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.     

Circadian pacemaker control of feeding in the rat,at dawn

Previous studies suggest that feeding at dusk is probably dependent on the rat's immediate energy requirements, while feeding at dawn may have an ‘anticipatory’ function. However, little is known regarding the relative contribution of habit, energy deficits and circadian pacemakers in the expression of feeding behavior. The aim of the present study was to investigate the involvement of, and interplay between, habit, pacemaker synchronization and energy deficits in the occurrence of dawn feeding in rats. Light onset exerts a strong control over the timing of dawn feeding. The motivation to feed at dawn persists even when access to food is prevented during this period, and shows accompanying shifts with changes in light onset. Rats compensate their caloric deficit imposed by food restriction at dawn by eating earlier in the light phase. This feeding probably occurs in response to feedback mechanisms signalling an energy deficit. The rapid shift in dawn feeding with changes in light onset and food restriction, and its quick reappearance after discontinuing food restriction, argue against habit formation as solely responsible for the occurrence and maintenance of dawn feeding. Habit formation does play a secondary role in the maintenance, but occurs within the ‘boundaries’ set by the pacemaker. These experiments show that the timing and maintenance of dawn feeding are under the control of a circadian pacemaker which can be shifted by light onset only.     

Circadian patterning of feeding,drinking and activity during diurnal food access in rats

Using an “ecologically” relevant approach, the present study investigated (1) the association between feeding and drinking patterns and their circadian organization and (2) changes in general activity (or life-style), during ad lib conditions (fa:LD) and in a situation where access to food was restricted to the light phase (fa:L). Rats were housed in large outcages with nest boxes. Feeding, drinking, activity, outcage and nest occupation were recorded automatically throughout the day-night cycle. Access to food was restricted by a sliding door situated in front of the food hopper. Under ad lib conditions rats were mainly nocturnal, eating 94% and drinking 95% of their daily intakes at night. The patterns of food and water intake were similar, showing a bimodal distribution over the dark phase. During fa:L rats showed an initial large peak in feeding with lights on, followed by a long pause. Thereafter, feeding activity was variable but remained at a low level. The nocturnal drinking pattern persisted. However, 17.5% of daily water intake was meal-associated, compared with 71% during ad lib. Diurnal activity was associated with feeding and nocturnal activity with drinking. Nocturnal outcage and nest occupation patterns were not shifted to the light phase. The experiment demonstrates that rats on an fa:L schedule reduce food and water intake, and body weight, while still retaining circadian characteristics in the temporal distribution of drinking, activity, outcage and nest occupation. Further, although feeding and drinking may be causally related they need not occur in close temporal association. The rapidity of re-establishment of the normal feeding pattern, on return to free-feeding, and the close association with drinking under normal conditions, suggest the existence of a single or two coupled oscillators controlling feeding and drinking.     

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.     

Meal pattern of hypophysectomized rats

The daily meal pattern of hypophysectomized rats was studied in order to investigate the possible role of hypophyseal hormones in the control of food intake. A 5-day recording of the ad lib feeding pattern in 10 hypophysectomized rats, compared to controls, confirmed earlier findings of a 50% reduction of the daily food intake. However the observation of maintained diurnal feeding cycle and of a prolongation of day-time meal-to-meal intervals suggests that the lack of hypophyseal hormones does not affect the basic feeding mechanisms. Particularly, no indication exists that the lack of growth hormone disrupts the day-time lipolysis and associated low food intake.     

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.     

Meal size as a determinant of food intake in normal and hypothalamic obese rats

Size of meals taken by normal rats was greatly increased by following each spontaneously initiated meal with gastric infusion of additional diet. In a second experiment, rats in the dynamic phase of hypothalamic obesity were limited to meals much smaller than the unusually large ones they usually take. In both experiments rats made precise adjustments in meal frequency which maintained daily food intakes at or close to pre-experimental levels.     

