of calories in liquid form may be a contributing factor, since some studies support the idea that caloric compensation is less adequate for liquid calories compared to calories delivered in more solid form. Work from our laboratory using rats has examined the role that differences in diet viscosity may play in altering energy intake and body weight regulation. This work has suggested that when offered diets matched on caloric density, macronutrient and micronutrient composition, and differing only in viscosity, adult rats fail to compensate for calories delivered in low-viscosity form in short-term intake tests. Further, long-term consumption of low-viscosity diets leads to enhanced weight gain in adult rats. In the present studies, we examined whether short- or long-term exposure to varying relationships between viscosity and calories led to altered food intake or body weight regulation in juvenile rats. The results demonstrated that animals given either short- or long-term experience with direct relationships between viscosity and calories (high viscosity, high calorie meals and low viscosity, low calorie meals) did not differ in food intake or body weight gain compared to animals given short- or long-term experience with indirect relationships between viscosity and calories (high viscosity, low calorie meals and low viscosity, high calorie meals). When juvenile rats were given long-term (9 weeks) exposure to a single, high or low viscosity diet supplement, matched on caloric density and differing only in viscosity, there were no effects on body weight gain. However, analysis of body composition using DEXA demonstrated that animals consuming the low viscosity supplements had significantly greater body fat than animals that consumed either the high viscosity supplement or no dietary supplement at all. These differences in body fat persisted for at least 3 months following the cessation of dietary supplements; during this 3-month period, animals previously exposed to the low viscosity supplement also gained significantly more weight than animals previously exposed to the high viscosity supplement. Taken together, the results suggest that consuming calories in low viscosity form may contribute to poor intake regulation over the short-term and to increased adiposity over the long term. When animals experience these diets as juveniles, these effects may persist into adulthood.     

Effects of alternations (10 days) of high-fat with normal diet on liver lipid infiltration, fat gain, and plasma metabolic profile in rats

The purpose of the present study was to test the hypothesis that short-term alternations of high-fat with normal chow feeding result in higher fat accumulation in liver than continuous intake of the same high-fat diet. Male Sprague-Dawley rats (7 weeks of age) were divided into 3 groups according to diet composition: standard chow (SD; 12,5% kcal as fat), high-fat (HF; 42% kcal as fat), and food cycles (FC) consisting of 10-day alternations between HF and SD diets beginning with the high-fat diet. Rats in each of these 3 groups were sacrificed after 10, 30, and 50 days (n = 10 rats/sub-groups). Energy intake, body weight, liver and muscle relative weights were not significantly (P > 0.05) different between FC- and HF-fed rats. Using the total energy intake for the 50-day period, it was calculated that approximately 30% less calories as fat was ingested in the FC- compared to the HF-fed rats. In spite of this, liver lipid infiltration as well as fat accretion in abdominal adipose tissues were increased (P < 0.01) similarly in FC- and HF-fed rats. Plasma FFA and insulin levels depicted strong tendencies (P < 0.07) to be higher in FC- than in continuous HF-fed rats at the end of the 50-day period. These results indicate that, despite a 30% reduction in ingested lipids, alternations of HF with normal chow diet compared to the continuous hyperlipidic diet caused the same level of infiltration of lipids in the liver and in the abdominal adipose tissues and, to a certain extent, may even result in a larger deterioration of the metabolic profile.     

Review of the effects of dilution of dietary energy with olestra on energy intake

The non-absorbable substitute for dietary triacylglycerol, olestra, has been marketed in the United States for fifteen years. Olestra is comprised of sucrose with six to eight of its hydroxyl groups forming ester links with long-chain fatty acids. Because olestra is not hydrolyzed by fat-splitting enzymes in the small intestine, it is not absorbed from the small intestine into blood and tissues, and therefore provides no energy that can be utilized by the body. The hedonic properties of olestra with a specific fatty acid composition are similar to those of a triacylglycerol with the same fatty acid composition. Its use by consumers has been restricted by federal regulation to the commercial preparation of savory snack food items, principally as a frying medium for potato chips. An important question about the substitution of olestra for absorbable fat in the diet is whether the consumer will sense that a smaller amount of energy has been ingested. If it is sensed, thereby providing no satiation, then consuming additional energy in later meals will compensate for the removal of absorbable energy from the diet. If it is not sensed at all, then there is no compensation, and the person reduces caloric intake. This review first summarizes studies with olestra that have focused on its effect on the physiology of appetite. In general these studies have demonstrated that olestra does not influence signals of satiation including cholecystokinin and stomach emptying. The review then discusses studies of food consumption in experimental animals in which olestra was substituted for fat in the diet. Rodents have been repeatedly observed to compensate completely for the substitution of olestra for normal fat by eating more total diet. Most studies of the effect of olestra on human satiation have found incomplete or no compensation through additional energy consumption when olestra was substituted for dietary fat. In two clinical studies, however, complete compensation was observed, suggesting that experimental conditions and individual variability influence the ability to sense the substitution of olestra for absorbable fat. There is no evidence that dietary olestra causes consumption of more energy than would have been consumed without olestra in the diet. The data from animals and humans strongly suggest that the rodent is not a satisfactory model for the human in the determination of the extent of compensation by substitution of olestra for dietary fat.     

