Central administration of ghrelin preferentially enhances fat ingestion

Ghrelin, a brain-gut peptide discovered from the stomach, stimulates growth hormone release, food intake, adiposity, and weight gain. Circulating ghrelin levels are modulated under conditions of positive and negative energy balance, however its effect on macronutrient selection is not known. The present experiment investigates the effect of ghrelin on single and two-diet feeding paradigms in high-carbohydrate (HC) and high-fat (HF) preferring rats. In the macronutrient selection test in which rats were given free access to either high-carbohydrate or high-fat diet, an intracerebroventricular (i.c.v.) administration of ghrelin potently enhanced fat intake over carbohydrate intake in both HC- and HF-preferring rats. In the diet preference test in which rats were given free access to both high-carbohydrate and high-fat diets simultaneously, an i.c.v. administration of ghrelin also preferentially enhanced fat consumption over carbohydrate in both HF- and HC-preferring rats. Intracerebroventricular administrations of galanin and neuropeptide Y enhanced fat and carbohydrate ingestion, respectively. Centrally administered ghrelin enhanced fat ingestion. These results provide further insights for the role of ghrelin in feeding behavior and the development of obesity.     

High-fat diets stimulate transient hyperphagia whereas wet diets stimulate prolonged hyperphagia in Fischer rats

The effectiveness of several different kinds of diets in stimulating hyperphagia in Fischer strain rats was compared. Of three different high-fat diets examined, only one stimulated significant hyperphagia and stimulated weight gain; this diet was high in both fat and carbohydrate. However, this hyperphagia and increased weight gain was transient, lasting less than four weeks. A high-sucrose diet stimulated energy intake for only one week. In contrast, adding water to a high-starch diet or adding saccharin to a wet diet stimulated energy intake and weight gain for at least ten weeks. Once water or saccharin were removed from these diets, hyperphagia subsided or even turned into hypophagia, until body weights approached control levels. The degree of hyperphagia during the first week did not correlate with subsequent hyperphagia or weight gain. These results suggest that wet diets act by different mechanisms than do dry high-fat and high-sucrose diets.     

Dietary fat content affects energy intake and weight gain independent of diet caloric density in rats

Recent considerations of high-fat diet hyperphagia have focused on fat's relatively high energy density as the critical variable which promotes overeating. However, a high-fat (HF) diet has been shown to enhance intake and weight gain relative to a high-carbohydrate (HC) diet when both energy density and palatability are equated [Am. J. Physiol. 269 (1995) R30]. The present studies investigated the generality of this finding across manipulations of diet caloric density, diet physical form, and chow availability. Separate groups of male rats were fed HF or HC at either 2.3 or 1.15 kcal/ml for 16 days; HF feeding enhanced weight gain relative to HC across both levels of energy density. HF hyperphagia also occurred when diets were presented in semisolid (gelled) form, and when chow was available in addition to liquid diet. These findings are consistent with previous observations that an HF diet can enhance daily kilocalorie intake and weight gain at least partly via a mechanism that is unrelated to caloric density.     

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.     

Effect of food deprivation and maintenance diet composition on fat preference and acceptance in rats

High-fat diets typically elicit greater kcal intake and/or weight gain than low-fat diets. Palatability, caloric density, and the unique postingestive effects of fat have each been shown to contribute to high-fat diet hyperphagia. Because long-term intake reflects the sum of many individual eating episodes (meals), it is important to investigate factors that may modulate fat intake at a meal. The present studies used high-fat (hi-fat) and high-carbohydrate (hi-carb) liquid diets (both 2.3 kcal/mL) to assess the effect of hunger level (0 versus 24-h food deprivation) and fat content of the maintenance diet (12 versus 48%) on fat preference (when a choice among foods is offered in a two-bottle test), and acceptance (only one food offered) in male rats. Preference for hi-fat relative to hi-carb (two-bottle test) was enhanced by 24-h food deprivation, and by a high-fat maintenance diet. In contrast, neither deprivation nor maintenance diet composition influenced relative meal size (one-bottle test) of hi-fat and hi-carb: irrespective of test conditions, meal size of hi-fat was bigger than meal size of hi-carb.     

