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.     

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.     

Sleep and feeding patterns in the ventromedial hypothalamic lesioned rat

Sleep and feeding patterns were recorded in Ventromedial Hypothalamic (VMH) lesioned rats during 10 to 15 consecutive days. During the dynamic phase of hyperphagia, daily Slow Wave Sleep (SWS) and Paradoxical Sleep (PS) were significantly increased compared to normal rats. These increases were exclusively due to a greater duration of SWS and PS episodes and a higher number of PS episodes during the dark period of the day. During the static phase, sleep amounts returned, as did food intake, to normal values but the equal circadian sleep distribution observed during the dynamic phase persisted. During the dynamic phase, the size of a meal was highly correlated with sleep events within the intermeal interval (IMI) preceding it, and only weakly correlated with sleep within the subsequent IMI. Unlike normal rats, there were no significant variations in deprivation and satiety ratios throughout the nycthemeron. These findings suggest that in VMH lesioned rats, physiological parameters determining sleep and feeding are perhaps more important than ecological ones. In addition, possible causative relations and common underlying factors between sleep and feeding are discussed.     

Meal patterns in “free-running” humans

The meal patterns of men who were initiating meals for extended periods in the absence of time cues were examined. A significant, positive correlation was found between the sizes (in kcals) of mixed and varied meals and the lengths of postprandial intervals. This quantitative relationship between meal size and meal timing is similar to patterns reported for freely feeding rats. If postprandial correlations reflect a short term mechanism for energy regulation then when humans schedule their meals due to social or time considerations they may negate this contribution to the regulation of their energy intake.     

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.     

1-Deoxy-glucosamine initiates, then effectively suppresses feeding in the rat

The deoxy analogues of D-glucose, 1-deoxy-D-glucosamine and D-glucosamine, are biochemically and structurally similar to 1-deoxy-D-glucose, so their direct effects on food intake were studied. Both 12 and 24 mumol 1-deoxy-glucosamine potently decreased feeding and body weight after an initial transient elicitation of food intake. The suppression included decreased meal size and prolonged postprandial intermeal interval which persisted for at least 3 days after injection. Ambulatory activity was unaffected. The initial elicitation of feeding was not accompanied by drinking episodes, and subsequent drinking suppression was persisted. These findings, plus other biochemical evidence, suggest that inversion with an amino group or removal of a hydroxyl group from C-2 and/or C-1 may affect feeding.     

Failure of 2-deoxy-D-glucose to stimulate feeding in deermice

Deermice (Peromyscus maniculatus) did not increase their food intake above baseline following treatment with 2-deoxy-D-glucose (2DG, 500 or 1000 mg/kg). They did eat more following food deprivation or treatment with insulin at a high dose (100 U/kg). House mice (Mus musculus) showed hyperphagia to 2DG, low dose of insulin (5 U/kg) and deprivation.     

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.     

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.     

Impaired feeding responses to intragastric, intraperitoneal, and subcutaneous injections of 2-deoxy-D-glucose in rats with zona incerta lesions

Rats with rostromedial zona incerta lesions displayed severely attenuated feeding responses to intragastric injections of 750, 1000, or 1250 mg/kg of 2-deoxy-D-glucose (2-DG) which elicited optimal feeding and few debilitating side effects in neurologically intact controls. Rats with ZI lesions also ate little, if at all, in response to low doses (200 and 300 mg/kg) of 2-DG administered subcutaneously which were approximately as effective in controls as the intragastric injections of much larger doses. The experimental animals also responded poorly to a standard (600 mg/kg) intraperitoneal (IP) injection of 2-DG but showed only a small impairment in responding to 8 Units/kg of insulin. These results suggest that less stressful routes of administration do not ameliorate the ZI rat's severe deficit in responding to 2-DG that has been reported after IP injections of large doses of the compound, and replicate the observation that insulin feeding is impaired relatively little if at all.     

Meal patterning in the streptozotocin-diabetic rat.

The free feeding behavior of 5 streptozotocin-diabetic rats and 5 sham injected rats was monitored both prior to and after the induction of diabetes. After streptozotocin injection, there was a marked increase in total intake resulting from the ingestion of large meals of long durations. Neither the postprandial relationship nor the circadian intake cycle were altered during diabetes. The results were interpreted as supporting the glucostatic theory of food intake regulation and suggest that the principal influence of insulin on feeding behavior is to promote the termination of feeding bouts.     

Metabolic correlates of the effects of 2-deoxy-D-glucose on meal size and post-meal satiety in rats

The effects of 2-deoxy-D-glucose (2-DG) (250 mg/kg) combined with 0 to 6 hours of food deprivation on the size of the first meal and the first post-meal interval have been studied in rats and compared for the two parts of the diurnal cycle. The observed changes in meal patterns have been correlated to plasma glucose (PG) and plasma free fatty acid (PFFA) levels determined 30 min after the first meal. It was shown that, compared to saline, 2-DG reduced the size of the first meal and did not change the post-meal interval at night, whereas during the day 2-DG reduced the latency to eat and the meal-to-meal interval without changing the meal size. At night, the reduction of the first meal was associated with the 2-DG induced high levels of PG and PFFA observed after the meal or in the absence of food. During the day, the acceleration of meal onsets by 2-DG was associated with a higher PG and lower PFFA levels 30 min after the meal than at the corresponding time at night. The results are discussed in terms of a dual and antagonistic action of 2-DG on feeding.     

