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.     

Dietary obesity: differential effects with self-selection and composite diet feeding techniques.

Female rats were given access to casein, cornstarch, and fat in separate cups or a composite diet identical in composition to that chosen by self-selecting rats. After 2 weeks, all rats received granular sucrose ad lib in addition to these other foods. Sucrose availability resulted in increased caloric intake and increased body weight gain in composite-fed rats, but not in self-selectors. Self-selectors maintained caloric homeostasis by decreased consumption of all foods initially, but protein intake recovered to control levels by the third week of treatment.     

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.     

Selection of protein and fat by diabetic rats following separate dilution of the dietary sources

Diabetic and normal rats were allowed to select their diets from separate sources of protein, carbohydrate and fat. Following the determination of baseline intakes, diabetic and normal rats received dietary components in which either the protein (Experiment 1) or fat source (Experiment 2) was diluted by 25% or 50% by the addition of cellulose. Diabetic rats failed to maintain protein intake at both dilution levels, but made up for the loss of protein-derived calories by consuming more fat. Diabetic rats maintained fat intake at both dilution levels. Dietary dilutions had no effect on total caloric intakes or body weight gain of diabetic rats. Diabetic status, measured by fasting plasma glucose levels and urinary glucose excretion rates, also was unaffected by diet dilutions. These data suggest that diabetic rats maintain total caloric intake following dilution of either the protein or fat source of their diets, but defend intake of fat-derived calories more readily than protein-derived calories. Normal rats maintained both protein and fat intake at the 25% but not at the 50% dilution level. These findings are discussed in terms of the ability of diabetic rats to solve the metabolic problems associated with their diabetic condition.     

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.     

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.     

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.     

Influence of dietary protein level on protein self-selection and plasma and brain amino acid concentrations

Control of protein intake was studied in young rats that were allowed to choose between either protein-free and 55% casein diets or 15% and 55% casein diets. Animals on the protein-free vs. 55% casein regimen exhibited a lower weight gain, a lower cumulative energy intake and a greater cumulative total protein intake during the 13-day study compared to rats selecting between 15% and 55% casein. The daily average proportion of total food selected as casein by animals choosing between protein-free and 55% casein diets increased from 15% to 38% during the course of the study. In contrast, rats choosing between 15% and 55% casein chose 18-22% of total food as protein throughout the entire study. Long-term protein intake or protein selection did not correlate significantly with whole-brain contents of 5-HT or 5-HIAA. Our results suggest that protein intake is not regulated at a constant proportion of total calories, but is controlled between a minimum level that will support rapid growth and a maximum that, if exceeded, would require the animal to undergo substantial metabolic adaptation. The mechanism controlling protein selection may involve diet-induced changes in the brain content of total free indispensable amino acids.     

Post-ingestive effects of quinine on intake of nutritive and non-nutritive substances

Intragastric intubation of 0.75 g QHCl had no effect on 4 hr food intake, but produced anorexia over a 24 hr period when paired with consumption of a novel sucrose octa acetate (SOA) treated chow. No anorexia developed when a familiar food was available. SOA adulteration alone produced a decrement in food intake that persisted for 4 hr; this effect was independent of quinine intubation. In a second experiment, intubation of 0.375 or 0.75 g QHCl, 0.3 M LiCl, or 0.6 g SOA did not alter 4 hr chow intake, but QHCl and LiCl both produced pica that lasted for 4 hr. SOA and 0.75 g QHCl produced a 24 hr hyperphagia, while LiCl and 0.375 g QHCl produced no change in 24 hr chow intake.     

Effects of monosodium glutamate on eating and drinking behavior in rats

The effect of monosodium glutamate (MSG) on eating and drinking behavior of rats was studied by means of preference tests in free choice situations. Low or high protein casein diets containing up to 3% MSG were selected indifferently compared to plain casein diet in 7 day trials. The acceptability of diets containing 7% MSG, particularly those of low protein level, was significantly reduced. Except for Day 1, the proportional intake (MSG flavored diet intake/total diet intake) of 9% protein--7% MSG diet was consistently lower than 18% protein--7% MSG diet. In brief exposure tests (10 min) solutions of MSG (0.02-8%) were preferred over deionized water. The acceptance of solutions containing higher concentrations of MSG was significantly reduced. The total volume intake of both choices (MSG flavored water and water) was significantly increased in tests using solutions containing 3-8% MSG. In long-term tests (1-14 days), solutions containing 0.05-1% MSG were preferred over water. The acceptance of a solution containing 5% MSG was significantly reduced. For maximum intake of moles of MSG, rats selected solutions which offered 3-5 x 10(-1) M MSG whether the experimental period was 10 min or up to 2 weeks. In a series of two choice preference tests using solutions where Na+ content and pH were kept equal, MSG was preferred over sodium acetate and over sodium glutamate but was less preferred than monosodium aspartate. The results of this study can be explained in terms of sensory quality.     

