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.     

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.     

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.     

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)     

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.     

Preference of rats for food flavors and texture in nutritionally controlled semi-purified diets

Preference for nutritionally controlled, semi-purified diets modified by the addition of potent food flavors was determined for Sprague Dawley rats using two-choice diet preference tests. Intake of each food cup was monitored after 1 hr and for each 24 hr period thereafter up to 5 days. Preference was also determined for the flavored diets prepared in three forms differing in texture: powdered, and pellets of two sizes. Rats easily detected minor amounts of the food flavors, and the tests provided a catalog of 12 preferred flavors. Exposure time to the diets altered preference for a minority of flavors; diets initially avoided in the first hour test were likely to become less aversive upon continued exposure. Whether or not a specific flavored diet was preferred, total food intake was not affected during the 5 day period monitored. Rats displayed strong preference for diets of a pelleted texture compared to the same diets in a powdered form.     

Learned preference for the limiting amino acid in rats fed a threonine-deficient diet.

Preference for an odor associated with protein is only seen in protein-deprived rats. We hypothesized that rats depleted of the amino acid threonine (THR) would prefer a flavored solution paired with a THR replete diet. Rats were given a flavored drink paired with THR-deficient (DEV) diet followed by a second flavor paired with a corrected (COR) diet. In choice testing, the animals clearly selected the COR-paired flavor, while control rats preferred the other flavor. This did not, however, differentiate between aversion to the DEV-paired flavor and learned preference for the COR-paired flavor. In subsequent tests, an unpaired flavor was given rather than the DEV-paired flavor. The COR-paired flavor was included in the test as before. In the second and third trials, using either DEV or a less profoundly deficient diet, animals depleted of THR selected the COR-paired flavor to a greater extent than control rats. We conclude that animals deficient in the essential amino acid, THR, show a learned preference for the flavor paired with repletion.     

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.     

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.     

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.     

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.     

Dietary soy protein and isoflavones have no significant effect on bone and a potentially negative effect on the uterus of sexually mature intact Sprague-Dawley female rats

OBJECTIVE: To evaluate the effect of dietary soy protein and isoflavones on bone and the reproductive tract in premenopausal rats. DESIGN: Three-month-old intact Sprague-Dawley female rats (N = 50) were fed diets containing casein, soy protein, or casein with isoflavone extract for 12 weeks. The amount of casein, soy protein, and extract (per kilogram diet) in each group was: (1) 200 g casein (control); (2) 100 g casein plus 100 g soy protein (low soy); (3) 200 g soy protein (high soy); 4) 200 g casein plus 17.2g extract (low extract); and (5) 200 g casein plus 34.4 g extract (high extract). Diet consumption, body weight, uterine wet weight, urinary deoxypyridinoline concentration, and bone mineral density of the femur and lumbar vertebrae were measured. Femur rigidity was evaluated by histomorphometry. The uterus and vagina were studied histologically. RESULTS: Rats in all treatment groups had lower body weights and lower deoxypyridinoline concentrations compared with controls, but none of the differences was statistically significant. There was no significant difference in femur and lumbar bone mineral density, uterine wet weights, or histomorphometry between the control and treatment groups. Histologically, uteri and vaginae were normal in all groups except that 1 of 10 rats in the high-soy group and 2 of 10 rats in the high-extract group showed extensive squamous metaplasia in the uterine gland. CONCLUSION: These results suggest that dietary isolated soy protein and isoflavones have no effect on bone and the vagina during premenopausal period, but may have an adverse effect on the uterus.     

The effect of parathyroidectomy and large doses of cholecalciferol on the ability of rats to adapt to changes in dietary intake of calcium.

1. Adaptation to different dietary levels of calcium was produced by feeding a low (0-2%) calcium diet or one of two high (1-6 or 0-8%) calcium diets for 4 or 6 weeks. Adaptive changes in true and apparent absorption of calcium, apparent absorption of phosphate and urinary excretion of calcium, were observed. 2. Parathyroidectomy performed prior to adaptation did not greatly impair the ability of rats to adapt to different levels of calcium in the diet. The response of the rats to parathyroidectomy was affected by their subsequent dietary history. 3. Six weeks after parathyroidectomy the plasma calcium was significantly higher than it had been immediately post-operatively. This rise in plasma calcium was seen in the rats adapted to the 1-6% calcium diet but not in those adapted to the 0-2% calcium diet. 4. Parathyroidectomy performed after adaptation had taken place did not abolish the adaptive changes. The response of rats to parathyroidectomy was affected by their previous dietary history. 5. Large doses of cholecalciferol given for 8 days after adaptation had taken place increased the absorption of calcium in rats adapted to the 0-8% calcium diet thereby abolishing or reducing the adaptive differences in absorption between these rats and rats adapted to the 0-2% calcium diet. The cholecalciferol increased urinary calcium excretion but did not abolish adaptive differences in urinary excretion of calcium between rats adapted to diets with different calcium levels. 6. It is concluded that parathyroid hormone does not play a major role in mediating adaptation to different dietary intakes of calcium. The possible role of 1-25 dihydroxycholecalciferol is discussed.     

