Dietary modulation of oral amphetamine intake in rats

The interaction of sucrose availability and oral self-administration of amphetamine was examined in 23 male Sprague-Dawley rats. Fourteen rats were given a 0.075 mg/ml amphetamine sulfate solution as their sole source of fluid and 9 rats were given water. Feeding conditions were alternated between weeks with both granulated sucrose and chow available and weeks with only chow present. Rats drank significantly less of the amphetamine solution when consuming sucrose and chow than when eating chow alone. Sucrose intake had a slight effect on water intake. Rats drinking the amphetamine solution consumed significantly less food, gained significantly less weight, and were significantly less efficient at using calories for weight gain than rats drinking water. However, when given access to sucrose, rats drinking the amphetamine solution chose a significantly greater proportion of their daily caloric intake as sucrose (60%) than rats drinking water (42.5%). The present results demonstrate that 1) amphetamine intake alters nutrient choice and 2) that dietary variables can profoundly affect drug self-administration.     

Overeating,dietary selection patterns and sucrose intake in growing rats

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

Flavor-cued modulation of intake in rats: role of familiarity and impact on 24-h intake.

Following training with distinctively flavored nutritive solutions that differ in concentration and thus in caloric value, rats demonstrate flavor-postingestive consequence learning by preferentially consuming one of the flavors in two-bottle tests (both flavors in nutrient-identical solutions.) Experiment 1 investigated whether the relative familiarity of the flavor-nutrient combinations encountered in two-bottle tests contributes to the observed preference. One of the training concentrations (rather than the customary intermediate concentration) was used to present the flavors in testing; thus, one of the flavors was in a familiar context while the other was in an unfamiliar context. The results of two independent trials (rats trained with 1 and 5% sucrose; rats trained with 5 and 40% sucrose) confirmed that two-bottle test preference was not a preference for the familiar flavor-nutrient combination. Experiment 2 examined whether caloric expectancies based upon a previously learned flavor-postingestive consequence association would affect total daily intake. On alternating days, rats consumed 30 mL of dilute (5%) and concentrated (40%) sucrose, each distinctively flavored. When given 30 mL of 22.5% sucrose containing each of the flavors on separate test days, they ate less chow and thus fewer total calories over 24 h when given the flavor previously paired with concentrated sucrose. Experiment 3 replicated the design of Experiment 2 except that fat calories were used instead of sucrose; no significant adjustment of chow intake in extinction tests was noted, even when the number of fat calories used in training was increased (Experiment 4). Thus, rats did not exhibit flavor-cued modulation of chow intake when trained with fat, in contrast to responsivity to flavor cues when trained with sucrose. This differential responding to fat versus carbohydrate calories is consistent with previous observations, in a variety of paradigms, that modulation of caloric intake is less energetically appropriate when ingested foods are high in fat relative to high-carbohydrate foods.     

Effect of diet consistency, taste and calories on food intake of weanling rats with dorsomedial hypothalamic lesions

Male weanling Sprague-Dawley rats with dorsomedial hypothalamic lesions (DMNL rats) primarily destroying the dorosomedial hypothalamic nuclei (DMN) showed significant hypophagia on lab chow chunks compared to sham-operated controls. When given a choice between lab chow in the form of chunks or powder, both controls and DMNL rats ate similar amounts of lab chow powder while DMNL rats ate less lab chow chunks. Total caloric consumption was the same as on chunks alone. When returned to lab chow chunks as the only source of calories, the pattern and magnitude of intake was again depressed for the DMNL rats. When offered a choice between Ginger Snaps cookies in chunk form versus powder, DMNL rats remained hypophagic in terms of chunk consumption while the intake from powder was similar in both controls and DMNL rats. When offered a choice between chunks of lab chow and Ginger Snaps, DMNL rats were again hypophagic on lab chow chunks, ate the same as the controls of the cookies, and the total caloric intake was of the same magnitude and pattern as observed in previous tests. The data suggest, but do not conclusively show, that DMNL rats are not hypophagic because they have an aversion to chewing hard food and that, when offered a diet similar in hardness to lab chow chunks i.e., hard cookies, will prefer the less tasty but nutritionally complete lab chow. They are apparently capable of choosing a diet for complete nutrition and, as previously reported, can meter calories competently.     

