Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB.

The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.

Control-theory models of body-weight regulation and body-weight-regulatory appetite.

Human body weight (BW), or some variable related to it, is physiologically regulated. That is, negative feedback from changes in BW elicits compensatory influences on appetite, which may be called BW-regulatory appetite, and a component of energy expenditure (EE) called adaptive thermogenesis (AdEE). BW-regulatory appetite is of general significance because it appears to be related to a variety of aspects of human appetite beyond just energy intake. BW regulation, BW-regulatory appetite and AdEE are frequently discussed using concepts derived from control theory, which is the mathematical description of dynamic systems involving negative feedback. The aim of this review is to critically assess these discussions. Two general types of negative-feedback control have been invoked to describe BW regulation, set-point control and simple negative-feedback control, often called settling-point control in the BW literature. The distinguishing feature of set-point systems is the existence of an externally controlled target level of regulation, the set point. The performance of almost any negative-feedback regulatory system, however, can be modeled on the basis of feedback gain without including a set point. In both set-point and simple negative-feedback models of BW regulation, the precision of regulation is usually determined mainly by feedback gain, which refers to the transformations of feedback into compensatory changes in BW-regulatory appetite and AdEE. Stable BW most probably represents equilibria shaped by feedback gain and tonic open-loop challenges, especially obesogenic environments. Data indicate that simple negative-feedback control accurately models human BW regulation and that the set-point concept is superfluous unless its neuroendocrine representation is found in the brain. Additional research aimed at testing control-theory models in humans and non-human animals is warranted.

RYGB and flavor-consequence learning.

Flavor-consequence learning refers to learned associations between flavor stimuli and post-oral consequences of food that affect food selection, amount eaten and affect. Forms of flavor-consequence learning include flavor aversions, flavor avoidance, conditioned satiety, expected satiety and appetition. Roux-en-Y gastric bypass surgery (RYGB) and other bariatric procedures alter gastrointestinal processing of food in a number of ways. Thus, it is plausible that these procedures alter post-oral unconditioned stimuli that support flavor-consequence learning, leading to altered food selection, amount eaten, and affect. Surprisingly, however, there is almost no research on the role of flavor-consequence learning in the effects of bariatric surgery on appetite. This issue urgently warrants investigation.

Deletion of leptin receptors in vagal afferent neurons disrupts estrogen signaling, body weight, food intake and hormonal controls of feeding in female mice.

Deletion of the leptin receptor from vagal afferent neurons (VAN) using a conditional deletion (Nav1.8/LepRfl/fl) results in an obese phenotype with increased food intake and lack of exogenous cholecystokinin (CCK)-induced satiation in male mice. Female mice are partially protected from weight gain and increased food intake in response to ingestion of high-fat (HF) diets. However, whether the lack of leptin signaling in VAN leads to an obese phenotype or disruption of hypothalamic-pituitary-gonadal axis function in female mice is unclear. Here, we tested the hypothesis that leptin signaling in VAN is essential to maintain estrogen signaling and control of food intake, energy expenditure, and adiposity in female mice. Female Nav1.8/LepRfl/fl mice gained more weight, had increased gonadal fat mass, increased meal number in the dark phase, and increased total food intake compared with wild-type controls. Resting energy expenditure was unaffected. The decrease in food intake produced by intraperitoneal injection of CCK (3 µg/kg body wt) was attenuated in female Nav1.8/LepRfl/fl mice compared with wild-type controls. Intraperitoneal injection of ghrelin (100 µg/kg body wt) increased food intake in Nav1.8/LepRfl/fl mice but not in wild-type controls. Ovarian steroidogenesis was suppressed, resulting in decreased plasma estradiol, which was accompanied by decreased expression of estrogen receptor-1 (Esr1) in VAN but not in the hypothalamic arcuate nucleus. These data suggest that the absence of leptin signaling in VAN is accompanied by disruption of estrogen signaling in female mice, leading to an obese phenotype possibly via altered control of feeding behavior.

RYGB increases the satiating effect of intrajejunal lipid infusions in female rats.

