INTRODUCTION

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THE RAT MODEL OF DIET-INDUCED obesity (DIO) has been useful in studying the role of the brain in regulating energy homeostasis. In the outbred Sprague-Dawley rat, there is a number of abnormalities of neural function in obesity-prone rats that are corrected only after these animals are allowed to become obese on a diet relatively high in fat, sucrose, and caloric content [high-energy (HE) diet] (14-16, 18). Once obesity is established and these neural abnormalities are "normalized," DIO rats avidly defend their higher body weight against chronic caloric restriction (14, 19, 23). Similarly, outbred rats that are resistant to DIO [diet-resistant (DR) rats] do not become obese on an HE diet but can be made obese by feeding them a highly palatable liquid diet (23). Parallel to DIO rats, DR rats defend their body weight against both chronic caloric restriction and overfeeding (19, 23).

We previously created substrains of the outbred Sprague-Dawley strain by selectively breeding the highest and lowest weight gainers on an HE diet (21). These selectively bred DIO and DR rats segregated out within three to five generations into genetically distinct substrains, suggesting a polygenic mode of inheritance. This selective breeding produced DIO and DR rats that breed 100% true to their weight gain phenotype. In addition, the selectively bred DIO rats begin to become obese at 4-5 wk of age, even when fed low-fat chow from weaning (21). By contrast, outbred DIO rats become obese only when fed an HE diet (14-16, 18). Thus it appears that the selective breeding process has concentrated the genes promoting obesity, producing a highly metabolically efficient rat.

Although the outbred DIO and DR rats have been well characterized with respect to both peripheral metabolic and central neural functions, few studies have been done to characterize the selectively bred DIO and DR rats. The current studies were undertaken to characterize the way in which selectively bred DIO and DR rats respond to diets low in fat and calories (chow); moderate fat, sucrose, and caloric content (HE diet); and high palatability (Ensure). In addition, we examined the neural correlates of chronic exposure to a highly palatable diet vs. their response to cycling among the three types of diets. These studies showed that selectively bred DIO rats will accurately defend differing, specific body weights that are dependent on the composition and/or palatability of the diet to which they are exposed.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Animals and experimental protocol. Male Sprague-Dawley rats from our in-house colony of rats selectively bred to develop DIO and DR (21) were used in each of three experiments beginning at 10-12 wk of age. All rats were kept at 23-24°C on a 12:12-h light-dark cycle and fed Purina rat chow (#5008) from weaning. Generally, food intake and body weight were monitored for the week before the onset of dietary manipulations and were then followed throughout.

Experiment 1: Effect of diet on energy homeostasis. Groups of six to eight selectively bred DIO and DR rats were maintained on either chow, HE diet, or chocolate-flavored Ensure (Ross Products) plus HE diet (this combination is referred to as "Ensure" throughout). Purina rat chow contains 3.30 kcal/g with 23.4% as protein, 4.5% as fat, and 72.1% as carbohydrate that is primarily in the form of complex polysaccharide (22). HE diet is composed of 8% corn oil, 44% sweetened condensed milk, and 48% Purina rat chow (Research Diets) and contains 4.47 kcal/g with 21% of the metabolizable energy content as protein, 31% as fat, and 48% as carbohydrate, 50% of which is sucrose (22). Ensure is a liquid diet that contains 1.06 kcal/ml with 14% of the metabolizable energy content as protein, 22% as fat, and 64% as carbohydrate. Rats were kept on their respective diets for 10 wk with body weight monitored weekly (Fig. 1). During the last week, food intake was monitored. At the end of 10 wk, rats were decapitated and trunk blood was taken for plasma glucose, insulin, and leptin levels. Epididymal, retroperitoneal, perirenal, and mesenteric fat pads were removed and weighed.

View larger version (27K): [in this window] [in a new window]   Fig. 1.   Body weight gain in selectively bred, 3-mo-old male Sprague-Dawley diet-resistant (DR) and diet-induced obesity (DIO) rats fed chow, high-energy (HE) diet, or Ensure + HE diet (En) for 10 wk in experiment 1. Data are means ± SE; n = 6-8/group.

