Effect of pre-feeding ammonium acetate on food intake of rats fed high protein diets

The effect on intake of a 75% casein diet after prefeeding for one week a 6% casein basal diet with additional 0%, 2%, 5%, 8% or 15% ammonium acetate was examined in rats trained to eat in three hours per day. Food intake was measured from 0-15, 15-30, 30-90, and 90-180 minutes for the first two days that the ammonium acetate diets were presented. Rats eating 5% and 8% or 15% ammonium acetate diet depressed their intake significantly for one day and for four days respectively. Rats eating 2%, 5%, 8%, or 15% ammonium acetate diets depressed their intake significantly from 0-30 minutes. When presented with the 75% casein diet, rats prefed 0% to 5% and 8% and 15% ammonium acetate diets ate 55% to 58% and 72% and 94% of their respective baseline intakes. It is suggested that prefeeding 15% ammonium acetate apparently induces sufficient metabolic adaptation to ammonia intake so that the rat is able to offset the metabolic consequences of intake of the 75% casein diet, thus preventing the usual food intake depressing effect of the high protein diet.     

Estrogen reduces total food and carbohydrate intake,but not protein intake,in female rats

Female rats were given simultaneous access to two isocaloric, isocarbohydrate diets that contained 5% or 45% protein. During four consecutive estrous cycles, rats reduced their total food and carbohydrate intake at estrus but maintained protein intake at levels found during the other stages of the estrous cycle. Administration of estradiol benzoate (EB) to ovariectomized rats given the same 5% or 45% protein diet choices also reduced total food and carbohydrate consumption while maintaining protein consumption. By contrast, administration of EB to ovariectomized rats given isocaloric, isonitrogenous diets containing 25% or 70% carbohydrate significantly reduced both total food and carbohydrate intake. The effects of estrogen on food and protein intake reported here are similar to those previously found after administration of drugs that enhance serotoninergic neurotransmission. Thus, the effects of estrogen on nutrient selection may depend, in part, on activation of a serotoninergic mechanism.     

The effects of early dietary experience on subsequent protein selection in the rat

Two series of experiments were designed to investigate the effects of protein composition of the first solid food consumed by young rat pups on their subsequent protein intake after weaning. In the first series of experiments, pups were allowed access to the maternal diet between 17 and 21 days of age, then weaned to a choice of 10% and 60% casein diets at 21 days. Pups with access to a maternal diet containing 39% casein selected high levels of protein (calculated as % of total calories selected as protein: 50 +/- 1% (access) vs. 29 +/- 3.5% (no access), p less than 0.05), whereas lower levels of protein were selected following access to a maternal diet containing 26% casein (16 +/- 1.1% (access) vs. 23 +/- 1% (no access), p less than 0.05) or 10% casein (27 +/- 3.9% (access) vs. 39 +/- 3.7% (no access), p less than 0.05). In a second series of experiments, the influence of early diet composition on subsequent protein selection was confirmed by prematurely weaning rats at 17 days of age to either 39% or a 10% casein diet. When given a choice of 10% and 60% casein diets at 21 days, pups selected higher levels of protein if weaned to a 39% casein diet as compared to a 10% casein diet (37 +/- 5.0% vs. 19 +/- 3.5% (p less than 0.05) for the first experiment and 43 +/- 3.0% vs. 26 +/- 4.0% (p less than 0.05) in a second experiment). It was concluded that the level of protein selected by young rats is positively correlated with the protein concentration of their first solid food.     

Protein selection by rats adapted to high or moderately low levels of dietary protein.

