Changes in rats' meal patterns as a function of the caloric density of the diet

Rats in a laboratory foraging paradigm were offered each of four diets which differed in caloric density, and intakes, meal frequencies, meal sizes, and eating rates were monitored. The rats maintained a constant daily caloric intake by eating more frequent, larger meals of the lower density foods. However, caloric meal size was not regulated, and significant correlations between meal size and the length of the post-meal interval were rarely found. The 24-hour pattern of calorie intake was the same regardless of diet. Higher-calorie foods were consumed at a faster rate within meals than were lower-calorie foods. The feeding patterns observed suggest that caloric intake may be regulated over a time frame of several meals rather than on a meal-to-meal basis.     

Role of hypothalamic norepinephrine in control of meal patterns

Feeding behavior has been shown to be strongly affected by central administration of catecholamines. In this study, we examined in freely-feeding rats the effect of hypothalamic norepinephrine (NE) injections on the basic parameters of spontaneous ingestion. Precise changes in feeding behavior in rats maintained on ad lib food and water intake were monitored by a PDP 8 computer connected to an apparatus capable of measuring licks and bites of food. Injections of NE were administered into the hypothalamic paraventricular nucleus, the most sensitive brain area for the elicitation of feeding through direct alpha-noradrenergic stimulation. In tests conducted under both light and dark conditions, NE facilitated food intake, primarily by an increase in meal size rather than meal frequency. The first meal after injection was increased in size and duration; the rate of eating was also enhanced. Whereas the following intermeal interval was significantly larger, subsequent meals and intermeal intervals appeared generally unaffected. This evidence is consistent with the proposal of a role for hypothalamic NE in the maintenance, rather than initiation, of feeding behavior in freely-feeding rats.     

Loss of glucagon suppression of feeding after vagotomy in rats

Infusion of pancreatic glucagon through the hepatic-portal vein decreased short-term food intake in sham-vagotomized but not in subdiaphragmatically vagotomized rats. Measurement of hepatic glycogen storage showed that vagotomized rats maintain a lower glycogen level than control animals over the four fasting periods evaluated. To determine whether the absence of a glucagon effect on feeding in vagotomized rats was the result of the reduced amount of substrate for glycogenolysis, vagotomized rats were not fasted and control animals were food deprived for 8h to produce comparable hepatic glycogen levels. Hepatic-portal infusion of glucagon into these differentially fasted animals suppressed feeding in control rats but not in vagotomized rats. It is concluded that the ineffectiveness of glucagon in suppressing feeding in vagotomized rats is not due to reduced concentration of hepatic glycogen. Instead, it is likely that glucagon induces glycogenolysis, but the glucose, or some other correlate of glycogen breakdown, loses its ability to produce satiety subsequent to vagotomy.     

CCK inhibits real and sham feeding in gastric vagotomized rats

We tested both sham feeding and real feeding in gastric vagotomized rats equipped with chronic gastric cannulas, either without pretest food deprivation or after 18 hr food deprivation. In each condition, 0.3-4 micrograms/kg CCK inhibited food intake similarly in control and vagotomized rats. Behavioral observations indicated the presence of normal postprandial satiety after CCK. We then tested real feeding in noncannulated rats after either total abdominal vagotomy, gastric vagotomy, or control operation. Doses of 0.5-4 micrograms/kg CCK had no effect on food intake after total vagotomy, but again inhibited feeding with equal potency in gastric vagotomized and control rats. The inhibitory effect of 6 micrograms/kg CCK was attenuated but not blocked by total vagotomy. Finally, we tested rats with gastric cannulas after gastric plus celiac vagotomy. CCK also inhibited both real and sham feeding after this lesion. These data confirm previous findings that abdominal vagal fibers mediate the satiety effect of moderate intraperitoneal doses of CCK, but fail to support the hypothesis that gastric branch fibers are the necessary vagal contribution.     

Nocturnal cyclic versus continuous total parenteral nutrition: food intake and feeding pattern in rats.