Experiment 2: Defense of a hyperphagia-induced body weight arrived at by intake of a palatable diet. Our prior studies (23) showed that outbred DR rats could be made obese by intake of Ensure but that they would not defend this higher body weight when placed back on a low-fat diet. This experiment was undertaken with the hypothesis that DIO rats would similarly not defend the higher body weight arrived at by hyperphagia induced by intake of a highly palatable diet when subsequently switched to a low-fat diet. Toward this end, selectively bred DIO male rats were placed on either an HE diet (n = 8) or Ensure plus HE diet (n = 16) at 12 wk of age. A final group (n = 8) was fed chow. Rats were kept on their respective diets for 10 wk with intake monitored during this week. All of the rats on HE diet and one-half of the rats on Ensure plus HE diet were then switched to chow ad libitum. At 3 wk after the diet switch (week 13), blood for leptin was taken by tail nip. During the 7th week after the diet switch (week 17), food intake was measured and urine for 24-h norepinephrine (NE) levels was collected. Animals were decapitated, and trunk blood was taken for leptin and insulin levels. Fat pads were removed and weighed. The brains were quickly removed, frozen on dry ice, and stored at 80°C until assayed for arcuate nucleus (Arc) neuropeptide Y (NPY), proopiomelanocortin (POMC), and prodynorphin (dynorphin) mRNA.

Experiment 3: Effect of diet switching on weight regain and defense of a palatability-induced body weight. The results of experiment 2 suggested that switching DIO rats from Ensure to chow had permanent effects on Arc POMC and dynorphin mRNA that might predispose them to regain weight more rapidly if exposed to an HE or Ensure diet. Also, there was the issue of whether DIO rats would defend a palatability-induced higher body weight if reexposed to the palatable diet. Thus two groups of selectively bred DIO male rats (n = 8/group) were started on an HE diet and three groups were placed on Ensure plus HE diet for 14 wk. At 14 wk, two of three Ensure groups and both of the HE diet groups were then switched to chow. The remaining Ensure group was left on Ensure plus HE diet. Food intake and body weight were monitored weekly. At 7 wk after this diet switch, one of the Ensure groups that had been switched to chow was reexposed to Ensure plus HE diet and the other was given an HE diet alone. One group of the HE diet rats that had been switched to chow was reexposed to an HE diet and the other was left on chow. A final group (n = 8) was fed chow for 21 wk and then exposed to Ensure. Rats were kept on their respective diets for a further 7 wk and then were decapitated for a collection of brains and weighing of fat pads. Thus diets during the three periods were: 1) Ensure-Ensure-Ensure; 2) Ensure-chow-Ensure; 3) Ensure-chow-HE diet; 4) HE diet-chow-chow; 5) HE diet-chow-HE diet; and 6) chow-chow-Ensure (Fig. 2).

View larger version (23K): [in this window] [in a new window]   Fig. 2.   Timeline for diet switching in selectively bred DIO rats in experiment 3. Selectively bred DIO rats were placed on an HE diet or En for 14 wk, and then all were switched to chow (Ch) except for 1 group that was maintained on En (En/En/En). Rats were maintained on their respective diets for 7 wk (week 21). At that time, 1 group of HE/Ch rats was switched to an HE diet (HE/Ch/HE). One group of En/Ch rats was switched to HE (En/Ch/HE), whereas another was switched back to Ensure (En/Ch/En). A final group of rats was kept on chow from weaning and then placed on Ensure at week 21 (Ch/Ch/En). Diets 1, 2, and 3 refer to the 3 different phases of diet manipulation; n = 8 rats/group.

In situ hybridization for NPY, dynorphin, and POMC mRNA. Brains were processed for in situ hybridization by minor modifications of previously described methods (17). Briefly, the 511-bp probe [derived from the original probe of Higuchi et al. (10)] for NPY, the 923-bp probe for POMC (provided by D. Richard), and the 700-bp prodynorphin probe (provided by C. Billington) were subcloned into a pBluescript SK(+) vector at an EcoRI site. Radiolabeled cRNA was synthesized in vitro from BamHI linearized plasmids. Sense and antisense probes were transcribed with T3 and T7 promoters, respectively, using ³⁵S UTP (1,000 Ci/mmol; New England Nuclear). The probes were hydrolyzed in 0.5 M NaHCO for 30 min. Frozen sections of the brain were freeze thawed onto gel-coated slides and fixed in 4% paraformaldehyde. They were treated with acetic anhydride for 10 min and dehydrated through six steps of graded ethanol solutions. Prehybridization was carried out at 50°C for 30 min, and then sections were hybridized with labeled sense and antisense probes at 50°C overnight. After treatment with RNAase A, sections were washed, dehydrated, dried, and opposed to SB-5 X-ray film (Kodak) for 3 days. The resulting autoradiograms were read by an experimentally "blinded" observer using computer-assisted densitometry (Drexel). Areal and optical density measures were made of that portion of the Arc showing maximal NPY, POMC, and dynorphin mRNA expression. Additional readings for dynorphin were taken through the paraventricular, ventrolateral, and ventromedial subnuclei of the ventromedial nucleus (VMN) and the lateral hypothalamic area. Readings from the sections with the three largest areas were averaged for comparison among groups.

