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.     

Effects of diet and handling on initiation of independent ingestion in rats

The present study examined the effects of dietary manipulation on the age of onset of weaning in rat pups. In Experiment 1, female rats were placed on a standard chow (SC) or high-fat (HF) diet 1 week following mating. Pups were weighed daily from birth to Day 12, then animals were placed into specialized cages for separate recording of food intake of pups and dams. Pups were offered the same diet as their dam, and food intake and body weight were determined twice daily until Day 25. The results demonstrated that pups reared by dams fed the HF diet initiated independent ingestion on Day 16, approximately 24 hr before pups reared by dams fed the SC diet. There were no differences in body weight in pups across the two diets. While few differences were noted across diets in pups' or dams' behavior, HF pups appeared to demonstrate a delay in the establishment of circadian patterns of food intake. In Experiment 2, all dams were maintained on an SC diet until the day after parturition. At that time, dams and litters were placed into specialized cages and divided into four groups: HF/HF, HF/SC, SC/SC, and SC/HF (dam's diet/pup's diet, respectively). The results demonstrated that dams given the HF diet had pups that initiated food intake approximately 2 days before the pups of dams given the SC diet. In addition, pups offered the HF diet, independent of the dam's diet, initiated food intake approximately 0.8 days prior to pups offered the SC diet. Further, by Day 12, HF dams had pups that were heavier than SC dams. The results suggest that the onset of weaning in rats is affected by maternal diet and the weaning diet available to the pup.     

Wheel running eliminates high-fat preference and enhances leptin signaling in the ventral tegmental area

Voluntary wheel running (WR) is a form of physical activity in rodents that influences ingestive behavior. This study examined the effects of WR on dietary preference and the potential role of leptin in mediating these effects. In a two-diet choice paradigm in which both palatable high-fat (HF) food and standard laboratory chow were provided ad libitum, rats displayed a strong preference for the former and chose to eat almost exclusively the HF diet over chow in sedentary conditions. With free access to running wheels, however, rats exhibited no preference for the HF food and consumed equal gram amounts of both chow and HF diets. The total daily caloric consumption during WR in the dietary choice protocol was equivalent to the amount of calories consumed daily by WR rats with only chow or only HF diet available, yet significantly less than sedentary chow caloric consumption. Two days after initiating WR, leptin signal transduction was examined in multiple selected brain sites following leptin injection into the third cerebral ventricle. The maximal leptin-stimulated STAT3 phosphorylation was enhanced only in the ventral tegmental area (VTA), but not in the arcuate nucleus, lateral hypothalamus, dorsal medial or ventral medial hypothalamus, or substantia nigra. In conclusion, wheel running appears to act either as an independent reinforcing factor or as a more favored activity to substitute for the consumption of a palatable HF diet, thus eliminating the preference for the HF food. Moreover, WR enhances leptin signaling specifically in the VTA, suggestive of a WR-evoked mechanism of heightened leptin function in the VTA to curb the drive to consume palatable HF foods.     

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.     

Diet composition alters the satiety effect of cholecystokinin in lean and obese Zucker rats

Although exogenous administration of the peptide cholecystokinin (CCK) has been shown to reduce food intake in a variety of experimental situations, few studies have examined the influence of dietary content upon CCK's effectiveness, particularly in obese states. To evaluate the effectiveness of CCK administration in animals consuming high fat diets, groups of obese and lean Zucker rats were maintained on laboratory chow (CH), a high fat diet isocaloric to chow (IF), or a hypercaloric fat diet (HF). After a 17 hr fast, rats were given intraperitoneal injections of saline or ascending doses of 0.06 to 2.0 micrograms/kg of the synthetic octapeptide of CCK. On all diets, obese rats required higher doses of CCK to significantly reduce feeding and showed smaller intake reductions than lean rats (p less than 0.001). Despite higher baseline caloric intakes (p less than 0.001), rats of both genotypes maintained on HF displayed larger reductions of intake than those fed IF or CH (p less than 0.001). Intake reductions by either genotype maintained on IF or CH were not reliably different. The manner in which the satiety effect of CCK was enhanced in rats consuming the calorically dense, palatable HF diet is unclear but may be related to orosensory and/or postingestive attributes of the diet.     