Appearance of glucoprivic control of feeding behavior in the developing rat

2-Deoxyglucose was administered to determine whether glucoprivation would shorten either suckling or nibbling latencies of fed pups studied on Days 10, 14, 17, 21 and 24. A dosage of 200 mg/100 g BW given IP 2 hr prior to latency studies, shortened suckling latencies on Day 24, but shortened nibbling latencies from Day 21 onward. It is concluded that glucoprivic feeding control appears in the developing rat by Day 21 and affects nibbling behavior to a greater extent than suckling behavior.     

Foraging cost and meal patterns in ferrets

The response of ferrets to changes in the cost of obtaining food was studied by systematically increasing the number of responses necessary to gain access to a feeder. The results obtained were consistent with an ecological analysis of feeding. As cost increased, meal frequency declined and meal size increased. These changes in feeding allowed the ferrets to obtain sufficient food intake to maintain growth, while conserving total time and energy spent procuring food.     

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.     

Meal patterns of lean and leptin-deficient obese mice in a simulated foraging environment

C57BL/6J lean and obese (lep -/-) mice were studied in a closed economy operant protocol that simulates foraging. A predetermined number of presses on a procurement lever (PFR) activated a consumatory lever on which presses would produce 20-mg food pellets. Mice could eat as much as they wished but, once no responding occurred for an elapsed 10-min period, the consumatory lever was inactivated and the procurement or foraging cycle began again. Under these conditions, as has been shown for rats and other species, mice initiated relatively discrete meals (about nine per day) at the lowest PFR, and the number of meals initiated per day decreased with increasing PFR. Meal size increased reciprocally, so that total intake was conserved across the range of PFR examined. Obese mice ate larger meals than lean mice at low PFR, and showed further increases but only at the highest PFRs. The small and inconsistent literature on meal patterns in mice is reviewed, and we discuss the utility of the present protocol to study the interactions between genetic and environmental economic factors, and their implications for the etiology of human obesity.     

A new approach of sleep and feeding behaviors in the laboratory rat

Sleep and feeding patterns were recorded in 24 rats housed in normal laboratory cages (control) and in niche equipped cages with either a foodcup continuously illuminated or not illuminated (experimental) for 10 to 15 consecutive days. The niche conditions brought about a complete disappearance of nibbling, a significant reduction in the daily number of meals (6.8 vs 9.3) and a significant increase in mean meal size. Though the daily food intake remained unchanged, body weight gain was significantly increased. When the foodcup is illuminated, meals were equally distributed between light and dark phases. Parallel to the latter, sleep durations (particularly Paradoxical Sleep) were increased during the dark phase tending towards an equal circadian distribution. Under the niche conditions, meal size was correlated with the post-prandial events (meal-to-meal intervals and sleep durations within them) during the dark phase as it was during the control period. However, unlike control animals, there were also correlations during the light phase. These results suggest that under the niche conditions, there is a stronger physiological (metabolic) motivation of meal taking and sleep occurrence. Henceforth, such an ecological artifact could be used in order to perform experiments aimed at a better understanding of behavioral regulatory mechanisms.     

Relationship between endogenous, natural feeding patterns and body composition in the rat.

The feeding patterns of rats were monitored during four 5 day periods from weaning to adulthood and correlated with the composition of the adult carcasses. Feeding patterns, immediately after weaning, had their major impact on carcass protein, while adult feeding patterns had their major impact on carcass fat and water. Gorging (large infrequent meals) was characteristic of heavy animals but not necessarily fat animals. The results suggest that the spontaneous gorging pattern was a function of the volume of intake and that, under natural conditions, nibbling or gorging are not significant factors affecting fat deposition.     

The effect of liver denervation on meal patterns,body weight and body composition of rats

Neural liver glucoreceptors have been proposed as a primary controller of food intake (FI). Male Sprague-Dawley rats were either sham operated or liver denervated (LD). LD rats had all tissue cut between the liver and the esophagus, stomach and upper 1 cm of the duodenum. The hepatic artery and surrounding tissue were also removed. Finally the hepatic portal vein and the bile duct were stripped clean and the former phenol treated. Three days after surgery animals were placed in modules for continuous computer monitoring of feeding behavior. At no time after surgery did the daily food intake or body weight of the groups differ significantly. Meal size and frequency (light-dark distribution) were determined for 6 days and averaged. Neither parameter was altered by LD. During the next 6 months food intake and body weights of the groups did not differ significantly. At sacrifice, body composition was directly determined with no significant differences observed between LD and sham operated rats. LD were confirmed histologically. Monoamine histofluorescence of the livers of rats subjected to liver denervation revealed an absence of the normal fluorescence seen on small blood vessels in liver parenchyma of sham operated rats. The data do not support the concept that liver glucoreceptors are a major controller of FI.     

Meal composition influences subsequent food selection in the young rat

Food selection one-half hour after the ingestion (2 g) of either a protein containing or a protein-free (carbohydrate) diet was studied in young rats. Following a 12 hr fast the rats were allowed 15 minutes to consume the meal (premeal). Thirty minutes later, they had access to two isocaloric diets that differed only in protein and carbohydrate content. During the first hour of ad lib feeding, protein intake and protein concentration selected were lower in the 45% casein prefed group compared to the carbohydrate group. A further reduction in protein intake and protein concentration occurred when the protein content of the premeal was increased to 70% from 45% casein. Rats prefed with the 70% casein diet significantly reduced their daily total food intake (12 hr) compared to either the 45% casein or carbohydrate prefed group. It is concluded that both quantity and composition of food selected is influenced by the composition of the preceding meal.