Total parenteral nutrition (TPN), providing the equivalent of 100% of mean daily oral intake for 24 hours, depressed spontaneous food intake (SFI) in rats by a mean greater than 80%. Whether TPN had the same effect on SFI when given either in 12-hour cycles (Cyclic-TPN) or in 24 hours (Continuous-TPN) for 8 days was tested in 16 rats with jugular catheters, placed in metabolic cages modified with an Automated Computerized Rat Eater Meter. This continuously measures food intake and its dependent indexes: meal number, meal size, meal duration, meal sniffs and intermeal sniffs. Both rat groups ate the same amounts of chow before TPN. On starting TPN, food intake decreased by greater than 80% in Cyclic-TPN and greater than 50% in Continuous-TPN during each first infusion period. A nadir in food intake was reached in 48 hours with Cyclic-TPN and in 72 hours with Continuous-TPN. Thereafter, there was no food intake during the 12 hours of Cyclic-TPN, with minimal food intake during the intervening 12 hours of non-TPN normal saline infusion period. In both forms of TPN infusion, a reduction in food intake was achieved by a reduction in meal number and size, and thus in meal duration. Feeding-related sniffing activity was decreased by similar amounts with Cyclic- or Continuous-TPN. Cyclic-TPN for 12 hours had no advantage over Continuous-TPN, in that there was no compensatory enhancement of food intake and meal-related behavior during the cyclic non-TPN normal saline period.     

Analysis of feeding patterns in normal and vagotomized rabbits.

Spontaneous meal sizes, intermeal intervals, and 24 hr feeding rhythms were monitored in normal and 60 day recovered vagotomized rabbits fed solid laboratory chow. Mean sizes of meals and intermeal intervals, and the circadian distribution of food intake did not differ between the two groups, but vagotomy was associated with increased frequencies of both smaller and larger than average meals. Positive meal to postmeal interval correlations were evident in intact but not vagotomized animals, whereas vagotomized animals displayed a meal to premeal interval correlation in the light phase that was not present in normal rabbits.     

Dynamics of oral intake resumption after general anesthesia and operation in rats.

The influence of general anesthesia and operation on dynamics of postoperative food intake resumption was investigated in eight rats. A laparotomy was performed on each rat under anesthesia induced by intraperitoneally injected chloral hydrate. Spontaneous food intake and feeding indexes were continuously measured using an Automated Computerized Rat Eater Meter (ACREM) before and after operation. Although spontaneous food intake and all feeding indexes were depressed immediately following anesthesia and operation, each feeding index was depressed to a greater degree during the dark vs. the light cycle. Initially, rats fully capable of eating ate fewer, smaller, and shorter meals. The return to normal of each feeding index differed temporally. Thus, although meal number normalized by the third postoperative day, meal size by the sixth postoperative day, and meal duration by the fourth postoperative day, normalization of meal number during the light cycle occurred sooner than during the dark cycle, while the converse occurred with meal size and meal duration.     

Threonine-imbalanced diet alters first-meal microstructure in rats

Diets limiting in an essential amino acid have long been known to suppress food intake. The purpose of this study was to examine the microstructure of feeding behavior of rats within the very first meal of an imbalanced diet. Rats were preconditioned for 12 days on a Baseline diet and were then given a test diet with either a corrected amino acid profile or a diet imbalanced with respect to the essential amino acid threonine. Overall, first-meal intake and first-meal duration were robustly and significantly reduced by the Imbalanced diet but not altered by the Corrected diet. The Corrected diet caused an increase in the number of feeding bouts during the first meal. The Imbalanced diet increased the duration of pauses during the first meal. Most rats in the Imbalanced group stopped eating after just 15 min of exposure to the diet, but those still eating after this time tended to have a lower rate of eating compared to those eating the Corrected diet. On the basis of these results, we conclude that changes in microstructure and meal duration contribute to the reduction in food intake upon exposure to amino-acid-deficient diets.     

Some determinants of intake and patterns of feeding in the guinea pig

Regulation of food and water intake was studied in young and adult Guinea pigs by continuously monitoring ad lib feeding and drinking patterns and by examining the response to levels of celluflour dilution that ranged from 20–75%. Meal size and the duration of the intermeal interval were not systematically related to the sizes of preceding meals or interval lengths. Variations in food intake were mediated almost entirely by changes in meal size. This parameter increased during growth, was elevated in the first meal following food deprivation, decreased when water was withheld and increased when water was returned. The probability of drinking within 20 min of eating was 0.80. Celluflour dilution led to a decrease in apparent caloric intake without a proportional decrease in body weight. Regulation of energy balance and meal to meal control of feeding in this monogastric, herbivorous rodent is discussed in relation to the Guinea pig's digestive physiology and ecological niche.     

