Caloric compensation to gastric loads in rats with hypothalamic hyperphagia

The physiology of experimental overfeeding in animals

BACKGROUND

Body weight is defended by strong homeostatic forces. Several of the key biological mechanisms that counteract weight loss have been unraveled over the last decades. In contrast, the mechanisms that protect body weight and fat mass from becoming too high remain largely unknown. Understanding this aspect of energy balance regulation holds great promise for curbing the obesity epidemic. Decoding the physiological and molecular pathways that defend against weight gain can be achieved by an intervention referred to as 'experimental overfeeding'.

SCOPE OF THE REVIEW

In this review, we define experimental overfeeding and summarize the studies that have been conducted on animals. This field of research shows that experimental overfeeding induces a potent and prolonged hypophagic response that seems to be conserved across species and mediated by unidentified endocrine factors. In addition, the literature shows that experimental overfeeding can be used to model the development of non-alcoholic steatohepatitis and that forced intragastric infusion of surplus calories lowers survival from infections. Finally, we highlight studies indicating that experimental overfeeding can be employed to study the transgenerational effects of a positive energy balance and how dietary composition and macronutrient content might impact energy homeostasis and obesity development in animals.

MAJOR CONCLUSIONS

Experimental overfeeding of animals is a powerful yet underappreciated method to investigate the defense mechanisms against weight gain. This intervention also represents an alternative approach for studying the pathophysiology of metabolic liver diseases and the links between energy balance and infection biology. Future research in this field could help uncover why humans respond differently to an obesogenic environment and reveal novel pathways with therapeutic potential against obesity and cardiometabolic disorders.

Feeding behavior during experimentally induced obesity in monkeys

In order to investigate the effects of the induction and remission of obesity on feeding behavior, male rhesus monkeys were made obese by sustained intragastric (IG) feeding of a complete liquid diet. Intragastric diet infusion levels of 100, 125, 145, and 165% of the baseline oral intake of each monkey were successively administered. During the initial overfeeding period (100% of the baseline oral intake), at least one week was required to reduce voluntary oral intake to less than 25% of the baseline levels and complete suppression of oral intake did not occur. This increased total caloric intake (IG infusion plus oral intake) resulted in a rapid rate of weight gain of at least 5 times the baseline rate. With successive increases in caloric infusion level, oral intake was eventually suppressed, and rapid weight gain was sustained. When the IG infusion was abruptly terminated after 50 to 130 days, 3 monkeys refused all food for 14 to 35 days. The monkeys' oral intakes stabilized three to ten weeks after the end of the overfeeding period. The length of this period prior to the resumption of normal oral intake was not related to length of overfeeding nor to the amount of weight gained. The monkeys' body weights dropped rapidly in the initial post-overfeeding period and then stabilized, sometimes at levels higher than their baseline body weights. In 2 monkeys, at the end of overfeeding the amounts infused were gradually reduced in order to determine the calories required to maintain their body weights at peak levels. Significantly fewer kcal/kg were required to maintain peak body weights than were ingested during the baseline periods.

Gastrointestinal factors in hunger and satiety

The evidence for the participation of signals from the gastrointestinal tract in hunger and satiety is reviewed. Various methods of subjecting only certain portions of the gastrointestinal tract to food are described including sham feeding, crossing of the intestines of two animals, and intragastric, intraduodenal and intravenous feeding. These methods have revealed that animals eat more when only the mouth is exposed to food, but consume less when food enters the stomach directly and still less when feeding themselves intravenously. The role of the stomach in satiety is discussed in relation to the results of the many experiments in which intragastric loads were administered. These experiments generally revealed that gastric loads suppressed intake in proportion to their caloric value, but that compensation was not precise. A similar analysis is made of the role of the intestines in satiety. These studies present evidence for osmoreceptors and glucoreceptors, distention, and pressure, all of which may participate in satiety. The numerous effects of vagotomy on food intake are reviewed and an attempt is made to separate afferent from efferent effects and vagal effects from nonspecific surgical injury. The role of other afferents is explored. A brief survey of the most recent studies on humoral factors indicates that cholecystokinin is probably acting as a satiety agent, but its mode of action remains unknown. The changes in the gastrointestinal tract that accompany an increase or decrease in food intake reveal adaptive changes that occur in the organ. The evidence for gastrointestinal factors in satiety of the suckling animal is presented. Finally, a hypothesis is presented in which the various factors that bring a meal to an end are included. Preabsorptive satiety may consist of multiple factors which probably include signals from osmoreceptors and possibly other chemoreceptors in the upper small intestine. The signals from these receptors are neural while other products of digestion stimulate the release of humoral agents, such as cholecystokinin, which may act locally, in the liver, or on the central nervous system. Stretch or distention of the intestines at various locations may also contribute to preabsorptive satiety.

Food intake after intragastric meals of short-, medium, or long-chain triglyceride

Food-deprived Sprague-Dawley rats were given equicaloric intragastric infusions of mixed meals consisting largely of short- (SCT), medium- (MCT), or long-chain triglyceride (LCT). When animals were allowed to feed 20 min after infusion, there was an immediate reduction of food intake that was sustained over the 2 hr feeding period. During the first hour of feeding, the SCT, which is digested and absorbed more rapidly than the MCT or the LCT, was more effective per calorie in reducing food intake than these longer-chain triglycerides. However, during the second hour, cumulative intakes after the different triglyceride infusions were not significantly different. Equicaloric infusions of the MCT and the LCT resulted in equivalent reductions of food intake at all times. The satiety effects of these two triglycerides appear to be related to their caloric properties rather than to chain length. Since the LCT reduced food intake before the absorbed fat could have entered the blood to stimulate satiety signals, this satiety effect may be mediated by a gastroenteric signal. None of the triglyceride infusions resulted in a conditioned taste aversion suggesting that food intake was reduced through normal satiety rather than through discomfort.

The enhancement of chlorpromazine-induced hypothermia by lesions in the anterior hypothalamus

1 Administration of chlorpromazine (Cpz), either systemically or centrally, to unanaesthetized rats at an environmental temperature of 23 degrees C caused dose-dependent hypothermia. 2 In order to achieve equivalent hypothermia, intraventricular administration required a total dose of 20 microgram Cpz and and intraperitoneal administration a dose of 9.7 mg/kg body weight. Accordingly, the dose-ratio between intraventricular and intraperitoneal administration was 1 to 110. Cpz apparently exerts its hypothermic effect by acting directly on central nervous structures rather than through peripheral sites. 3 Cpz-induced hypothermia was potentiated by preoptic anterior hypothalamic (POAH) lesions but not by lesions of the ventromedial nucleus (VMN) of the hypothalamus. It was found that Cpz induced hypothermia most readily in rats with large POAH lesions (-10.4 degrees C), less so in rats with spinal lesions (-5.5 degrees C) at least with control rats (-2.9 degrees C).

Satiety: a graded behavioural phenomenon regulating caloric intake

Rhesus monkeys inhibited their feeding in response to intragastric nutrient preloads, so as to maintain a constant caloric intake. There is a definite dose- response relationship to calories which does not depend on the volume, caloric concentration, or nature of the nutrient. This accurate control is accomplished even though some of the preload remains in the stomach throughout the feeding period.