The role of the gut sweet taste receptor in regulating GLP-1, PYY, and CCK release in humans

The role of nutrient sensing dysregulation in anorexia of ageing: The little we know and the much we don't

The age-related decline in appetite and food intake - termed "anorexia of ageing" - is implicated in undernutrition in later life and hence provides a public health challenge for our ageing population. Eating behaviour is controlled, in part, by homeostatic mechanisms which sense nutrient status and provide feedback to appetite control regions of the brain. Such feedback signals, propagated by episodic gut hormones, are dysregulated in some older adults. The secretory responses of appetite-related gut hormones to feeding are amplified, inducing a more anorexigenic signal which is associated with reduced appetite and food intake. Such an augmented response would indicate an increase in gut sensitivity to nutrients. Consequently, this review explores the role of gastrointestinal tract nutrient sensing in age-related appetite dysregulation. We review and synthesise evidence for age-related alterations in nutrient sensing which may explain the observed hormonal dysregulation. Drawing on what is known regarding elements of nutrient sensing pathways in animal models, in other tissues of the body, and in certain models of disease, we identify potential causal mechanisms including alterations in enteroendocrine cell number and distribution, dysregulation of cell signalling pathways, and changes in the gut milieu. From identified gaps in evidence, we highlight interesting and important avenues for future research.

Identification of Sweetness Preference-Related Single-Nucleotide Polymorphisms for Polygenic Risk Scores Associated with Obesity

Our study aimed to identify sweetness preference-associated single-nucleotide polymorphisms (SNPs), characterize the related genetic loci, and develop SNP-based polygenic risk scores (PRS) to analyze their associations with obesity. For genotyping, we utilized a pooled genome-wide association study (GWAS) dataset of 18,499 females and 10,878 males. We conducted genome-wide association analyses, functional annotation, and employed the weighted method to calculate the levels of PRS from 677 sweetness preference-related SNPs. We used Cox proportional hazards modeling with time-varying covariates to estimate age-adjusted and multivariable hazard ratios (HRs) and 95% confidence intervals (CIs) for obesity incidence. We also tested the correlation between PRS and environmental factors, including smoking and dietary components, on obesity. Our results showed that in males, the TT genotype of rs4861982 significantly increased obesity risk compared to the GG genotype in the Health Professionals Follow-up Study (HPFS) cohort (HR = 1.565; 95% CI, 1.122-2.184; p = 0.008) and in the pooled analysis (HR = 1.259; 95% CI, 1.030-1.540; p = 0.025). Protein tyrosine phosphatase receptor type O (PTPRO) was identified as strongly associated with sweetness preference, indicating a positive correlation between sweetness preference and obesity risk. Moreover, each 10 pack-year increment in smoking was significantly associated with an increased risk of obesity in the HPFS cohort (HR = 1.024; 95% CI, 1.000-1.048) in males but not in females. In conclusion, significant associations between rs4861982, sweetness preference, and obesity were identified, particularly among males, where environmental factors like smoking are also correlated with obesity risk.

Taste and Appetite at Altitude: A Comprehensive Review of Sensory and Hunger Modulation in High-Altitude Environments

Introduction: Individuals living or working at high altitudes typically experience altered taste perceptions and reduced appetite. These changes can lead to nutritional deficiencies, affecting the energy balance and body composition. Methods: We conducted a nonsystematic review of PubMed to explore these phenomena and expound on their findings to offer additional insights. Results: Changes in taste and perception are common and typically lead to loss of mass. There are limited practical solutions to mitigate these challenges. Discussion: Gradual acclimatization and tailored nutritional strategies are required to enhance health and performance in high-altitude environments. This review provides critical insights into the intersection of altitude, nutrition, and health.

