Reversible obesity and plasma fat metabolites

Impact of ketosis on appetite regulation-a review

To reduce the health burden of obesity, it is important to identify safe and practical treatments that are effective for weight loss while concurrently preventing weight regain. Diet-induced weight loss is usually followed by a concomitant increase in ghrelin secretion and feelings of hunger, which may compromise weight loss goals and increase the risk of weight regain. The aim of this review is to describe the status of knowledge regarding the impact of ketosis, induced by diet or exogenous ketones (ketone esters), on appetite and the potential mechanisms involved. Ketogenic diets (KDs) have been shown to prevent an increase in ghrelin secretion, otherwise seen with weight loss, as well as to reduce hunger and/or prevent hunger. However, the exact threshold of ketosis needed to induce appetite suppression, as well as the exact mechanisms that mediate such an effect, has yet to be elucidated. Use of exogenous ketones may provide an alternative to KDs, which have poor long-term adherence due to their restrictive nature. Ketone esters have been shown to have concentration-dependent effects on food intake and body weight in rodent models, with effects becoming apparent when 30% of total dietary energy comes from ketone esters (threshold effect). In humans, acute consumption of a ketone ester drink reduced feelings of hunger and increased satiety compared to a dextrose drink. With the emerging widespread acceptance of KDs and exogenous ketones in mainstream media and the diet culture, it is important to fully understand their role on appetite control and weight management and the potential mechanisms mediating this role.

Integration of metabolism and intake regulation: a review focusing on periparturient animals

There has been great interest in dry matter intake regulation in lactating dairy cattle to enhance performance and improve animal health and welfare. Predicting voluntary dry matter intake (VDMI) is complex and influenced by numerous factors relating to the diet, management, housing, environment and the animal. The objective of this review is to identify and discuss important metabolic factors involved in the regulation of VDMI and their integration with metabolism. We have described the adaptations of intake and metabolism and discussed mechanisms of intake regulation. Furthermore we have reviewed selected metabolic signals involved in intake regulation. A substantial dip in VDMI is initiated in late pregnancy and continues into early lactation. This dip has traditionally been interpreted as caused by physical constraints, but this role is most likely overemphasized. The dip in intake coincides with changes in reproductive status, fat mass, and metabolic changes in support of lactation, and we have described metabolic signals that may play an equally important role in intake regulation. These signals include nutrients, metabolites, reproductive hormones, stress hormones, leptin, insulin, gut peptides, cytokines, and neuropeptides such as neuropeptide Y, galanin, and corticotrophin-releasing factor. The involvement of these signals in the periparturient dip in intake is discussed, and evidence supporting the integration of the regulation of intake and metabolism is presented. Still, much research is needed to clarify the complex regulation of VDMI in lactating dairy cows, particularly in the periparturient animal.

The effect of triacylglycerol chain length on food intake in domestic fowl

Triacylglycerols of varying chain lengths were tested for their effects on food intake in fasted Single-Comb White Leghorn cockerels 9-12 weeks of age. Tributyrin, tricaprylin, triolein, or saline were intubated intragastrically or infused intrahepatically. All triacylglycerol administrations were in 10 ml volumes with 10 kcal being infused intrahepatically and 10 or 20 kcal being infused intragastrically. When administered intragastrically, the short-chained triacylglycerol tributyrin decreased food intake at most times tested; the medium chain-length tricaprylin and the long chain-length triolein affected food intake sporadically. When infused into the liver, tributyrin and tricaprylin decreased food intake at most times tested, whereas triolein decreased food intake at 60 and 180 minutes only. Satiating effectiveness was related to triacylglycerol chain length when intubated, which may reflect rate of absorption from the gut; however, hepatic satiety receptors were also differentially sensitive to triacylglycerol chain length indicating that more than rate of assimilation from the gut must be considered.

Does dietary hyperphagia contradict the lipostatic theory?

