High-fat diet preference in developing and adult rats

Diet preference tests in rats have yielded equivocal results, as some investigators have reported a strong preference for diets high in fat over those containing less fat, while others have failed to see this preference. To further explore this unresolved problem, two diet preference experiments were conducted. In Experiment 1, adult rats were maintained for at least three months on one of three powdered diets (control, high-carbohydrate or high-fat). Rats were then given a preference test with all three diets available. Animals from each group overwhelmingly preferred the high-fat diet. To determine whether this preference was also present in younger, developing rats, in Experiment 2, weanling animals were tested with the same three diets as in Experiment 1. As observed with adult animals, weanling rats also showed a strong preference for the high-fat diet. The idea that rats prefer a diet with a relatively high level of fat is supported. Possible explanations for these findings are discussed.     

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

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

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.     

Effects of monosodium glutamate on eating and drinking behavior in rats

The effect of monosodium glutamate (MSG) on eating and drinking behavior of rats was studied by means of preference tests in free choice situations. Low or high protein casein diets containing up to 3% MSG were selected indifferently compared to plain casein diet in 7 day trials. The acceptability of diets containing 7% MSG, particularly those of low protein level, was significantly reduced. Except for Day 1, the proportional intake (MSG flavored diet intake/total diet intake) of 9% protein--7% MSG diet was consistently lower than 18% protein--7% MSG diet. In brief exposure tests (10 min) solutions of MSG (0.02-8%) were preferred over deionized water. The acceptance of solutions containing higher concentrations of MSG was significantly reduced. The total volume intake of both choices (MSG flavored water and water) was significantly increased in tests using solutions containing 3-8% MSG. In long-term tests (1-14 days), solutions containing 0.05-1% MSG were preferred over water. The acceptance of a solution containing 5% MSG was significantly reduced. For maximum intake of moles of MSG, rats selected solutions which offered 3-5 x 10(-1) M MSG whether the experimental period was 10 min or up to 2 weeks. In a series of two choice preference tests using solutions where Na+ content and pH were kept equal, MSG was preferred over sodium acetate and over sodium glutamate but was less preferred than monosodium aspartate. The results of this study can be explained in terms of sensory quality.     

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

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

Flavor preferences conditioned by intragastric fat infusions in rats

The present series of experiments determined if rats would learn to prefer cue flavors that were paired with intragastric (IG) infusions of fat. Adult female rats were fitted with gastric catheters and were trained to drink a flavored (CS+) solution, e.g., cherry-water, associated with IG infusions of a corn oil emulsion. On other days, an alternately flavored (CS-) solution, e.g., grape-water, was paired with IG infusions of water. The preference between the CS solutions was then assessed with 2-bottle choice tests. In Experiment 1, food-restricted rats were given saccharin-sweetened CS solutions for 10 min/day followed by IG infusions of a 7.1% oil emulsion or water. In subsequent choice tests they displayed a small (60%) but significant preference for the CS+ solution. In Experiments 2-5, rats had access to the CS solutions for 20 or 23 hr/day; drinking the CS solutions automatically triggered IG infusions of oil or water (electronic esophagus preparation). Infusions of 7.1% oil failed to produce a significant preference for the CS+ solution in rats given ad lib access to chow, and offered unsweetened CS flavors. Increasing the oil concentration (14.5% and 29%) did not facilitate the formation of a preference, nor did infusing the rats with a preingested oil emulsion. However, rats restricted to 2 hr/day access to chow, and 20 hr/day access to the CS solutions acquired a significant (85%) preference for the CS+ flavor; subsequent training and testing with saccharin-sweetened CS solutions increased the preference for the CS+ flavor to 95%. Ad lib fed rats trained with saccharin-sweetened CS solutions also developed a reliable preference (76%) for the CS+ flavor paired with IG oil; this preference persisted during four extinction tests when both CS+ and CS- flavors were paired with IG water. These results demonstrate that, as with other macronutrients (carbohydrate, protein), the postingestive effects of fats can condition flavor preferences, and suggest that the postingestive actions of nutrients play an important role in food selection and reward.     

