A preprandial intake pattern in weanling rats ingesting a high fat diet

Six male albino rats were weaned at 16 days and their intake of a high fat liquid diet was subsequently monitored with drinkometers for a 25-day period. Intake increased during growth via an increase in meal size and duration; meal frequency declined and intake rate remained stable. Light-dark differences were found only on the meal frequency measure. Immediately after weaning, there was a significant positive correlation between the duration of the preceding interval and the size of the meal; preprandial correlation. This relationship subsequently declined while the magnitude of the correlation between the meal size and the following interval (postprandial correlation) increased. These results indicate that there is a changeover in the mode of food intake regulation during the early postweaning period.     

Meal patterns in the genetically obese zucker rat

Meal patterns of genetically obese rats and their lean littermates were compared. Total food intake of the genetically obese rats exceeded that of nonobese rats by approximately 30%. Differences in the obese animals' meal patterns included enlarged meal size and decreased meal frequency. The normal pattern of predominantly nocturnal feeding was absent in obese animals.     

Effects of age,sex, and prior body weight on the development of dietary obesity in adult rats

Adult male and female rats were fed either lab chow (Groups 2 and 3) or lab chow and an assortment of palatable supermarket foods (Group 1) during Days 1–60 of the experiment. All rats were maintained on only lab chow during Days 61–90. Group 1 and 3 rats were then given the supermarket diet during Days 91–150, while Group 2 rats continued on lab chow only. The rats fed the supermarket diet significantly more body weight than did the lab chow fed rats, and this dietary obesity was greater in the older rats (i.e., during the Days 91–150 of the experiment) than in the younger rats (Days 1–60). Male rats gained as much or more weight on the supermarket diet as did the females, but compared to the same-sexed chow fed rats females displayed greater weight gains than did males. Finally, during Days 91–150 Group 1 and 3 rats gained similar amounts of weight on the supermarket diet despite the fact that the Group 1 rats had previous experience with the diet and had been overweight as a result. The findings demonstrate that age and sex, but not prior experience with palatable foods and overweight, are important factors in the development of dietary obesity in adult rats. The similarity between dietary obesity and hypothalamic obesity with respect to these three factors is discussed.     

Dietary obesity in exercising or cold-exposed Syrian hamsters

High-fat diet-feeding increases body weight and adiposity in Syrian hamsters (Mesocricetus auratus), effects due in part to decreased energy expenditure. The effects of voluntary exercise- or cold exposure-induced increases in energy expenditure were examined in fat- or chow-fed, female Syrian hamsters. In Experiment 1, voluntary exercise (10 weeks) caused a moderate hyperphagia and actually increased body weight in both diet groups through increases in lean body mass. Carcass lipid was not affected by by exercise in chow-fed hamsters and only slightly reduced in fat-fed animals. In Experiment 2, chronic (8 weeks) cold exposure (5 degrees C) increased energy intake to the same extent in both dietary groups relative to the warm-exposed (23 degrees C) controls. High-fat diet-induced obesity was largely prevented by cold exposure. Cold exposure reduced lean body mass in chow-fed hamsters, but this carcass component was spared by fat-feeding. These results indicate that the increased metabolic demands of cold exposure were more effective in preventing this form of diet-induced obesity than those of voluntary exercise (80% and 17% reductions in carcass lipid, respectively). These results are discussed in terms of possible beneficial effects of eating a lipid-rich diet prior to winter.     

The small intestine and the control of meal patterns of the rat

Preabsorptive stimuli associated with a meal may be responsible for short term control of feeding in ad lib fed rats. It has been proposed that one of these stimuli may be feedback associated with intestinal distension. Mannitol, a nonabsorbable sugar, was added to a mash diet; meal size was found to be relatively constant regardless of the concentration of mannitol used to prepare the diet, but the length of time that elapsed before the onset of the next meal increased as the mannitol concentration increased. Consumption of diets containing concentrated mannitol solutions was associated with an accumulation of water, and possibly food, in the upper small intestine but not the stomach or lower intestine. It was also found, using a conditioned taste aversion procedure, that discomfort induced by mannitol ingestion was unlikely to be related to delaying the next meal. These results suggested that intestinal distension may function under natural conditions to delay the onset of a meal or prolong a postmeal interval rather than to stop an ongoing meal.     