The role of T1r3 and Trpm5 in carbohydrate-induced obesity in mice

We examined the role of T1r3 and Trpm5 taste signaling proteins in carbohydrate-induced overeating and obesity. T1r3, encoded by Tas1r3, is part of the T1r2+T1r3 sugar taste receptor, while Trpm5 mediates signaling for G protein-coupled receptors in taste cells. It is known that C57BL/6 wild-type (WT) mice are attracted to the tastes of both Polycose (a glucose polymer) and sucrose, whereas Tas1r3 KO mice are attracted to the taste of Polycose but not sucrose. In contrast, Trpm5 KO mice are not attracted to the taste of sucrose or Polycose. In Experiment 1, we maintained the WT, Tas1r3 KO and Trpm5 KO mice on one of three diets for 38days: lab chow plus water (Control diet); chow, water and 34% Polycose solution (Polycose diet); or chow, water and 34% sucrose solution (Sucrose diet). The WT and Tas1r3 KO mice overconsumed the Polycose diet and became obese. The WT and Tas1r3 KO mice also overconsumed the Sucrose diet, but only the WT mice became obese. The Trpm5 KO mice, in contrast, showed little or no overeating on the Sucrose and Polycose diets, and gained less weight than WT mice on these diets. In Experiment 2, we asked whether the Tas1r3 KO mice exhibited impaired weight gain on the Sucrose diet because it was insipid. To test this hypothesis, we maintained the WT and Tas1r3 KO mice on one of two diets for 38 days: chow, water and a dilute (1%) but highly palatable Intralipid emulsion (Control diet); or chow, water and a 34% sucrose+1% Intralipid solution (Suc+IL diet). The WT and Tas1r3 KO mice both exhibited little or no overeating but became obese on the Suc+IL diet. Our results suggest that nutritive solutions must be highly palatable to cause carbohydrate-induced obesity in mice, and that palatability produces this effect in part by enhancing nutrient utilization.     

Consumption of a high-fat diet alters the homeostatic regulation of energy balance

Humans in many countries are currently experiencing what has been called an epidemic of obesity. That is, the average body weight (and amount of fat stored in the body) is increasing over years, carrying with it a multitude of associated medical, psychological, and economic problems. While there is no shortage of possible causes of this epidemic, increased availability and consumption of high-fat (HF), calorically dense and generally quite palatable food is often touted as a likely culprit. In order to better assess the impact of consuming a diet with those qualities, we have developed a well-controlled animal model in which the effects of chronic consumption of a high-fat diet can be dissociated from those of becoming obese per se. Long-Evans rats are fed one of two semipurified pelleted diets, a HF diet that contains 20% fat by weight and a low-fat (LF) diet that contains 4% fat by weight. Pair-fed animals consume the HF diet but are limited to the daily caloric intake of LF rats. Another group receives pelleted chow. Relative to animals consuming diets low in fat, HF animals weigh more, have more carcass fat, are hyperinsulinemic and hyperleptinemic, and are insulin resistant. HF-fed animals, independent of whether they become obese or not, also have central insulin and MTII insensitivity. Finally, HF rats have a down-regulated hypothalamic apo A-IV system that could contribute to their hyperphagia.     

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.     

Feeding and drinking patterns in young pigs

Feeding and drinking patterns of six immature female pigs, weighing from 10 to 130 kg, operantly obtaining feed and water at a fixed ratio of 10, were determined. It was found by log survivorship analysis that 10 min was the minimum interbout interval defining separate eating bouts. As the pigs grew through this weight range, daily feed intake increased nearly threefold, while eating bout frequency fell from 14 to 7 per day; consequently both eating bout size and interbout interval increased. However, bout size was increased primarily by an increased rate of eating during bouts without any consistent increase in bout duration. Neither premeal nor postmeal intervals were correlated with meal size. Of the pigs' daily water intake, 75% was closely associated with eating bouts and over 1/3 of this (25%) was preprandial. Sixty-four percent of daily food intake and 68% of water intake was during the 12-hr light period. Nocturnal eating bouts were less frequent, but larger.     

Palatability and meal patterns

Investigations of palatability have often been confounded by nutritional, pharmacological or methodological problems. The present experiment examined the free-feeding meal patterns of two differentially preferred yet nutritionally equivalent diets. When the two diets were offered sequentially (Experiment 1), no differences were found in meal frequency, meal size, or meal duration. However, when the diets were offered simultaneously in Experiment 2, the preferred diet was consumed more frequently, in larger meals, and at a faster feeding rate than the less preferred diet. Further, the meal patterns in Experiment 2 of the preferred diet were identical with those found for both diets in Experiment 1. Therefore, the meal patterns of two diets with differing palatability depend upon the method of presentation with differences appearing when the diets are offered simultaneously but not when they are offered sequentially.     