The effect of bariatric surgery on the expression of gastrointestinal taste receptors: A systematic review

Gastrointestinal nutrient sensing via taste receptors may contribute to weight loss, metabolic improvements, and a reduced preference for sweet and fatty foods following bariatric surgery. This review aimed to investigate the effect of bariatric surgery on the expression of oral and post-oral gastrointestinal taste receptors and associations between taste receptor alterations and clinical outcomes of bariatric surgery. A systematic review was conducted to capture data from both human and animal studies on changes in the expression of taste receptors in oral or post-oral gastrointestinal tissue following any type of bariatric surgery. Databases searched included Medline, Embase, Emcare, APA PsychInfo, Cochrane Library, and CINAHL. Two human and 21 animal studies were included. Bariatric surgery alters the quantity of many sweet, umami, and fatty acid taste receptors in the gastrointestinal tract. Changes to the expression of sweet and amino acid receptors occur most often in intestinal segments surgically repositioned more proximally, such as the alimentary limb after gastric bypass. Conversely, changes to fatty acid receptors were observed more frequently in the colon than in the small intestine. Significant heterogeneity in the methodology of included studies limited conclusions regarding the direction of change in taste receptor expression induced by bariatric surgeries. Few studies have investigated associations between taste receptor expression and clinical outcomes of bariatric surgery. As such, future studies should look to investigate the relationship between bariatric surgery-induced changes to gut taste receptor expression and function and the impact of surgery on taste preferences, food palatability, and eating behaviour.Registration code in PROSPERO: CRD42022313992.

Recent advancements in the taste transduction mechanism, identification, and characterization of taste components

In the realm of human nutrition, the phenomenon known as taste refers to a distinctive sensation elicited by the consumption of food and various compounds within the oral cavity and on the tongue. Moreover, taste affects the overall comfort in the oral cavity, and is a fundamental attribute for the assessment of food items. Accordingly, clarifying the material basis of taste would be conducive to deepening the cognition of taste, investigating the mechanism of taste presentation, and accurately covering up unpleasant taste. In this paper, the basic biology and physiology of transduction of bitter, umami, sweet, sour, salty, astringent, as well as spicy tastes are reviewed. Furthermore, the detection process of taste components is summarized. Particularly, the applications, advantages, and distinctions of various isolation, identification, and evaluation methods are discussed in depth. In conclusion, the future of taste component detection is discussed.

Sweet Taste Receptor T1R3 Expressed in Leydig Cells Is Closely Related to Homeostasis of the Steroid Hormone Metabolism Profile

Taste receptor type 1 subunit 3 (T1R3) is initially expressed in mammal tongue for recognition and response of sweet/umami tastants and is critical to nutrient absorption, even endocrine. In this study, down-regulation of related steroidogenic enzymes such as StAR, 3β-HSD, CYP17A1, and 17β-HSD with the decrease of T1R3 expression was found in Leydig cells treated by a T1R3 inhibitor (lactisole). The abundances of progesterone, 17a-hydroxyprogesterone, androstenedione, testosterone, and deoxycorticosterone were down-regulated by 2.3, 3.5, 1.4, 1.6, and 2.2 times, respectively, after T1R3 inhibition. In addition, opposite results were found in saccharin sodium treatment. T1R3 activation contributed to intracellular cyclic adenosine monophosphate (cAMP) accumulation (14.41 ± 0.58 vs 20.21 ± 0.65) and increased testosterone (20.31 ± 3.49 vs 50.01 ± 7.44) and steroidogenic metabolite levels. Coadministration of human chorionic gonadotropin and saccharin sodium resulted in elevating the testosterone and cAMP levels and enhancing the expression levels of steroidogenic-related factors. Similarly, intratesticular injection of lactisole and saccharin sodium further confirmed that T1R3 inhibition/activation affected the expression of related steroidogenic enzymes and the testosterone levels in mice. The above findings suggest that T1R3 plays a role in testicular steroidogenesis.