Comparison of androgenic effects on food intake and body weight in adult rats

In order to investigate the interrelationships between sex hormones, food intake, and body weight, three androgens (testosterone propionate (TP), dihydrotestosterone (DHT), and androstenedione (ANDR)) were administered to separate groups of gonadectomized adult rats of both sexes. In males TP increased body weight to the greatest degree, while the effects of DHT, a non-aromatizable androgen, and ANDR were approximately equal to each other. The relationship between each androgen and food intake partly paralleled that of body weight. In females, however, DHT exerted a stronger effect on body weight than TP, and ANDR produced no significant increases: food intake was stimulated by both TP and DHT. As others have indicated, aromatization of testosterone to estrogen then is not critical to the effect of androgens on either food intake or body weight. Furthermore, the fact that increases in body weight are not always accompanied by concomitant changes in food intake suggests that gonadal hormones may exert their effects on body weight primarily through various metabolic processes, not simply through changes in food intake.     

Homeostatic competition between food intake and temperature regulation in rats

Rats obliged to leave a thermoneurtral box to feed at air temperatures (T_a) of 25°, 5 or ?15°C reduced the total time spent feeding and the duration of each meal as T_a fell, but increased their food intake by eating faster. Increasing the palatability of the food offered at ?15°C T_a did not prolong feeding but further increased food intake and the speed of eating. The estimated maximum fall in rectal temperature during feeding at ?15°C was small (0.48°±0.15°C, S.E.) but skin temperatures of ears and tail tip fell to near 0°C. These rats were able to maintain near-normal balances of food intake and body temperature by reallocating the times spent feeding and sheltering and by altering the speed of eating; they thus resolved a conflict between hunger and cold discomfort with little evidence of a strain on homeostasis.     

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.     

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.     

Diurnal rhythms of leptin and ghrelin in the systemic circulation and in the gastric mucosa are related to food intake in rats

We investigated diurnal changes in leptin and ghrelin levels in the stomach and in the systemic circulation and their relation to food intake rhythms in Wistar rats housed at 22 °C with a 12-h light/dark cycle and free access to food and water. Animals were sacrificed every 3 h over a 24-h period. Leptin and ghrelin levels in serum and in the gastric mucosa were analysed by immunoassay. Leptin mRNA levels were determined in the gastric mucosa by RT-PCR and in different adipose tissue depots (epididymal, retroperitoneal and mesenteric) by Northern blot. Ghrelin mRNA levels were determined by Northern blot. Gastric and serum leptin levels displayed similar diurnal rhythms, rising during the dark phase and decreasing gradually during the light phase. Leptin expression in the different adipose tissue depots correlated positively with circulating leptin levels (P<0.05), although there were some depot-associated differences. Leptin mRNA levels in the mesenteric depot correlated positively with food intake (P<0.05). In blood, ghrelin levels rose sharply just before the onset of the dark phase and dropped suddenly just after. In the stomach, ghrelin levels were high during the fasting period of light and low during the night, and correlated inversely with food intake, gastric contents and serum leptin levels (P<0.05). Leptin and ghrelin in the stomach and in the systemic circulation thus show diurnal variations that are influenced by food intake rhythms. The results agree with a role for ghrelin as a stimulant of meal initiation.     

Comparison of voluntary and foraging running wheel activity on food demand in mice

The effects of running wheel activity on food intake and meal patterns were measured under several cost conditions for food in CD1 mice. In a first experiment, voluntary wheel running activity increased daily food intake relative to a sedentary group, and runners consumed bigger but fewer meals. Although they ate more, runners had significantly lower body fat than sedentary mice. In a second experiment, running was used as an approach cost and food access was contingent on running wheel activity. Mice were able to emit more wheel revolution responses compared to a condition in which nose poking was the approach response. In both voluntary and foraging running protocols mice had inelastic demand functions compared to the non-running groups. When running was voluntary (experiment 1), the day-night cycle for activity was more pronounced compared to when running was a foraging or approach activity (experiment 2).     

Feeding patterns of theophyllinized rats and effects of dextrose on their food intake.

Chronically theophylline administered rats did not show normal increase in food intake during dark periods, while the total amount of intake per 24 hr was not affected. These ingestive results were consistent with changes in arterial-venous blood glucose differences: i.e., those in the control group were markedly dependent on a light-dark feeding cycle, but not those in the theophylline group. Food containing relatively high concentrations of dextrose was found to facilitate food consumption of theophyllinized rats, an effect opposite to that on the controls. These observed changes in the feeding behavior of theophyllinized rats may reflect impaired feeding motivation and circadian desynchronization of feeding.     