We used a novel rat model to investigate the physiological bases of early satiation after Roux-en-Y gastric bypass surgery (RYGB). Female rats were subjected to RYGB or sham surgery. Chronic infusion catheters were placed in the Roux limb of RYGB rats and the corresponding anatomical locus of the jejuna of sham-RYGB rats. Rats were also ovariectomized and chronically treated with either estradiol (E2; 2 µg each 4th day SC) or the oil vehicle. Testing was begun 10-12 wk after surgery. Intrajejunal lipid infusions (10 min, 4.4 mL, 8.8 kcal) were performed just before test meals of a low-energy artificially sweetened gel diet (0.1 kcal/g) that RYGB rats ingest avidly. Intrajejunal lipid infusions reduced test-meal size more in RYGB rats than sham-operated rats, indicating that, at least after prolonged adaptation to surgery, the satiating actions of lipids acting intra- or post-jejunally are increased by RYGB and that accelerated meal appearance in the intestines after RYGB is not necessary for this effect. The satiating effects of intrajejunal lipid infusions were similar in E2-and oil-treated rats, suggesting that the effect was not dependent on an activational effect of estrogens. In a second experiment, pretreatment with the cholecystokinin A-receptor antagonist devazepide reduced the satiating effect of intrajejunal lipid infusions in E2-treated RYGB rats. Although these data are preliminary due to the smaller numbers of rats than in the first experiment, they suggest that cholecystokinin-mediated jejunal satiation contributes to early satiation after RYGB in ovariectomized rats with peri-ovulatory levels of estradiol. The results of these experiments may be relevant to understanding RYGB outcome in pre- and postmenopausal women.

Estrogenic suppression of binge-like eating elicited by cyclic food restriction and frustrative-nonreward stress in female rats.

Because binge eating and emotional eating vary through the menstrual cycle in human females, we investigated cyclic changes in binge-like eating in female rats and their control by estrogens. Binge-like eating was elicited by three cycles of 4 days of food restriction and 4 days of free feeding followed by a single frustrative nonreward-stress episode (15 min visual and olfactory exposure to a familiar palatable food) immediately before presentation of the palatable food. Intact rats showed binge-like eating during the diestrous and proestrous phases of the ovarian cycle, but not during the estrous (periovulatory) phase. Ovariectomized (OVX) rats not treated with estradiol (E2) displayed binge-like eating, whereas E2-treated OVX rats did not. The procedure did not increase signs of anxiety in an open-field test. OVX rats not treated with E2 that were subjected to food restriction and sacrificed immediately after frustrative nonreward had increased numbers of cells expressing phosphorylated extracellular signal-regulated kinases (ERK) in the central nucleus of the amygdala (CeA), paraventricular nucleus of hypothalamus (PVN), and dorsal and ventral bed nuclei of the stria terminalis (BNST) compared with nonrestricted or E2-treated rats. These data suggest that this female rat model is appropriate for mechanistic studies of some aspects of menstrual-cycle effects on emotional and binge eating in human females, that anxiety is not a sufficient cause of binge-like eating, and that the PVN, CeA, and BNST may contribute to information processing underlying binge-like eating.

RYGB progressively increases avidity for a low-energy, artificially sweetened diet in female rats.

Weight re-gain within 2 y after Roux-en-Y gastric bypass (RYGB) is significantly associated with increased intake of and cravings for sweet foods. Here we describe a novel model of this late increase in sweet appetite. Ovariectomized RYGB and Sham-operated rats, with or without estradiol treatment, were maintained on Ensure liquid diet and offered a low-energy, artificially sweetened diet (ASD) 2 h/d. First, we tested rats more than six months after RYGB. ASD meals were larger in RYGB than Sham rats, whereas Ensure meals were smaller. General physical activity increased during ASD meals in RYGB rats, but not during Ensure meals. Second, new rats were adapted to ASD before surgery, and were then offered ASD again during 4-10 wk following surgery. Estradiol-treated RYGB rats lost the most weight and progressively increased ASD intake to >20 g/2 h in wk 9-10 vs. ∼3 g/2 h in Sham rats. Finally, the same rats were then treated with leptin or saline for 8 d. Leptin did not affect body weight, Ensure intake, or activity during meals, but slightly reduced ASD intake in estradiol-treated RYGB rats. Food-anticipatory activity was increased in estradiol-treated RYGB rats during the saline-injection tests. Because increased meal-related physical activity together with larger meals is evidence of hunger in rats, these data suggest that (1) RYGB can increase hunger for a low-energy sweet food in rats and (2) low leptin levels contribute to this hunger, but are not its only cause. This provides a unique rat model for the increased avidity for sweets that is significantly associated with weight recidivism late after RYGB.