After preliminary studies on flavor acceptability, patterns and indices of subsequent feeding behavior were monitored by computer in young rats which were adapted to 15% or 70% casein diets before being offered, sequentially, choices between flavored diet pairs in which the proportions of percentage casein were 5/65, 5/55, 5/45, 5/35 and 5/25. Similarly adapted rats received these choices in the reverse sequence. Rats adapted to 15% casein usually ate randomly from the diet pairs and selected approximately 15-30% casein; individual behaviors were prominent. The 70% casein groups avoided the higher casein diet, often within minutes (except for the first-offered 5/25 choice), and seldom selected more than 10% casein; individual differences were infrequent. Such rats also distinguished between flavored 70% and 65% casein diets. Sizes and numbers of meals and rates of eating differed for the paired diets, especially for rats adapted to 70% casein. A flavor added to the 70% casein adaptation diet was not avoided when present only in the 5% casein diet of a 5/65 choice. Rats adapted to 70% soy protein before receiving flavored 5/65 to 5/25 choices selected 20-28% soy protein, a level far above those of casein selections by rats adapted to 70% casein. Dietary adaptation and type of protein thus affect subsequent diet selection and feeding patterns and indices.     

Pancreatic glucagon's effects on satiety and hepatic glucose production are independently affected by diet composition

We tested whether pancreatic glucagon's effects on satiety and hepatic glycogenolysis depend on variations in the macronutrient composition of the diet. Rats were kept on high carbohydrate, high fat, or high protein diets (HC-, HF- or HP-rats, respectively) and adapted to a 5 hour feeding (11:00-16:00)-19 hour deprivation (16:00-11:00) schedule. Glucagon (540 mcg/kg body weight) was intraperitoneally injected at 14:00. Glucagon stimulated similar amounts of hepatic glycogenolysis in HC- and in HF-rats and less glycogenolysis in HP-rats, but glucagon decreased food intake only in HC-rats. When HF- or HP-rats were prefed the HC-diet once for 1 hour (11:00-12:00) prior to injection, glucagon stimulated hepatic glycogenolysis similarly in both groups but decreased food intake only in HF-rats. Therefore, stimulation of hepatic glycogenolysis by glucagon is not sufficient to elicit satiety under all feeding conditions. The results also suggest that glucagon-induced satiety is not limited to carbohydrate intake.     

Food intake and selection after peripheral tryptophan

Two studies investigated the effects of peripheral (IP) administration of the dietary indispensible amino acid tryptophan, on food intake and macronutrient selection in rats adapted to a 12 hr nocturnal feeding period and a choice of 10% and 60% casein diets. In a dose-response study (35, 55, 75, 95, 115 mg/kg), the threshold dose of 75 mg/kg produced a significant reduction in total food intake (3.6 to 2.3 g, p less than 0.05) during the first hour of feeding. The reduction in carbohydrate intake (2.1 vs. 1.2 g, p less than 0.05) was greater than that for protein intake (1.6 vs. 1.1 g, p less than 0.05). Twelve hr total food intake was also decreased (20.9 to 19.5 g, p less than 0.05) and this was attributable to decreased carbohydrate intake (13.2 to 11.8 g, p less than 0.05). In a second study designed to determine if tryptophan's effects were mediated by the central nervous system, brain tryptophan uptake was blocked by co-injecting valine with tryptophan. The significant reduction in first hour total food intake by tryptophan was not prevented by co-injection of an equal quantity of valine (3.5 to 1.8 g, p less than 0.05). Again the suppression of carbohydrate intake (2.0 to 0.9 g p less than 0.05) was greater than that for protein intake (1.5 to 0.9 g, p less than 0.05). This dose of valine significantly reduced brain tryptophan uptake by 16% (21.3 to 17.8 micrograms/g, p less than 0.05) and when administered alone did not affect first hour total food intake (3.1 vs 3.2 g).(ABSTRACT TRUNCATED AT 250 WORDS)     

Food intake and behavioral caloric compensation after protein repletion in the rat