Overcompensation of food intake following brief periods of food restriction

Four adult female Sprague-Dawley rats, maintained on an ad lib feeding schedule were deprived for either 0, 0.5, 1, 2, 4, 6, or 8 hours during the dark phase of the day-night cycle. It was found that the latency to initiate the first meal following the deprivation was independent of the previous deprivation interval. The animals were found to overcompensate for the periods without food by eating a large initial meal that increased proportionately in size with the duration of food restriction. Furthermore, the animals continued to overeat throughout the day. This deprivation-induced overeating by the animals resulted in an overcompensation in total food consumption that was 21–56% greater than on control days. The role of food intake as a regulator of body weight is discussed.     

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.     

Meal patterns and foraging in melanocortin receptor knockout mice

We report the meal patterns of mice with the deletion of either the melanocortin type 3 or 4 receptors (MC3RKO or MC4RKO) compared with that of the wild type (WT) under conditions of varying foraging costs. Mice lived in two-lever operant chambers; the completion of a designated number of responses (termed procurement fixed ratio or PFR) on the "foraging" lever activated the other lever. On this second lever, the completion of a designated number of responses (termed consumatory fixed ratio or CFR) caused the delivery of a 20-mg food pellet. Animals could complete as many CFRs as they wished to constitute a meal, but whenever 10 min elapsed without pressing on this second lever, the meal was terminated and pressing on the "foraging" lever was again required to initiate a new meal. At lower PFRs, mice of all three genotypes took 5-7 well-defined meals per day of approximately 35 pellets/meal. At the highest PFR, mice of all three groups took about half this number of meals, with some increase in meal size, and total intake was slightly reduced. MC4RKO mice were obese compared with WT or MC3RKO but failed to eat more food in the operant chambers and, as a consequence, lost weight, regardless of PFR. Thus, changes in meal-taking strategies as a function of imposed foraging cost are not critically dependent on either MC3 or MC4 receptors, but these conditions did not allow us to study meal patterns in MC4RKO mice that are hyperphagic.     

Decreased food intake of rats kept under adiurnal feeding cycles: effect of suprachiasmatic lesions

The circadian feeding rhythm and food intake under restricted feeding conditions, including adiurnal feeding cycles, were examined in rats with bilateral lesions of the suprachiasmatic nucleus (SCN). Although rats with SCN-lesions ate nearly as much food per day as those with control-lesions, their feeding pattern did not show circadian rhythmicity. When rats with control-lesions were fed for 5 hr once every 20 hr or for 7 hr once every 28 hr, they ate less than when they were fed for 6 hr once every 24 hr, probably due to some effect of desynchronization between the feeding cycle and an endogenous circadian oscillator. Decreased food intake under adiurnal feeding cycles was also observed in rats with SCN-lesions as in those with control-lesions. It is suggested that the circadian rhythm entrained by food is related to an endogenous time-keeping system that does not include the SCN.     

Feeding behavior responses of Zucker rats to naloxone

The effects of naloxone on feeding patterns were studied in both obese and lean Zucker rats during both light and dark phases of the diurnal cycle. Eight female obese (471±9 g) and lean (225±6 g) Zucker rats were trained to bar press for food. They were administered 0, 0.5, 1.0 or 2.0 mg/kg naloxone at the initiation of the light or dark phase of the diurnal cycle and feeding behavior was recorded for the subsequent 12 hr using an automated real-time data collection system. First meal size and duration were decreased and first postmeal interval was increased by naloxone and responses did not vary with phenotype or phase of the diurnal cycle. Naloxone decreased food intake during the 12-hr period by decreasing average meal size but meal frequency was not affected. Overall, the feeding behavior responses of obese rats to naloxone were greater than those of lean rats, supporting the hypothesis of an association between opioid peptides and obesity. Opioid involvement in diurnal control of food intake is also supported by the greater responses generally demonstrated in the light compared with dark phases.     

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.