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.     

Combination of soy protein and high dietary calcium on bone biomechanics and bone mineral density in ovariectomized rats

OBJECTIVE: To determine if feeding soy in combination with a high-calcium diet would preserve bone mineral density and biomechanical bone strength to a greater extent than either soy or calcium alone. DESIGN: Rats were sham-operated (n = l0) and fed a control diet (AIN93G containing 0.2% calcium, 20% casein) or ovariectomized (n = 40) and randomized to one of the following diets (n = 10 per group): control, high calcium (2.5% calcium, 20% casein), soy (0.2% calcium, 20% soy protein), or soy plus high calcium (2.5% calcium, 20% soy protein) for 8 weeks. Bone mineral density of femurs and lumbar vertebrae 1 through 6 were measured by dual energy x-ray absorptiometry. Biomechanical strength properties of femurs and the fifth lumbar vertebrae were measured by three-point bending and compression, respectively. RESULTS: The dietary combination of soy and high calcium did not result in higher femur bone mineral density compared with other ovariectomized groups, and there were no differences in femur yield load or peak load among groups. In contrast, soy plus high calcium resulted in a higher (P < 0.05) vertebral bone mineral density compared with all other ovariectomized groups. Vertebral strength was preserved among rats fed either soy plus high calcium, soy, or high calcium whereas the ovariectomized group fed a control diet had lower (P < 0.05) vertebral strength than the sham-operated group. CONCLUSION: Bone mineral density of the lumbar spine was the only bone outcome that significantly benefited from the combination of soy and high calcium compared with soy or high calcium alone.     

Flavor preferences conditioned by sucrose depend upon training and testing methods: two-bottle tests revisited

In confirmation of prior work, rats given one-bottle training with flavored 5% and 30% sucrose solutions (CS5 and CS30) strongly preferred the CS5 when both flavors were presented in intermediate 17.5% sucrose solutions. The CS5 preference has been attributed to a conditioned satiety response to the CS30 flavor, but equal intakes of CS5 and CS30 in one-bottle tests did not support this view. To determine if sweetness differences between training and test solutions contributed to the CS5 preference, new rats were trained and tested with flavored 10% sucrose solutions. One flavor (CS5) was paired with matched intragastric (ig) water infusions (=net 5% solution) and another flavor (CS30) was paired with matched infusions of 50% sucrose (=net 30% solution) during one-bottle training. In two-bottle tests with both flavors paired with an intermediate infusion (25%=net 17.5%), the rats initially showed no overall preference for the CS5 or CS30. Following additional training, the rats significantly preferred the CS30 to the CS5. The intragastric data suggested that a change in sweet taste context between training and testing might have accounted for the strong CS5 preference obtained in the first experiment. This was confirmed in a third experiment in which rats were trained with flavored 5% and 30% sucrose solutions and then given two-bottle tests with both flavors presented either in 5% sucrose or 30% sucrose. Rats tested with 30% sucrose strongly preferred the CS5 flavor, whereas rats tested with 5% sucrose significantly preferred the CS30 flavor. Thus, the outcome of two-bottle flavor preference tests and presumably other tests of conditioned flavor reward may be greatly influenced by the solutions used in the tests. The impact of this variable may be greatest when the training solutions do not substantially differ in their net postingestive reinforcing actions. This appears to be the case with 5% and 30% sucrose solutions because the satiating effect of the concentrated solution tends to counteract its nutrient reinforcing action.     

Dietary soy protein and isoflavones: minimal beneficial effects on bone and no effect on the reproductive tract of sexually mature ovariectomized Sprague-Dawley rats