The gut odorant receptor and taste receptor make sense of dietary components: A focus on gut hormone secretion

Odorant receptors (ORs) and taste receptors (TRs) are expressed primarily in the nose and tongue in which they transduce electrical signals to the brain. Advances in deciphering the dietary component-sensing mechanisms in the nose and tongue prompted research on the role of gut chemosensory cells. Acting as the pivotal interface between the body and dietary cues, gut cells "smell" and "taste" dietary components and metabolites by taking advantage of chemoreceptors-ORs and TRs, to maintain physiological homeostasis. Here, we reviewed this novel field, highlighting the latest discoveries pertinent to gut ORs and TRs responding to dietary components, their impacts on gut hormone secretion, and the mechanisms involved. Recent studies indicate that gut cells sense dietary components including fatty acid, carbohydrate, and phytochemical by activating relevant ORs, thereby modulating GLP-1, PYY, CCK, and 5-HT secretion. Similarly, gut sweet, umami, and bitter receptors can regulate the gut hormone secretion and maintain homeostasis in response to dietary components. A deeper understanding of the favorable influence of dietary components on gut hormone secretion via gut ORs and TRs, coupled with the facts that gut hormones are involved in diverse physiological or pathophysiological phenomena, may ultimately lead to a promising treatment for various human diseases.

Colonic Delivery of Nutrients for Sustained and Prolonged Release of Gut Peptides: A Novel Strategy for Appetite Management

Obesity is one of the major global threats to human health and risk factors for cardiometabolic diseases and certain cancers. Glucagon-like peptide-1 (GLP-1) plays a major role in appetite and glucose homeostasis and recently the USFDA approved GLP-1 agonists for the treatment of obesity and type 2 diabetes. GLP-1 is secreted from enteroendocrine L-cells in the distal part of the gastrointestinal (GI) tract in response to nutrient ingestion. Endogenously released GLP-1 has a very short half-life of <2 min and most of the nutrients are absorbed before reaching the distal GI tract and colon, which hinders the use of nutritional compounds for appetite regulation. The review article focuses on nutrients that endogenously stimulate GLP-1 and peptide YY (PYY) secretion via their receptors in order to decrease appetite as preventive action. In addition, various delivery technologies such as pH-sensitive, mucoadhesive, time-dependent, and enzyme-sensitive systems for colonic targeting of nutrients delivery are described. Sustained colonic delivery of nutritional compounds could be one of the most promising approaches to prevent obesity and associated metabolic diseases by, e.g., sustained GLP-1 release.

Oral Erythritol Reduces Energy Intake during a Subsequent ad libitum Test Meal: A Randomized, Controlled, Crossover Trial in Healthy Humans

The impact of oral erythritol on subsequent energy intake is unknown. The aim was to assess the effect of oral erythritol compared to sucrose, sucralose, or tap water on energy intake during a subsequent ad libitum test meal and to examine the release of cholecystokinin (CCK) in response to these substances. In this randomized, crossover trial, 20 healthy volunteers received 50 g erythritol, 33.5 g sucrose, or 0.0558 g sucralose dissolved in tap water, or tap water as an oral preload in four different sessions. Fifteen minutes later, a test meal was served and energy intake was assessed. At set time points, blood samples were collected to quantify CCK concentrations. The energy intake (ad libitum test meal) was significantly lower after erythritol compared to sucrose, sucralose, or tap water (p < 0.05). Before the start of the ad libitum test meal, erythritol led to a significant increase in CCK compared to sucrose, sucralose, or tap water (p < 0.001). Oral erythritol given alone induced the release of CCK before the start of the ad libitum test meal and reduced subsequent energy intake compared to sucrose, sucralose, or tap water. These properties make erythritol a useful sugar alternative.

Response of blood glucose and GLP-1 to different food temperature in normal subject and patients with type 2 diabetes

Background

Eating behavior is a major factor in type 2 diabetes. We investigated the different responses of glucose-regulating hormones to cold and hot glucose solutions in normal subjects and patients with type 2 diabetes.

Methods

In this crossover, self-controlled study, normal subjects (N = 19) and patients with type 2 diabetes (N = 22) were recruited and randomly assigned to a hot (50 °C) or a cold (8 °C) oral glucose-tolerance test (OGTT). The subsequent day, they were switched to the OGTT at the other temperature. Blood glucose, insulin, GIP, glucagon-like peptide-1 (GLP-1), and cortisol were measured at 0, 5, 10, 30, 60, and 120 min during each OGTT. After the hot OGTT, all subjects ingested hot (>42 °C) food and water for that day, and ingested food and water at room temperature (≤24 °C) for the day after cold OGTT. All participants had continuous glucose monitoring (CGM) throughout the study.

