Dietary selection for lysine by the chick

Broiler chicks were provided choices of synthetic diets (a) adequate or low in lysine, and (b) adequate in or devoid of lysine. In each case, chicks consumed some of each diet offered, but preference was shown for the adequate lysine diet. Growth rates of chicks given choices ranged from 80% of that of chicks fed an adequate lysine diet with no choice for two weeks, then growth rates fell to about 60% of those fed adequate lysine. In another study, chicks were fed a diet devoid of lysine but were offered pure L-lysine HCl in a separate feeder. These chicks selected some of the supplementary lysine, but their body weights were only 68% of the body weight of chicks fed an adequate lysine diet after 21 days. Chicks given a choice of diets prepared with an adequate quantity of either L- or D-lysine preferred with L-lysine diet but did not select sufficient quantity to reach normal growth. These observations indicate that chicks can discern the presence of L-lysine in diets or separately, but will not select sufficient quantity for maximum growth potential. A diet prepared with D-lysine was more acceptable than one completely devoid of lysine, suggesting some sensory recognition for lysine.     

Dietary self-selection and the regulation of protein and energy intake in chicks

Studies were conducted to determine whether chicks could regulate their protein intake independent of total energy intake in self-selection feeding trials. Day-old White Mountain cockerels were reared in electrically-heated battery brooders and given access to either a 23% protein control ration (no choice) or two diets containing 10% or 60% protein with or without supplemental amino acids. The latter were added to either improve the dietary amino acid balance or to alter plasma and brain levels of free large neutral amino acids (tryptophan, isoleucine, leucine, valine, phenylalanine, and tyrosine) which have been implicated in the neuroregulation of feed intake. Both feed and water were provided ad lib, and the location of the feed troughs within each pen were changed daily. Body weights and feed intakes were measured daily, and total calorie and protein intakes were calculated. Chicks offered 10% and 60% protein diets with no supplemental amino acids exhibited reduced weight gains and markedly higher protein intakes as compared to birds fed either the control ration or those given a choice between 10% and 60% protein diets supplemented with methionine. The higher protein consumption by chicks fed the unsupplemented diets most likely was a result of an attempt to compensate for a dietary methionine deficiency. Chicks fed the 10% and 60% protein diets supplemented with amino acids grew at a slower rate than those fed the 23% protein control diet. In general, plasma and brain data did not support a proposed relationship between certain large neutral amino acid ratios and protein or energy intake.     

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.     

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.     

Ambient temperature effects on energetic relations in growing chicks

The effects of different ambient temperature conditions on the diet selection, intake, growth, body temperature, and activity of immature domestic chicks were assessed in two experiments. In Experiment 1, the ambient temperature either remained warm during both the light and dark phases of the photoperiod, as is characteristic in laboratory settings, or was warm during the light phase and cold during the dark phase. The latter condition reflects the daily temperature pattern in natural settings. Chicks exposed to low nocturnal ambient temperatures had lower body temperatures in both phases of the photoperiod, were less active, ate more, selected a higher percentage of carbohydrate in their diets, and grew faster but were less feed-efficient than warm-reared controls. In Experiment 2, the ambient temperature was either cool in both phases of the photoperiod or cool in the light phase and warm in the dark phase. Chicks reared continuously in the cold had lower body temperatures, selected a high-carbohydrate diet, and grew faster, but both rearing groups were relatively inactive. These results show that an animal's body temperature, diet composition, food intake, feed efficiency, and activity reflect its 24-hr energy requirements and are a part of a general strategy of maximizing energy income and minimizing energy expenditure in response to energetic challenges to growth.     

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.     

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.     