Enterostatin (Val-Pro-Asp-Pro-Arg), the activation peptide of procolipase, selectively reduces fat intake

Valine-proline-aspartate-proline-arginine (VPDPR), the amino terminal pentapeptide of pancreatic procolipase, produced a dose-dependent reduction in food intake when injected intraperitoneally into Osborne-Mendel rats that had been starved overnight. This inhibition of feeding was observed when the rats were fed a high-fat diet but not in rats fed a high-carbohydrate, low-fat diet. At higher doses of VPDPR, the inhibition of feeding was maintained for over 6 hours. An equimolar mixture of the free amino acids had no effect on food intake. In rats adapted to a three-choice macronutrient diet, VPDPR inhibited fat intake but had no effect on carbohydrate or protein intake. This selective inhibition of fat intake was observed in both overnight-fasted rats presented with food and in ad-lib-fed rats at the beginning of the dark-onset feeding period. It is suggested that this peptide may be a feedback signal to regulate the intake of dietary fat.     

Dietary influences on the acute effects of anorectic drugs

The effects of acute administration of d-amphetamine sulfate (0.0, 0.5, 1.0 and 2.0 mg/kg) and dl-fenfluramine hydrochloride (0.0, 1.5, 3.0 and 6.0 mg/kg) on food intake were examined in male Sprague-Dawley rats fed either a high-carbohydrate diet (carbohydrate equaled 65% of total calories) or a high-fat diet (fat equaled 65% of total calories). Animals were given ad lib access to the diets throughout the experiment. Drug injections were given at 0900 on experimental days and food intakes were measured at 1, 3 and 6 h postinjection. Amphetamine led to dose-related decreases in food intake for animals on both diets. The effects of amphetamine were most noticeable at 1 and 3 h postinjection. No differences in amphetamine's effects on food intake were found as a function of diet. Fenfluramine injections also led to dose-related reductions in food intake for animals in both dietary conditions. In contrast to amphetamine, however, fenfluramine led to greater reductions in food intake for rats fed the high-fat diet than for rats fed the high-carbohydrate diet. These data demonstrate that dietary variables must be considered when evaluating the anorectic actions of psychopharmacological agents.     

Effects of 2-deoxy-D-glucose on food intake of rats are affected by diet composition.

The effect of various doses of 2-deoxy-D-glucose (2-DG) on food intake in rats fed either a medium fat diet with a moderate carbohydrate content (MF-rats) or a carbohydrate-free high fat diet (HF-rats) was tested. Injections were given intraperitoneally either in the middle of the bright phase or 1 h after onset of the dark phase. During the light phase 2-DG induced a transient hyperphagia in both HF- and MF-rats, but the hyperphagia was somewhat less pronounced in HF-rats. During the dark phase 2-DG produced a hyperphagia in the MF-rats and a long-term hypophagia in the HF-rats. Since 2-DG elicited feeding in HF-rats during the light phase, the feeding response to 2-DG not only reflects hunger for carbohydrate as previously suggested but also seems to produce hunger for energy.     

Relative contribution of fat, protein, carbohydrate, and ethanol to intestinal hyperemia

The relative contribution of dietary fat, protein, carbohydrate, and ethanol to postprandial intestinal hyperemia was assessed by comparing the vascular and metabolic effects of luminal placement of various solutions prepared from standard high -fat, high-protein, and high-carbohydrate test diets, corn oil, and ethanol in the jejunum of anesthetized dogs. The high-fat diet (45% fat, 18% protein, 29% carbohydrate) produced the greatest hyperemia (+30.4% of control), followed by high-protein (22% fat, 64% protein, 4% carbohydrate) (+24.1%) and high-carbohydrate (8% fat, 18% protein, 68% carbohydrate) (+18.2%) diets. When the fat content of the high-carbohydrate diet was raised to equal that of the high-protein diet, the two diets produced the same degree of hyperemia. All three diets produced a significantly greater hyperemia than the solutions containing the same amount of fat. All these dietary solutions increased intestinal oxygen consumption. Ethanol, however, increased blood flow without altering oxygen consumption. Thus, on weight basis, fat produces the greatest hyperemia, but the contribution of protein and carbohydrate to postprandial intestinal hyperemia cannot be considered as insignificant. The hyperemia results from a synergistic effect of all three dietary components.     