It has frequently been suggested that body weight or fat somehow exerts an inhibitory influence on food intake in a way that acts to maintain a stable body weight or fat. The principal evidence supporting this idea is that animals that have been induced to overeat and become overweight by various means, eat less than control rats when they are permitted to eat freely. If the degree of suppression of appetite by overweight is as large as several experiments suggest, then dietary hyperphagia should be self-limiting. Any overeating induced by dietary treatments should disappear after animals become moderately overweight. Animals fed some kinds of hyperhagia-promoting diets do show this pattern. However, animals fed other kinds of diets do not show this pattern, and with most diets, dietary hyperphagia continues for extended periods. This implies that either 1) overweight does not suppress appetite as much as suggested by various authorities, 2) dietary manipulations can override normal regulatory mechanisms, or 3) certain diets induce irreversible changes in body fat that are not evident from changes in body weight.

Brain 3-hydroxybutyrate, glutamate, and GABA in a rat model of dietary obesity

Whole brain concentrations of 3-hydroxybutyrate, glutamate and gamma-aminobutyric acid (GABA) have been measured in two strains of rats with differing susceptibility to obesity. S 5B/Pl rats are resistant to developing obesity when eating a high-fat diet, whereas Osborne-Mendel rats readily develop obesity when eating the same diet. We tested the hypotheses that brain 3-hydroxybutyrate, glutamate and GABA differ between S 5B/Pl rats and Osborne-Mendel rats, and that these substrates/neuroregulators are altered when eating a high-fat diet primarily in S 5B/Pl (resistant) rats. Blood and brain 3-hydroxybutyrate concentrations were higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.05) but diet effects were not significant. Brain glutamate concentration, like 3-hydroxybutyrate, was higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.01) and was not affected by adding fat to the diet. Brain GABA differed only slightly between strains but increased after adding fat to the diet (p less than 0.05) in both strains with a greater increase occurring in S 5B/Pl rats. The brains of S 5B/Pl rats are chronically exposed to higher levels of 3-hydroxybutyrate and glutamate than are those of Osborne-Mendel rats. Thus, 3-hydroxybutyrate is a potential signal in the regulation of body weight. Brain GABA increases with fat feeding, especially in S 5B/Pl rats, suggesting that the ability to adjust to an energy dense diet may be through suppression of food intake by elevated brain GABA.

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.

Effect of intrahypothalamic hydroxybutyrate on sympathetic firing rate

Following the microinjection of 3-hydroxybutyrate into either the paraventricular or ventomedial nucleus of the hypothalamus, there was a significant increase in firing rate of the sympathetic nerves to brown adipose tissue, with a peak increase in activity occurring three minutes following the injection. The response in both hypothalamic regions was dose-dependent. When injected into both regions at the same time, the increase in firing rate was additive. These studies suggest that ketone bodies in the central nervous system may activate the sympathetic nervous system.

Brain uptake of ketones in rats with differing susceptibility to dietary obesity

The effect of a low- and high-fat diet on the transport of 3-hydroxybutyrate and glucose across the blood brain barrier has been measured in two strains of animals that have a marked difference in the degree of obesity that develops when they eat a high fat diet. The S 5B/Pl rats are resistant to dietary obesity whereas the Osborne-Mendel rats readily develop obesity when eating a high-fat diet. The transport of 3-hydroxybutyrate and glucose across the blood-brain barrier was measured as the ratio of radioactive compound (3-hydroxybutyrate or glucose) to radioactively labeled water by the technique of Oldendorf. The uptake of 3-hydroxybutyrate was significantly higher in the S 5B/Pl rats than in the Osborne-Mendel rats when they were eating either the low-fat diet or the high-fat diet. In addition, there was a significant increase in the transport of 3-hydroxybutyrate in the animals of both strains when eating the high-fat diet as compared to the low-fat diet. However, there was no difference in the transport of glucose between the two strains of rats whether they ate a low-fat or high-fat diet. These data are consistent with the hypothesis that resistance to dietary obesity is associated with increased transport of 3-hydroxybutyrate across the blood-brain barrier.