Effects of estradiol on food intake and meal patterns for diets that differ in flavor and fat content

Apart from the well known inhibitory effects of estradiol on food intake, meal size, and body weight in female rats that have been documented over the past thirty years, a more recent report presents the opposite finding; that a large dose of estradiol can increase food intake and weight gain in gonadally intact female rats presented with a palatable diet. The purpose of the present experiment was to further examine this hypothesis by evaluating the ability of estradiol to influence feeding behavior in ovariectomized rats presented with diets that differ in flavor and fat content. Female rats were given a cyclic regimen of estradiol benzoate treatment (5.0 or 20.0 µg) or the oil vehicle and were presented with the standard chow diet or a diet with a higher fat content and chocolate flavor. Food intake, meal size, and meal number were monitored three days after the first injection of estradiol or oil. Compared to the chow diet, food intake increased when animals had access to the chocolate/fat diet during the vehicle treatment condition. Both doses of estradiol significantly decreased food intake, meal size, and body weight gain when animals were presented with either the standard chow diet or the chocolate/fat diet. These findings indicate that estradiol does not stimulate the intake of a palatable diet in ovariectomized rats, and suggest that previous results showing that estradiol enhanced eating and weight gain stemmed from a disruption of the hypothalamic-pituitary-gonadal axis when intact females received a large dose of exogenous estradiol.     

Alcohol as a Food: A Commentary on Richter

The present study examined the maintenance of voluntary alcohol intake in male Long-Evans rats. A microstructural analysis of consummatory behaviors (food, alcohol, water) was carried out using a computerized drinkometer system. In this sample of animals, there was no association (r = 0.07) between total food intake and total alcohol intake. There was no compensation for the extra calories ingested in the form of alcohol via a reduction in total food intake, or a reduction in food bout sizes associated with pre- or postprandial alcohol consumption. Further microstructural analyses determined that there were no significant difference between water and alcohol in terms of their distribution in relation to food (non-, pre-, or postprandial bouts). Of the total of 586 bouts of fluid intake analyzed, 45.6% were consumed postprandially, with a similar number (43.2%) consumed nonprandially. A comparison of the size of food bouts associated with different fluid bout types (pre- or postprandial) indicated that food bouts were the same size regardless of whether they were accompanied by water or alcohol. A final analysis determined that 55% of the total daily alcohol intake was consumed postprandially, and that the sizes of non-, pre-. or postprandial fluid bouts were significantly different for water vs. alcohol. Post hoc pairwise comparisons found that alcohol postprandial bouts were significantly larger than all types of water bouts. Alcohol and water bouts ranged in size from <0.5 ml to >5.5 ml There was a significant difference in the distribution of bout sizes with more alcohol bouts at the high end of the distribution. Only 24% of the water bouts were >2.5 ml compared to 48.4% of the alcohol bouts. The results of this study demonstrate that rats organize their consummatory behavior in many discrete, short bouts. There were considerable individual differences in alcohol preference, alcohol-bout frequency, duration, and size, as well as the prandial distribution of bouts. All of these variables together produce the “pattern” of alcohol intake in individual animals, and is likely to influence the level of intoxication achieved. Although rats do not dissociate their alcohol intake from normal feeding patterns, alcohol bouts occurring postprandially are significantly larger than other bouts of fluid consumption, suggesting that animals perceive the pharmacological effects of and are affected by the alcohol they consume. In animals with a preference for alcohol solutions, it is unlikely that alcohol is consumed as a food.     

Failure of periodic presentation of palatable diet to entrain feeding,drinking and activity rhythms under constant conditions