Effects of dietary fat, nutrition labels, and repeated consumption on sensory-specific satiety

This study investigated whether energy from fat, nutrition information, and/or repeated consumption of a palatable snack food affects the development of sensory-specific satiety (SSS). Participants (51 males and 44 females) ate an afternoon snack of potato chips in a laboratory for two 10-day (Monday-Friday) sessions in a repeated measures, cross-over design. In one 10-day session, participants were given regular, full-fat potato chips (22.2 kJ/g; 150 kcal/oz) and, in the other, they were given potato chips made with olestra (11.8 kJ/g; 80 kcal/oz), a non-absorbable fat replacer. Information about the fat and energy content of the chips was provided to half the participants, while the other half was not informed. In both sessions, participants were instructed to consume the potato chips ad libitum. Initial ratings of sensory properties of the two types of chips did not differ significantly. In SSS tests, participants rated sensory properties of the chips and four test foods (turkey, strawberry yogurt, cookie, and carrot) on days 1, 5, and 10 of the 10-day sessions. Following consumption, ratings of pleasantness of taste and texture and prospective consumption of both types of chips declined compared to the test foods. Further analyses showed that the development of SSS was not affected by the fat and energy content of the chips, the provision of nutrition information, or repeated consumption.     

Consumption of a high-fat diet induces central insulin resistance independent of adiposity

Plasma insulin enters the CNS where it interacts with insulin receptors in areas that are related to energy homeostasis and elicits a decrease of food intake and body weight. Here, we demonstrate that consumption of a high-fat (HF) diet impairs the central actions of insulin. Male Long-Evans rats were given chronic (70-day) or acute (3-day) ad libitum access to HF, low-fat (LF), or chow diets. Insulin administered into the 3rd-cerebral ventricle (i3vt) decreased food intake and body weight of LF and chow rats but had no effect on HF rats in either the chronic or the acute experiment. Rats chronically pair-fed the HF diet to match the caloric intake of LF rats, and with body weights and adiposity levels comparable to those of LF rats, were also unresponsive to i3vt insulin when returned to ad libitum food whereas rats pair-fed the LF diet had reduced food intake and body weight when administered i3vt insulin. Insulin's inability to reduce food intake in the presence of the high-fat diet was associated with a reduced ability of insulin to activate its signaling cascade, as measured by pAKT. Finally, i3vt administration of insulin increased hypothalamic expression of POMC mRNA in the LF- but not the HF-fed rats. We conclude that consumption of a HF diet leads to central insulin resistance following short exposure to the diet, and as demonstrated by reductions in insulin signaling and insulin-induced hypothalamic expression of POMC mRNA.     

Dietary fat-induced hyperphagia in rats as a function of fat type and physical form

The influence of dietary fat on food intake and weight gain was assessed by feeding adult female rats diets that differed in the type and form of fat, as well as in the availability of other macro- and micronutrients. Compared to chow-fed controls, the various fat diets increased total food intake by 4% to 27%. Specifically, rats fed chow and a separate source of fat (fat option diet) consumed more fat and total calories, and gained more weight when the fat source was emulsified corn oil rather than pure corn oil or was vegetable shortening rather than corn oil. However, corn oil and shortening had similar effects on caloric intake and weight gain when presented as emulsified gels. Also, pure and emulsified-gel forms of shortening did not differ in their effects on caloric intake and weight gain. Supplementing the vegetable shortening with micronutrients, however, enhanced its hyperphagia-promoting effect. The results of two-choice tests revealed that the rats' preferences for the orosensory properties of the various fat sources did not account for the differential hyperphagias obtained. Rather, it appears that long-term fat selection and caloric intake are influenced primarily by postingestive factors. Fat selection and total intake were determined not only by the fat source itself, but also by the other diet options. That is, rats selected more fat and consumed more calories when chow was the alternative food than when separate sources of carbohydrate and protein were available.     