Results

Compared to cold OGTT, blood glucose was significantly higher with hot OGTT in both groups (both P < 0.05). However, insulin and GLP-1 levels were significantly higher in hot OGTT in normal subjects only (both P < 0.05). The GIP and cortisol responses did not differ with temperature in both groups. CGM showed that normal subjects had significantly higher 24-h mean glucose (MBG) (6.11 ± 0.13 vs. 5.84 ± 0.11 mmol/L, P = 0.021), and standard deviation of MBG with hot meals (0.59 ± 0.06 vs. 0.48 ± 0.05 mmol/L, P = 0.043), T2DM patients had higher MBG only (8.46 ± 0.38 vs. 8.88 ± 0.39 mmol/L, P = 0.022).

Conclusions

Food temperature is an important factor in glucose absorption and GLP-1 response. These food temperatures elicited differences are lost in type 2 diabetes.

The Ile191Val Variant of the TAS1R2 Subunit of Sweet Taste Receptors Is Associated With Reduced HbA1c in a Human Cohort With Variable Levels of Glucose Homeostasis

The Ile191Val variant of the TAS1R2 gene of sweet taste receptors causes a partial loss-of-function and is associated with reduced glucose excursions in a healthy lean cohort. However, it is unclear whether this polymorphism contributes to the regulation of glucose homeostasis in metabolically unhealthy individuals. Thus, we used participants with variable glycemic profiles and obesity to assess the effects of the TAS1R2-Ile191Val variant. We found that the Val minor allele carriers had lower HbA1c at all levels of fasting glucose and glucose tolerance. These effects were not due to differences in beta-cell function or insulin sensitivity assessed with a frequently sampled intravenous glucose tolerance test. This study extends our previous findings and provides further evidence that sweet taste receptor function may contribute to glucose regulation in humans.

Nutrient Sensing via Gut in Drosophila melanogaster

Nutrient-sensing mechanisms in animals' sense available nutrients to generate a physiological regulatory response involving absorption, digestion, and regulation of food intake and to maintain glucose and energy homeostasis. During nutrient sensing via the gastrointestinal tract, nutrients interact with receptors on the enteroendocrine cells in the gut, which in return respond by secreting various hormones. Sensing of nutrients by the gut plays a critical role in transmitting food-related signals to the brain and other tissues informing the composition of ingested food to digestive processes. These signals modulate feeding behaviors, food intake, metabolism, insulin secretion, and energy balance. The increasing significance of fly genetics with the availability of a vast toolbox for studying physiological function, expression of chemosensory receptors, and monitoring the gene expression in specific cells of the intestine makes the fly gut the most useful tissue for studying the nutrient-sensing mechanisms. In this review, we emphasize on the role of Drosophila gut in nutrient-sensing to maintain metabolic homeostasis and gut-brain cross talk using endocrine and neuronal signaling pathways stimulated by internal state or the consumption of various dietary nutrients. Overall, this review will be useful in understanding the post-ingestive nutrient-sensing mechanisms having a physiological and pathological impact on health and diseases.

The Role of D-allulose and Erythritol on the Activity of the Gut Sweet Taste Receptor and Gastrointestinal Satiation Hormone Release in Humans: A Randomized, Controlled Trial

BACKGROUND

Glucose induces the release of gastrointestinal (GI) satiation hormones, such as glucagon-like peptide 1 (GLP-1) and peptide tyrosine tyrosine (PYY), in part via the activation of the gut sweet taste receptor (T1R2/T1R3).

OBJECTIVES

The primary objective was to investigate the importance of T1R2/T1R3 for the release of cholecystokinin (CCK), GLP-1, and PYY in response to D-allulose and erythritol by assessing the effect of the T1R2/T1R3 antagonist lactisole on these responses and as secondary objectives to study the effect of the T1R2/T1R3 blockade on gastric emptying, appetite-related sensations, and GI symptoms.