Prenatal flavor exposure affects growth, health and behavior of newly weaned piglets

Young animals can learn about flavors from the maternal diet that appear in the amniotic fluid and mother's milk, which may reduce neophobia for similarly flavored food types at weaning. Flavor learning may be beneficial for piglets, which after the rather abrupt weaning in pig husbandry frequently show a period of anorexia, reduced health, and stress-induced behaviors. We investigated the effects of pre- and postnatal flavor exposure through the maternal diet on acceptance of a similarly flavored food and subsequent growth, health and behavior of newly weaned piglets.Sows were offered anise-flavored (F) or control (C) food during late gestation. Piglets were cross-fostered after birth, with each sow fostering 5 piglets from an F sow and 5 from a C sow. During lactation, sows were offered F or C food, resulting in FF, CF, FC and CC piglets. Piglets were weaned on day 25 and were given both control and flavored food for two weeks using a double food choice approach.The flavored food was not preferred. Yet, prenatally exposed animals showed a higher food intake and a higher body weight in the first days after weaning, and a lower occurrence of diarrhoea than non-exposed piglets. Prenatal exposure also increased the latency to fight, and reduced oral manipulation of pen mates and mounting during the first two weeks after weaning.Prenatal exposure, but not postnatal exposure alone, to anisic flavor through the maternal diet reduced weaning-associated problems in piglets and enhanced their health and welfare in the period after weaning.     

Self-selection of copper-containing diets by copper-deficient and overloaded rats

To determine the effect of copper status on the preference for copper-containing food, male Sprague-Dawley rats were weaned to a copper-deficient, copper-adequate or high-copper diet. Four weeks later, alterations in copper status were confirmed by measurement of liver copper concentrations. Rats (n=10) were then given the choice between a copper-adequate or a copper-restricted diet of similar composition. Preference scores indicated that control rats preferred copper-deficient food. Preferences of rats on the high-copper diet were not different from control rats, suggesting that copper overload did not alter the pattern of selection. However, 7 of the 10 copper-deficient rats ate 80% or more of their intake as copper-adequate food resulting in preference scores that were significantly different from controls (p<0.03). This alteration in preference was corrected within 1 day of access to copper-adequate food.     

Facilitation of hypothalamic obesity by greasy diets: Palatability vs lipid content

Rats with obesifying lesions of the ventromedial hypothalamus (VMH), obesifying hypothalamic knife cuts, or sham operations, were given a synthetic powdered high-fat diet and/or an isocaloric and preoperatively preferred synthetic powdered control diet. Postoperatively the knife-cut rats preferred, overate, and became obese on the high fat diet. The electrolytically lesioned rats did not show a switch in preference, but they did overeat and became obese on the synthetic high fat diet when it was the only diet available. Thus, postingestive aspects of high-fat diets contribute to overeating. When a standard Purina chow diet was subsequently presented as pellets instead of as powder, overeating again occurred. In this case preingestive factors alone facilitated feeding. When firm consistency, high fat content, and high caloric density were combined in a Purina/Crisco high fat diet, the greatest intakes and weight gains of all three tested diets were obtained. Thus, both pre- and postingestive changes in response to the diet appear to contribute to the mediation of hypothalamic obesity. These factors not only persist, but are exaggerated in the obesity elicited by hypothalamic knife cuts.     

Flavor preferences conditioned by intragastric polycose infusions: a detailed analysis using an electronic esophagus preparation

The present series of experiments examined the rat's conditioned preference for flavors associated with intragastric (IG) Polycose infusions. Adult female rats were fitted with two chronic intragastric catheters and were trained to drink flavored water (CS+; e.g., cherry-water) paired with IG infusions of 32% Polycose. On alternate days a different flavor (CS-; grape-water) was paired with IG water infusions. The flavored water and chow were available 23 hr/day. In subsequent two-choice tests the rats displayed strong preferences for the CS+ flavor (up to 98%). The CS+ preference persisted for several weeks during extinction tests when both the CS+ and CS- were paired with IG water or with no infusions. The rats also preferred the CS+ to plain water which contrasts with the mild aversion naive rats display to the flavored water. The acquired preference for the CS+ flavor was not as strong, however, as the rats' innate preferences for the taste of saccharin or Polycose. Also, unlike their response to saccharin and Polycose, the rats' acceptance (absolute intake) of the CS+ flavor was not elevated. CS+ intake was increased, though, when saccharin was added to the flavored water. The rats reversed their flavor preference when the reinforcement contingencies were reversed. Also, in the absence of unique flavor cues, the rats learned to prefer, apparently based on somatosensory cues, the sipper tube that was paired with IG Polycose infusions. The effects of flavor and IG Polycose infusions on drinking patterns and caloric intake are also described.     