The food intake response of rats to intragastrically infused or orally consumed protein was characterized and compared to the response to similarly administered carbohydrate. In Experiment 1, protein hydrolysate loads administered at the beginning of the dark phase via chronic intragastric cannulae led to rapid (complete within 3–4 hr), dose-related suppressions of food intake in comparison to sham loads or to urea loads controlling for volume, concentration and pH. The suppression remained evident in cumulative intake records through 24 hr and represented a greater than caloric compensation for the amount of metabolizeable energy delivered (1.38 and 1.12 kcal/kcal in Experiments 1a and 1b, respectively). In Experiment 2, protein hydrolysate loaded by gavage suppressed food intake significantly more than isocaloric glucose loads (1.32 vs 0.81 kcal/kcal after 24 hr). Water intake in both experiments was elevated by protein loads, although this occurred after the food intake suppression. Differential satiety responses to protein and carbohydrate repletion were shown in Experiment 3 to extend to orally consumed, naturally occurring macronutrients—casein vs starch and disaccharide diets. These data suggest that satiety in the rat is not solely graded with respect to the energy delivered by the preceding meal but rather that repletion with different macronutrients leads to quantitatively (or perhaps qualitatively) different satieties.     

Maternal malnutrition in the rat: Effects on food intake and body weight

The effects of dietary protein level on food intake and body weight were examined in adult female rats during a 35-day pre-mating period and during gestation and lactation. During the pre-mating period, no differences in daily food intake were observed among rats fed a 6% casein, 8% casein or 25% casein diet. However, during this period, rats fed the 6% casein diet gained significantly less weight than those with ad lib access to the 8% or 25% casein diets or than rats pair-fed the 25% casein diet in amounts equivalent to that consumed by rats in the 6% or 8% casein groups. Additionally, rats fed the 6% casein diet displayed decreased efficiency of energy utilization, calculated as weight gain per 100 kilocalories consumed, relative to rats fed the 8% or 25% casein diets. No differences in food intake were observed among the groups during gestation. However, rats fed the 6% casein diet gained less weight than rats fed the 8% or 25% casein diets. During lactation rats fed either the 6% or 8% casein diet consumed significantly less food than animals given the 25% casein diet ad lib. During the second week of lactation, rats receiving ad lib access to the 25% casein diet gained weight while those receiving the 6% or 8% casein diets continued to lose weight. At parturition, body weights of pups did not differ as a function of dietary condition. However, by day 12 of life, pups whose dams had ad lib access to the 25% casein diet weighed significantly more than pups whose dams consumed the 6% or 8% casein diet or whose dams were pair-fed the 25% casein diet in amounts equivalent to those consumed by rats fed the 6% or 8% casein diet.     

Physiological, environmental, and subjective determinants of food intake in humans: a meal pattern analysis

The spontaneous food intake of 31 adult humans was investigated with diary self-reports of ingestive behaviors and subjective hunger over 7 consecutive days. Meals were identified, using five different definitions, and their composition of total calories, carbohydrate, fat, and protein was estimated from a computer file of the nutritive value of foods. These data were analyzed by intercorrelating meal sizes with intermeal intervals, estimated stomach contents and self-reported hunger with univariate and multiple correlation techniques. Human feeding was found to be regulated on the basis of preprandial factors; the premeal interval, estimated premeal stomach content, and self-reported hunger significantly correlating with the meal size. These correlations were significant regardless of whether the meal was evaluated as spontaneous or constrained by external factors, but were stronger when the meals occurred without other people being present. The postprandial relationship between the meal size and the duration of the postmeal interval, which is present in nonhuman species, was only present in humans when meals that were eaten alone were considered separately. This suggests that the species differences may be due to the social context of observation. The intake of carbohydrate, fat, and protein was found to have a suppressive effect on subsequent intake and subjective hunger through their contributions to total food energy ingested. Protein, however, was found to suppress subsequent intake and subjective hunger independent of its contribution to total calories, suggesting that the macronutrients have differing satiating properties.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of a noxious taste in determining food intake in the rat

Adult female rats were given access to cornstarch, fat, and 2 cups of casein ad lib. Sucrose octaacetate was added to one or both casein cups in concentrations of 0, 0.1, or 2.5%. The lower concentrations of SOA had no effect on total caloric intake or food choices, although a taste aversion experiment indicated that the rats could detect SOA at this level. The 2.5% concentration had no effect on total caloric intake. If only one casein cup was treated with SOA, the animals consumed 70% of their protein from the unadulterated casein supply, but total protein intake was unchanged. If both cups were treated with 2.5% SOA, protein intake decreased, but carbohydrate consumption showed a compensatory increase. Fat intake was unchanged.     