Caloric regulation in normal-weight men maintained on a palatable diet of concentional foods

The spontaneous food intake of six normal-weight male volunteers was measured for 24 days while the subjects were inpatients on a metabolic unit. They were fed a palatable diet of conventional foods and were kept unaware that their food intake was being measured. On days 7–18 the caloric content of their diet was covertly reduced by 25% by substituting aspartame-sweetened analogues for all menu items containing sucrose. Subjects did not alter their food intake for 3 days. Then between days 4–6 on the aspartame diet, they increased their intake to compensate for 40% of the missing calories. Food intake stabilized at 85% of baseline and remained the same for the rest of the 12-day dilution period. Subjects did not show a shift in either sweetened or unsweetened food choices while their diet was being diluted. In adjusting for the missing calories, they simply ate more of their customary diet. The replacement of sucrose by aspartame tended to curb the weight gain observed on the baseline diet.     

A detailed analysis of sucrose drinking in the rat

The present report represents an initial attempt to examine and quantify the eating and drinking patterns of rats presented with water, laboratory chow, and sucrose solution for 23 hours. The concentration of the sucrose solution was systematically increased (0.10 M, 0.25 M, 0.5 M, 1.0 M) with a single concentration being presented to rats in four-day blocks. As has been previously shown, total intake (ml) of sucrose solution increased with concentration to a peak at 0.25 M and then decreased with further rises in concentration. Calories consumed from sucrose monotonically increased with concentration, reaching a maximum at 0.50 M. As calories consumed from sucrose increased with rising concentration, chow intake monotonically decreased. This compensatory decrease in chow intake was primarily attributable to decreases in nighttime chow consumption when the concentration of sucrose available was less than or equal to 0.25 M; when the concentration was greater than 0.25 M, further reductions in chow intake occurred during the day. Moreover, the decrease in chow intake was due solely to a reduction in the number of chow bouts. As the concentration of sucrose increased, the day-to-night ratio of sucrose intake approached unity. Bout volume increased with concentration to a broad peak spanning 0.25-0.5 M, and then decreased with 1.0 M. Bout duration changed with sucrose concentration such that the bout drinking rate (ml/min) was seen to monotonically increase, reaching a stable maximum at 0.5 M. Since the caloric intake per sucrose bout progressively increased with each rise in concentration, the asymptotic portion of the curve describing calories consumed from sucrose was attributable to alterations in sucrose bout number and not sucrose bout size.     

Sex differences in the effects of voluntary activity on sucrose-induced obesity

Sedentary, adult rats of both sexes fed Purina chow and a 32% sucrose solution overate, gained excess weight and had higher Lee Indexes of obesity than control animals fed only Purina chow. The magnitude of these effects was similar in the males and females. Animals of both sexes fed the sucrose diet showed a slower rate of weight loss during food deprivation than the chow controls. Access to an activity wheel led to a reduction in caloric intake and the elimination of obesity in male rats. In the chow fed male rats activity led to a smaller, transient suppression in caloric intake and a slightly lower level of body weight than the sedentary chow controls. Access to activity did not affect body weight in the female rats in either dietary condition. Rather, both active groups of female rats appeared to compensate for the energy cost of voluntary activity by a small increase in food consumption. Long-term exposure to activity was associated with more rapid weight loss during food deprivation in both males and females. These data reveal that high levels of activity and obesity can co-exist when normal female rats are fed a palatable diet but that activity eliminates this form of obesity in the male rat.     

Dietary self-selection in insulin-injected hamsters

Adult male golden hamsters were given access to a variety of nutrient sources and were observed following the administration of regular insulin. It was hypothesized that if insulin produced hyperphagia in hamsters by the activation of a glucoprivic feeding mechanism, a selective increase in carbohydrate consumption would be observed. All animals received subcutaneous injections of 10, 30, 50 and 100 units/kg of insulin as well as a control injection of saline. Food consumption was recorded at +3, +6 and +24 hours after injections. In Experiment 1 hamsters having continuous access to Purina lab chow, fat (Crisco) and sucrose (sugar cubes) increased their total caloric consumption in response to insulin, but did not do so by selectively increasing their carbohydrate intake. In Experiment 2 hamsters maintained on Purina chow and sugar cubes consistently increased their carbohydrate intake as well as their total caloric consumption in response to insulin, but again the increase in carbohydrate intake was not selective; increased consumption of both sugar cubes and Purina chow occurred, and neither the proportion of total calories derived from carbohydrate nor the proportion of total calories derived from sugar cubes was affected by insulin administration. The results support the conclusion that insulin-induced hyperphagia in hamsters results from the activation of a non-glucoprivic feeding mechanism.     