Flavor preference conditioning as a function of fat source

Rats learn to prefer foods based, in part, on postingestive nutrient actions. This study compared the effectiveness of intragastric (IG) infusions of fat emulsions which varied in their fatty acid composition (chain length and saturation) to condition preferences for flavored saccharin solutions. In Experiment 1, food-restricted rats were trained (30 min/day) with one flavor (CS+CO) paired with IG corn oil (CO) and a second flavor (CS+MCT) paired with IG medium chain triglyceride (MCT); the fats were prepared as isocaloric emulsions. A third flavor (CS-) was paired with IG water. The rats subsequently showed a strong preference for the CS+CO (84%) and a weaker preference for the CS+MCT (65%) relative to the CS-. In a direct choice test, the CS+CO was preferred to the CS+MCT by 75%. In Experiment 2, new rats trained with flavors paired with IG corn oil and beef tallow (BT) infusions learned to prefer both the CS+CO (89%) and the CS+BT (82%) relative to the CS-, and preferred the CS+CO to the CS+BT by 67%. The same rats were trained with three new flavors paired with IG infusions of corn oil, vegetable shortening (VS), and water. The rats strongly preferred both the CS+CO (91%) and CS+VS (86%) over the CS-, and they preferred the CS+CO to the CS+VS by 64%. In Experiment 3, new rats trained with corn oil and safflower oil (SO) paired flavors preferred both the CS+CO and CS+SO to the CS-, and equally preferred the CS+CO and CS+SO in two-bottle tests. The rats were also given one-and two-bottle tests with the various fat emulsions and their preference profile was consistent with their conditioned preferences for the flavored saccharin solutions. These findings demonstrate that many different fat sources can condition flavor preferences. Fats with high polyunsaturated content and/or lower saturated fat content are the most reinforcing.     

Nutrient-conditioned flavor preference and incentive value measured by progressive ratio licking in rats

Rats develop strong preferences for flavors associated with the postingestive actions of glucose and other nutrients. This preference may involve changes in both the hedonic appeal and incentive salience of the nutrient-paired flavor. The present study used a progressive ratio (PR) operant licking task to evaluate the degree to which nutrient conditioning changes the incentive value of flavors. Food restricted rats were trained to associate one flavored saccharin solution (CS+) with intragastric glucose infusions and another flavored solution (CS-) with water infusions. The rats subsequently showed a strong preference for the CS+ in two-bottle tests and also licked more for the CS+ than CS- in PR tests. PR licking for the CS+ was similar to that for an 8% fructose solution. Together with earlier data indicating that the CS- is isopreferred to a 3% fructose solution, these findings indicate that IG glucose conditioning enhances the hedonic and incentive value of the CS+ solution so that the animal responds as if the solution had a sweeter taste.     

Food color, flavor, and conditioned avoidance among red-winged blackbirds

The relationship between food flavors and postingestive feedback enables mammalian herbivores to procure nutrients and avoid toxins within ever-changing environments. We conducted four experiments with red-winged blackbirds (Agelaius phoeniceus) in captivity to determine the relative roles of color and flavor cues paired with negative postingestive feedback. We first conducted baseline tests to assess preferences for colors and flavors. All blackbirds preferred red- to blue-colored food, and they preferred umami- (l-alanine) flavored to bitter/astringent food (tannic acid). We observed no difference in consumption of salty (NaCl) vs sour (citric acid) foods during baseline tests (i.e., neutral flavors). We then conditioned experimentally naïve blackbirds with intraperitoneal injections of lithium chloride (LiCl) to avoid food treated with red and l-alanine, or red and NaCl (n = 30 birds per conditioning group). Subsequent to conditioning with LiCl, three test groups were established from each conditioned group to evaluate color and flavor preferences, and preferences for novel color–flavor pairings (e.g., red/tannic acid vs blue/l-alanine). Blackbirds avoided red and salty food throughout the 4-day test. Avoidance conditioned with LiCl extinguished for preferred flavors, but not for colors, of food. Conditioning affected indifference for the otherwise preferred flavor and avoidance for the otherwise neutral flavor. Relative to the neutral-flavor conditioning group, the group conditioned with a preferred flavor exhibited stronger conditioned avoidance of colored food. Unlike conditioned flavor avoidance, birds were conditioned to avoid red food only when blue food was made familiar prior to conditioning. Collectively, these results illustrate that blackbirds used affective processes (flavor–feedback relationships) to shift preference for both novel and familiar flavors, and cognitive associations (colors) to avoid food, subsequent to toxin exposure. We discuss the opportunities afforded by affective and cognitive processing for reducing agricultural damage caused by blackbirds.     