Feeding, drinking and activity of two groups of male laboratory rats were studied across three experimental stages: LD 12:12; LL; and LD 12:12. In the first group (Control), ordinary diet was availabel ad lib for the entire experiment. In the second group (Experimental), palatable diet was available for the first 12 hr of each day and ordinary diet for the remainder of the day in stages 2 and 3. It was found that in stage 1 all behaviors were highly nocturnal. The nocturnality of feeding was achieved by the ingestion of more meals in the D phase as compared to the L phase. Dark ingested meals were not significantly larger than meals in the L phase. In stage 2, circadian rhythms in all three variables were damped and meals became evenly spaced across the day in both groups. In the Control group, meal sizes were also of the same magnitude across the day. However, periodic presentation of palatable diet resulted in larger meals and, therefore, the ingestion of 60% of total daily food intake during the 12 hr period when palatable food was available. Total daily food intake in LL did not change compared to stage 1 and did not differ between the groups. The size and number of significant meal size/post-meal interval correlations increased over the duration of the experiment but remained relatively small. In stage 2, water intake and activity were similarly evenly distributed over the 24 hr period in both groups and were not influenced by periodic presentation of palatable diet. No marked decreases in total daily water intake, but a substantial decrease in daily activity, were found. Under LL, daily body weight increments in both groups were maintained, but subtle changes were observed. In stage 3 when the LD cycle was reinstated, both groups ate a greater number of meals in the D phase despite the availability of palatable food in the L phase for the Experimental group. These animals ate fewer but larger meals of palatable diet than ordinary food resulting in 50% of their daily intake being ingested during the L phase. Activity and water intake returned to stage 1 levels and the distributions of these behaviors were similar to those found in stage 1 in both groups. It was concluded that palatable diet influenced meal size only and that the timing of meals depended on the illumination conditions only. Further, periodic presentation of palatable diet failed to entrain the damped rhythms of feeding, drinking and activity in LL. These results do not rule out the possibility that such an entraining effect of periodic presentation of palatable diet might be found in conditions of DD.     

The effect of liver denervation on the consumption of various diets by rats

Liver afferents have been proposed to influence food intake control, however, previous studies have shown that chow (pellet) intake is apparently not altered in total liver denervated rats. The present study explored whether total liver denervation could alter the rats' intake of various diets other than chow pellets. Total liver denervations were verified using staining histological and monoamine histofluorescence techniques. The denervated and sham operated rats were given short-term (4-6 days) exposure to four diets: (diet 1, chow plus a 32% w/v sucrose-water solution; diet 2, 1:1 mixture of powdered chow and granular glucose; diet 3, 33% w/w Crisco and powdered chow mixture and diet 4, a 5% w/v glucose-water solution plus chow. Body weight gains were not affected-by either surgery or diet exposure. Daily consumptions of the diets were similar in both groups, nevertheless, there was a trend for the denervated rats to consume slightly more of a high fat diet, which lends support for one hypothesized liver satiety mechanism. Also, the denervated rats consumed less (an average 5 kcal/day) of the 5% glucose solution (one hypothesis tested would predict an increase consumption of glucose by the denervated rats). Thus the liver may play a role, albeit small, through several ill defined mechanism(s) in the regulation of feeding.     

Peripheral control of food intake: interaction between test diet and postingestive chemoreception

The experiments reported here were designed to investigate the role of the duodenum in conjunction with the palatability of the diet in the control of meal size. Rats fitted with chronic cannula resting in the duodenal cap were infused with 15 ml of 5.05% glucose, 10.1% glucose, 1.6% urea, 3.2% urea, 15.15% glucose and 4.8% urea at a rate of 0.2 ml/min, or were not infused at all. In one experiment, intake of sweetened condensed milk, and in a second experiment, intake of a 3.2% glucose solution was recorded over a 5 $\text{1}\text{2}$ hour period. Only infusions of very concentrated solutions had significant effects on milk ingestion. On the other hand glucose infusions isosmotic to body fluids led to a decrease in the size of the first meal of a 3.2% glucose solution and a lengthening of the subsequent intermeal interval. These effects were directly related to the concentration of the glucose infused. Equiosmolar urea infusions had no consistent effect on glucose drinking. Intake of moderately palatable substances can be altered by the presence of small amounts of glucose in the duodenum, but intake of highly palatable substances remains largely unaffected unless high concentrations of glucose are infused.     

Sham-feeding of corn oil by rats: sensory and postingestive factors

Previous research indicates that rats fed a high-fat (HF) diet increase their intake and preference for oil compared with rats fed a high-carbohydrate (HC) diet. To assess whether this increased intake was due to the sensory or postingestive properties of oil, rats were adapted to either the HF or HC diet and then allowed to sham-feed pure corn oil daily for 30 min. During the first 4 trials, rats fed the HF diet sham-fed more oil than did rats fed the HC diet; however, this difference diminished with repeated testing and was absent after 8 trials. In both diet groups, 4-5 calories (approximately 25%) of sham-fed oil could not be recovered and may have escaped to the intestine. These results suggest that, compared with rats fed a HC diet, rats fed a HF diet are initially attracted to the sensory properties of oil, but that the differential oil intakes of rats fed the HF or HC diet are maintained by postingestive, rather than sensory factors.     