Dietary fat-induced hyperphagia in rats as a function of fat type and physical form

The influence of dietary fat on food intake and weight gain was assessed by feeding adult female rats diets that differed in the type and form of fat, as well as in the availability of other macro- and micronutrients. Compared to chow-fed controls, the various fat diets increased total food intake by 4% to 27%. Specifically, rats fed chow and a separate source of fat (fat option diet) consumed more fat and total calories, and gained more weight when the fat source was emulsified corn oil rather than pure corn oil or was vegetable shortening rather than corn oil. However, corn oil and shortening had similar effects on caloric intake and weight gain when presented as emulsified gels. Also, pure and emulsified-gel forms of shortening did not differ in their effects on caloric intake and weight gain. Supplementing the vegetable shortening with micronutrients, however, enhanced its hyperphagia-promoting effect. The results of two-choice tests revealed that the rats' preferences for the orosensory properties of the various fat sources did not account for the differential hyperphagias obtained. Rather, it appears that long-term fat selection and caloric intake are influenced primarily by postingestive factors. Fat selection and total intake were determined not only by the fat source itself, but also by the other diet options. That is, rats selected more fat and consumed more calories when chow was the alternative food than when separate sources of carbohydrate and protein were available.     

Refeeding after various times of ingestion of a low protein diet: effects on food intake and body weight in rats

Rats born of protein-deprived mothers were fed on a low protein (LP) diet (5% casein) from weaning. In each time sequence (0, 1, 3, 5, 8 and 16 weeks after weaning), 12 of them were refed on an isocaloric well-balanced diet (18% casein) for 2 weeks. Food intake, body and adipose tissue weights and protein efficiency ratio (PER) were measured in the refed rats as well as in 12 LP rats. At weaning and after one week, refed (RF) rats immediately increased their food intake. This increase was delayed at weeks 3, 5 and 8 occurred during the second week of refeeding only. At week 16, there was a significant decrease during the first week when compared with LP rats. Body weight increased regularly during each refeeding period without any significant augmentation of the proportion of adipose tissue. During all the experiment (except at week 16), PER in the RF group remained high (about 3 g body weight/g protein) during the first week of refeeding, and fell to 2.0-2.5 g/g during the second week. It was particularly significantly greater than that of the LP rats between week 3 and 5 where an important decrease was observed in this group (1.99 +/- 0.36 vs. 3.23 +/- 0.58 g body weight/g protein during the 1-3 weeks period). It appeared therefore that protein restriction during gestation and lactation in dams had no effect on the mechanisms controlling food intake of their offspring at weaning.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Changes in macronutrient selection as a function of dietary tryptophan

It has been hypothesized that the serotonergic system is involved in the regulation of carbohydrate and/or protein intake. Tests of this hypothesis using added dietary tryptophan and diets varying in the ratio of carbohydrate/protein resulted in depressed intakes of high carbohydrate/low protein diets, elevated intakes of low carbohydrate/high protein diets, and a reduction of total caloric intake. The present studies gave rats increased options for adjusting to added tryptophan by providing them with separate sources of protein, carbohydrate, and fat. The results showed the expected decrease in carbohydrate intake, but also increases in fat intake and, to a lesser extent, protein intake. Total caloric intake was conserved. Hypothalamic concentrations of serotonin and 5-hydroxyindole acetic acid indicated increased activity of the serotonergic system. These results lend support to serotonin's involvement in nutrient selection, in that carbohydrate consumption decreased in response to tryptophan loading, but indicate that other nutrients may also be affected. Given the option of altering fat intake, the animals maintained a constant caloric intake despite the reduction of carbohydrate consumption.     