METHODS

In this randomized, controlled, double-blind, crossover study, 18 participants (5 men) with a mean ± SD BMI (in kg/m2) of 21.9 ± 1.7 and aged 24 ± 4 y received an intragastric administration of 25 g D-allulose, 50 g erythritol, or tap water, with or without 450 parts per million (ppm) lactisole, respectively, in 6 different sessions. 13C-sodium acetate was added to all solutions to determine gastric emptying. At fixed time intervals, blood and breath samples were collected, and appetite-related sensations and GI symptoms were assessed. Data were analyzed with linear mixed-model analysis.

RESULTS

D-allulose and erythritol induced a significant release of CCK, GLP-1, and PYY compared with tap water (all PHolm < 0.0001, dz >1). Lactisole did not affect the D-allulose- and erythritol-induced release of CCK, GLP-1, and PYY (all PHolm > 0.1). Erythritol significantly delayed gastric emptying, increased fullness, and decreased prospective food consumption compared with tap water (PHolm = 0.0002, dz = -1.05; PHolm = 0.0190, dz = 0.69; and PHolm = 0.0442, dz = -0.62, respectively).

CONCLUSIONS

D-allulose and erythritol stimulate the secretion of GI satiation hormones in humans. Lactisole had no effect on CCK, GLP-1, and PYY release, indicating that D-allulose- and erythritol-induced GI satiation hormone release is not mediated via T1R2/T1R3 in the gut.

Acesulfame potassium induces dysbiosis and intestinal injury with enhanced lymphocyte migration to intestinal mucosa

BACKGROUND AND AIM

The artificial sweetener acesulfame potassium (ACK) is officially approved as safe for intake and has been used in processed foods. However, ACKs have been reported to induce metabolic syndrome, along with alteration of the gut microbiota in mice. In recent years, studies have suggested that this artificial sweetener promotes myeloperoxidase reactivity in Crohn's disease-like ileitis. We aimed to investigate the effect of ACK on the intestinal mucosa and gut microbiota of normal mice.

METHODS

Acesulfame potassium was administered to C57BL/6J mice (8 weeks old) via free drinking. Intestinal damage was evaluated histologically, and messenger RNA (mRNA) levels of TNF-α, IFN-γ, IL1-β, MAdCAM-1, GLP1R, and GLP2R were determined with quantitative reverse transcription polymerase chain reaction (qRT-PCR). Immunohistochemistry was performed to examine the expression of MAdCAM-1 in the small intestine. The composition of gut microbiota was assessed using high-throughput sequencing. We performed intravital microscopic observation to examine if ACK altered lymphocyte migration to the intestinal microvessels.

RESULTS

Acesulfame potassium increased the expression of proinflammatory cytokines, decreased the expression of GLP-1R and GLP-2R, and induced small intestinal injury with an increase in intestinal permeability, and ACK treatment induced microbial changes, but the transfer of feces alone from ACK mice did not reproduce intestinal damage in recipient mice. ACK treatment significantly increased the migration of lymphocytes to intestinal microvessels.

CONCLUSION

Acesulfame potassium induces dysbiosis and intestinal injury with enhanced lymphocyte migration to intestinal mucosa. Massive use of non-caloric artificial sweeteners may not be as safe as we think.

Luminal Chemosensory Cells in the Small Intestine

In addition to the small intestine's well-known function of nutrient absorption, the small intestine also plays a major role in nutrient sensing. Similar to taste sensors seen on the tongue, GPCR-coupled nutrient sensors are expressed throughout the intestinal epithelium and respond to nutrients found in the lumen. These taste receptors respond to specific ligands, such as digested carbohydrates, fats, and proteins. The activation of nutrient sensors in the intestine allows for the induction of signaling pathways needed for the digestive system to process an influx of nutrients. Such processes include those related to glucose homeostasis and satiety. Defects in intestinal nutrient sensing have been linked to a variety of metabolic disorders, such as type 2 diabetes and obesity. Here, we review recent updates in the mechanisms related to intestinal nutrient sensors, particularly in enteroendocrine cells, and their pathological roles in disease. Additionally, we highlight the emerging nutrient sensing role of tuft cells and recent work using enteroids as a sensory organ model.