Hepatic vagotomy disrupts somatotropin-induced protein selection

Rats treated with somatotropin (STH) and allowed to self-select between diets varying in protein content will consume more of the high-protein diet. The objective of this study was to determine the role of the hepatic vagus nerve in this ability to select protein. Female Sprague-Dawley rats (n=40) received a hepatic vagotomy (HVAGX) or a sham surgery. Postsurgery, the rats were maintained on pelleted diets for 2 weeks, after which the rats were adapted to selecting between powdered diets with 5% casein and 30% casein. After a 7-day adaptation to diet selection, rats in each surgical treatment group were treated with STH (4 mg/day) or physiological saline for 14 days. Body weight and intake were recorded daily. STH treatment increased growth rate to a similar degree in both sham and HVAGX groups. Despite causing an increase in total food intake, there was no effect of HVAGX alone on body weight. Relative to the sham-saline group, sham-STH in treated rats had greater total food intake that was accounted for entirely by increased consumption of the 30% protein diet and no change in intake of the 5% diet. In contrast, HVAGX+STH rats exhibited 20-30% increases in consumption of both the 5% and 30% protein diets. Thus, the HVAGX+STH rats recognized an increased need for protein, but were unable to distinguish between the high- and low-protein diets and selected more of both. The data suggest that the ability to alter diet selection in response to a stimulation of protein accretion is at least partially mediated through the liver and hepatic branch of the vagus nerve.     

Dietary self-selection vs. complete diet: body weight gain and meal pattern in rats.

Food intake and body weight gain of male adult Wistar rats were examined in two groups of animals. One group (n = 14) was allowed to select its diet from separate sources of protein (casein, 3.1 kcal/g), fat (lard and sunflower oil, 7.9 kcal/g) and carbohydrate (CHO, starch and sucrose, 3.3 kcal/g). Another group (n = 10) received a nutritionally complete diet (3.3 kcal/g). After 2 weeks of adaptation to the diets, body weights and meal patterns were recorded for at least 4 days. The total caloric intake was nearly identical for the two groups of rats. Rats given dietary choice gained less weight over 4 days than rats fed chow and showed reduced feed efficiency. During the 24-h period, self-selecting rats consumed 20.8% of calories as proteins, 21% as fats and 58.2% as CHO. Self-selecting rats ate significantly less calories during the day than did rats given chow. The chow diet consisting of 17.3% calories as protein, 7.6% as fat and 75.1% as CHO. When comparing the self-selecting group nutrient intakes to those of chow-fed group it was observed that 24-h protein calorie intakes were identical in both groups. Fat intake was significantly higher and CHO reduced as compared to chow-fed rats. During the day, CHO intake was higher in self-selecting rats, and fat intake was not significantly reduced. During the night, protein and fat intakes were significantly higher in self-selecting rats, while CHO intake was significantly decreased, particularly in the last periods of the night.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dietary protein on food and water intake in spontaneously hypertensive rats

We have been studying the development of hypertension in spontaneously hypertensive rats (SHR) fed a low protein diet. The effects of a low protein diet upon food and water intake were examined. Body weight gain, food and water intake were measured in three to twenty-three week-old SHR and Wistar Kyoto rats (WKY) fed diets containing 8%, 15% or 25% casein. Body weights of SHR and WKY fed an 8% casein diet were significantly lower at 23 weeks than rats on the higher protein diets, although both groups on the 8% diet consumed more food and water per g of body weight. In addition, SHR fed an 8% casein diet drank less water per gram of food than WKY or SHR fed 15% and 25% casein diets. These results indicate that changes in food and water intake, as a consequence of low protein diets, should be an additional consideration when examining the effects of dietary protein on the development of hypertension.     