Sparrows discriminate between diets differing in valine or lysine concentrations

White-crowned Sparrows (WCS) were given free access to pairs of semisynthetic diets that were either adequate or subadequate (25% of requirement) in valine or lysine. Within 2 to 4 days WCS chose a ratio of the paired diets that allowed them to maintain body mass or restore any losses quickly. On the initial choice days the birds transiently reduced total daily food intake (TDFI) roughly in proportion to their intake of the subadequate diet. The initial decrease of TDFI was greater and the latency in choosing an effective ratio of the paired diets was 2-3 days longer with valine than with lysine diets in well-nourished test birds. In malnourished birds fed only the subadequate test diet for 3 days, valine-deficient birds increased TDFI and body mass more promptly than did lysine-deficient birds when offered a choice of adequate and subadequate diets. The form of the test amino acid (CAA = crystalline, PB = protein-bound) had little effect on choice behavior, but sudden transfer of WCS from a PB acclimation diet to test diets with a large total CAA concentration increased the latency of effective choice by 2-3 days. A brief acclimation (2-3 days) to a CAA diet precludes any bias between nutritionally equivalent CAA and PB diets. The small differences in choice dynamics between valine and lysine and between dietary forms may help to identify mechanisms involved in food choice but are probably ecologically insignificant to free-living WCS. These birds are very adept at selecting diets that satisfy their amino acid requirements.     

Purified diets: some cautions about casein

Growth and intake of weanling rats on five casein-based purified diets and matched cereal-based diets were compared. The results support Lát's conjecture that the failure of animals to exhibit normal growth on some purified diets may be attributed to their failure to consume sufficient casein. Given a choice, the rats consumed both types of food, composing a diet in which the ratio of purified to cereal-based diet was 2:1, and they grew normally. Although the growth-promoting characteristics of caseins from different sources varied, the preference ratio between casein- and cereal-based diets was the same with one exception: a reduced preference for purified diet was shown by animals who had previously experienced subnormal growth for an extended period of time on a purified diet. These results show that all caseins are not equivalent in their growth-promoting properties. Further, they show that rats are exquisitely sensitive to the nutritional properties of their diets and, given the opportunity, can compose a diet which yields normal growth.     

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.     

Restriction of amino acids extends lifespan in Drosophila melanogaster

Dietary restriction extends adult Drosophila melanogaster life span when the concentration of dietary yeast is diluted in a media with abundant carbohydrates. Here we vary the concentration of casein as a source of amino acids in adult diet to uncover a quality of nutrient yeast responsible for longevity control. Longevity is maximized upon diet with intermediary levels of casein. Differences in survival are not caused by elevated age-independent mortality; the longevity maximum at intermediate casein does not arise because casein is non-specifically harmful at higher concentrations. Furthermore, fecundity increases when the level of dietary casein is elevated. The demographic phenotypes of adult Drosophila maintained on intermediate levels of casein resemble their response to limited dietary yeast. Dietary restriction through dilution of yeast may extend longevity because this limits the intake of amino acids.     

Avoidance of GABA-containing diets by olfactory bulbectomized rats

In order to test the importance of olfaction in the avoidance of GABA-containing diets by intact rats, olfactory bulbectomized and sham-operated control rats were allowed to choose between a low protein control diet and this diet supplemented with 2.5% GABA; the specificity of the response was examined by also testing for responses to other amino acids. Both groups of rats markedly avoided the GABA diet initially; the bulbectomized rats later tended to increase their intake of this diet. The bulbectomized rats chose similar amounts from the control diet and one supplemented with alanine whereas the control rats ultimately strongly preferred the latter diet. Both groups only moderately avoided a threonine-supplemented diet. The results suggest that (1) the odor of GABA is not critical in the avoidance of diets containing this amino acid; and (2) patterns of food selection from diets containing GABA differ from choices from diets containing other small neutral amino acids such as alanine or threonine.     

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.     

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.     