Hypophagia following dietary obesity

Two experiments examined the hypophagia that occurs when rats are switched from a high-fat to a low-fat diet. In the first experiment, rats fed a high-fat diet for eight weeks weighed 79 g more than rats fed a low-fat diet. Removal of the high-fat diet led to reduced food intake for at least four weeks. Reducing the body weight of the rats by a 24 hour fast did not alter the time course of the hypophagia. Plasma levels of free glycerol, free fatty acids and ketones were elevated during and after feeding the high-fat diet; suggesting that feeding a high-fat diet increases fat oxidation even after the high-fat diet is withdrawn. In the second experiment, feeding rats the high-fat diet for four weeks increased body weight and body fat. Starving the rats for two days after feeding the high-fat diet did not alter subsequent hypophagia and did not alter the percentage of body fat. This pattern of results is similar to that previously seen following termination of obesity-inducing insulin treatment. The results are consistent with the idea that a persistent increase in fat oxidation is responsible for the hypophagia.     

Differential reinforcing and satiating effects of intragastric fat and carbohydrate infusions in rats.

Food intake and preferences are modulated by the postingestive satiating and reinforcing actions of nutrients. This experiment compared the feeding effects of isocaloric intragastric (i.g.) carbohydrate (maltodextrin) and fat (corn oil) infusions in food-restricted rats fed low-fat (12% fat kcal) or high-fat (48% fat kcal) diets. In Experiment 1, the rats were given one flavored saccharin solution (CS+C) paired with i.g. carbohydrate infusions, a second flavor (CS+F) paired with i.g. fat infusions, and a third flavor (CS-) paired with i.g. water infusions during 30-min one-bottle training sessions. In subsequent two-bottle tests, the rats preferred both CS+s to the CS- (68-83%) and the CS+C to the CS+F (68-70%). In Experiment 2, the feeding inhibitory effects of the nutrient infusions on an ongoing meal (satiation test) or a subsequent meal (satiety test) were compared. The intake of a palatable Polycose+saccharin solution was suppressed by a concurrent carbohydrate infusion but not by a fat infusion. Also, i.g. carbohydrate preloads suppressed the intake of a subsequent (30-180 min) mixed carbohydrate+fat test meal more than did i.g. fat preloads. The satiety effects of the fat preloads were more pronounced in rats fed the low-fat diet than in rats fed the high-fat maintenance diet. Diet composition did not reliably influence the preference conditioning and satiation effects of the nutrient infusions. These results confirm prior reports that fat is less satiating than carbohydrate, and further demonstrate that i.g. carbohydrate infusions condition a stronger flavor preference than fat infusions.     

Diet composition determines course of hyperphagia in developing Zucker obese rats

Previous observations from this laboratory indicate that, during growth, the hyperphagia of the male genetically obese Zucker rat reaches a peak or "breakpoint" and then declines. To examine the effect of dietary macronutrient content on the course of hyperphagia, groups of male lean and obese rats were maintained from 5-28 weeks of age on powdered chow, or isocaloric diets (3.6 kcal/g) containing 72% of calories as corn oil, dextrose, or soy isolate protein (n = 5 lean and obese rats/diet). On chow, hyperphagia was maintained at a level of 7-8 g above lean control intake until a "breakpoint" was reached at 17 weeks, and obese intake declined to lean control level. On the fat diet, hyperphagia was increased to 10 g/day when a breakpoint was reached at 8 weeks. On the dextrose and protein diets, hyperphagia at a level of 3-4 g/day reached breakpoints at weeks 18 and 16, respectively. On all diets, the intakes of obese rats were precisely equal to the intakes of lean control rats by weeks 19-20. These data show that the magnitude and duration of hyperphagia in the developing obese rat are influenced by diet composition. Previously, we have proposed that the obese rat's hyperphagia arises from rapid adipocyte filling. Since high-fat diets facilitate adipocyte enlargement, the early "breakpoint" of hyperphagia seen with the high-fat diet may indicate that this feeding stimulation decreases as the fat cells of the obese rat approach maximal size.     

Energy density and macronutrient composition determine flavor preference conditioned by intragastric infusions of mixed diets

In prior studies rats preferred a flavor (CS+HF) paired with intragastric (IG) infusions of a high-fat diet to a flavor (CS+HC) paired with a high-carbohydrate diet, yet just the opposite preference was observed with pure-nutrient infusions. The present study tested the hypothesis that variations in nutrient density as well as composition influence flavor learning. Animals were trained (22 h/day) with IG infusion of milk-based high-fat and high-carbohydrate liquid diets paired with intakes of flavored, noncaloric CS+ solutions. A third flavor, the CS-, was paired with water infusion. Standard chow was available ad libitum. The rats preferred both CS+ flavors to the CS-, whether the infused diets were dense (HF and HC, 2.1 kcal/ml) or dilute (hf and hc, 0.5 kcal/ml), indicating that all diet infusions were reinforcing. They consumed the CS+hc and CS+hf equally in training, and preferred the CS+hc, showing that at low-energy density carbohydrate was more reinforcing than fat. In contrast, CS+HF intake exceeded that of CS+HC in training, and the rats preferred the CS+HF to the CS+HC. In further tests the rats preferred the CS+HF to the CS+hc, the CS+HF to the CS+hf, and the CS+HC to the CS+hc; i.e., when the diets differed in energy density the flavors associated with the more concentrated infusions were preferred. In the absence of influence by flavor cues from the nutrients themselves, rats' preferences for flavors associated with diets high in fat or carbohydrate are dependent on energy density. The differential satiating effects of fat and carbohydrate may contribute to these density-dependent preferences.     