Differential effects of sucrose and fructose on dietary obesity in four mouse strains

We examined sugar-induced obesity in mouse strains polymorphic for Tas1r3, a gene that codes for the T1R3 sugar taste receptor. The T1R3 receptor in the FVB and B6 strains has a higher affinity for sugars than that in the AKR and 129P3 strains. In Experiment 1, mice had 40 days of access to lab chow plus water, sucrose (10 or 34%), or fructose (10 or 34%) solutions. The strains consumed more of the sucrose than isocaloric fructose solutions. The pattern of strain differences in caloric intake from the 10% sugar solutions was FVB > 129P3 = B6 > AKR; and that from the 34% sugar solutions was FVB > 129P3 > B6 ≥ AKR. Despite consuming more sugar calories, the FVB mice resisted obesity altogether. The AKR and 129P3 mice became obese exclusively on the 34% sucrose diet, while the B6 mice did so on the 34% sucrose and 34% fructose diets. In Experiment 2, we compared total caloric intake from diets containing chow versus chow plus 34% sucrose. All strains consumed between 11 and 25% more calories from the sucrose-supplemented diet. In Experiment 3, we compared the oral acceptability of the sucrose and fructose solutions, using lick tests. All strains licked more avidly for the 10% sucrose solutions. The results indicate that in mice (a) Tas1r3 genotype does not predict sugar-induced hyperphagia or obesity; (b) sucrose solutions stimulate higher daily intakes than isocaloric fructose solutions; and (c) susceptibility to sugar-induced obesity varies with strain, sugar concentration and sugar type.     

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)     

Developmental aspects of sucrose-induced obesity in rats

Daily caloric intakes and body weights were measured from weaning to 70 days of age in male Sprague-Dawley rats given access to either a standard laboratory diet and water, or the standard diet, a 32% sucrose solution and water. Lee index of obesity (3 square root body weight/naso-anal length) and fasting blood glucose levels were determined at 46, 57, and 70 days of age. Animals were sacrificed at 70 days, and body composition analyses were performed. Aniamls given access to the sucrose solution consumed significantly more calories per day than animals given only the standard diet. Sucrose animals took approximately 50 to 60% of their daily caloric intake from the sugar solution. Despite the greater caloric intakes of the sucrose animals, sucrose and control animals did not differ in body weight. While there were no differences in body weights between the two groups, the Lee Index of obesity was significantly greater in the sucrose animals than in controls as early as 46 days of age. Fasting blood glucose levels were significantly lower in sucrose animals than in controls at both 46 and 57 days of age. Direct determinations of body compositions when animals were 70 days of age revealed that animals with access to sucrose had significantly greater percentages of body fat and lower percentages of body protein than controls.     

Liquid diet preference in weanling rats with dorsomedial hypothalamic lesions

Two groups of weanling rats received bilateral electrolytic lesions in the ventromedial and dorsomedial hypothalamic nuclei, respectively. Sham-operated animals served as controls. During 14 days post-operation, the intake of standard laboratory chow was measured. The rats then received, in addition to laboratory chow and tap water, a sweet, eggnog-type liquid diet for 17 days, during 14 of which food intake from both diets was measured daily. The liquid diet was then withdrawn and for two weeks laboratory chow was again the sole source of calories. During the availability of the liquid diet, both groups of rats with hypothalamic lesions profoundly reduced their intake of laboratory chow but conspicuously increased their consumption of liquid diet. The rats with dorsomedial lesions consumed as much liquid diet as the controls, while on laboratory chow alone they were highly significantly hypophagic compared with the controls. During the availability of the liquid diet the rats with dorsomedial lesions also became obese, but after the return to a period in which laboratory chow was the sole source of calories they lost this extra fat while again being hypophagic.It is suggested that the hypophagia observed in rats lesioned in the dorsomedial hypothalamic nucleus is not due to a deficiency in caloric metering but rather, alternatively, to (a) aversion to dry food, (b) taste preference for a sweet, liquid diet, (c) a motivational deficit or (d) a motor deficit.     

Sucrose-induced hyperphagia and obesity in rats fed a macronutrient self-selection diet

Adult female rats were allowed to self-select their diet from separate sources of fat, protein, and carbohydrate (starch). Other rats were fed a composite diet that matched the nutrient composition chosen by the self-selecting rats (50% fat, 28% protein, 22% carbohydrate) or a low-fat, high-carbohydrate chow diet. Half of the rats in each diet condition were given access to a 32% sucrose solution for 30 days. Sucrose availability increased total caloric intake (approximately 20%) and body weight gain in all three groups compared to control groups not fed the sucrose solution. The selection animals compensated for their sucrose intake by reducing their fat intake, and to a lesser degree, their starch intake; protein intake was the least affected by sucrose availability. The selection rats consumed less sucrose than the chow-fed rats and displayed a smaller increase in weight, relative to controls, than the chow-fed rats. These differences were attributed to the high-fat intake of the selection animals since similar results were obtained with the rats fed the composite diet. In particular, both the selection and composite diets produced mild obesity in the absence of sucrose. The results demonstrate that sucrose-induced overeating and overweight is not an artifact of restraining the diet choices of rats to a pure sugar and a nutritionally complete diet.     