Understanding synergy.

Analysis of the interactive effects of combinations of hormones or other manipulations with qualitatively similar individual effects is an important topic in basic and clinical endocrinology as well as other branches of basic and clinical research related to integrative physiology. Functional, as opposed to mechanistic, analyses of interactions rely on the concept of synergy, which can be defined qualitatively as a cooperative action or quantitatively as a supra-additive effect according to some metric for the addition of different dose-effect curves. Unfortunately, dose-effect curve addition is far from straightforward; rather, it requires the development of an axiomatic mathematical theory. I review the mathematical soundness, face validity, and utility of the most frequently used approaches to supra-additive synergy. These criteria highlight serious problems in the two most common synergy approaches, response additivity and Loewe additivity, which is the basis of the isobole and related response surface approaches. I conclude that there is no adequate, generally applicable, supra-additive synergy metric appropriate for endocrinology or any other field of basic and clinical integrative physiology. I recommend that these metrics be abandoned in favor of the simpler definition of synergy as a cooperative, i.e., nonantagonistic, effect. This simple definition avoids mathematical difficulties, is easily applicable, meets regulatory requirements for combination therapy development, and suffices to advance phenomenological basic research to mechanistic studies of interactions and clinical combination therapy research.

Estradiol increases body weight loss and gut-peptide satiation after Roux-en-Y gastric bypass in ovariectomized rats.

Despite the fact that ∼85% of bariatric operations are performed in women, the effects of the reproductive axis function on outcome of bariatric surgery remain to be determined. Here we developed the first published model of Roux-en-Y gastric bypass (RYGB) in female rats. We show in ovariectomized rats receiving estradiol or control treatment that (1) RYGB-induced body weight loss and (2) the satiating efficacy of endogenous glucagon-like peptide-1 and cholecystokinin satiation were significantly increased in estradiol-treated rats. These data are relevant to the care of obese women, in particular perimenopausal women, undergoing bariatric surgery.

Sex differences in the physiology of eating.

Hypothalamic-pituitary-gonadal (HPG) axis function fundamentally affects the physiology of eating. We review sex differences in the physiological and pathophysiological controls of amounts eaten in rats, mice, monkeys, and humans. These controls result from interactions among genetic effects, organizational effects of reproductive hormones (i.e., permanent early developmental effects), and activational effects of these hormones (i.e., effects dependent on hormone levels). Male-female sex differences in the physiology of eating involve both organizational and activational effects of androgens and estrogens. An activational effect of estrogens decreases eating 1) during the periovulatory period of the ovarian cycle in rats, mice, monkeys, and women and 2) tonically between puberty and reproductive senescence or ovariectomy in rats and monkeys, sometimes in mice, and possibly in women. Estrogens acting on estrogen receptor-α (ERα) in the caudal medial nucleus of the solitary tract appear to mediate these effects in rats. Androgens, prolactin, and other reproductive hormones also affect eating in rats. Sex differences in eating are mediated by alterations in orosensory capacity and hedonics, gastric mechanoreception, ghrelin, CCK, glucagon-like peptide-1 (GLP-1), glucagon, insulin, amylin, apolipoprotein A-IV, fatty-acid oxidation, and leptin. The control of eating by central neurochemical signaling via serotonin, MSH, neuropeptide Y, Agouti-related peptide (AgRP), melanin-concentrating hormone, and dopamine is modulated by HPG function. Finally, sex differences in the physiology of eating may contribute to human obesity, anorexia nervosa, and binge eating. The variety and physiological importance of what has been learned so far warrant intensifying basic, translational, and clinical research on sex differences in eating.

Energy homeostasis from Lavoisier to control theory.

The intellectual history of energy homeostasis, focusing on food intake and energy storage, is briefly reviewed. Physiological energetics was founded by Lavoisier, who in the late eighteenth century invented direct and indirect calorimetry and discovered the role of oxygen in combustion and respiration. Energy was understood well enough by the mid-nineteenth century to realize the physiological energy-balance equation, that energy intake - energy expenditure = energy storage, but this did not greatly influence physiological research for another century. Homeostasis, the concept that many vital physiological variables are actively regulated in narrow envelopes, was developed by Bernard and Cannon between approximately 1870-1940 and remains a central principle of physiology. Kennedy coined the term lipostasis in 1953 to refer to the constancy of fat mass, which Mayer argued was the mechanism regulating body weight. A parameterized control-theory model suggests that a proportional negative-feedback control system incompletely compensates weight loss during persistent negative energy balance, suggesting that Cannon's idea of constancy within a narrow envelope may not fit body-weight regulation well. This modelling encourages further application of control theory to issues in energy homeostasis, including to the development of obesity. It also sets the stage for understanding the underlying neuroendocrine mechanisms. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part I)'.