Effects of dietary caloric density on feeding behavior in Mongolian gerbils (Meriones unguiculatus)

Total daily energy intake, water intake, body weight, and meal patterns were studied in Mongolian gerbils as a function of dietary caloric density. On diets ranging in caloric density from 2.25 kcal/g to 6.09 kcal/g, gerbils consumed an average of 40 kcal per 100 g of body weight per day. In comparison to gerbils fed ground Purina Laboratory Chow (4.2 kcal/g), gerbils presented with diets diluted with nonutritive cellulose increased food intake in proportion to the percentage of cellulose added. Gerbils given diets in which the caloric density was increased by the addition of fat, decreased food intake as a direct function of the added fat. Water intake was increased on the calorically diluted diets, and decreased on the concentrated diets. Body weight did not vary as a function of dietary conditions. On a standard pelleted diet (Noyes), gerbils ate approximately 18 meals a day. Average meal size was 0.4 g. When presented with calorically diluted pellets, gerbils maintained daily energy intake by increasing both meal frequency and meal size. There were no differences in food intake, meal frequency or meal size between the light and dark portions of the 24-hr cycle.     

Vagotomy in the rat reduces meal size of diets containing fat

This study was designed to examine the possibility that the presence of rat in a diet is responsible for preventing the small post vagotomy meals that occur with chemically defined low fat liquid diets but not with lab chow diets. Rats were thoroughly adapted to four liquid diets varying in fat content from virtually 0 to 30 g per 100 ml. They were then subjected to an operation designed to destroy the vagus nerves. All animals who were shown in post mortem tests to have suffered vagal damage also showed reduced meal sizes on all the diets. The meal sizes of animals for whom there was no evidence of vagal damage were normal. It was concluded that bulk rather than fat content is responsible for preventing the reduced post vagotomy meals.     

Hyperactivity of the HPA axis is related to dietary restraint in normal weight women

The objective of our study was to investigate the relationship between hypothalamus/pituitary/adrenal (HPA) axis functioning and dietary restraint in normal weight (BMI between 20 and 25 kg/m²) men and women. We therefore assessed in 38 men and 38 women HPA axis functioning, through measuring 5-hour cortisol exposure and cortisol feedback functioning through a dexamethasone (4 mg) suppression test. Eating behavior was assessed through the Three Factor Eating Questionnaire and body composition through hydro densitometry and deuterium dilution method. No relationship between HPA axis functioning and dietary restraint was found in men. Normal weight women with a restraint score ≥ 9 showed increased cortisol concentrations over a 5-hour time period, increased cortisol concentrations after a dexamethasone (4 mg) suppression test, higher BMI, and higher body fat percentage, when compared to women with a restraint score < 9. Moreover, a positive relationship was found between cortisol concentrations over a 5-hour time period and dietary restraint in combination with the disinhibition score (R² = 0.23, p < 0.001). We conclude that in normal weight women hyperactivity of the HPA-axis is related to dietary restraint especially in combination with disinhibition.     

Ontogeny of feeding behavior in the Zucker obese rat

Increased food intake in Zucker obese rats has been reported as early as 16 days of age. To determine if the feeding behavior of Zucker obese rats differs from that of lean rats by weaning, or if the differences develop with age, feeding patterns of Zucker obese and lean rats were compared from 3–10 weeks of age. Increased food intakes of obese rats at 3 weeks of age were due to a trend toward increased frequency while at 4 weeks of age were due to increased meal size. Meal size subsequently increased at a faster rate in obese than lean rats and meal frequencies did not differ. While always greater in obese than lean rats meal durations decreased and rates of eating increased with age and were not different for obese and lean rats. The adult patterns of diurnal variation in obese and lean rats were apparent by 3 weeks of age. Thus, in the Zucker obese rat the characteristic of increased meal size did not occur until 4 weeks of age after increased food intake and body weight were evident, and the characteristic of decreased meal frequency did not occur by 10 weeks of age. While increased meal size is associated with early differences in feeding behaviors, decreased meal frequency may be a consequence of obesity.     

The light/dark difference in meal size in the laboratory rat on a standard diet is abolished during REM sleep deprivation

Using a modified flowerpot technique, by which it was possible to use test animals as their own controls, the meal patterns of eight adult male rats were recorded before, during and after rapid eye movement sleep (REMs) deprivation. The results demonstrated that the prominent light/dark (LD) difference in meal size, typical of the baseline meal pattern, was abolished during REMs deprivation. This equalization was entirely due to reduction in meal size during the dark hours. After termination of REMs deprivation the size of the dark meals increased almost to the baseline level. However, the LD difference was not restored since the size of the light meals was also significantly increased. Changes in meal frequency were confined to the period of REMs deprivation. The possible role of REMs as a modulator of meal size in relation to the LD cycle of illumination is discussed.     