A role for sweet taste: calorie predictive relations in energy regulation by rats

Animals may use sweet taste to predict the caloric contents of food. Eating sweet noncaloric substances may degrade this predictive relationship, leading to positive energy balance through increased food intake and/or diminished energy expenditure. These experiments were designed to test the hypothesis that experiences that reduce the validity of sweet taste as a predictor of the caloric or nutritive consequences of eating may contribute to deficits in the regulation of energy by reducing the ability of sweet-tasting foods that contain calories to evoke physiological responses that underlie tight regulation. Adult male Sprague-Dawley rats were given differential experience with a sweet taste that either predicted increased caloric content (glucose) or did not predict increased calories (saccharin). We found that reducing the correlation between sweet taste and the caloric content of foods using artificial sweeteners in rats resulted in increased caloric intake, increased body weight, and increased adiposity, as well as diminished caloric compensation and blunted thermic responses to sweet-tasting diets. These results suggest that consumption of products containing artificial sweeteners may lead to increased body weight and obesity by interfering with fundamental homeostatic, physiological processes.     

Obesity by choice revisited: effects of food availability, flavor variety and nutrient composition on energy intake

Recent work suggested that the energy intake and weight gain of rats maintained on chow and 32% sucrose solution could be increased by simply offering more sources of sucrose [Tordoff M.G. Obesity by choice: the powerful influence of nutrient availability on nutrient intake. Am J Physiol 2002;282:R1536-R1539.]. In Experiment 1 this procedure was replicated but the effect was not: rats given one bottle of sucrose and five bottles of water consumed as much sucrose as those given five bottles of sucrose and one of water. Adding different flavors to the sucrose did not increase intakes further in Experiment 2. The relative potency of sucrose and other optional foods was studied in Experiment 3. Sucrose solution stimulated more overeating and weight gain than fat (vegetable shortening), and offering both sucrose and shortening did not generate further increases in energy intake. Finally, foods commonly used to produce overeating and weight gain were compared. Sucrose was less effective than a high-fat milk diet, and offering cookies in addition to the milk did not increase energy intake further. The nature of optional foods (nutrient composition and physical form) was markedly more important than the number of food sources available to the animals, and is a better contender as the reason for "obesity by choice".     

Dietary self-selection of golden hamsters in response to acute food deprivation and chronic food restriction

Adult male golden hamsters were maintained on either Purine Rat Chow (Chow diet) or a self-selection diet consisting of high-protein chow, pure fat, and pure carbohydrate (Choice diet). In Experiment 1, animals were deprived of food for single periods of up to 48 hr. Animals on the Chow diet did not increase intake at any time after deprivation; animals on the Choice diet selectively increased their consumption of fat-derived calories and increased their total caloric intake during the first 6 hr of refeeding, but not thereafter. The nature of the diet did not influence the rate at which animals regained weight following deprivation. In Experiment 2, hamsters were placed on food-restriction schedules (access to food either for 1 hr/day only or on alternate days only) until they lost 20% of starting body weight. Chow-fed animals demonstrated little or no change in food intake either during or after food restriction. Hamsters on the Choice diet consumed more calories and lost weight more slowly than did chow-fed animals during 1-hr/day feeding; intake of fat-derived calories was elevated during restriction. Choice hamsters increased total caloric intake only towards the end of the alternate-days restriction schedule. Choice hamsters were hyperphagic following both types of food-restriction schedules, but no increased preference for fat-derived calories was observed. Factors influencing food consumption of hamsters in response to deprivation and restriction are discussed.     