Peter JV. Sugars and Sweet Taste: Addictive or Rewarding?

The notion of food "addiction" often focuses on the overconsumption of sweet tasting foods or so-called sugar "addiction". In the extreme, some have suggested that sugar and sweet tastes elicit neural and behavioral responses analogous to those observed with drugs of abuse. These concepts are complicated by the decades long uncertainty surrounding the validity and reproducibility of functional magnetic resonance imaging (fMRI) methodologies used to characterize neurobiological pathways related to sugar and sweet taste stimuli. There are also questions of whether sweet taste or post-ingestion metabolic consequences of sugar intake would lead to addiction or excessive caloric intake. Here, we present a focused narrative review of literature related to the reward value of sweet taste which suggests that reward value can be confounded with the construct of "addictive potential". Our review seeks to clarify some key distinctions between these constructs and questions the applicability of the addiction construct to human over-eating behaviors. To adequately frame this broad discussion requires the flexibility offered by the narrative review paradigm. We present selected literature on: techniques used to link sugar and sweet tastes to addiction neurobiology and behaviors; sugar and sweet taste "addiction"; the relationship of low calorie sweetener (LCS) intake to addictive behaviors and total calorie intake. Finally, we examined the reward value of sweet tastes and contrasted that with the literature describing addiction. The lack of reproducibility of fMRI data remains problematic for attributing a common neurobiological pathway activation of drugs and foods as conclusive evidence for sugar or sweet taste "addiction". Moreover, the complicated hedonics of sweet taste and reward value are suggested by validated population-level data which demonstrate that the consumption of sweet taste in the absence of calories does not increase total caloric intake. We believe the neurobiologies of reward value and addiction to be distinct and disagree with application of the addiction model to sweet food overconsumption. Most hypotheses of sugar "addiction" attribute the hedonics of sweet foods as the equivalent of "addiction". Further, when addictive behaviors and biology are critically examined in totality, they contrast dramatically from those associated with the desire for sweet taste. Finally, the evidence is strong that responses to the palatability of sweets rather than their metabolic consequences are the salient features for reward value. Thus, given the complexity of the controls of food intake in humans, we question the usefulness of the "addiction" model in dissecting the causes and effects of sweet food over-consumption.

Intestinal alteration of α-gustducin and sweet taste signaling pathway in metabolic diseases is partly rescued after weight loss and diabetes remission

Carbohydrates and sweeteners are detected by the sweet taste receptor in enteroendocrine cells (EECs). This receptor is coupled to the gustducin G-protein, which α-subunit is encoded by GNAT3 gene. In intestine, the activation of sweet taste receptor triggers a signaling pathway leading to GLP-1 secretion, an incretin hormone. In metabolic diseases, GLP-1 concentration and incretin effect are reduced while partly restored after Roux-en-Y gastric bypass (RYGB). We wondered if the decreased GLP-1 secretion in metabolic diseases is caused by an intestinal defect in sweet taste transduction pathway. In our RNA-sequencing of EECs, GNAT3 expression is decreased in patients with obesity and type 2 diabetes compared with normoglycemic obese patients. This prompted us to explore sweet taste signaling pathway in mice with metabolic deteriorations. During obesity onset in mice, Gnat3 expression was downregulated in EECs. After metabolic improvement with enterogastro anastomosis surgery in mice (a surrogate of the RYGB in humans), the expression of Gnat3 increased in the new alimentary tract and glucose-induced GLP-1 secretion was improved. To evaluate if high-fat diet-induced dysbiotic intestinal microbiota could explain the changes in the expression of sweet taste α-subunit G-protein, we performed a fecal microbiota transfer in mice. However, we could not conclude if dysbiotic microbiota impacted or not intestinal Gnat3 expression. Our data highlight that metabolic disorders were associated with altered gene expression of sweet taste signaling in intestine. This could contribute to impaired GLP-1 secretion that is partly rescued after metabolic improvement.NEW & NOTEWORTHY Our data highlighted 1) the sweet taste transduction pathway in EECs plays pivotal role for glucose homeostasis at least at gene expression level; 2) metabolic disorders lead to altered gene expression of sweet taste signaling pathway in intestine contributing to impaired GLP-1 secretion; and 3) after surgical intestinal modifications, increased expression of GNAT3, encoding α-gustducin contributed to metabolic improvement.

