Self-selection and the obese Zucker rat: the effect of dietary fat dilution

Intermittent access to sweet high-fat liquid induces increased palatability and motivation to consume in a rat model of binge consumption

Binge eating disorders are characterized by discrete episodes of rapid and excessive food consumption. In rats, giving intermittent access to sweet fat food mimics this aspect of binge eating. These models typically employ solid food; however, the total amount consumed depends on motivation, palatability and satiety, which are difficult to dissociate with solid food. In contrast, lick microstructure analysis can be used to dissociate these parameters when the ingestant is a liquid. Therefore, we developed a binge model using a liquid emulsion composed of corn oil, heavy cream and sugar. We show that rats given intermittent access to this high-fat emulsion develop binge-like behavior comparable to that previously observed with solid high-fat food. One feature of this behavior was a gradual escalation in consumption across 2.5 weeks of intermittent access, which was not apparent in rats given lower-fat liquid on the same access schedule. Lick microstructure analysis suggests that this escalation was due at least in part to increases in both motivation to consume and palatability-driven consumption.

Feeding response of rats to no-fat and high-fat cakes

The nutritional effects of high-fat diets have been extensively studied in laboratory animals, but as yet few experiments have examined the feeding response of animals to newly developed fat substitutes. The present study used commercially available no-fat (0% fat, 92% carbohydrate) and high-fat (41% fat, 54% carbohydrate) cake to determine the effects of fat substitutes on food preference and caloric intake in rats. The first experiment showed that nondeprived rats found the high-fat and no-fat cakes equally palatable and highly preferred to lab chow. Food deprived rats, however, preferred the high-fat cake to the no-fat cake, which may be related to its higher caloric density. In the second experiment, rats fed high-fat cake, in addition to chow, for 30 days consumed more calories and gained more weight than did rats fed no-fat cake and chow. The no-fat cake group, however, overate and gained more weight than chow-only controls. The hyperphagic response to the no-fat cake can be attributed to its carbohydrate content, moisture, and high palatability. Thus, removing fat from the cake reduced, but did not eliminate, its obesity-promoting effect. Obviously, low-fat foods must be consumed in moderation if used for weight control.

Nutrient preference and diet-induced adiposity in C57BL/6ByJ and 129P3/J mice

Purified carbohydrates and fats are usually palatable to humans and other animals, and their consumption often induces weight gain and accumulation of fat. In this study, we examined consumption of complex carbohydrates (cornstarch and Polycose) and fats (soybean oil and margarine) in mice from two inbred strains, C57BL/6ByJ and 129P3/J. At lower concentrations of liquid nutrients tested using two-bottle tests, when the amounts consumed had negligible energy content, the C57BL/6ByJ mice had higher acceptance of Polycose and soybean oil. This was probably due to strain differences in chemosensory perception of Polycose and oil. At higher concentrations, the mice consumed a substantial part of their daily energy from the macronutrient sources, however, there were no or only small strain differences in nutrient consumption. These small differences were probably due to strain variation in body size. The two strains also did not differ in chow intake. Despite similar energy intakes, access to the nutrients resulted in greater body weight (BW) gain in the C57BL/6ByJ mice than in the 129P3/J mice. The diet-induced weight gain was examined in detail in groups of 2-month-old C57BL/6ByJ and 129P3/J mice given ether chow, or chow and margarine to eat. Access to margarine did not increase total energy consumption of either strain. It increased BW and adiposity of the C57BL/6ByJ mice, but only after they reached the age of approximately 3 months. There were no differences in BW and adiposity between control and margarine-exposed 129P3/J mice. The results suggest that diet-induced adiposity in the B6 mice depends on age and does not depend on hyperphagia.

Calcium-deprived rats avoid sweet compounds

To characterize the link between calcium status and sweet solution intake, rats fed a diet containing 25 mmol Ca2+/kg (Ca-25, low calcium) or 150 mmol Ca2+/kg (Ca-150, control) were given 48-h two-bottle tests with a choice between water and various concentrations of a nutrient (sucrose, Polycose, ethanol and/or corn oil). Rats fed the Ca-25 diet had significantly lower sucrose intakes and preferences over the entire range tested (10-320 g/L) even though the same (Experiment 1), or identically treated (Experiment 2a) rats had normal Polycose and ethanol intakes and normal (Experiment 1, 2b) or significantly greater (Experiment 2a) corn oil intakes. In additional tests, rats fed the Ca-25 diet had significantly lower intakes relative to rats fed the Ca-150 diet of other sweeteners (30 mmol/L D-phenylalanine, 1 mmol/L saccharin and 0.3 mmol/L aspartame), significantly higher intakes of 0.5 mg/L capsaicin and 300 mmol/L monosodium glutamate, and normal intakes of 10 g/L or 80 g/L safflower oil and 10 g/L peanut oil. In a three-cup macronutrient selection experiment (Experiment 3), calcium-deprived rats ate significantly less of a high sucrose carbohydrate source and significantly more of a protein source than did controls. These results suggest that calcium deficiency reduces the rat's liking for sweetness, irrespective of the type or form of sweetener, and that this is not due to a general reduction in energy intake.

Heritable variation in food preferences and their contribution to obesity

What an animal chooses to eat can either induce or retard the development of obesity; this review summarizes what is known about the genetic determinants of nutrient selection and its impact on obesity in humans and rodents. The selection of macronutrients in the diet appears to be, in part, heritable. Genes that mediate the consumption of sweet-tasting carbohydrate sources have been mapped and are being isolated and characterized. Excessive dietary fat intake is strongly tied to obesity, and several studies suggest that a preference for fat and the resulting obesity are partially genetically determined. Identifying genes involved in the excess consumption of dietary fat will be an important key to our understanding of the genetic disposition toward common dietary obesity.