Increased sucrose intake and corresponding c-Fos in amygdala and parabrachial nucleus of dietary obese rats

The intake-excitatory effects of caloric foods are mainly due to the palatable taste and the ensuing positive postingestive effects. Dietary obese individuals are inclined to overeat high caloric foods. However, it is still unclear whether the taste or postingestive reinforcement mainly contributes to the excessive intake by obese individuals. In the present study, we measured 10- or 120-min sucrose solution drunk by dietary obese rats and measured c-Fos expression following 120-min tests in the central nucleus of amygdala (CeA), a forebrain nucleus involved in the hedonic reward and craving, and the parabrachial nucleus (PBN), a taste relay area responsive to positive postingestive effects. Dietary obese rats, compared with those fed normal chow, ingested larger amounts of sucrose solution (0.25 M) in the 120-min test, but not in the 10-min test. In addition, significantly more sucrose-induced c-Fos positive cells were found in the CeA, but much less in the external lateral subnucleus of the PBN of dietary obese rats. Our results demonstrate that increased sucrose intake in dietary obese rats is mainly due to the alteration of postingestive effects. The differences in these postingestive effects in obesity may involve greater positive/excitatory signals in which the CeA may play a role, and less negative/inhibitory signals in which the el-PBN may be involved.     

Drosophila diet restriction in practice: do flies consume fewer nutrients?

Although many studies of Drosophila melanogaster report an effect of diet upon lifespan, it is assumed rather than know that adults maintained on restricted diet acquire fewer nutrients. Diet is restricted in practice by feeding flies a medium where nutrients are diluted and some early reports suggest that flies on diluted medium compensate with increased food intake to consume equal calories on all diets. Here we measure the feeding rate of adult Drosophila upon a yeast-restricted diet that increases survival and reduces fecundity. We directly assessed food intake from the volume of consumed dye-marked medium and from the quantity of marked fecal pellets. Females were longest lived on diet with intermediate yeast concentration but food intake was greatest on diet with abundant yeast. Rather than compensatory feeding upon diluted diet, females increased food intake on the diet with elevated nutrient concentration. Consequently, females on diluted yeast-limited media will consume fewer calories as well as less yeast. To understand the importance of specific nutrients relative to calories as mediators of Drosophila aging, we must directly measure food intake and control for the feeding stimulation of nutrients.     

Quantifying food intake in socially housed monkeys: social status effects on caloric consumption

Obesity results from a number of factors including socio-environmental influences and rodent models show that several different stressors increase the preference for calorically dense foods leading to an obese phenotype. We present here a non-human primate model using socially housed adult female macaques living in long-term stable groups given access to diets of different caloric density. Consumption of a low fat (LFD; 15% of calories from fat) and a high fat diet (HFD; 45% of calories from fat) was quantified by means of a custom-built, automated feeder that dispensed a pellet of food when activated by a radiofrequency chip implanted subcutaneously in the animal's wrist. Socially subordinate females showed indices of chronic psychological stress having reduced glucocorticoid negative feedback and higher frequencies of anxiety-like behavior. Twenty-four hour intakes of both the LFD and HFD were significantly greater in subordinates than dominates, an effect that persisted whether standard monkey chow (13% of calories from fat) was present or absent. Furthermore, although dominants restricted their food intake to daylight, subordinates continued to feed at night. Total caloric intake was significantly correlated with body weight change. Collectively, these results show that food intake can be reliably quantified in non-human primates living in complex social environments and suggest that socially subordinate females consume more calories, suggesting this ethologically relevant model may help understand how psychosocial stress changes food preferences and consumption leading to obesity.     

Role of caloric homeostasis and reward in alcohol intake in Syrian golden hamsters

The Syrian golden hamster drinks alcohol readily, but only achieves moderate blood alcohol levels, and does not go through withdrawal from alcohol. Because the hamster is a model of caloric homeostasis, both caloric content and reward value may contribute to the hamster's alcohol consumption. The current study examines alcohol consumption in the hamster when a caloric or non-caloric sweet solution is concurrently available and caloric intake in the hamster before, during, and after exposure to either: alcohol, sucrose or saccharin. In Experiments 1 and 2, hamsters were given access to alcohol (15% v/v) and water; once alcohol consumption steadied, a bottle containing an ascending concentration of sucrose (99-614 mM) or saccharin (2-10 mM), or water was added. In Experiment 3, hamsters were given access to alcohol (15% v/v), sucrose (614 mM), saccharin (4 mM), or a second water bottle for 14 days. After the second bottle was removed, measurements continued for 14 days. Sucrose exposure suppressed alcohol consumption at concentrations lower in calories than the alcohol solution. Saccharin exposure failed to suppress alcohol consumption. Exposure to sucrose and alcohol but not saccharin decreased food intake. Decreased alcohol consumption in response to a caloric sweetener and decreased food intake during alcohol exposure support that alcohol consumption by the hamster is mediated by caloric content. However, suppression of alcohol intake by a sucrose solution of lower caloric content and the equivalent intake of individual alcohol, sucrose and saccharin solutions support a role for reward value in alcohol consumption.