Somatic and metabolic responses of mature female rats with dietary obesity to dorsomedial hypothalamic lesions: Effects of diet palatability

Caloric intake, body weight, obesity status (Lee Index) and incorporation of U-14 C-glucose into liver and retroperitoneal fat pad glycogen and lipid were studied in mature female rats that had received bilateral lesions or sham-operations in the dorsomedial hypothalamic nuclei (DMN) after dietary obesity was well established. Their diet consisted of a high-fat-sucrose chow mix, chocolate chip cookies and a drinking fluid of 32% sucrose in tap water. Comparable groups of DMN lesioned rats (DMNL rats) and sham-operated controls were maintained on lab chow pellets and tap water. Prior to the hypothalamic operation, the animals on the high-caloric regimen consumed significantly more calories than the rats on lab chow and also attained commensurately higher body weights and obesity indices. The bulk of the calories consumed during this time was derived from the cookies. Following DMNL, the animals maintained on lab chow became hypophagic and had lower body weights than the sham-operated rats, as has been previously reported. In rats on the high-caloric regimen, DMNL resulted in hyperphagia in comparison to all other groups. The greatest percentage of the calories during this time was derived from the high-fat-sucrose chow mix and sugar water. Correspondingly, DMNL rats on the high-caloric regimen had higher body weights and obesity indices than all other groups. At sacrifice, both a diet and lesion effect were noted in an elevated incorporation of U 14-C glucose in both fat pad and liver lipid and glycogen. The data are interpreted to mean that (1) when a highly palatable, high-caloric diet is available, DMNL do not exert their usual hypophagic and weight-lowering effects; (2) DMNL and control rats show excessive caloric intake when both groups are fed a highly palatable, high-caloric diet in comparison to their chow-fed counterparts. However, DMNL rats fed high-caloric diet also consume significantly more than controls fed this diet; (3) This excessive caloric intake of the DMNL rats possibly predisposes these animals to exaggerated lipogenesis in liver and adipose tissue; (4) the sham-operated controls on the high-caloric regimen also show greater lipogenesis but at a level intermediate between the chow-fed controls and the DMNL rats on the high-caloric diet.     

Dietary obesity and exercise in young rats

Food intake, body weight, body composition, resting metabolic rate (RMR) and the thermic effect of food (TEF) were measured in young rats, some of which were fed a high energy (HE) diet and some of which were forced to swim daily. In general, high energy feeding as compared to chow feeding, resulted in higher food intake, higher body weight, higher body fat, and a slightly lower TEF. In many cases, however, the specific effects varied with the age and sex of the animals. Animals forced to swim weighed less; were leaner; and had higher RMR and TEF than sedentary animals. The effects of exercise on energy balance were greatest in males, while the effects of the high energy diet on energy balance were greatest in females. All HE-fed rats were switched to lab chow at 104 days of age. Body weights of sedentary HE-fed rats returned to control levels but those of exercised HE-fed rats did not. Both HE-fed groups remained fatter than chow-fed controls, even two months after the diet switch.     

Dietary obesity in adult rats: Similarities to hypothalamic and human obesity syndromes

Normal adult female rats fed a variety of supermarket foods in addition to lab chow rapidly gained weight and became obese compared to rats fed only lab chow. Group housing the animals in an enriched environment did not alter the development of dietary obesity, but housing the rats in activity wheels reduced, although did not prevent, the obesity. The dietary obese rats did not normally defend their excessive weights since they were less willing to eat quinine diets, worked less for food, failed to increase their activity when deprived, and regained their weight at a slower rate following a fast than did controls. The similarity between this behavioral pattern and that displayed by hypothalamic obese rats and overweight humans is discussed.     

The ecological determinants of reinforcement in the rat

The intake of a freely feeding animal can be characterized by four parameters: frequency of meals, duration of meals, rate or intensity of eating, and choice of items eaten. The values of these parameters characterize species and the ecological niche they occupy. Constraints on any one of these parameters leads to compensatory changes in other parameters over the feeding cycle. Any change in performance of the instrumental behavior ancillary to feeding which causes the values of these parameters to recover their optimum or privileged value, defined by their niche, will be strengthened. These changes in performance reflect what is usually meant by motivation and the process of reinforcement.     

Sex differences in the effects of high-fat feeding on behavior and carcass composition

Male and female Sprague-Dawley rats were fed a high-fat diet (40% by weight) for 11 weeks beginning at 70–80 days of age. At the end of the 11th week, high-fat fed rats of both sexes were significantly heavier than chow-fed controls. All rats were then food deprived and were trained to bar-press in an operant chamber for Noyes pellets. Testing on fixed ratio (FR) schedules started when their body weights reached 85% of pre-deprivation levels and they pressed bar steadily. At the end of operant testing, all rats were refed their previous diet until body weights returned to pre-deprivation levels. The animals were then sacrificed. Fat pads from retroperitoneal, inguinal and gonadal regions were dissected out and cellularity determined. Carcass composition was analyzed by chemical methods. On the operant apparatus, the high-fat fed female rats (F-HF) behaved more like VMH lesioned obese rats, i.e. decreased bar pressing responses when compared with controls. No difference in operant responding was found between males fed high-fat diet and chow. Fat cell number and size were increased in retroperitoneal and inguinal fat pad for rats fed high-fat diet. In gonadal pads, only cell size was increased. Females on the high-fat diet had higher percentages of body fat than males on the high-fat diet. The behavioral difference in F-HF rats could be attributed to their higher adiposity. The results support previously reported findings on the behavior and adipose tissue cellularity of dietary obese rats.     

