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.     

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.     

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.     

L-tryptophan injection fails to alter nutrient selection by rats

Nutritional studies on rats given a choice between two diets differing in protein content have led to the proposal that brain 5-HT content regulates protein intake [2]. Pharmacologic studies under similar conditions of dietary self-selection suggest that brain 5-HT controls carbohydrate intake [41]. We tested the effect of elevating brain 5-HT via tryptophan injection (100 mg/kg) on short-term food intake and selection by rats choosing between two diets differing in protein and carbohydrate content. Under these conditions neither total food intake nor protein and carbohydrate selection were affected despite increases of 50% in brain concentrations of 5-HT and 5-HIAA. The effect of Trp administration was selective to serotonin metabolism as brain concentrations of NE, DA and DOPAC were not affected. These results suggest that alterations in brain 5-HT content which may occur following meal ingestion may not be of physiological importance in regulating nutrient intake and selection.     

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.     

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.     

Dietary protein level alters gentamicin-induced nephrotoxicity in rats

Aminoglycosides (AG) such as gentamicin are antimicrobial drugs widely used in the hospital setting due to their efficacy in the treatment of severe gram-negative bacterial infections. However, all AG have the potential to cause nephrotoxicity. Two studies have been conducted (1) to assess the protein level of a diet that would give the best renal outcome with gentamicin administration, and (2) to get a better idea about the rhythms of food ingestion associated with the different protein levels. Adult female Sprague–Dawley rats fully adapted to a standard chow diet, the standard chow with 20% or 55% added casein were chronically treated for 10 days with a nephrotoxic dose of gentamicin sulfate (40 mg/kg/day, i.p.) or a saline solution. Food ingestion patterns of rats were recorded every hour using a Diet Scan system and gentamicin nephrotoxicity indices were measured. The second study used rats that were fed the same diets and given a sham injection. Corticosterone was assayed to quantify the stress of the animals. Results showed that chronic gentamicin treatment leads to a decrease in food intake and flattening of the rhythms of food ingestion. Also, chow feeding and the 20% casein diet were found to be more protective against gentamicin-induced nephrotoxicity than the 55% casein diet. Therefore, while a protein-rich diet can be protective against gentamicin-induced nephrotoxicity, the present study demonstrates that a diet too high in protein might rather be harmful to the kidneys.     

Self-selection of dietary casein and soy-protein by the cat

Growing specific-pathogen-free kittens were fed for two weeks a choice between two complete diets differing only in protein content. When casein diets containing 18, 36 and 54% protein were offered in the three possible combinations, the kittens consistently avoided the higher casein diets and kittens offered the two highest levels of casein significantly reduced their total food intake. In one soy-protein choice study, 16, 31 and 63% protein diets were each offered with a protein-free (PF) diet. When diets were similar in physical consistency, kittens selected similar amounts of both diets with the result that the PF:16% group consumed below their requirement of protein. In another soy-protein experiment the 16, 31 and 63% protein diets were offered in their three possible combinations. Kittens in all three groups selected similar amounts of both diets. Except for their avoidance of casein, the kittens did not regulate in a consistent manner their intake of protein and therefore, behaved very differently from the rat in the self-selection of dietary protein.     

Cingulate lesions and behavioral adaptation to amino acid imbalanced diets

The effect of anterior cingulate cortex lesions on dietary intake and adaptation of disproportionate amounts of amino acids was examined. Rats with bilateral electrolytic lesions in the anterior cingulate cortex and sham-operated rats were fed, in turn, amino acid basal, imbalanced or devoid diets involving threonine and isoleucine as the growth limiting amino acids, and then a low protein (6% casein) followed by a high protein (75% casein) diet. Lesions of the anterior cingulate cortex did not prevent the initial depression in food intake of the amino acid imbalanced diets, but shortened the duration of anorexia associated with dietary amino acid imbalances. Cingulate lesions did not influence the food intake of rats fed amino acid devoid diets. When switched from a low protein to a high protein diet, animals bearing lesions and sham-operated controls reduced markedly their initial food intake and adapted to the high protein diet in similar manner. It was concluded that the initial food intake depression associated with a dietary amino acid imbalance is a direct response to postingestive cues which influence food intake. Moreover, that the difference in adaptive intakes of the cingulate cortex lesioned animals who ingested a diet of imbalanced amino acids or of high protein, indicates that separate mechanisms act to control food intake of animals fed diets containing imbalanced amino acid mixtures or diets with excessive amounts of protein.     