Urine NE and plasma leptin, insulin, and glucose levels. Urine was collected in metabolic cages at 12-h intervals over 24 h and assayed by high-performance liquid chromatography with electrochemical detection (15). Tail blood was collected in EGTA-coated capillary tubes, and the plasma was assayed for glucose by an automated glucose oxidase method (Beckman) and for insulin and leptin by radioimmunoassays (Linco) using antibodies to authentic rat insulin and leptin, respectively.

Statistics. Food intake and body weights were measured two times per week over the entire period. Therefore, ANOVA for repeated measures was used initially to compare intergroup differences. When significant intergroup differences were found (P 0.05), post hoc Scheffé's multiple comparison tests were carried out at each time point where significant differences across a given phase occurred. One-way ANOVA was used for single-point measures of terminal body and adipose depot weights, plasma glucose, insulin, and leptin at the end of each phase.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Experiment 1. This study was designed to characterize the long-term effects of three diets of varying composition and palatability on energy homeostasis in rats selectively bred for the DIO and DR genotypes. As in the outbred Sprague-Dawley male rat (23), chronic exposure of selectively bred DR rats to an HE diet had no effect on either body weight gain, food intake, total fat pad weights, leptin, or insulin levels compared with comparable DR rats fed chow (Fig. 1, Tables 1 and 2). Unlike outbred Sprague-Dawley rats where DIO- and DR-prone rats have the same body weights, intakes, and metabolic parameters at 2-3 mo of age (19, 23), the selectively bred 12-wk-old DIO rats used here were 25% heavier than selectively bred DR rats fed chow from weaning. The hyperphagia produced by feeding selectively bred DR rats Ensure produced a final body weight, body weight gain, and food intake that were greater than DR rats fed either chow or an HE diet. Their final body weight was comparable to DIO rats fed chow, and their body weight gain was midway between DIO rats fed chow and those fed an HE diet (Table 1). However, it is likely that total body fat mass and insulin resistance were greater in the Ensure-fed DR rats than the chow-fed DIO rats because their leptin and insulin levels were 90-100% higher. In addition, total fat pad weights and the ratio of adipose depot to body weights were midway between that for DIO rats fed chow and an HE diet. This demonstrates that selectively bred DR rats, parallel to the outbred DR rats, will become obese if given a highly palatable diet. Both DR and DIO rats preferred Ensure to an HE diet. When offered both diets simultaneously, both groups ate ~90% of their total calories in the form of Ensure during the last week of testing. This and the marked hyperphagia induced suggest that Ensure was more palatable than the HE diet.