Leptin resistance does not induce hyperphagia in the rat

Leptin has been thought to work as a mediator for body weight control by inhibiting food intake. Leptin, however, cannot prevent obesity induced by a high-fat diet (HFD) probably because of leptin resistance. We investigated daily feeding and weight gain when ordinary chow (OC) was changed to a HFD in male rats. Food intake, by weight, significantly increased the next day, but gradually decreased until at 20 days the HFD intake contained the same calories as consumed by the OC-fed control rats. The reduction in food intake occurred only during the night without change of preference for the HFD, even after leptin resistance had developed. Nonetheless, the HFD-fed rats gained more weight than the controls. From the present experiment, it is concluded that leptin resistance does not induce hyperphagia, and suggested that body weight is not regulated to be constant.     

Consumption of a high-fat diet alters the homeostatic regulation of energy balance

Humans in many countries are currently experiencing what has been called an epidemic of obesity. That is, the average body weight (and amount of fat stored in the body) is increasing over years, carrying with it a multitude of associated medical, psychological, and economic problems. While there is no shortage of possible causes of this epidemic, increased availability and consumption of high-fat (HF), calorically dense and generally quite palatable food is often touted as a likely culprit. In order to better assess the impact of consuming a diet with those qualities, we have developed a well-controlled animal model in which the effects of chronic consumption of a high-fat diet can be dissociated from those of becoming obese per se. Long-Evans rats are fed one of two semipurified pelleted diets, a HF diet that contains 20% fat by weight and a low-fat (LF) diet that contains 4% fat by weight. Pair-fed animals consume the HF diet but are limited to the daily caloric intake of LF rats. Another group receives pelleted chow. Relative to animals consuming diets low in fat, HF animals weigh more, have more carcass fat, are hyperinsulinemic and hyperleptinemic, and are insulin resistant. HF-fed animals, independent of whether they become obese or not, also have central insulin and MTII insensitivity. Finally, HF rats have a down-regulated hypothalamic apo A-IV system that could contribute to their hyperphagia.     

Chronic food restriction and reduced dietary fat: risk factors for bouts of overeating

The purpose of this study was to determine the effect of chronic food restriction and reduced dietary fat on feeding behavior and body weight. Young female rats were fed ad lib or food restricted on a low-fat (LF) or a fat-free (FF) diet for 4 weeks. Rats then received 24-h free access to 2 diets, the maintenance diet (LF or FF) plus a novel high-fat (HF) diet (24-h intake test). After the test, all the rats were allowed chronic free access to the HF diet until body weight was stable. During the 24-h test, the restricted groups ate significantly more calories than the ad lib groups, and the FF-restricted rats ate significantly more total food, carbohydrate and protein than the LF-restricted rats; there were no differences between the two ad lib groups. During chronic free access to the HF diet, the formerly restricted rats achieved and defended lower body weights than the formerly non-restricted rats. Throughout the experiment, the ad lib groups had more body fat than the restricted groups independent of the dietary subgroup. Hence, a history of chronic food restriction predisposes to consuming more food in acute feeding situations, particularly when dietary fat is reduced, and lowers the level of body weight maintained and defended. Chronic food restriction accompanied by reduced dietary fat may increase risk for bouts of overeating.     

Effects of abuse pattern of gestational toluene exposure on metabolism, feeding and body composition

AIMS: Inhalant abuse during pregnancy lowers birth weight and impedes early development. These studies explored the effects of brief, repeated, prenatal toluene exposures in pregnant female rats on body weight, metabolic rate, body composition, and food intake in their offspring. METHOD: Rats were exposed to 0, 8000, 12,000, or 16,000 ppm of toluene twice daily for 15 min from gestational days 8 to 20. The effects of such exposures on post-weaning litter weights, oxygen consumption, carbon dioxide output, and body fat content were determined in 2 cohorts (n=23, n=24) of offspring. Food intakes and weight changes in response to 3 different diets (regular chow, purified diet, purified high fat diet) were examined in another cohort (n=24) from postnatal days 72 to 116. RESULTS: Litter weights showed a significant linear decrease as a function of toluene dose. Offspring exposed to the 16,000 ppm toluene dose displayed statistically lower energy expenditures than control rats. Male rats exposed to 8000 or 16,000 ppm toluene had significantly greater percentage of body fat as well as total body fat than the other groups. Toluene also significantly suppressed weight gain over the time chow was consumed compared to the 0 ppm control group. Finally there were trends for a main effect of toluene dose on food intake during chow and during high fat diet consumption, with rats in the 12,000 ppm group consuming more than the 0 ppm group on both diets. DISCUSSION: These data suggest that, in addition to other previously documented abnormalities in neurological development and behavior, the physiological regulation of metabolism and body composition in males as well as food intake and weight gain in both sexes may be altered by prenatal exposure to toluene.     