Lack of diet-induced thermogenesis in brown adipose tissue of obese medial hypothalamic-lesioned rats

Radiofrequency heat lesions were made in the medial hypothalamus of 12-week old male and female Holtzman rats. Two to three days later rats were offered a palatable cafeteria diet in addition to chow or were fed chow alone for the next 3-4 weeks. Male lesioned rats were only slightly hyperphagic on the chow diet and gained little extra weight. When fed the cafeteria diet, energy intake of male lesioned rats almost doubled in comparison with chow-fed lesioned rats and a very rapid extra weight gain occurred. Despite the marked hyperphagia, thermogenesis in brown adipose tissue was suppressed in the cafeteria-fed lesioned rats, as indicated by low mitochondrial guanosine diphosphate (GDP) binding. In female rats, lesions induced much greater hyperphagia and body weight gain than in male rats, particularly when they ate the cafeteria diet. Again, thermogenesis in brown adipose tissue was suppressed in the cafeteria-fed female lesioned rats. The proportion of energy derived from carbohydrate was not altered by the cafeteria diet in either male or female rats, whether lesioned or not, but there was an increase in the proportion of energy derived from fat at the expense of protein. No sex differences in food selection were observed. The accumulation of body fat was always greater in female lesioned rats than in male lesioned rats for similar food intakes. It is concluded that medial hypothalamic lesions prevent the normal occurrence of diet-induced thermogenesis in brown adipose tissue despite extreme overeating by the rats of a palatable cafeteria diet.     

Effects of alternations (10 days) of high-fat with normal diet on liver lipid infiltration, fat gain, and plasma metabolic profile in rats

The purpose of the present study was to test the hypothesis that short-term alternations of high-fat with normal chow feeding result in higher fat accumulation in liver than continuous intake of the same high-fat diet. Male Sprague-Dawley rats (7 weeks of age) were divided into 3 groups according to diet composition: standard chow (SD; 12,5% kcal as fat), high-fat (HF; 42% kcal as fat), and food cycles (FC) consisting of 10-day alternations between HF and SD diets beginning with the high-fat diet. Rats in each of these 3 groups were sacrificed after 10, 30, and 50 days (n = 10 rats/sub-groups). Energy intake, body weight, liver and muscle relative weights were not significantly (P > 0.05) different between FC- and HF-fed rats. Using the total energy intake for the 50-day period, it was calculated that approximately 30% less calories as fat was ingested in the FC- compared to the HF-fed rats. In spite of this, liver lipid infiltration as well as fat accretion in abdominal adipose tissues were increased (P < 0.01) similarly in FC- and HF-fed rats. Plasma FFA and insulin levels depicted strong tendencies (P < 0.07) to be higher in FC- than in continuous HF-fed rats at the end of the 50-day period. These results indicate that, despite a 30% reduction in ingested lipids, alternations of HF with normal chow diet compared to the continuous hyperlipidic diet caused the same level of infiltration of lipids in the liver and in the abdominal adipose tissues and, to a certain extent, may even result in a larger deterioration of the metabolic profile.     

Effect of a high-fat diet on firing rate of sympathetic nerves innervating brown adipose tissue in anesthetized rats