Experiment 2. In this study, only selectively bred DIO rats were used to assess the defense of an obese body weight arrived at by chronic intake of either an HE diet or Ensure plus HE diet (Fig. 3). After 10 wk, DIO rats fed Ensure gained 61% more weight than those fed an HE diet. During the 10th week, the caloric intake of Ensure-fed rats was 23% greater than those on an HE diet (Table 3). When HE diet-fed rats were switched to chow, their body weights plateaued and they gained no further weight during the next 7 wk (Fig. 3). On the other hand, when Ensure-fed rats were switched to chow, they spontaneously reduced their intakes by 50-60% and, within 3 wk, their body weights fell to the level of HE rats that had been switched to chow. At this time, the plasma leptin levels of rats switched from Ensure to chow were also equal to rats switched from an HE diet to chow. This suggests that the loss of body weight in the rats switched from Ensure to chow was due to a loss of fat mass (Table 3). Their body weights remained at the level of rats switched from an HE diet to chow for the remaining 4 wk (Fig. 3). Despite this reequilibration of body weights for 4 wk, the intake of rats switched from Ensure to chow was still only 48% of those rats switched from an HE diet to chow during the last week of the study. In the face of this reduced intake, rats switched from Ensure to chow had 24-h urine NE levels that were almost twofold greater than those switched from an HE diet to chow (Table 3). These urine NE levels were comparable to rats that had remained on Ensure for the entire 14 wk, even though these rats weighed 125-130% more and their intake was 63% greater than rats switched from an HE diet to chow and 132% greater than those switched from Ensure to chow (Table 3). Terminally, the sum of the four adipose depot weights taken from rats maintained on Ensure throughout was 230% greater, the ratio of total fat pad to terminal body weights was 80% higher, and leptin levels were 147% greater than the other two groups of rats (Table 4). Finally, 24-h urine NE levels obtained during the last week of study were comparable in both groups of rats originally fed Ensure, and these were 94% higher than those switched from an HE diet to chow. Thus, even though the switch from Ensure to chow produced persistent reductions in body weight, fat mass, and caloric intake, these rats maintained an increased sympathetic nervous system activity.

View larger version (29K): [in this window] [in a new window]   Fig. 3.   Effect of prolonged exposure to a palatable diet on the defended body weight in experiment 2. Selectively bred DIO rats were fed chow (n = 8), an HE diet (n = 8), or En (n = 16) for 10 wk. At that time, 8 of the rats on En and all of the rats on an HE diet were switched to chow. Then all rats were maintained on their respective diets for an additional 7 wk. Data are means ± SE.

View larger version (31K): [in this window] [in a new window]   Fig. 4.   Effect of prolonged exposure to a palatable diet on the hypothalamic neuropeptide Y (NPY), proopiomelanocortin (POMC) and prodynorphin (dynorphin) mRNA expression. Rats were treated as in Fig. 2 and their brains processed for in situ hybridization for arcuate nucleus (Arc) NPY and POMC and dynorphin. Dynorphin expression was measured in the Arc, paraventricular (PVN), the ventrolateral (VMNvl) and dorsomedial (VMNdm) subnuclei of the ventromedial hypothalamic nuclei, and the lateral hypothalamic area (LH). Data are means ± SE of the product of area x optical density (OD). Bars in a given brain area for a specific peptide with differing superscripts differ from each other at the P < 0.05 level by post hoc Scheffé's test after ANOVA showed significant intergroup differences.

Experiment 3. This study was carried out to assess the effect of diet switching on subsequent weight gain on reexposure to Ensure or an HE diet 7 wk after the rats had been switched to chow. Figure 2 gives the timeline for the various combinations of diet switches among the six experimental groups. Given the reduction in Arc POMC and dynorphin mRNA expression in rats switched from Ensure to chow, we postulated that they would gain weight more rapidly than rats never exposed to Ensure when subsequently fed an HE diet or if they were reexposed to Ensure plus HE diet. In fact, prior diet exposure had no effect on subsequent weight gain. Rats previously fed Ensure and then switched to chow gained weight at exactly the same rate on Ensure exposure as those kept on chow for the entire time (Fig. 5). Both groups gained weight rapidly on Ensure. After only 2-3 wk, they matched the weight gain trajectory and then maintained the same weight gain curve of the rats maintained on Ensure for an additional 4 wk (Fig. 5). Intake generally followed weight gain for the respective groups (Fig. 6). At 7 wk after they were switched to their respective diets, terminal body and fat pad weights were comparable among the three groups of Ensure-fed rats. Their terminal body weights were 12% heavier than the two groups on an HE diet and 23% heavier than the group on chow [F(2,41) = 10.97; P = 0.0001]. The total fat pad weights of Ensure-fed rats were 29% heavier than those on an HE diet and 125% heavier than the chow-fed rats [F(2,41) = 22.31; P = 0.0001; Table 5]. Neither did prior cycling from Ensure to chow predispose to greater weight on subsequent exposure to an HE diet. These rats gained weight at the same rate as those switched from an HE diet to chow and then back to an HE diet. Both of these groups gained significantly more weight than rats maintained on chow after their switch from an HE diet.