The economics of food choice in chicks

Growing chicks offered diets containing either surfeit or inadequate protein in the day but adequate protein at night adopted a pattern of nocturnal feeding but did not eliminate diurnal meals. Their weight gain surpassed that of corresponding dietary controls who received continuous access to surfeit or inadequate protein but was less than that of chicks with continuous access to a standard diet. Chicks receiving the standard diet at night with no food available in the light phase also grew but did so at a slower rate than the diurnally feeding control group. The shift in feeding patterns was not accompanied by a shift in body temperature, nor was nocturnal antipredator behavior, characterized by sustained motoric inhibition, disrupted by nocturnal feeding. These data demonstrate that feeding specializations which have been selected over a species' evolutionary history are not rigidly fixed but can be modified by the economic relations in the current habitat.     

Dietary self-selection and intake of protein and energy is altered by the form of the diets

The technique of dietary self-selection has been widely used to investigate proposed mechanisms of protein and carbohydrate regulation and to substantiate theories relating nutrients, brain chemistry and behaviour. These experiments have produced conflicting results which might be accounted for by differences in experimental variables or in the different techniques used to measure self-selection. The possibility that the sensory qualities of diets might alter dietary self-selection was examined in three groups of rats self-selecting protein and carbohydrate from pairs of isocaloric diets in which the physical form (finely-milled casein/corn starch powder, granular casein/corn starch powder and gelled finely-milled casein/corn starch), but not the nutrient composition of each pair was varied. In addition, since the serotonergic drug fenfluramine has been frequently used to influence dietary selection, the effects of this drug on the selection of protein and carbohydrate from the three diets were also examined. The selection of particular diets and consequently protein and carbohydrate intake was markedly altered by the form of the diet. Furthermore, the effects of fenfluramine on diet selection were also modified significantly by the form of the diet. These results suggest that intake of certain nutrients is not strictly regulated at a particular fixed value and that drug-induced changes of self-selection behaviour must be interpreted cautiously.     

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.     

Macronutrient self-selection through demand-feeders in rainbow trout

The objective of this research was to test the ability of rainbow trout to feed on, and select from, three "pure" macronutrient diets formulated to contain only one macronutrient (protein, fat, and carbohydrate). The three diets were offered to fish in three different self-feeders, the fish having to choose from them in order to compose a nutritionally balanced diet. A total of 12 trout (115 g initial body weight) were held individually in 57 liter tanks at a constant 14 degrees C. The results demonstrated that the trout were capable of composing their own complete diet, with a higher preference for protein than fat and carbohydrate. Food intake was regulated to balance energy intake and maintain steady growth. Trout showed a strict diurnal feeding rhythm, which free run under constant light with an endogenous period ranging from 18.2 to 27.7 h. These results provide the first insight into macronutrient self-selection in trout and valuable information on their feeding preferences, which may be taken into consideration when investigating fish nutrition, feeding regulation, and the design of adequate diets.     

Observations on some changes during acute S. gallinarum infection in chicks

Haematological, serological, morbid anatomical and bacteriological observations were made daily for the 9 consecutive days that followed oral infection of chicks aged 15 days with Salmonella gallinarum. The pathological changes among chicks fed with a nutritionally adequate diet containing only fish meal as the protein supplement were similar to, but in some respects more marked than, those in chicks fed with a corresponding meat-meal diet. The gradual development of a microcytic anaemia in all the infected animals probably indicated an alteration in iron absorption or metabolism in the acute stages of the infection.     

Effects of dietary nutrients and foraging costs on meal patterns of rats

Rats were fed either a cereal-based or a purified casein-based diet in a foraging paradigm in which the costs of procurement and consumption were varied. The group offered the cereal-based diet consumed about 10% more calories than the group offered the casein-based diet, but both groups grew at the same rate. The intake of a control group offered a choice between the two diets was approximately 80% from the casein diet, and the growth of this group did not differ from that of the experimental groups. Variations in the cost of procurement and the cost of consumption affected the patterning of meals differentially for the two diets: changes in meal patterns tended to control the time and/or energy spent feeding. These results show that (1) meal patterns in the foraging paradigm are sensitive to subtle differences in diets, and (2) the amount of diet consumed (acceptance) and the choice between diets (preference) are determined by the economics of feeding and the nutritive quality of the foods, as well as by their palatability.