Hungry, but not thirsty, rats prefer flavors paired with ethanol

The present experiments examined the reinforcing effects of an ethanol (EtOH) unconditioned stimulus (UCS) on conditioned flavor preferences in food-deprived rats and in water-deprived rats. In Experiment 1A food and water deprived animals received distinct conditioning treatments. One half of the animals were intragastrically intubated with EtOH (0.5 g/kg), and thereafter allowed 20 min free access to similar flavored drinking solutions. Remaining animals were intubated with distilled water. All animals received 15 presentations of an EtOH-paired flavor. A two-bottle preference test was subsequently used to evaluate preferences or aversions to flavors paired with EtOH in food-deprived and water-deprived animals. Results of Experiment 1A showed that food-deprived animals preferred the flavor associated with EtOH. Conversely, preferences for EtOH-paired flavors were not established in water-deprived animals. In Experiment 1B deprivational states of animals used in Experiment 1A were reversed without further drug training. Following a two week habituation period to deprivation state animals again received a two-bottle preference test to re-evaluate preferences or aversions to the EtOH-paired flavors. Results of those manipulations indicated that an ethanol aversion was established in the water-deprived animals. Those results indicated that water-deprived animals of Experiment 1B reversed their EtOH-paired flavor preference when the caloric need associated with food deprivation conditions was eliminated. Since deprivational state determined the development of EtOH preferences, the present results indicate that caloric need may play an initial role in establishing conditioned preferences for EtOH.     

Dietary preference behavior in rats fed bitter tasting quinine and sucrose octa acetate adulterated diets

Rats were fed a bitter and presumably toxic quinine adulterated diet and a more bitter, but non-toxic sucrose octa acetate (SOA) adulterated diet. In two-diet preference tests the animals initially preferred the quinine diet over the SOA diet. After being fed only the SOA diet for four days the rats switched their preference to this diet during a subsequent two-diet preference test. In 10 day single diet tests the rats ate significantly more of the SOA diet than of the quinine diet. The results are consistent with the hypothesis that the toxic effects of quinine result in the formation of a conditioned taste aversion to the diet, and indicate that rats can distinguish between two bitter diets and develop a preference for the more bitter, but non-toxic one.     

Flavor preferences conditioned in C57BL/6 mice by intragastric carbohydrate self-infusion

This study determined the feasibility of conditioning flavor preferences in mice by self-administered intragastric (IG) nutrient infusions. Male C57BL/6J mice were surgically fitted with an IG catheter that was attached by a tether system to an infusion pump. The mice were given ad-libitum access to chow and a flavored solution 23 h/day. Drinking was monitored with a computerized lickometer system that controlled the infusion pumps. In Experiment 1, drinking one flavored solution (CS+, e.g., grape-saccharin) was paired with matched infusions of 8% maltodextrin, whereas drinking another solution (CS-, e.g., cherry-saccharin) was matched with water infusions across 6 one-bottle training days. During training, the mice drank more CS+ than CS-; this was due to an increase in bout size but not bout frequency. In subsequent two-bottle choice tests, the mice strongly preferred (91%) the CS+ to the CS-. Experiment 2 obtained a significant but less robust (71%) CS+ preference in mice trained with unsweetened CS solutions. These data indicate that mice, like rats, acquire an increased acceptance and preference for flavors paired with the postingestive actions of nutrients. Our understanding of flavor-nutrient learning can be advanced by studying this process in selected mouse strains and genetically modified animals.     