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.     

Dietary self-selection vs. complete diet: body weight gain and meal pattern in rats.

Food intake and body weight gain of male adult Wistar rats were examined in two groups of animals. One group (n = 14) was allowed to select its diet from separate sources of protein (casein, 3.1 kcal/g), fat (lard and sunflower oil, 7.9 kcal/g) and carbohydrate (CHO, starch and sucrose, 3.3 kcal/g). Another group (n = 10) received a nutritionally complete diet (3.3 kcal/g). After 2 weeks of adaptation to the diets, body weights and meal patterns were recorded for at least 4 days. The total caloric intake was nearly identical for the two groups of rats. Rats given dietary choice gained less weight over 4 days than rats fed chow and showed reduced feed efficiency. During the 24-h period, self-selecting rats consumed 20.8% of calories as proteins, 21% as fats and 58.2% as CHO. Self-selecting rats ate significantly less calories during the day than did rats given chow. The chow diet consisting of 17.3% calories as protein, 7.6% as fat and 75.1% as CHO. When comparing the self-selecting group nutrient intakes to those of chow-fed group it was observed that 24-h protein calorie intakes were identical in both groups. Fat intake was significantly higher and CHO reduced as compared to chow-fed rats. During the day, CHO intake was higher in self-selecting rats, and fat intake was not significantly reduced. During the night, protein and fat intakes were significantly higher in self-selecting rats, while CHO intake was significantly decreased, particularly in the last periods of the night.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dietary hyperphagia in rats: role of fat, carbohydrate, and energy content

Dietary energy, fat and carbohydrate content were varied to determine the nutritional factors responsible for hyperphagia induced by feeding rats high-fat diets. In the first experiment, rats were fed isoenergetic high-fat or high-carbohydrate diets for 2 weeks. Weight gain and energy intake were lower in rats given the high-fat diet. When some of the rats were switched to a diet that was high in fat, carbohydrate and energy, gram food intake was initially unchanged, resulting in a substantial increase in energy intake and weight gain. Energy intake gradually declined over the 4 weeks following the switch to the high-energy diet. In the second experiment, rats were fed high-fat diets that were either high or low in carbohydrate content and either high or low in energy content (kcal/g). Rats fed a high-fat diet that was high in energy and carbohydrate ate the most energy and gained the most body weight and carcass fat. In the third experiment, rats were fed high-carbohydrate diets varying in fat and cellulose content. Energy intake and body weight gain varied directly as a function of caloric density regardless of the fat or cellulose content of the diets. It is concluded that hyperphagia induced by feeding high-fat diets is not due to the high dietary fat content alone. Rather, high levels of fat, carbohydrate, and energy interact to produce overeating and obesity in rats fed high-fat diets.     

Effect of dietary macronutrients on food intake, body weight, and tail width in the marsupial S. crassicaudata

The role of dietary fat, as opposed to total energy intake, in the etiology of obesity is controversial. The aim of this study was to determine the effect of macronutrient content, specifically changes in dietary fat on body weight, fat stores, and food intake in S. crassicaudata, a marsupial that stores about 25% of total body fat in its tail. Female animals were divided into three groups (n = 7-9) matched for food intake per gram of body weight. Each group of animals was fed, ad lib an isocaloric diet (1.01 kcal/g), which contained either 10, 20, or 40% of calories from fat. Body weight, food intake, and tail width (an index of body fat stores) were measured daily. Over 21 days, cumulative energy intake was less (p = 0.026) in the 40% fat group compared to the 10% fat group. Despite the differences in food intake, body weight in each group remained stable throughout the study, so that at day 21 there were no differences in the body weights between the three groups. In contrast, tail width increased in the animals who received the 40% fat diet compared to either the 10% (p = 0.016) or 20% (p = 0.001) fat intake groups, whereas there was no significant change in tail width in either of these two groups. These observations indicate that macronutrient composition has a role, independent of total calories in the regulation of food intake and body fat stores, specifically that dietary fat promotes adiposity, independent of total caloric intake.     