Geometric analysis of macronutrient selection in the rat

A conceptual framework is introduced which has been derived from work on insects. The scheme is intended to integrate studies of diet selection, regulation of amounts eaten, nutrient utilization, body composition and animal performance. Aspects of framework are illustrated with published data on macronutrient selection in the rat. An animal is viewed as moving through a multidimensional nutrient space, which is bounded by axes representing each required nutrient and within which lie optimal points of intake and nutrient allocation ("targets"). The aim is first to estimate the location of these functional optima experimentally, and then to interpret the responses of animals which are constrained from reaching them ("decisions of best compromise"). The framework can then be used to interpret data from animals reared under differing environmental conditions and to compare animals of differing developmental stage, genotype or nutritional state.

Macronutrient choice following food deprivation: effect of dietary fat dilution

Under standard laboratory conditions rats given access to three separate macronutrient sources compose a diet yielding 31% of their total daily calories as protein, 34% as carbohydrate, and 34% as fat. This selection pattern is dramatically altered with restored access following a 48 h fast. During the first hour of refeeding, rats composed a diet that was low in protein and high in carbohydrates and fat. By the end of 24 h, no difference in selection pattern was found, though intake of all three macronutrients was higher than baseline. A separate group given access to three macronutrient sources of equal caloric density specifically increased fat intake during the period of restored access. Another group, familiarized with a concentrated fat source, was given access to a diluted fat source during refeeding. Similarly, a fourth group, familiarized with a diluted fat source during the baseline condition was given access to a concentrated fat source during refeeding. Results from these experiments suggest that prior experience with a diluted fat source promotes a significant increase in fat intake and a suppression of carbohydrate intake during initial refeeding following a 48 h fast. In a second experiment, rats that were given a choice of both fat sources preferred the concentrated source; 72% of all fat (g) and 82% of all fat calories were consumed from the concentrated fat source. These results suggest that fat intake increases following deprivation not solely due to its inherent relatively increased caloric density but also possibly due to its role in energy metabolism.

Orosensory stimulation of feeding by oils in preweanling and adult rats

To determine the orosensory control of ingestion by oils, we investigated the acceptance and preference of corn oil and of mineral oil in a series of experiments with preweanling rat pups, adult male, Sprague-Dawley rats, and adult female lean and obese (fa/fa) Zucker rats. Emulsions of corn oil in water (greater than or equal to 25%), administered by anterior intraoral catheter, elicited more intake than water at 14, but not 7, days of age. By 21 days of age, intake was a sensitive function of emulsified corn oil concentrations (greater than or equal to 6.25%). In experiments with adult rats, the sham feeding technique was used to maximize the relationship between orosensory stimulation and intake. Rats sham fed approximately similar volumes of 100% corn oil and 100% mineral oil. Maximal rate of intake was about 1 ml/min. The large intake of mineral oil is compelling evidence for the potency of the orosensory stimuli of oil to control intake in the absence of postingestive metabolic effects. The orosensory control is very sensitive in the adult rat--0.78% corn oil produced a larger intake than water. The orosensory control of acceptance (1-bottle tests) did not correlate with preference (2-bottle tests): 100% corn oil and 100% mineral oil were equally accepted, but rats showed a very strong preference for 100% corn oil. The peripheral sensory mechanism of this orosensory control is unknown. As a working hypothesis, we suggest that oral tactile mechanisms of the trigeminal afferent nerves provide the necessary sensory pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Corn oil and mineral oil stimulate sham feeding in rats

To determine the orosensory effects of oils on ingestion, we measured the 1-bottle intake of corn oil and of mineral oil during 30 minutes of sham feeding in rats that were food deprived overnight or nondeprived. Rats sham fed both oils. Food-deprived rats ingested significantly more of both oils than nondeprived rats. Rats discriminated corn oil from mineral oil and as little as 0.78% corn oil emulsion from water. When rats sham fed 8 dilutions of corn oil, intake was an inverted-U function of concentration with maximal intakes produced by 12.5%, 25% and 50% corn oil emulsions. Despite similar, sometimes equal, intakes of corn oil and mineral oil in 1-bottle tests, food-deprived and nondeprived rats showed a strong preference for corn oil in 2-bottle, sham-feeding, preference tests. The sensory mechanisms that mediate the oral effects of oil on intake and preference are not known, but the olfactory and trigeminal sensory systems are the most likely candidates. Further work is required to characterize the potency, sensitivity, and discriminability of the orosensory effects of oils, the mechanisms that mediate them, and their role in the control of fat intake.

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.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Nutritional influences on dietary selection patterns of obese and lean Zucker rats

Little is known about the behavioral, nutritional, and metabolic events that control dietary intake quality. Two experiments are described that manipulate nutritional conditions that have been hypothesized to affect dietary item choice so as to assess what effect, if any, the added factor of genetic obesity has in modifying the response to these manipulations. In the first experiment, 5 week old male obese and lean Zucker rats were fed a diet that varied in protein content (10%, 20%, or 60% casein by weight) for ten weeks. They were then allowed to select a diet from three separate macronutrient sources (casein, starch, or corn oil). Although body weights at the end of the 10 week maintenance period were markedly different, selection patterns were not influenced by pre-feeding different levels of protein. Obese rats selected a diet that was higher in fat and lower in protein than the diets composed by lean rats. In the second experiment, four groups of 7 month old obese and lean Zucker rats were given access to one of four diets that varied in protein content (5%, 10%, 15%, or 20% casein by weight). In addition, each rat was periodically given access to a 32% sucrose solution. Access to sucrose promoted increases in total caloric intake, independent of the protein content of the diet. Obese rats typically ate more calories per day than did their lean littermates. Results from these experiments suggest that food item selection is determined more by factors associated with obesity than by factors associated with dietary history.