Increased Meal Size but Reduced Meal-Stimulated Plasma Cholecystokinin Concentrations in Women With Obesity.

To better understand the physiological basis of obesity in women, we investigated whether obesity or menstrual cycle phase affects laboratory test-meal size or meal-stimulated plasma cholecystokinin (CCK) concentration. Women with healthy weight (body mass index [BMI] of 18.5-24.9 kg/m2, N = 16) or obesity (BMI 30-39.9 kg/m2, N = 20) were tested once in the late-follicular or peri-ovulatory phase (LF/PO) and once in the mid-luteal phase (ML). Meals of ham sandwiches were offered and blood was sampled. Menstrual cycle phases were verified with participants' reports of menses and measurements of progesterone and luteinizing hormone (LH) concentrations. Women with obesity ate significantly larger meals than women with healthy weight, (mean, 711 [95% CI, 402-1013] kJ, P = 0.001, during the LF/PO and 426 [105-734] kJ, P = 0.027, larger during the ML). Women with healthy weight ate smaller meals during LF/PO than ML (decrease, 510 [192-821 kJ], P = 0.008), but women with obesity did not (decrease, 226 [-87-542] kJ, P = 0.15). CCK concentrations 18 to 30 minutes after meal onset were lower in women with obesity than in women with healthy weight during LF/PO (3.6 [3.1-4.1] vs 6.1 [4.5-7.7] pmol/L; P = 0.004), but not during ML, with a significant interaction effect (1.8 [1.2-2.4] pmol/L, P = 0.048). Women with obesity consumed larger meals than women with healthy weight but displayed reduced meal-stimulated plasma CCK concentrations. These data are consistent with the hypothesis that a defect in CCK secretion compromises satiation in obese women and contributes to the development or maintenance of obesity.

Is It Worth It? Obesity Affects Snack Food Valuation Across the Menstrual Cycle.

BACKGROUND: The importance of menstrual cycle physiology in appetite and obesity is poorly understood. We investigated the effects of body mass index (BMI), menstrual cycle phase and sweet and salty taste on monetary valuation of snack foods. METHODS: We recruited 72 women and after the application of in- and exclusion criteria 31 participants with healthy weight and 25 with obesity remained. The participants completed a willingness to pay (WTP) task to measure subjective value of 30 snack food items in the pre-ovulatory and mid-luteal cycle phases. RESULTS: Generalized linear mixed model (GLMM) analysis revealed that BMI, cycle phase and snack taste interacted to influence WTP (-0.15 [-0.22, -0.03], p = 0.002). Hence, WTP was inversely related to BMI, but the strength of the relation depended on cycle phase and taste. The WTP of participants with healthy weight for salty taste changed across cycle phase but the WTP for sweet taste was not affected by cycle phase. Moreover, the cycle effect for the salty snacks ceased in participants with obesity. CONCLUSION: The inverse effect of BMI on WTP valuation of snack foods contrasts with the positive effect of BMI on pleasantness ratings for milkshakes by the same women that we previously reported. This indicates that the two measures reflect different aspects of food-related valuative processing in obesity. Furthermore, the WTP data suggest that the selection of salty snacks may differ from that of sweet snacks in the pre-ovulatory phase of the menstrual cycle for individuals of healthy weight. The cycle phase does not seem to affect food valuation of participants with obesity. These findings are relevant to understanding and treating obesity in women.

Beneficial effects on glucose metabolism of chronic feeding of isomaltulose versus sucrose in rats.