Food intake in the domestic fowl: effect of intrahepatic lipid and amino acid infusions

Five experiments were conducted to determine the effect of hepatic and jugular infusions of amino acid and lipid solutions on short-term food intake of cockerels. Neither jugular nor hepatic infusions of amino acids had any effect on food intake of Single-Comb White Leghorn (SCWL) cockerels. Lipid infusions had no effect on SCWL cockerels when administered intrajugularly but decreased food intake significantly when infused intrahepatically. In contrast, no effect on food intake was observed when Rock-Cornish (RC) cockerels were infused intrahepatically with lipid. Thus, it appears that the liver of the SCWL cockerel is not responsive to plasma amino acid concentrations but is sensitive to circulating lipid levels. This lipid sensitive mechanism may be important in the peripheral control of food intake in a chicken such as the SCWL which has been bred for egg production but may be desensitized in fowl such as the RC broiler which has been selected for large body size and feed consumption.

Selective hepatic vagotomy does not prevent compensatory feeding in response to body weight changes

The effects of selective hepatic vagotomy on compensatory adjustments of feeding in response to experimentally induced changes in body weight were investigated in rats. Repeated injections of slow-acting insulin increased food intake and body weight in vagotomized as well as in sham-vagotomized rats. When the treatment was stopped, the rats of both surgical groups displayed a compensatory hypophagia of similar magnitude and body weight returned to preinjection levels. In turn, after 7 days of restricted feeding (5 g/day) with a concomitant loss of body weight, rats became hyperphagic and body weight approached the normal level. Again, the feeding responses or body weight changes did not differ between vagotomized and sham-vagotomized rats. The results demonstrate that selective hepatic vagotomy does not disrupt the putative feedback-loop between body weight and feeding, and raise questions concerning the role of the liver in the lipostatic control of food intake.

Hypophagia following dietary obesity

Two experiments examined the hypophagia that occurs when rats are switched from a high-fat to a low-fat diet. In the first experiment, rats fed a high-fat diet for eight weeks weighed 79 g more than rats fed a low-fat diet. Removal of the high-fat diet led to reduced food intake for at least four weeks. Reducing the body weight of the rats by a 24 hour fast did not alter the time course of the hypophagia. Plasma levels of free glycerol, free fatty acids and ketones were elevated during and after feeding the high-fat diet; suggesting that feeding a high-fat diet increases fat oxidation even after the high-fat diet is withdrawn. In the second experiment, feeding rats the high-fat diet for four weeks increased body weight and body fat. Starving the rats for two days after feeding the high-fat diet did not alter subsequent hypophagia and did not alter the percentage of body fat. This pattern of results is similar to that previously seen following termination of obesity-inducing insulin treatment. The results are consistent with the idea that a persistent increase in fat oxidation is responsible for the hypophagia.

Is regulation of body weight elucidated

A new perspective on mechanisms involved in the regulation of a constant fat body mass and its relation to body energy balance is presented on the basis of a series of experiments. A study of the neuroendocrine conditions underlying the daily weight gain-weight loss cycle in rat and man and experimentally induced over and underweight, leads to the notion that lipogenesis and lipolysis above and below a range of physiological fluctuations of body fat develop a counter-regulatory tendency to correcting lipolysis and lipogenesis respectively. This development is attributed to a chronic central action of plasma insulin concentration on hypothalamic insulin receptors. This liporegulatory system which controls and regulates the filling and emptying of fat stores modulates the feeding system which controls and regulates the filling of a gastrointestinal store by eating and its emptying by metabolic food utilization.