Overeating,dietary selection patterns and sucrose intake in growing rats

Weanling rats were allowed access to food for either 5 hours or 24 hours per day. Within each food availability condition one group had access to a complete diet and one group had access to the complete diet and a 32% solution of sucrose. Caloric intake and rates of growth were considerably higher in the 24 hour access groups. The availability of sucrose led to a small (10–15%) but consistent elevation in caloric intake in the ad lib condition but did not influence growth in either food availability condition. Absolute levels of sucrose intake and the proportion of calories taken from the sucrose solution were consistently higher in the ad lib group and increased with increases in body size for both groups. Dilution of the chow component or the sucrose component of the diet did not alter dietary selection patterns in either food availability condition. It appears that access to a palatable carbohydrate solution can lead to overeating in the rat but these solutions do not induce the rat to select imbalanced diets that compromise growth.     

A carbohydrate diet rich in sucrose increased insulin and WAT in macronutrient self-selecting rats

In order to evaluate the influence of a carbohydrate (CHO) diet rich in sucrose (37%) on food choice and body composition, Wistar rats received a food selection diet (protein, CHO, fat) from the time of weaning to 13 weeks of age. Three groups of animals were examined: the first received a CHO diet containing 37% sucrose; the second, a diet containing only 10% sucrose; and the third, control group, received a complete standard diet (14% protein, 72% CHO including 10% sucrose). Food intakes and body weight (BW) were recorded four times a week. No differences in total food intake were observed between the two self-selecting groups and the control group, and no differences were observed between the two self-selecting groups in terms of their protein intake (about 45% of the total calorie intake). The latter groups modified their selections during the 10-week period, but the variations were similar. BW gain in the 37% group was lower but the white adipose tissue (WAT)/total BW ratio was significantly higher than those seen in the control and 10% groups. Insulinemia was higher in 37% and control groups. In conclusion, the high preferences for protein and fat were identical, whatever the CHO diet composition. The sucrose level in the diet was an essential factor for the development of hyperinsulinemia, leptin resistance and thus a higher prevalence of obesity. These results confirm the importance of the quality of CHO sources in the diet.     

Food selection during recovery from protein restriction in lactation.

Lactating rats were fed a 12% or 25% casein diet. After the pups were weaned on Day 21, dams were given ad lib access to three food cups containing protein, carbohydrate, and fat. Both groups were hyperphagic for three to four weeks. Previously, protein-restricted rats consumed more protein than controls during Weeks 2 and 3, but protein intake returned to control levels as body weight recovered.     

Effect of type of protein on food intake of rats fed high protein diets

The effects of type of protein on intake of a high protein diet after adapting rats to a low protein diet were examined in rats trained to eat a 5.2% (N X 6.25) protein diet containing a mixture of casein, lactalbumin, egg white and soy protein, in a single 3-hour period per day. Food intake was measured from 0-15, 15-30, 30-90, and 90-180 minutes. After a 2-week adjustment period, rats were presented with a 40% (N X 6.25) protein purified diet containing only one of the 4 proteins mentioned above or a mixture of these 4 proteins. During the first 15-minute interval, rats eating diets containing protein mixture, lactalbumin, egg white or soy protein depressed their intake significantly compared with the average intake of the 3-day pre-test period, whereas rats eating casein diet increased their intake. During the last 90-minute interval of the first day, all rats depressed their intake, those rats eating casein the least and those rats eating egg white the most. On the second day, rats offered lactalbumin depressed their intake 52.5% for the 3-hour period and rats offered casein depressed their intake 34.3%. Rats eating soy protein, egg white and protein mixture increased their intake from day 1 to day 2. These experiments show that type of protein affects rats' initial intake when they are offered a high protein diet.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protein selection, food intake, and body composition in response to the amount of dietary protein