The stomach energy content governs meal patterning in the rat

The feeding behavior of 38 male rats was continuously monitored for 14 days. The meal sizes and intermeal intervals were then used in conjunction with a mathematical model of stomach emptying to predict the energy content of the stomach at the times of meal initiation and termination. A correlational analysis was then performed to determine what if any role the stomach energy content played in the determination of the size of meals or the intermeal interval (IMI). Significant correlations between the meal size and the following IMI (postprandial correlation) were obtained. The energy content of the stomach before the meal was found to be as good a predictor of the following IMI as the meal size and the energy content of the stomach after the meal was found to be even a better predictor, equalling the variance accounted for by the combined influence of the premeal stomach energy content and the meal size. Additionally, the postprandial correlation was found to be negatively correlated to the variance of the premeal stomach energy content, suggesting that the weakness of the postprandial correlation in the past was due to a lack of consideration of the energy present in the stomach at the time of meal initiation. The results indicate that satiety is a strong influence on the meal pattern under ad lib conditions and that satiety is governed by a factor associated with the energy content of the stomach.     

Interactions between nocturnal feeding and wheel running patterns in the rat

The purpose of this experiment was to investigate the relationship between daily patterns of wheel running activity and feeding behavior. Wheel running and bar pressing for food were continuously recorded from rats which had concurrent access to both wheels and food. In a second condition, wheel running and (non-reinforced) bar pressing were recorded during food deprivation. In a third condition, bar pressing for food was recorded while wheel running was prevented. During ad lib access to wheels and food, both behaviors occurred primarily during the dark phase of the light-dark cycle. However, the nocturnal distribution of these responses differed: the early dark hours were primarily devoted to running, while feeding occurred in small, frequent meals. Maximal food intake occurred during the late dark hours, when meals were larger and less frequent, and there was little running. During three days of complete food deprivation, both wheel running and meal initiation attempts (as indicated by nonreinforced bar pressing) maintained their normal daily patterns. However, during wheel deprivation, changes occurred in the temporal distribution of meals which equalized early and late dark food intake. These results show that the availability of an alternative activity, like wheel running, can be an important determinant of daily feeding patterns. However, the daily pattern of wheel running is not influenced by food availability. Different behaviors may show differential plasticity in the coordination of an overall daily behavioral program.     

A cost-benefit analysis of nocturnal feeding in the rat

Rats are nocturnal feeders. The present results show that when either the cost of procuring a meal or the cost of consuming it increases differentially in the dark relative to the light, rats shift their feeding activity to the light phase of the light/dark cycle. These results suggest that some of the same factors that presumably provided the selection pressure for nocturnal feeding in the phylogeny of the rat are still capable of modifying its current patterns of feeding.     

Meal patterns in the genetically obese Zucker rat: A reexamination

The meal patterns of four obese and four lean female Zucker rats were analyzed using five different intermeal interval definitions in combination with four minimum meal size definitions. Results from these 20 analyses revealed that obese rats typically initiate fewer but larger meals than their lean littermates. However, these general findings were not always observed with the use of particular combinations of intermeal interval and minimum meal size definitions. The interaction between meal pattern analysis parameters and rat genotype led to a second series of statistical analyses. Results from these procedures revealed that unlike the “nibble-eat” pattern of intake that characterizes lean rats, obese rats seem to initiate three classes of meals; the very small meal, the typical meal, and the “super-meal.” It is suggested that the analysis of the meal patterns of Zucker rats incorporate relatively large minimum meal size definition with at least two intermeal interval definitions.     

Early weight gain and behavioral responsivity as predictors of dietary obesity in rats

Variance in diet-induced weight gain was examined for possible relationships with variations in early weight gain and three tests of behavioral responsivity. Female Sprague-Dawley rats were reared in litters of 4, 8, or 20. When animals reached adulthood, each animal's acoustic startle reflex, tail-pinch feeding responses and activity in an open field containing a palatable food were assessed. After completing these behavioral tests, rats were exposed to either palatable foods or a control diet for 59 days, following which all subjects were maintained on the control diet for 66 days. Body weights, food intakes, and naso-anal lengths were measured. Preweaning body weight gain for all rats correlated positively with later diet-induced weight gain. Rats reared in litters of 4 or 20 both gained less weight after exposure to palatable foods than did rats reared in litters of 8. Diet-induced weight gain correlated positively with magnitude of acoustic startle reflex and with latency to eat in response to tail pinch. These results tentatively identify specific predictive factors which may be useful in future studies of dietary obesity.