Effect of amino acid imbalance and deficiency on dietary choice patterns of rats

Detailed dietary choice patterns were determined with a computerized feeding monitoring system in groups of Sprague Dawley rats kept on a 12:12 hr light-dark cycle and offered in sequence a series of dietary choice regimens involving amino acid-imbalanced or deficient diets with threonine as the most limiting amino acid. Animals established their preference for a threonine-basal diet over a threonine-imbalanced or a threonine-devoid (devoid of threonine) diet shortly (within 2-3 hr) after the consumption of small quantities of either diet in the beginning of the first dark-cycle. An intensive sampling process characterized by frequent small bouts was evident throughout the light period. Both the meal size and the meal frequency of the imbalanced or devoid diet were curtailed after prolonged choices. Animals preferred the threonine-corrected (imbalance corrected by threonine supplementation) over the threonine-basal diet initially with an increase in meal frequency. But no clear choice for either diet was observed thereafter. Animals did not establish their preference for the threonine-corrected diet when paired with the threonine-devoid diet until after 5 days with a steady decrease in the meal size of the devoid diet but not the meal frequency. When the protein-free diet was introduced as an alternative for the threonine-imbalanced diet, animals selected the protein-free diet during the first dark-cycle after consuming a small amount of the imbalanced diet. Initially there was a drastic reduction in meal size of the imbalanced diet and subsequently a decrease in meal frequency as well. Nevertheless, animals immediately rejected the protein-free diet and chose the threonine-basal diet when it replaced the imbalanced diet as an alternative. The almost exclusive preference for the basal diet occurred in the beginning of the first dark-cycle with an increase in meal size but no change in meal frequency. The sampling bouts of small quantities, which followed the first introduction of the diets in the choice regimens, may be an inherent investigative behavior whereby the physical or oropharyngeal properties of the diets are recognized. The establishment of the choices for the alternative diets in the present experiments provides additional information about the rapid time course of the food intake control mechanisms in rats fed amino acid-imbalanced or deficient diets.     

Flavor preferences conditioned by intragastric nutrient infusions in food restricted and free-feeding rats

The role of deprivation state in flavor preference conditioning by nutrients was investigated in rats fitted with intragastric (IG) catheters. In different experiments, food restricted (FR) and food ad libitum (AL) groups were trained to drink one flavored solution (CS+) paired with IG infusions of maltodextrin, corn oil, or casein and another flavored solution (CS-) paired with IG water infusions. Training intakes of the CS solutions were limited to equate the exposure of the FR and AL groups. The IG nutrient infusions conditioned flavor preferences in FR and AL groups which, in three of four experiments, were of similar magnitude. Food restriction did, however, increase the overall intake of the CS+ solutions during testing. Rats trained with one CS+ while food restricted and a second CS+ while food unrestricted showed similar preferences for the two CS+ flavors. Prefeeding AL rats to satiety with chow prior to daily training sessions did not prevent them from developing a preference for a CS+ paired with IG maltodextrin. These findings indicate that the postoral actions of nutrients are reinforcing in food sated as well as hungry rats.     

Protein selection by rats adapted to high or moderately low levels of dietary protein.

After preliminary studies on flavor acceptability, patterns and indices of subsequent feeding behavior were monitored by computer in young rats which were adapted to 15% or 70% casein diets before being offered, sequentially, choices between flavored diet pairs in which the proportions of percentage casein were 5/65, 5/55, 5/45, 5/35 and 5/25. Similarly adapted rats received these choices in the reverse sequence. Rats adapted to 15% casein usually ate randomly from the diet pairs and selected approximately 15-30% casein; individual behaviors were prominent. The 70% casein groups avoided the higher casein diet, often within minutes (except for the first-offered 5/25 choice), and seldom selected more than 10% casein; individual differences were infrequent. Such rats also distinguished between flavored 70% and 65% casein diets. Sizes and numbers of meals and rates of eating differed for the paired diets, especially for rats adapted to 70% casein. A flavor added to the 70% casein adaptation diet was not avoided when present only in the 5% casein diet of a 5/65 choice. Rats adapted to 70% soy protein before receiving flavored 5/65 to 5/25 choices selected 20-28% soy protein, a level far above those of casein selections by rats adapted to 70% casein. Dietary adaptation and type of protein thus affect subsequent diet selection and feeding patterns and indices.     

Overeating, overweight and obesity induced by an unpreferred diet

Rats fed diets containing 50-71% added water (liquid diets) eat more energy and gain more weight than rats fed the same diets without added water (solid diets). The present experiments examined the effects of making a liquid diet less palatable. The first experiment examined the effects of sucrose octaacetate on diet preference. Rats, given a choice of a liquid diet containing 0.5% sucrose octaacetate and a plain solid diet, preferred the plain solid diet for three weeks. When the concentration of sucrose octaacetate was reduced to 0.05%, the rats did not show a reliable preference for either the sucrose octaacetate liquid or plain dry diet. In subsequent experiments, each rat was given only one diet at a time. In the second experiment, rats were fed 0.5% sucrose octaacetate liquid diet for three weeks followed by 0.05% sucrose octaacetate liquid diet for another four weeks. The rats fed the sucrose octaacetate liquid diet overate and became obese compared to the rats fed plain solid diet throughout. In the third experiment, rats fed 0.5% sucrose octaacetate liquid diet for six weeks became obese compared to rats fed plain solid diet throughout. Thus, the overeating and obesity induced by liquid diets cannot be attributed solely to their high palatability.