Feeding and drinking behavior responses of adult Zucker obese rats to cholecystokinin

Increased meal size in obese animals may occur because of decreased sensitivity to satiety factors. Feeding and drinking behavior responses of Zucker obese and lean rats to two forms of cholecystokinin (CCK), a putative satiety factor, were compared in these experiments. Octapeptide of CCK (CCK-8) in both obese and lean rats decreased meal size (47 and 65%) and rate of eating but not meal duration, and increased satiety ratio but not postmeal interval. Impure CCK decreased first meal size after a 6-hr fast similarly in obese and lean rats (33 and 40%, respectively); however, meal duration was decreased only in lean rats, and rate of eating was decreased only in obese rats. Postmeal interval was decreased in lean rats, while satiety ratio was increased only in obese rats. In spite of decreased first meal size daily food intakes were increased by both impure CCK and CCK-8 in lean rats. In 6-hr water-deprived rats injected with CCK-8, decreased water intake was associated with decreased food intake. However, while impure CCK in lean rats elicited similar responses as CCK-8, impure CCK in obese rats decreased food but not water intake. Feeding behavior response to the putative satiety agent, cholecystokinin, depended on form of the peptide administered and phenotype.     

Caloric regulation and patterns of food choice in a patchy environment: the value and cost of alternative foods

Rats in a laboratory foraging paradigm had 24-hr-per-day access to a feeder where they could search, by completing a fixed number of bar presses, for an opportunity to eat one of a pair of foods differing in caloric density (2.5, 3, 3.5, or 4 kcal/g) and, in Experiment 2, the price of food pellets (10 to 50 bar presses per pellet). The rats could either accept the opportunity, and eat a meal, or reject it in favor of further search. Daily caloric intake was relatively constant. The rats always included both foods in their diet, but, for any particular food, the degree of inclusion in the diet and of acceptance of meal opportunities, the meal size, and the rate of eating were all functions not only of the price and caloric value of that food but also of the price and value of the alternately-available food. The patterns of intake for one food relative to those for the other available food were strongly correlated with the relative rate of calorie intake during consumption of that food compared to the other. Although the rats appeared to be sensitive to the local rates of calorie flow, they did not maximize daily calories consumed per time spent feeding.     

The body weight loss of young rats caused by hepatic branch vagotomy is influenced by environmental temperature

Changes in body weight after hepatic branch vagotomy or sham vagotomy were examined in rats of three different body weights (about 100, 200 and 280 g), and it was noted that the weight loss was evident only in 100 g rats. In a second experiment, 100 g animals were hepatic branch vagotomized or sham operated and housed at three different room temperatures (12-17, 17-22, 22-27 degrees C). Subsequently, the reduction in weight of the vagotomized animal was correlated with the lowest room temperature in the vagotomized animal, and food intake was also decreased in direct relation to the temperature. The results suggest that there might be a hepatic vagal mechanism modulating food intake in young animals, and such a mechanism may be modulated by the environmental temperature.     

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.     

Meal patterning in the lactating rat

The present study was undertaken to investigate feeding behaviour of the lactating rat over the day-night cycle. Food intake was recorded continuously in six pregnant and subsequently lactating female rats with 10 pups each. Although there was a twofold increase of food intake during the first post partum week, the meal frequency did not increase above the level of previous weeks. Only the mean meal size increased. After this week food intake increased to three times the normal intake, meal size did not change but meal frequency increased in favour of daytime meals. It is suggested that with moderate caloric demands food intake regulation in the rat occurs through changes in meal size. With higher energy requirements, however, meal frequency is also affected. The possible causal factors involved in the change in feeding are discussed.     

Effect of chronic caloric restriction on physiological variables related to energy metabolism in the male Fischer 344 rat

In the present study, a number of physiological and behavioral measures that are related to metabolism were continuously monitored in 19-month-old male Fischer 344 rats that were fed ad libitum or fed a caloric restricted diet. Caloric restricted rats ate fewer meals but consumed more food during each meal and spent more time eating per meal than did rats fed ad libitum. Therefore, the timing and duration of meals as well as the total number of calories consumed may be associated with life extension. Average body temperature per day was significantly lower in restricted rats but body temperature range per day and motor activity were higher in restricted rats. Dramatic changes in respiratory quotient, indicating rapid changes in metabolic pathway and lower temperature, occurred in caloric restricted rats when carbohydrate reserves were depleted. Lower body temperature and metabolism during this time interval may result in less DNA damage, thereby increasing the survival potential of restricted rats. Nighttime feeding was found to synchronize physiological performance between ad libitum and caloric restricted rats better than daytime feeding, thereby allowing investigators to distinguish the effects of caloric restriction from those related solely to the time-of-day of feeding.     

