Control of food intake and meal patterns in monkeys

Feeding patterns have been studied in 10 adult male rhesus monkeys (Macaca mulatta) adapted to primate restraint chairs for physiological studies. Using an automated computer-monitored liquid diet feeding system, feeding behavior was studied under 8 hour and 24 hour feeding schedules. Calorie/kg intake was significantly reduced with the 8 hour schedule (p<0.05). Average rate of weight gain and feeding efficiency were also lower with 8 hour feeding, but not significantly different from 24 hour ad lib feeding (p>0.10). Meal size was negatively correlated with meal frequency across monkeys, but not within monkeys. Although monkeys ingested 78% of their voluntary intake during the light hours, there was no difference in meal size, meal duration, or rate of feeding between light and dark periods. With feeding restricted to 8 hours during the light period, meal size was positively correlated with the length of the interval preceding the meal in 4 of 6 monkeys, and meal size was also positively correlated with length of the postmeal interval in 3 of 6 monkeys. In contrast, 24 hour ad lib fed monkeys showed no relationship between premeal interval and subsequent meal size, but a positive correlation between meal size and postmeal interval in 3 of 4 monkeys. We conclude that feeding schedule or deprivation state may alter the relative roles of “hunger” and “satiety” signals in regulating food intake amounts and patterns.     

Feeding conditions and intermeal relationships

The free feeding pattern of rats was examined as a function of decreasing the accessibility of the meal. Meal frequency decreased and meal size increased proportionally resulting in a constant food intake per unit time. Various characteristics of the feeding pattern were dramatically affected by these procedures. A diurnal variation in meal size and meal frequency was observed when food was most accessible, but the variation in meal size and meal frequency disappeared when accessibility was slightly hampered. Moreover, no correlation between meal size and postmeal interval was observed in the free feeding situation, but did occur when mean accessibility was decreased. Finally, a regular periodic feeding pattern emerged when accessibility was maximally hindered. A second experiment showed that these changes in the characteristics of the meal eating pattern produced in Experiment 1 were more related to the increase in meal size than to the particular feeding situation. The data also suggested that a minimum calorie intake at a meal of approximately seven calories was necessary to obtain a significant meal size-postmeal interval correlation for dry diets and a minimum intermeal interval of four hours was necessary to obtain a meal size-premeal interval relationship.     

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.     

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.     

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.     

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.     

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.     

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.     

Ovarian influences on the meal patterns of female rats.

A series of experiments examined the effects of estrous cycles, ovariectomy, and estradiol-replacement on free-feeding meal patterns of female rats maintained on a liquid diet. The proestrous decrease in food intake was accomplished by a decrease in meal size and a less than fully compensatory increase in meal frequency. The hyperphagia induced by ovariectomy was reflected in an increase in meal size and a decrease in meal frequency. When food intake returned to preoperative levels, meal size remained elevated while frequency decreased further. Estradiol benzoate (2 μg/day) permanently decreased meal size in long-term ovariectomized rats. The subsequent return to food intake levels of controls was due primarily to an increase in meal frequency. These results suggest that the transient changes in food intake caused by estradiol withdrawal and replacement are accomplished by permanent changes in meal size followed by compensatory changes in the number of meals consumed per day. They suggest that the decrease in meal size at proestrus is due to a direct effect of estradiol on the mechanisms that terminate short-term food intake and are not secondary to changes in the level of total daily food intake.     

Meal to meal energy balance in rats

Meal to meal energy balance was examined in thirty-eight simultaneous recordings of feeding pattern and O₂ consumption in six rats. The mean difference between energy intake in a meal and energy expenditure until the onset of the next meal was found positive at night and negative during day time. At night the excess of meal intake over meal to meal expenditures was decreasing from the beginning to the end of the night and was strongly correlated to meal sizes. During day time meal to meal deficit was decreasing from the beginning to the end of the period but was not correlated to meal sizes. These meal location and size effects on the meal to meal energy balance were not determined by an effect of these factors on metabolic rate. No indication was provided that meal to meal energy balance was influenced by a “meal induced thermogenesis.” Rather an evolution from the beginning to the end of the night of the correlation between meal size and durations of meal to meal intervals was found to be parallel to the evolution of positive meal to meal energy balance throughout the night. From these data it is concluded that at night a dual utilization of meal caloric intake (current energy metabolism plus fat storage) and a dual source of fuel during the day (food plus mobilized fats) determine time and mechanism of meal onset.     

Effect of Water Deprivation on Eating Patterns of Lactating Cows Fed Grass and Corn Pellets Ad Lib

The present study investigated the effect of 48 h of water deprivation on eating behavior of 12 lactating cows fed grass and corn pellets ad lib. Water deprivation reduced grass and corn pellet intake significantly, with corn pellet intake decreasing earlier than grass intake. The reduction of feed intake was entirely due to a reduction of meal size for both grass and corn-pellet meals. The size of the first meal of the water deprivation period was already reduced by about 30%. Grass-meal frequency increased, whereas corn-pellet meal frequency decreased during water deprivation. Water deprivation also decreased live weight and milk yield by about 12 and 30%, respectively. All parameters returned to baseline values with rehydration. With ad lib access to feed and water, 77% of drafts occurred in relation to meals, but the number of meals clearly exceeded the number of drafts. The results demonstrate that water deprivation rapidly leads to premature meal termination. This is in line with the assumption that an enhanced prandial increase in ruminal fluid osmolality contributes to dehydration-induced hypophagia. Yet, further studies are necessary to prove this assumption and to better understand the complex relationships between eating and drinking in ruminants.     

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.     

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.     