View larger version (30K): [in this window] [in a new window]   Fig. 5.   Effect of diet switching on body weight gain in selectively bred DIO rats in experiment 3. The protocol for diet switching and abbreviations are given in Fig. 2. Data are means ± SE.

View larger version (26K): [in this window] [in a new window]   Fig. 6.   Effect of diet switching on caloric intake in selectively bred DIO rats in experiment 3. Groups and methods are as described in Fig. 5.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The outbred Sprague-Dawley rat strain contains two distinct DIO and DR substrains with the characteristics of high vs. low weight gain on an HE diet, respectively (24). Phenotypic differences in body weight gain, adiposity, and insulin sensitivity are only expressed between these outbred substrains when they are placed on a diet of moderate fat, sucrose, and caloric content (24). We selectively bred for the high vs. low weight gain phenotypes to produce two genetically distinct substrains of DIO and DR rats (21). The current studies were carried out to characterize more fully these substrains with regard to their weight gain and defense of body weight on diets of differing macronutrient content and palatability. As previously reported (21), when fed chow from weaning, the selectively bred DIO rats here gained more weight and so were heavier at the onset of the studies than DR rats. This differs from outbred Sprague-Dawley DIO-prone and DR rats that show no differences in body weight before exposure to an HE diet (24). Similar to outbred rats, selectively bred DIO, but not DR, rats increased their body weight gain when chronically exposed to an HE diet. Unlike outbred DIO rats that eat approximately the same amount as DR rats on an HE diet over 2-3 mo (24), selectively bred DIO rats consumed more calories than DR rats fed chow, HE diet, or Ensure plus HE diet, respectively. This is presumably because of higher metabolic demands associated with their larger size and lean body mass (21). Outbred DR rats become as obese as DIO rats on an HE diet when made hyperphagic on a highly palatable liquid diet such as Ensure (23). Here, selectively bred DR rats fed Ensure gained only as much body weight as chow-fed DIO rats, probably because they were so much lighter than DIO rats at the onset of the study. However, Ensure-fed DR rats and HE diet-fed DIO rats had comparable plasma leptin levels, suggesting that they were comparably obese.

Both selectively bred DR and DIO rats became hyperphagic and excessively obese when chronically exposed to Ensure. It is possible that the hyperphagia produced by Ensure might simply be due to the fact that it is a liquid diet. Because of its lower caloric density than solid chow or HE diets, rats might overeat simply because of poor caloric compensation for this lower caloric density. However, there are important reasons to believe that palatability is the predominant force driving the hyperphagia. First, when given the choice between Ensure and an HE diet, both DR and DIO rats consumed ~90% of their calories from Ensure. Second, neither DIO nor DR rats will overeat on either vanilla- or strawberry-flavored Ensure. These facts strongly support the contention that they perceive Ensure as highly palatable and overeat on it because of its palatability.

As with outbred DR rats made obese on Ensure (23), the Ensure-fed selectively bred DIO rats would not defend their higher body weight when diet palatability was reduced. They spontaneously reduced their intake and rapidly lost weight and adipose mass over 3 wk to the level of rats switched from an HE diet to chow. By contrast, the switch from an HE diet to chow resulted only in a plateau in body weight gain. Outbred DIO rats will remain obese for more than 3 mo after such a switch to a low-fat diet (22). This plateau weight was rapidly reached and maintained here by DIO rats switched from Ensure to chow, although their intake persisted at only one-half that of rats switched from an HE diet to chow. Furthermore, this reduced intake was associated with a paradoxical increase in tonic sympathetic activity as assessed by 24-h urine NE levels. Increased sympathetic activity is expected when rats become hyperphagic on a highly palatable diet (13). There is no obvious explanation for the curious combination of reduced intake and increased sympathetic activity in rats switched from Ensure to chow. The minimally reduced Arc POMC mRNA expression at 7 wk after the switch from Ensure to chow should have increased intake and decreased sympathetic activity due to reduced -melanocyte-stimulating hormone synthesis and release (8).