A maternal low protein diet during pregnancy and lactation in the rat impairs male reproductive development

Nutrient restriction during pregnancy and lactation impairs growth and development. Recent studies demonstrate long-term programming of function of specific organ systems resulting from suboptimal environments during fetal life and development up to weaning. We determined effects of maternal protein restriction (50% control protein intake) during fetal development and/or lactation in rats on the reproductive system of male progeny. Rats were fed either a control 20% casein diet (C) or a restricted diet (R) of 10% casein during pregnancy. After delivery mothers received either C or R diet until weaning to provide four groups: CC, RR, CR and RC. We report findings in male offspring only. Maternal protein restriction increased maternal serum corticosterone, oestradiol and testosterone (T) concentrations at 19 days gestation. Pup birth weight was unchanged but ano-genital distance was increased by maternal protein restriction ( P < 0.05). Testicular descent was delayed 4.4 days in RR, 2.1 days in CR and 2.2 days in RC and was not related to body weight. Body weight and testis weight were reduced in RR and CR groups at all ages with the exception of CR testis weight at 270 days postnatal age (PN). At 70 days PN luteinizing hormone and T concentrations were reduced in RR, CR and RC. mRNA for P450 side chain cleavage (P450scc) was reduced in RR and CR at 21 days PN but was unchanged at 70 days PN. Fertility rate was reduced at 270 days PN in RC and sperm count in RR and RC. We conclude that maternal protein delays sexual maturation in male rats and that some effects only emerge in later life.     

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)     

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.     

The comparative effects of abrupt vs. stepwise discontinuation of TPN in rats.

The comparative effects of discontinuing total parenteral nutrition (TPN: caloric ratio of glucose:fat:amino acid = 50:30:20) abruptly or in a stepwise manner on spontaneous food intake were investigated in two studies. Study 1: In 16 rats, TPN was given for 4 days, then stopped abruptly in eight rats. In the other eight rats, TPN was tapered; they received TPN at 75%, 50%, and 25% of their mean daily energy requirements per day for 3 consecutive days, and then switched to normal saline. Total parenteral nutrition induced a significant 60% reduction in spontaneous food intake (SFI) in both groups during the first TPN day. After 4 days of TPN, an 80% decrease in SFI had occurred in both groups. Resumption of SFI was significantly sooner in the abruptly-stopping group than in the stepwise-stopping group. But, in the latter group, there was a significantly greater cumulative caloric intake during the entire study. Study 2: In 32 rats, TPN providing either 100%, 50%, or 25% of their mean daily caloric requirements was given to three groups each of eight rats, for 3 days, then abruptly changed to normal saline; control rats received normal saline throughout. The TPN-induced decrease in SFI was proportional to the caloric density of the solution infused. Three days of 100%, 50%, or 25% TPN infusion led to an approximate 85%, 60%, or 35% decrease in SFI, respectively. Spontaneous food intake recovery was independent of the caloric density of TPN.(ABSTRACT TRUNCATED AT 250 WORDS)     

Caloric regulation and patterns of food choice in a patchy environment: the value and cost of alternative foods

Rats in a laboratory foraging paradigm had 24-hr-per-day access to a feeder where they could search, by completing a fixed number of bar presses, for an opportunity to eat one of a pair of foods differing in caloric density (2.5, 3, 3.5, or 4 kcal/g) and, in Experiment 2, the price of food pellets (10 to 50 bar presses per pellet). The rats could either accept the opportunity, and eat a meal, or reject it in favor of further search. Daily caloric intake was relatively constant. The rats always included both foods in their diet, but, for any particular food, the degree of inclusion in the diet and of acceptance of meal opportunities, the meal size, and the rate of eating were all functions not only of the price and caloric value of that food but also of the price and value of the alternately-available food. The patterns of intake for one food relative to those for the other available food were strongly correlated with the relative rate of calorie intake during consumption of that food compared to the other. Although the rats appeared to be sensitive to the local rates of calorie flow, they did not maximize daily calories consumed per time spent feeding.     

Hypothalamic hyperphagic rats overeat bitter sucrose octa acetate diets but not quinine diets.

Female rats were made hyperphagic with knife cuts or lesions in the ventromedial hypothalamus (VMH) and their aversion to food made bitter by adulteration with quinine or sucrose octa acetate (SOA) was examined. In short-term choice tests VMH obese rats, as well as controls, strongly preferred a 0.1% quinine diet to a 1.0% SOA diet. Yet, in 24 hr intake tests VMH obese rats overate, relative to controls, the 1% SOA diet, but underate the 0.1% quinine diet. VMH rats in both dynamic and static stages also overate SOA diets in concentrations up to 16%. However, VMH obese rats underate a 1% SOA diet when previously fed a 0.1% quinine diet. The results indicate that the VMH rat's finickiness to quinine diets may not be due to bitter taste alone, but may result from toxic postingestive effects of quinine and the development of a conditioned taste aversion.