[Satiation mechanism]

Food intake in man and animals occurs as meals. Because changes in daily food intake in laboratory animals are often associated with corresponding changes in meal size, while meal frequency is less affected, food intake appears to be regulated mainly by the satiety mechanism. Meal size is controlled by feedback or satiety signals originating from the stomach, the small intestine, the liver and the fat depots. Both neural and hormonal (Bombesin?, Cholecystokinin?) feedback signals are elicited in the stomach and intestine. The feedback signals of the liver apparently originate from hepatic chemoreceptors, whose activity is modulated by certain metabolites, in particular glucose. The satiety effect of pancreatic glucagon, whose plasma concentration in the portal vein increases during a meal, also appears to be mediated by the hepatic chemoreceptors. Unmyelated nerve endings are probably the morphological substrate of the hepatic chemoreceptors. There is some evidence that circulating fat metabolites (fatty acids, ketone bodies, glycerol), whose plasma levels are related to the size of the fat depots, function as lipostatic feedback signals. The signal transfer from the gastrointestinal chemo- and mechanoreceptors and from the hepatic chemoreceptors to the central nervous system occurs mainly through vagal afferents. The first relay for this afferents is the nucleus tractus solitarii in the medulla oblongata. The next relay is at the pons, from where neurons project to the hypothalamus and other forebrain areas. The hypothalamus has important integrative functions in the control of food intake.

Different effects of subcutaneous D,L-3-hydroxybutyrate and acetoacetate injections on food intake in rats

Cumulative food intake following subcutaneous injection of D,L-3-hydroxybutyrate (DL3HB) or acetoacetate (AcAc) was investigated in rats, because ketone bodies might contribute to food intake regulation according to Kennedy's lipostatic hypothesis. In addition, the metabolic effects of DL3HB-injections were studied by measuring the levels of plasma D3HB, plasma non-esterified fatty acids (NEFA), plasma glycerol, blood glucose, and liver glycogen. Subcutaneously injected DL3HB (10 mmoles/kg body weight) significantly reduced feeding while equimolar AcAc did not. DL3HB-injection increased plasma D3HB and decreased plasma NEFA and plasma glycerol 1-2 hours after the injection but did not affect blood glucose or liver glycogen content. The data suggest that oxidation of D3HB to AcAc contributes to the inhibition of feeding following subcutaneous DL3HB-injection in rats.

Early streptozotocin diabetes and hunger

The initial effect of streptozotocin diabetes is not hyperphagia, but reduced food intake. Diabetic hyperphagia reaches maximum only after many days. Utilization of body fat may account for the delayed appearance of diabetic hyperphagia; this effect may be mediated by plasma fat metabolites. Plasma levels of free fatty acids (FFAs), ketone bodies, and glycerol are greatly elevated following STZ treatment, but return nearly to normal as diabetic hyperphagia appears.

Plasma fat metabolites and hunger

In order to test the hypothesis that the fat metabolites are the blood-borne signals which suppress hunger during recovery from reversible obesity, experiments were designed to manipulate plasma fat metabolite levels directly. In order to elevate plasma glycerol levels, glycerol was infused intravenously into relatively unrestrained rats for 36 hours; this treatment greatly increased plasma glycerol levels but reduced voluntary food intake only slightly. Similar results were obtained when glycerol was mixed with powdered rat food. These results suggest that glycerol is not the "lipostatic hormone" although it may contribute to regulation. Similar experiments with a synthetic precursor of the ketone bodies (1,3 butanediol), suggest that the ketone bodies contribute to the decrease in food intake after reversible obesity, but cannot be a complete explanation. Dietary fat consumption raised plasma free fatty acid (FFA) levels to the range seen during recovery from reversible obesity, suggesting that plasma FFAs may be a blood-borne signal of fat utilization in both cases. Intralipid, a synthetic triglyceride emulsion designed for intravenous administration, also increased plasma FFA levels but suppressed food intake by less than predicted. However, Intralipid may tend to cause spuriously high plasma FFA readings for reasons which are discussed. These results suggest that plasma fat metabolites, especially FFAs, may be blood-borne signals which contribute to the voluntary dieting after reversible obesity.