BACKGROUND/AIMS: Isomaltulose (alpha-D-glucosylpyranosyl-1,6-D-fructofuranose) is a natural disaccharide used in human nutrition. It is structurally related to sucrose, but more slowly hydrolyzed and absorbed. Because this sugar's metabolic effects are poorly characterized, we compared the effects of chronic ad libitum access to high-isomaltulose and high-sucrose diets on glucose metabolism in rats. METHODS: Adult male rats were offered 62% isomaltulose, sucrose or starch diets ad libitum for 26 (trial 1) or 56 (trial 2) days. After 2- to 3-week adaptation, plasma glucose, fructose and insulin were measured after test meals of the adaptation diet. RESULTS: The main finding was that both plasma glucose and plasma insulin concentrations were transiently but markedly increased after sucrose test meals compared to isomaltulose or starch meals. These differences were not associated with consistent differences in food intake, body weight gain or adiposity. CONCLUSIONS: Chronic isomaltulose feeding has beneficial effects on postprandial glucose metabolism in comparison to sucrose feeding in rats, although the effects are modest. Further work is warranted to determine whether substitution of isomaltulose for sucrose or other sweet carbohydrates might be therapeutically useful in patients with, or at risk for, insulin resistance or type 2 diabetes mellitus.

Hindbrain administration of estradiol inhibits feeding and activates estrogen receptor-alpha-expressing cells in the nucleus tractus solitarius of ovariectomized rats.

17beta-estradiol (E2), acting via estrogen receptor (ER)-alpha, inhibits feeding in animals. One mechanism apparently involves an increase in the satiating potency of cholecystokinin (CCK) released from the small intestine by ingested food. For example, the satiating potency of intraduodenal lipid infusions is increased by E2 in ovariectomized rats; this increased satiation is dependent on CCK, and it is accompanied by increases in the numbers of ERalpha-positive cells that express c-Fos in a subregion of the caudal nucleus tractus solitarius (cNTS) that receives abdominal vagal afferent projections. To test whether direct administration of E2 to this area of the hindbrain is sufficient to inhibit food intake, we first implanted 0.2 microg estradiol benzoate (EB) in cholesterol or cholesterol alone either sc or onto the surface of the hindbrain over the cNTS. Food intake was significantly reduced after hindbrain EB implants but not after sc EB implants. Next we verified that equimolar hindbrain implants of E2 and EB had similar feeding-inhibitory effects and determined that only small amounts of E2 reached brain areas outside the dorsal caudal hindbrain after hindbrain implants of (3)H-labeled E2. Neither plasma estradiol concentration nor plasma inflammatory cytokine concentration was increased by either hindbrain or sc EB implants. Finally, hindbrain EB implants, but not sc implants, increased c-Fos in ERalpha-positive cells in the cNTS after ip injection of 4 microg/kg CCK-8. We conclude that E2, acting via ERalpha in cNTS neurons, including neurons stimulated by ip CCK, is sufficient to inhibit feeding.

Estradiol-dependent decrease in the orexigenic potency of ghrelin in female rats.

Ghrelin, the only known orexigenic gut hormone, is secreted mainly from the stomach, increases with fasting and before meal initiation in humans and rats, and increases food intake after central or peripheral administration. To investigate sex differences in the action of ghrelin, we assessed the effects of exogenous ghrelin in intact male and female rats, the effects of exogenous ghrelin in ovariectomized (OVX) and estradiol (E2)-treated female rats, as well as the effects of OVX on plasma ghrelin and hypothalamic orexigneic neuropeptide expression in rats and on food intake and weight gain in transgenic mice lacking the ghrelin receptor (Ghsr(-/-) mice). Male and OVX female rats were significantly more sensitive than intact female rats to the orexigenic effects of both centrally (intra-third ventricular, i3vt, 0.01, 0.1, and 1.0 nmol) and systemically (ip, 3, 6, and 9 nmol) administered ghrelin. This difference is likely to be estradiol dependent because E2 attenuated the orexigenic action of ghrelin in OVX female and male rats. Furthermore, OVX increased food intake and body weight in wild-type mice, but not in Ghsr(-/-) mice, suggesting that OVX increases food intake by releasing ghrelin from a tonic inhibitory effect of estradiol. In addition, following OVX, there was an increase in plasma ghrelin that was temporally associated with increased food intake, body weight, and hypothalamic neuropeptide Y and Agouti-related protein mRNA expression. Collectively, these data suggest that estradiol inhibits the orexigenic action of ghrelin in females, that weight gain associated with OVX is ghrelin mediated, and that this endocrine interaction may account for an important sex differences in food intake and the regulation of body weight.