Though not universally observed, moderately low-protein diets have been found to increase caloric intake and body fat. It appears that animals overeat in calories in order to obtain more dietary protein. For animals to control protein intake, they must be able to distinguish between two isocaloric diets containing different percentages of protein and make the appropriate dietary selection on the basis of their previous history of protein intake. Experiment 1 examined the 24-h diet selection (5 vs. 35% casein) of Sprague-Dawley rats that had been previously fed diets containing various percentages of dietary protein (5, 10, 20, 35, or 60% casein). Animals fed 5, 10, or 20% dietary protein showed a preference for the higher protein selection diet. In contrast, no significant diet preference was found in animals pre-fed the two higher levels of dietary protein (35 or 60% casein). In this study, daily food intake and body fat of rats fed the low-protein diets (5 and 10% casein) were similar to rats fed the 20% casein diet. Experiment 2 examined the effects of the level of methionine supplementation on rats fed 10% casein. In this study, food intake and body fat were increased by approximately 20% in rats fed 10% casein diets, regardless of the level of methionine supplementation (0.3 vs. 0.15%). Together, the results suggest that the presence of low-protein-induced hyperphagia helps maintain body protein levels in the face of moderately low dietary protein and promotes an increase in the amount of body fat and energy.     

Selective effects of CGS 10686B, dl-fenfluramine or fluoxetine on nutrient selection

We compared the effects of CGS 10686B (a new drug that blocks serotonin reuptake), on nutrient selection and total food consumption with those of two other serotoninergic drugs, dl-fenfluramine and fluoxetine. The animals were given simultaneous free access to two isocaloric 40%-carbohydrate diets in separate food pans; one of these diets (5% protein) was shown to enhance brain serotonin synthesis by raising brain tryptophan levels; the other (45% protein) did not. CGS 10686B (4-7.5 mg/kg) markedly decreased (60-70%) consumption of the 5% protein diet, with a smaller effect (20-30%) on consumption of the 45% protein diet. Hence, it increased the ratio of protein to carbohydrate in the total food consumed. Higher doses (12.5-15 mg/kg) were no longer nutritionally-specific. Fluoxetine, which also blocks serotonin reuptake, and dl-fenfluramine, which both releases serotonin and suppresses its reuptake, had similar effects on nutrient intake; dl-fenfluramine was most potent and fluoxetine least. None of the drugs selectively affected carbohydrate or protein intake if the composition of the test diets provided was such that neither diet, eaten alone, would increase brain serotonin. These observations affirm that drugs which enhance serotoninergic neurotransmission selectively suppress the intake of high-carbohydrate, low-protein meals which increase brain serotonin synthesis.     

Effects of hippocampal lesions on adaptive intake of diets with disproportionate amounts of amino acids

Bilateral double electrolytic overlapping lesions were placed in dorsal-lateral hippocampus of male 230 g rats, and their food intake responses to the ingestion of diets containing disproportionate amounts of amino acids were examined. Rats with such lesions and intact control rats maintained their normal intakes of the 6% casein basal diet or a threonine basal amino acid diet postoperatively. However, they exhibited marked initial food intake depression, similar to that of intact rats, when fed the threonine imbalanced amino acid diet. Also, animals with lesions in certain areas of the dorsal-lateral hippocampus showed facilitated adaptation to the amino acid imbalanced diet. Similar severe reduction in food intake with relative lack of adaptation were observed in both the intact controls and rats with hippocampal lesions when fed amino acid diets completely devoid of threonine. Initial food intake of rats with hippocampal lesions was inhibited drastically as was the case with the intact controls when fed a 75% casein high protein diet. All rats, either intact or lesioned, showed similar slow adaptation patterns with the prolonged ingestion of the high protein diet. The initial food intake responses and facilitated adaptation of the animals bearing lesions in certain areas of the hippocampus suggest that such areas are not crucially involved in the inhibition of food intake of rats fed disproportionate amounts of dietary amino acids. Rather, such areas of lesions in the hippocampus may play a role in a system governing the behavioral adaptation of the intake of amino acid imbalanced diets but not of diets containing amino acids in general excess. This would also indicate that different mechanisms control the intake of amino acid imbalanced diets and diets containing amino acids in excess.