Three experiments have examined the effects of ad lib and forced intake of a high-fat diet on sympathetic firing rate to brown adipose tissue. Seven days after beginning of ad lib intake of either a low-fat or high-fat diet, sympathetic activity was not significantly different in either group nor was it significantly different from the values obtained in animals measured at the switch from the chow to a semisynthetic high- or low-fat diet. After 22 days on the semisynthetic diet, however, the sympathetic firing rate of animals eating the high-fat diet had decreased nearly 25% and was significantly lower than the animals maintained on the semisynthetic low-fat diet or animals studied at the transition from the chow to the low-fat diet. In a second experiment animals were tube-fed for 3, 6 or 9 weeks on a high- or low-fat diet. Sympathetic firing rate of the rats eating the low-fat diet was higher at all three times, but the difference decreased with longer feeding. To eliminate differences in food intake, animals were tube-fed a moderate- or high-fat liquid diet three times a day for six days. The 80 kcal/day intake produced a steady weight gain in both groups. Liver weight, retroperitoneal white adipose tissue weight, and interscapular brown adipose tissue weight were all significantly greater in the animals fed the high-fat diet. Sympathetic firing rate, however, was significantly lower in the animals fed the high-fat semisynthetic diet as compared to animals fed the moderate-fat diet. These data show the high-fat diets are associated with a reduction in sympathetic activity to brown adipose tissue.     

Chronic food restriction and reduced dietary fat: risk factors for bouts of overeating

The purpose of this study was to determine the effect of chronic food restriction and reduced dietary fat on feeding behavior and body weight. Young female rats were fed ad lib or food restricted on a low-fat (LF) or a fat-free (FF) diet for 4 weeks. Rats then received 24-h free access to 2 diets, the maintenance diet (LF or FF) plus a novel high-fat (HF) diet (24-h intake test). After the test, all the rats were allowed chronic free access to the HF diet until body weight was stable. During the 24-h test, the restricted groups ate significantly more calories than the ad lib groups, and the FF-restricted rats ate significantly more total food, carbohydrate and protein than the LF-restricted rats; there were no differences between the two ad lib groups. During chronic free access to the HF diet, the formerly restricted rats achieved and defended lower body weights than the formerly non-restricted rats. Throughout the experiment, the ad lib groups had more body fat than the restricted groups independent of the dietary subgroup. Hence, a history of chronic food restriction predisposes to consuming more food in acute feeding situations, particularly when dietary fat is reduced, and lowers the level of body weight maintained and defended. Chronic food restriction accompanied by reduced dietary fat may increase risk for bouts of overeating.     

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.     

Effects of diet and obesity on body weight regulation during pregnancy and lactation in the rat

The effects of obesity level and cafeteria feeding were studied in rats during pregnancy and lactation. The non-fetal weight gain in pregnancy was three times greater with the cafeteria diet than with chow, indicating that fat deposition is not regulated at an optimal level during pregnancy. There was a strong negative correlation between postpartum weight and weight change during lactation. Obese rats were finicky in that their weight changes in lactation were exaggerated when the diet was changed between pregnancy and lactation. Pup growth rate was proportional to maternal energy intake but in this experiment not related to maternal protein, fat or carbohydrate intake. In obese rats switched to chow, intake was inadequate for normal pup growth. Thus, the weight gains in pregnancy are not regulated at a set level, and the weight change in lactation appears to compensate for the weight gain in pregnancy.     

Effects of higher dietary protein intake on energy balance and metabolic control in children with long-chain 3-hydroxy acyl-CoA dehydrogenase (LCHAD) or trifunctional protein (TFP) deficiency

The incidence of overweight and obesity is increasing among children with long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) or mitochondrial trifunctional (TFP) deficiency. Traditional treatment includes fasting avoidance and consumption of a low-fat, high-carbohydrate diet. A diet higher in protein and lower in carbohydrate may help to lower total energy intake while maintaining good metabolic control. To determine the short-term safety and efficacy of a high protein diet, subjects were admitted to the General Clinical Research Center and fed an ad-libitum high-protein diet and a high-carbohydrate diet for 6 days each using a randomized, crossover design. Nine subjects with LCHAD or TFP deficiency, age 7-14 were enrolled. Body composition was determined by DEXA. Total energy intake was evaluated daily. Resting energy expenditure and substrate utilization were determined by indirect calorimetry. Post-prandial metabolic responses of plasma glucose, insulin, leptin, ghrelin, acylcarnitines, and triglyceride were determined in response to a liquid meal. Subjects had a higher fat mass, lower lean mass and higher plasma leptin levels compared to reference values. While on the high protein diet energy consumption was an average of 50 kcals/day lower (p = 0.02) and resting energy expenditure was an average of 170 kcals/day higher (p = 0.05) compared to the high carbohydrate diet. Short-term higher protein diets were safe, well tolerated, and resulted in lowered energy intake and increased energy expenditure than the standard high-carbohydrate diet. Long-term studies are needed to determine whether higher protein diets will reduce the risk of overweight and obesity in children with LCHAD or TFP deficiency.