Chronic stress, energy balance and adiposity in female rats

Stress preferentially increases the consumption of high fat foods in women, suggesting the interaction of these two factors may disproportionately predispose women toward excess weight gain. In the present study, female rats were exposed to a chronic high fat or chow diet and were exposed to 4 weeks of chronic variable stress (CVS) or served as home cage controls. Control females exposed to a high fat diet displayed many symptoms of the metabolic syndrome including increased body weight gain, total and visceral adiposity and insulin and leptin concentrations relative to all groups. However, CVS-high fat, CVS chow and control chow groups had similar body weight gain and caloric efficiency. This finding suggests that CVS increases energy expenditure much more in females exposed to a high fat diet relative to those fed a standard chow diet. The CVS-high fat group had increased adiposity and increased circulating leptin and insulin concentrations, despite the fact that their body weight did not differ from the controls. These results underscore the importance of assessing the degree of adiposity, rather than body weight alone, as an index of overall metabolic health. Overall, the data indicate that in female rats, chronic stress prevents high fat diet related increases in body weight, but does not prevent high fat diet induced increases in adiposity when compared to chow-fed females.     

Sucrose intake as a function of its cost and the cost of chow

The avid consumption of pure carbohydrate solutions, which often results in a distortion of nutrient balance, is generally presumed to be driven by their taste. In the first of two experiments, we examined the effect of consumption cost on rats' intake of three concentrations of sucrose solution (8%, 16%, and 32%) when a nutritionally complete chow was concurrently freely available. In the second experiment, we examined the intake of 24% sucrose solution and chow as the consumption costs of both were varied. Increasing the cost of sucrose resulted in a reduction in the percent calories taken from sucrose; the steepness of the decline in intake with price was inversely related to the sucrose concentration and to the cost of chow. Chow calories were substituted for relatively expensive sucrose calories. An increase in the cost of chow resulted in a reduction in the percent of calories taken from chow and a protein-poor diet. The cost of sucrose did not affect the slope of the chow intake curve, presumably because, despite its sweet taste, sucrose was not a substitute for the protein, fat, and micronutrients in chow. Total caloric intake was conserved in all cases.

Thus, the avid consumption of sucrose solution is curtailed when it is costly; but the degree of change in intake with cost depends on the cost of an alternative food. These results suggest that diet selection involves a comparison not only of the taste and post-ingestive consequences of available foods, but also of the cost of calories and nutrients in the foods. Selection appears to be guided first by caloric requirements and the relative cost of calories, then by nutrient requirements and the relative cost of nutrients, and finally by taste.     

Dietary modulation of oral amphetamine intake in rats

The interaction of sucrose availability and oral self-administration of amphetamine was examined in 23 male Sprague-Dawley rats. Fourteen rats were given a 0.075 mg/ml amphetamine sulfate solution as their sole source of fluid and 9 rats were given water. Feeding conditions were alternated between weeks with both granulated sucrose and chow available and weeks with only chow present. Rats drank significantly less of the amphetamine solution when consuming sucrose and chow than when eating chow alone. Sucrose intake had a slight effect on water intake. Rats drinking the amphetamine solution consumed significantly less food, gained significantly less weight, and were significantly less efficient at using calories for weight gain than rats drinking water. However, when given access to sucrose, rats drinking the amphetamine solution chose a significantly greater proportion of their daily caloric intake as sucrose (60%) than rats drinking water (42.5%). The present results demonstrate that 1) amphetamine intake alters nutrient choice and 2) that dietary variables can profoundly affect drug self-administration.     

Quantifying food intake in socially housed monkeys: social status effects on caloric consumption

Obesity results from a number of factors including socio-environmental influences and rodent models show that several different stressors increase the preference for calorically dense foods leading to an obese phenotype. We present here a non-human primate model using socially housed adult female macaques living in long-term stable groups given access to diets of different caloric density. Consumption of a low fat (LFD; 15% of calories from fat) and a high fat diet (HFD; 45% of calories from fat) was quantified by means of a custom-built, automated feeder that dispensed a pellet of food when activated by a radiofrequency chip implanted subcutaneously in the animal's wrist. Socially subordinate females showed indices of chronic psychological stress having reduced glucocorticoid negative feedback and higher frequencies of anxiety-like behavior. Twenty-four hour intakes of both the LFD and HFD were significantly greater in subordinates than dominates, an effect that persisted whether standard monkey chow (13% of calories from fat) was present or absent. Furthermore, although dominants restricted their food intake to daylight, subordinates continued to feed at night. Total caloric intake was significantly correlated with body weight change. Collectively, these results show that food intake can be reliably quantified in non-human primates living in complex social environments and suggest that socially subordinate females consume more calories, suggesting this ethologically relevant model may help understand how psychosocial stress changes food preferences and consumption leading to obesity.