OBJECTIVE: The present study was conducted to determine the effects of dietary soy protein and isoflavones on bone and the reproductive tract in the absence of the ovary. DESIGN: Three-month-old Sprague-Dawley rats (n = 56) were either sham-operated or ovariectomized and then fed diets containing casein or soy protein +/- isoflavone extract for 12 weeks. The amounts of casein, soy protein, and extract (per kg diet) in each group were as follows: (1) Ovariectomy, 200 g of casein; (2) Ovariectomy+low soy, 100 g of casein + 100 g of soy protein; (3) Ovariectomy+high soy, 200 g of soy protein; (4) Ovariectomy+low extract, 200 g of casein + 17.2 g of extract; (5) Ovariectomy+high extract, 200 g of casein + 34.4 g of extract; (6) Ovary intact, 200 g of casein; (7) Ovariectomy+estradiol-17beta, 200 g of casein. Diet consumption, body weight, uterine weight, urine deoxypyridinoline, and bone mineral density of the femur and lumbar vertebrae were measured. The femur rigidity was evaluated by histomorphometry. The reproductive tract (uterus, vagina, and cervix) was studied histologically. RESULTS: The Ovariectomy group showed significant increases in body weight, diet consumption, and deoxypyridinoline, decreases in uterine weight and bone mineral density, and negative changes in histomorphometry compared with the Ovary intact group. Neither soy protein nor extract diets abrogated these alterations, except for the Ovariectomy+high extract group that showed statistically significant positive changes in histomorphometric parameters. There were no histological differences in the reproductive tract among Ovariectomy, Ovariectomy+soy, and Ovariectomy+extract groups. The estradiol-17beta replacement abrogated ovariectomy-induced alterations. CONCLUSION: Dietary intake of isoflavones by sexually mature ovariectomized rats has a minimal beneficial effect on bone with no effect on the reproductive tract.     

The preference-aversion behavior of rats for nutritionally-controlled diets containing oil or fat

The preference-aversion behavior of Sprague-Dawley rats for semi-purified diets containing various levels of either corn oil, animal fat (lard) or hydrogenated vegetable oil (Crisco) was determined in brief and long-term, two-choice preference tests. After the first day of exposure, rats selected more calories from the control-unadulterated diet than from experimental diets containing either oil or fat. Only in the first hour of exposure (with diets containing either 5% Crisco, 15 or 25% lard) or in the first day (with diets containing 5% Crisco, 5 or 15% lard) was a preference for experimental diets observed. There were significant effects of oil and fat concentration and of exposure time upon preference for experimental diets, such that preference for the experimental diets was reduced as levels of oil or fat and as the time of experiment increased. Changes in diet selection had no major effect on total caloric intake measured as the sum of both experimental and control dietary choices, although total intake on a daily basis tended to decrease during the middle portion (2-4 days) of the study, then recover by the end of the study. The results suggest that postingestional factors related to the dietary oil and fat (rather than the sensory properties of the diets) directed the long-term preference behavior of rats for semi-purified diets adulterated with various levels of either corn oil, animal fat (lard) or hydrogenated vegetable oil (Crisco).     

Dietary composition alters gentamicin-induced nephrotoxicity in rats

Previous studies have shown temporal variations in gentamicin-induced renal toxicity characterized by a peak when administered during the resting period and a trough during the active period. This time-dependent toxicity was also altered according to the macronutrient composition of dietary regimens offered to female rats. In the present study, adult female Sprague-Dawley rats were adapted to semipurified isocaloric diets containing 20% casein or soy-protein (10% fat each) or to a standard chow diet (18.1% mixed proteins; 4.5% fat). The animals were then chronically treated for 10 days with a nephrotoxic dose of gentamicin sulfate (40 mg/kg/day ip) or a saline solution administered in the middle of their resting period (1200 h) or in the middle of their activity period (0000 h). Body weights of rats injected in the middle of their resting period decreased over the last 6 days of gentamicin treatment. Total 12-h light and 12-h dark food intakes were decreased in gentamicin-treated rats. Rats fed the standard chow diet had significantly lower corticocellular regeneration, serum creatinine and blood urea nitrogen compared to those fed the casein- and soy-containing diets. The present study demonstrates that chronic gentamicin-induced renal toxicity varies temporally according to the time of administration and that a mixed protein diet containing a lower fat level can protect against gentamicin-induced nephrotoxicity.     

Increase in plasma ammonia and amino acids when rats are fed a 44% casein diet

Rats were trained to eat a 6% casein basal diet during a 3-hour period per day. They were then fed either the same 6% casein diet or a 44% casein diet for 3 hours. No food intake depression was observed in the rats eating 44% casein diet during the 3-hour period. Plasma ammonia and amino acids and brain amino acids were measured at 0, 4, 12 and 24 hours after presentation of the 6% or 44% casein diets. Plasma ammonia rose to 134 (p less than 0.01) and 110 micromolar (p less than 0.05) in the 44% casein fed rats at 4 and 12 hours, respectively, as compared to 67 and 53 micromolar, respectively, for the 6% casein fed rats. All plasma amino acid concentrations except methionine and glutamate were elevated (p less than 0.05) at 4 hours. In the brain, threonine, glutamine and tyrosine concentrations were elevated (p less than 0.05) at 4 hours after diet presentation. At 24 hours, valine, isoleucine, leucine, phenylalanine, and methionine concentrations were also elevated (p less than 0.05). Because intake of the 44% casein diet decreases the second day of its presentation, as noted in an earlier experiment, the increases in plasma ammonia and its possible entry into the brain as reflected by increased brain glutamine together with changes in amino acid concentrations should be considered collectively among possible metabolic signals affecting intake of high protein diets.