The effect of Type I diabetes on the eating patterns of free-living French: a diet diary study

In order to investigate the changes produced by Type I diabetes on the ad libitum eating behavior of free-living humans, 56 French participants with diabetes and 28 healthy controls were paid to maintain detailed food intake diaries for four 7-day periods. The participants with diabetes ate more protein and more frequent meals, ate slightly later and with fewer other people, were less hungry, thirsty, and depressed, but more elated than the healthy participants. Responses to social facilitation, the diurnal rhythm, subjective hunger, the palatability of the food, and the weekend were not affected by the presence of diabetes. The participants with diabetes had significantly larger correlations and regression coefficients for the relationship between meal size and the duration of the before-meal interval. The relationship between meal size and the after-meal interval was strong and positive in all participants. The recorded behavioral characteristics of diabetics may, in part, be accounted for by the nutritional education that is provided as an integral part of treatment. Based upon these results, it was theorized that the glucose regulatory system is an influence on intake, but only one of many that are responsible for the coordination, control, and regulation of nutrient intake in free-living humans.     

Patterns of food intake in the monkey during free or restricted feeding and in response to 2-DG or insulin injection.

When given free access to food, the restrained macaque monkey establishes a pattern of ingestion characterized by frequent meals and a stable caloric intake of 26–33 g/kg/day. When the availability of food is restricted to eight hours per day, the number of meals decreases but the mean daily intake as well as the first meal size increases. No consistent correlation is found between meal size and either the pre- or post-meal interval. A subcutaneous injection of 300 mg/kg 2-deoxy-D-glucose (2-DG) causes a 100% increase in the size of the monkey's first meal but a reduction in the number of meals eaten with no change in its daily food intake. This compensatory adjustment to the large first meal is made through a decrease in the number of meals eaten with the insertion of one extremely long post-meal interval. Insulin (1.0 or 5.0 U/kg) similarly injected evokes an increase in both the size of the first meal and the monkey's daily food intake; however, this occurs only in the female macaque. The failure to regulate caloric intake accurately in this case is due to an increase in the magnitude of at least two meals. Thus, although both insulin and 2-DG do augment the size of the following meal, only insulin enhances the total daily intake of food. The differential patterning of food intake after an injection of either 2-DG or insulin suggests that a meal eaten under the influence of 2-DG results in a greater satiation than normal, whereas a meal consumed during insulin-induced hypoglycemia could be less satiating.     

Effects of duodenal osmotic loads on spontaneous meals in pigs

Fourteen young Large White pigs were trained to press a panel switch to obtain reinforcements of food (FR 10–15). Duodenal injections (5 ml/kg) were made automatically via implanted catheters shortly after the beginning of every other meal 24 hours a day. Solutions of varying osmoconcentration and nutrient content were injected. Isosmotic solutions of glucose (5, 20 and 40%) and NaCl (0.9, 3.25 and 6.5%) equally depressed the size of ongoing meals in proportion to their hypertonicity. For example, 20% glucose and 3.25% NaCl depressed meal size to 62% of control, and 40% glucose and 6.5% NaCl to 34%. Xylose (33%) was less effective than its glucose osmotic equivalent. Tetracaine (0.5%) blocked most of the reduction of meal size. The reduced meal size was due to a decrease in meal duration. Neither rate of eating during the meal nor intermeal interval changed. The results support the hypothesis that a duodenal osmoreceptive system participates normally in the control of meal size.     

Caractéristiques des prises d'eau et d'aliments spontanées chez des chiens en situation ad lib

The alimentary behavior in three male dogs, having free access to food and water, was studied. The results show that: (a) two dogs consumed the majority of their water and food in the light phase; (b) for two dogs, in the light phase and for one dog in the dark phase, there is a correlation between the weight of each meal and the quantity of water ingested between this meal and the next one; (c) for two dogs, in the dark phase, there is a correlation between the weight of each drink and the post drinking interval until the next water intake; (d) for the three dogs, during the light phase, there is a correlation between the weight of each meal and the duration of the next intermeal interval; (e) for each dog the water and food intake speed (g/s) is approximately the same during the light and dark phases. In ad lib situation there are, except for certain species' communal relationships, striking differences between the feeding behavior pattern of each dog.     

CCK(1) receptor is essential for normal meal patterning in mice fed high fat diet

Cholecystokinin (CCK), released by lipid in the intestine, initiates satiety by acting at cholecystokinin type 1 receptors (CCK(1)Rs) located on vagal afferent nerve terminals located in the wall of the gastrointestinal tract. In the present study, we determined the role of the CCK(1)R in the short term effects of a high fat diet on daily food intake and meal patterns using mice in which the CCK(1)R gene is deleted. CCK(1)R(-/-) and CCK(1)R(+/+) mice were fed isocaloric high fat (HF) or low fat (LF) diets ad libitum for 18 h each day and meal size, meal frequency, intermeal interval, and meal duration were determined. Daily food intake was unaltered by diet in the CCK(1)R(-/-) compared to CCK(1)R(+/+) mice. However, meal size was larger in the CCK(1)R(-/-) mice compared to CCK(1)R(+/+) mice when fed a HF diet, with a concomitant decrease in meal frequency. Meal duration was increased in mice fed HF diet regardless of phenotype. In addition, CCK(1)R(-/-) mice fed a HF diet had a 75% decrease in the time to 1st meal compared to CCK(1)R(+/+) mice following a 6 h fast. These data suggest that lack of the CCK(1)R results in diminished satiation, causing altered meal patterns including larger, less frequent meals when fed a high fat diet. These results suggest that the CCK(1)R is involved in regulating caloric intake on a meal to meal basis, but that other factors are responsible for regulation of daily food intake.     

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.