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.     

Palatable daily meals entrain anticipatory activity rhythms in free-feeding rats: dependence on meal size and nutrient content

Circadian wheel-running rhythms were monitored continuously in 3 groups of female Sprague-Dawley rats under different palatable food availability schedules. All rats had free access to standard rat chow and water throughout the study. In addition, Group 1 rats received a palatable nutrient-rich mash for 2 hr each day for 28 days, beginning 3 hr after light onset of a 12:12 LD cycle. Group 2 rats received the same mash but were limited to 4 g daily. Group 3 rats received a palatable non-nutritive mash. Ten of 13 Group 1 rats, 2 of 13 Group 2 rats, and 0 of 13 Group 3 rats showed anticipatory running prior to the daily palatable meal. Palatable mash intake was generally lower among Group 3 rats than among Group 1 rats. However, several Group 3 rats consumed non-nutritive mash in amounts which equalled or exceeded the nutritive mash intake of Group 1 rats showing anticipatory running. The results indicate that temporally limited daily access to a palatable food can entrain anticipatory wheel-running in rats that are not food-deprived. They also indicate that entrainment to periodic food availability depends on stimuli associated with the concentrated intake of nutrients rather than on the absolute size or palatability of a meal.     

Feeding behavior in rats fed diets containing medium chain triglyceride.

The effect of dietary medium chain triglyceride (MCT) on short-term food intake was compared with the effect of long chain triglyceride (LCT) in rats. Corn oil and glyceryl tricaprylate were used as LCT and MCT sources, respectively. Rats were given diets containing 200 g MCT/kg diet (MCT diet), 100 g MCT + 100 g LCT/kg diet (ML diet), or 200 g LCT/kg diet (LCT diet) in Experiment 1. Cumulative food intake was determined every h for the first 12 h, then at 2-h intervals thereafter during the subsequent 12 h. As early as 1 h after feeding, cumulative food intake significantly decreased in MCT-fed animals in a dose-dependent fashion. In Experiment 2, rats were given a choice between MCT and LCT diets for 1 h to confirm whether or not the palatability of diets was influenced by dietary fat sources. There was no difference in food intake between the two diets. In Experiment 3, the responsibility of endogenous cholecystokinin (CCK) for the difference in food intake between the two diets was investigated for 6 h by using a CCK-A receptor antagonist, Devazepide (DVZ, 1 mg/kg b. wt.). Food intake in the MCT diet and also in the LCT diet was improved by DVZ. It is concluded that the satiety, but not the palatability, is affected by carbon chain length in dietary triglyceride sources, although the responsibility of endogenous CCK is very small.     

Flavor preferences conditioned by high-fat versus high-carbohydrate diets vary as a function of session length.

Intragastric (i.g.) infusions of fat and carbohydrate condition flavor preferences in rats, but different results have been obtained in studies using pure and mixed nutrient infusions. This experiment compared the preference conditioning effects of mixed high-carbohydrate (HC) and high-fat (HF) diet infusions during short-term and long-term sessions. In Experiment 1 food-deprived rats were given one flavored saccharin solution (CS+HC) paired with i.g. infusions of an HC liquid diet, a second flavor (CS+HF) paired with HF diet infusions, and a third flavor (CS-) paired with i.g. water infusions during 30-min one-bottle training sessions. In subsequent two-bottle tests (30 min/day), the rats preferred both CS+s to the CS- and preferred the CS+HC to the CS+HF. In Experiment 2, the same rats were trained and tested with the CSs and paired infusions during 22 h/day sessions with chow available ad lib. Both CS+s were again preferred to the CS-, but now the CS+HF was preferred to the CS+HC. When given additional 30-min choice sessions in Experiment 3 the rats showed no reliable preference for the CS+HC versus CS+HF under food-deprived or ad lib conditions. In Experiment 4, the rats were given 22-h CS+HC versus CS+HF choice sessions every other day. They showed no reliable CS preference during the first 30 min of each session, but reliably preferred the CS+HF during the remaining 21.5 h. These findings indicate that previously reported differences in preferences conditioned by pure versus mixed nutrient infusions are due to training procedures (session length, deprivation state) rather than to the type of nutrient infusions per se. The rats displayed different CS+HF versus CS+HC preferences as a function of test duration even after being given both short- and long-term training. Thus, short-term choice tests do not always predict the long-term intakes and preferences for high-fat and high-carbohydrate foods.