Effect of amygdaloid lesions on dietary intake of disproportionate amounts of amino acids

Male rats with bilateral electrolytic lesions in the anterior, medial or posterior aspects of the ventral amygdala and groups of intact rats were fed, in turn, basal, imbalanced or deficient amino acid diets involving threonine or isoleucine as the limiting amino acid, and then a low protein (6% casein) followed by a high protein (75% casein) diet. No change in food intake was observed in animals fed the threonine basal diet postoperatively. When the threonine or isoleucine imbalanced diet was substituted for the respective basal diet, animals with lesions in certain areas of the medial amygdala showed little or no depression in food intake of the imbalanced diets, while all other rats with amygdala lesions reduced their food intake markedly, as did intact controls, when fed such diets. All animals, however, curtailed their food intake of the deficient or high protein diets. The lack of responsiveness of the animals with medial amygdaloid lesions to the imbalanced diets suggests that these areas may be involved in a system regulating food intake of animals fed diets containing imbalanced amino acid mixtures.     

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.     

Acquisition of dietary self-selection in rats with normal and impaired oral sensation

To study the mechanisms of dietary self-selection, the dietary choice behavior of rats was observed for up to 5 weeks, beginning on the first day of exposure to two nutritionally different diets, a high-protein diet (44% protein) and a protein-free carbohydrate diet. In Experiment 1 normal rats selected equal amounts of the two selection diets at first and over a 7-day period gradually modified the choice ratio until a stable protein intake of 14.6% (SE,0.4) of total intake was reached. In Experiment 2 rats were subjected to partial trigeminal deafferentation, which impairs oral somatosensory input (touch, temperature, pain), before the two selection diets were introduced. The deafferented animals did not develop a stable selection pattern; their protein ratio varied over the entire possible range (0-44%) throughout the experiment. It is hypothesized that quantitative protein/carbohydrate selection involves an associative learning process in which somatosensory inputs from the feeding activity and/or from the properties of the food link dietary choice behavior to later metabolic consequences.     

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.     

Intact regulation of protein intake during the development of hypothalamic or genetic obesity in rats

Adult female rats made hyperphagic with bilateral lesions of the medial hypothalamus as well as spontaneously hyperphagic adult female rats of the Zucker strain were studied for their abilities to regulate dietary protein and energy intakes. This was accomplished by permitting both models of animal obesity to select freely between two isocaloric diets containing 10% vs 60% by weight casein (protein) for at least one month. Compared to appropriate control groups, both experimental groups became obese by consuming more of the low protein diet and less of the high protein diet. The overall effect of this selection pattern was an excessive intake of energy in the form of carbohydrate and fat while maintaining control levels of protein intake. These data imply that the stimulus for hyperphagia in both animal models of obesity is some physiological and/or behavioral error in energy regulation but not protein regulation.     

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.     

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.     

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.     

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)     

Responses to protein dilution in the adult female rat

Adult female rats were given ad lib access to 3 food cups containing casein, cornstarch, and a fat mixture, or a composite diet formulated from these nutrients. One-half of the animals in each diet condition were subjected to 25, 50, 150, and 500% sequential dilutions of the casein or the composite diet. Self-selecting rats decreased intake of the diluted casein, but increased consumption of cornstarch, so that total caloric intake remained constant. Composite-fed rats compensated for dilution by increasing the volume of diet consumed, but caloric regulation was less precise than in self-selectors.     

Maternal diet affects feeding behaviour of self-selecting weanling rats

Weanling rats have been shown to regulate protein intake when provided the opportunity. The objective of these studies was to determine the influence of protein concentration in the maternal diet on protein selection of the offspring. Dams were fed single diets containing 10, 20, 30, or 40% casein throughout gestation and lactation. Pups from all groups were then allowed to select from diets containing 10 and 60% casein for two weeks. Protein selection in the weanling rat was found to correlate with protein concentration in the maternal diet expressed either as g/100g BW (r=0.48, p<0.05) or protein concentration (r=0.65, p<0.01). In order to differentiate between a gestational and lactational effect a cross-over study was performed. Pups from dams fed 10% casein during gestation and/or lactation selected 17% protein concentration, lower than the 25.1% selected by pups from dams receiving 40% casein through gestation and lactation. Therefore, feeding behaviour, as indicated by protein selection, is influenced by maternal diet during both gestation and lactation. The use of the self-selection paradigm has been proposed as a behavioural model for the investigation of the functional significance of maternal nutrition.