Vagotomy fails to block the satiating effect of food in the stomach

Normal rats and rats with bilateral subdiaphragmatic vagotomy, each equipped with a pyloric noose, ate liquid food after 3-hr food deprivation. When the noose was open, ingested food accumulated in the stomach and entered the intestine in normal fashion. When the noose was closed, ingested food was trapped in the stomach and did not enter the intestine. Normal and vagotomized rats ate the same size meal with the noose closed as they ate with the noose open and all rats displayed a postprandial sequence of satiety behavior that culminated in resting. Thus, vagotomy failed to block the satiating effect of food in the stomach. This result suggests that gastric distension mediated by vagal afferents is not a necessary stimulus for satiety elicited by food in the stomach.     

Energetics of meal patterns in rats

Meal patterns in rats were examined as functions of the caloric density and availability of the diet. Three diets were used, a standard laboratory diet (3.6 kcal/g), a calorically diluted diet (2.7 kcal/g), and a calorically concentrated diet (4.5 kcal/g). After obtaining ad lib measures of meal patterns on each diet availability of food was constrained by requiring the rats to complete fixed ratio requirements of barpresses to obtain access to a meal. On all 3 diets, meal frequency decreased, while meal size and duration increased as functions of the ratio requirement. Under ad lib conditions and low ratio requirements, in comparison to the standard diet, meal frequency was greater on the diluted diet and less on the concentrated diet. Meal size did not vary as a function of diet on low ratio schedules. At high ratio requirements, rats continued to maintain caloric intake on the diluted diet by increasing meal frequency. On the concentrated diet, however, rats maintained intake at high ratio values by decreasing meal size rather than meal frequency. The results indicate that the rat can adopt a variety of strategies to solve the problem of controlling energy intake constant across the daily feeding cycle.     

Drinking to intracellular dehydration following vagotomy in rats

Abdominally vagotomized rats maintained on a solid diet drank less and had longer latencies to drink than sham vagotomized rats following IP injection of an osmotic load (0.75 M NaCl, 1% BW). However, these two groups did not differ in latency or water intake following injection of isotonic saline. Since both vagotomized and control rats drank more water and had shorter latencies following injection of hypertonic saline than after isotonic saline, vagotomy apparently attenuated but did not abolish osmotic drinking. Maintenance on a liquid diet and a brief fast prior to testing (to ensure an empty stomach) did not alter these results, indicating that the impairment of gastric emptying of solid food that accompanies total abdominal vagotomy cannot account for the attenuation of osmotically induced drinking. Furthermore, this deficit was seen even when intracellular dehydration was produced at different times during the circadian cycle and when water presentation was delayed 0.5 hr postinjection. In addition, vagotomized rats drank less than control rats following 16-hr water deprivation and exhibited a lower water-food ratio on ad lib regimen. However, vagotomized and sham vagotomized rats exhibited the same relative day-night difference in water consumption as well as short latency response to thermal pain, which with other results indicates that vagotomy did not result in a general impairment of behavior. These findings suggest that osmotic perturbations are detected by the viscera and the information conveyed to the brain via afferent vagus nerves.     

Effects of vagotomy on entrainment of activity rhythms to food access

Subdiaphragmatic vagotomy had little effect on anticipatory wheel running in rats maintained on 4 hr of food access per day. In ad lib conditions, sections of the anterior trunk of the vagus reduced total activity, but did not substantially affect its nocturnal distribution. During restricted feeding, rats with anterior trunk sections showed amounts of anticipatory wheel running comparable to controls, while vagotomized rats with suprachiasmatic nucleus (SCN) lesions showed a reduced amount of activity. After the posterior trunk was sectioned, as well, or after combined sections, there was little change in the amount of anticipatory activity but a significant increase (1 hr) in the duration. In control rats, the duration of anticipatory activity showed a nonsignificant decrease over 50 days on restricted feeding. Neither the vagus nerve, nor the SCN, are essential for the entrainment of activity to restricted feeding.     

Automated computerized rat eater meter: description and application

A real-time Automated Computerized Rat Eater Meter was developed by modifying commercially available metabolic cages. Food access via a feeding tunnel was monitored by photocells. Food consumption was measured by an electronic scale. The signals thus generated were processed by a computer. This allowed us to continuously measure the spontaneous feeding behavior of free-feeding nondeprived Fischer rats for a sum total of 35 study days. Based on our data, we defined a meal as an episode of food consumption preceded and followed by at least 5 minutes of no feeding. Fischer rats showed periodic nychthemeral eating behavior. Food consumption, number of meals, meal sniffs, intermeal sniffs, and, consequently, eating activity were greater during the dark cycle than the light cycle. Meal duration, meal size, and thus food consumption rates remained constant throughout both cycles. Our modification of commercially available metabolic cages provides unique data for continuously monitoring rat feeding patterns over prolonged periods of time.