On the other hand, -endorphin is also produced by Arc POMC neurons (2). This endogenous opioid, along with dynorphin, appears to be involved in the intake of palatable foods (6, 7, 29, 30). The reduced Arc dynorphin expression in rats switched from Ensure to chow led to the original, erroneous prediction that reexposure to Ensure would cause them to gain weight more rapidly than those naive to Ensure. Despite the failure to confirm this hypothesis, the altered dynorphin expression in these rats was quite interesting. Expression was reduced selectively in the ventrolateral subdivision of the VMN of all rats chronically fed Ensure, independent of prior dietary history. The consistent and selective diet-induced depression of dynorphin expression suggests that this subnucleus is particularly responsive to the intake of a palatable diet. Besides dynorphin, the ventrolateral VMN subnucleus also contains somatostatin and enkephalin neurons, many of which have estrogen receptors and project to the parabrachial nucleus (9, 25). Neurons in the ventrolateral VMN are also activated when the forebrain is perfused with glucose (3), suggesting that they might be involved in the detection and regulation of nutrient balance.

On the other hand, diet cycling was associated with plastic change of Arc POMC and Arc paraventricular nucleus dynorphin expression. The switch from Ensure to chow selectively reduced expression of these peptides in these areas 7 wk after the diet switch. Dynorphin expression in the region of the Arc is particularly sensitive to changes in palatability (29), so that reduced expression might have contributed to their decreased intake of a diet of low palatability. Reexposure to Ensure normalized Arc dynorphin and POMC expression, whereas exposure to an HE diet resulted in raised paraventricular nucleus dynorphin expression. Such plasticity is reminiscent of the changes in ₂-adrenoceptors associated with diet cycling in outbred DIO and DR rats (14). Similar to outbred DIO rats, such diet cycling had no effect on subsequent weight gain or metabolic efficiency in selectively bred DIO rats reexposed to Ensure or an HE diet. On the other hand, such diet cycling does markedly increase the weight gain and metabolic efficiency of outbred DR rats (14). Although the underlying mechanisms for these genotype differences are unknown, they represent another important example of the interactions among diet and genetic backgrounds and the neural systems regulating energy homeostasis.

The other critical point of these studies is that dietary composition and palatability produced at least three different defended rates of body weight gain. Because Sprague-Dawley rats gain weight throughout their lives, regaining a prior body weight means hitting a moving target when weight gain is altered by changing diet composition and palatability. In both outbred (14) and selectively bred DIO rats, switching from an HE diet to chow produces a weight gain plateau, whereas switching back to an HE diet again resets an elevated rate of weight gain. In both outbred DR (23) and selectively bred DIO rats, switching from Ensure to chow produces hypophagia and a rapid weight loss to the level of comparable chow-fed controls. Although DR rats match control intakes as soon as their excess weight is shed (23), DIO rats here remained hypophagic for a full 4 wk after matching the weight gain curves of the controls. Finally, when chow-fed, outbred DIO and DR rats are food restricted to ~85% of their body weight, they rapidly recover the weight gain trajectory of chow-fed controls of the same phenotype when allowed to eat ad libitum (19, 23).

In summary, genetic background, diet composition, and palatability have differential effects on both brain neuropeptide systems and the food intake and body weight gain patterns that these neuropeptide systems control. Our results suggest that there may be at least two parallel but interacting neural systems that regulate energy homeostasis. One is responsive to metabolic factors related to the nutritional content of the diet. The other responds to motivational factors relating to texture, palatability, and reinforcing properties of the diet.

Perspectives

What, if anything, do such studies tell us about the potential treatment of obesity in humans? First, it suggests that even those individuals with a genetic predisposition toward leanness can become obese if the palatability of the diet is maintained at sufficiently high levels. One might predict that such individuals would be most successful in their attempts to lose weight if the psychosocial contributions to their obesity could be modified. On the other hand, our data suggest that individuals with a genetic predisposition toward obesity would face a difficult problem in our current environment where foods of high palatability and energy content are abundant. These studies suggest that genetically predisposed animals are born with a neural template that predisposes them to weight gain if sufficient energy is made available (14, 16, 18). Once such a preprogrammed weight is achieved, neural systems controlling energy homeostasis function normally and the new body weight is avidly defended (14, 19, 23). Such individuals must thus fight against an elevated metabolic and palatability-driven set point if weight loss is to be accomplished. The combination of heritable factors and environmental excess could certainly account for the extremely low success rate in the treatment of obesity in humans. If humans resemble DIO rats at all, it seems likely that drugs designed to alter both metabolic and motivational aspects of body weight maintenance will be required for long-term treatment of such individuals.