Olfactory and Gustatory Function before and after Laparoscopic Sleeve Gastrectomy

Background and Objectives: Bariatric surgery is the gold standard for the treatment of morbid obesity, and current evidence suggests that patients undergoing surgery can show changes in their sense of taste and smell. However, no definitive conclusions can be drawn given the heterogeneity of the studies and the contrasting results reported in the literature. Materials and Methods: We enrolled 18 obese patients undergoing laparoscopic sleeve gastrectomy (LSG) and 15 obese controls. At baseline (T0) and 6 months after enrollment/surgery (T1), both groups underwent Sniffin’ Sticks and whole mouth test. Post-operative qualitative taste variations were also analyzed and SNOT-22, VAS for taste and smell, and MMSE were administered. Results: An improvement in the olfactory threshold was observed in the treatment group (p = 0.03) at 6 months. At multivariate analysis, the olfactory threshold differences observed correlated with MMSE (p = 0.03) and T0 gustatory identification (p = 0.01). No changes in sense of taste were observed between the two groups at 6 months, even though nine subjects in the treatment group reported a worsening of taste. This negatively correlated with age (p < 0.001), but a positive marginal correlation was observed with the olfactory threshold difference between T0 and T1 (p = 0.06). Conclusions: Olfaction can improve after LSG, and this seems to be the consequence of an improved olfactory threshold. Although we did not observe any change in gustatory identification, food’s pleasantness worsened after bariatric surgery.

Slow digestion-oriented dietary strategy to sustain the secretion of GLP-1 for improved glucose homeostasis

Dysregulated glucose metabolism is associated with many chronic diseases such as obesity and type 2 diabetes mellitus (T2DM), and strategies to restore and maintain glucose homeostasis are essential to health. The incretin hormone of glucagon-like peptide-1 (GLP-1) is known to play a critical role in regulating glucose homeostasis and dietary nutrients are the primary stimuli to the release of intestinal GLP-1. However, the GLP-1 producing enteroendocrine L-cells are mainly distributed in the distal region of the gastrointestinal tract where there are almost no nutrients to stimulate the secretion of GLP-1 under normal situations. Thus, a dietary strategy to sustain the release of GLP-1 was proposed, and the slow digestion property and dipeptidyl peptidase IV (DPP-IV) inhibitory activity of food components, approaches to reduce the rate of food digestion, and mechanisms to sustain the release of GLP-1 were reviewed. A slow digestion-oriented dietary approach through encapsulation of nutrients, incorporation of viscous dietary fibers, and enzyme inhibitors of phytochemicals in a designed whole food matrix will be implemented to efficiently reduce the digestion rate of food nutrients, potentiate their distal deposition and a sustained secretion of GLP-1, which will be beneficial to improved glucose homeostasis and health.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Nutrient-Induced Cellular Mechanisms of Gut Hormone Secretion

The gastrointestinal tract can assess the nutrient composition of ingested food. The nutrient-sensing mechanisms in specialised epithelial cells lining the gastrointestinal tract, the enteroendocrine cells, trigger the release of gut hormones that provide important local and central feedback signals to regulate nutrient utilisation and feeding behaviour. The evidence for nutrient-stimulated secretion of two of the most studied gut hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), along with the known cellular mechanisms in enteroendocrine cells recruited by nutrients, will be the focus of this review. The mechanisms involved range from electrogenic transporters, ion channel modulation and nutrient-activated G-protein coupled receptors that converge on the release machinery controlling hormone secretion. Elucidation of these mechanisms will provide much needed insight into postprandial physiology and identify tractable dietary approaches to potentially manage nutrition and satiety by altering the secreted gut hormone profile.

Gut-adipose tissue crosstalk: A bridge to novel therapeutic targets in metabolic syndrome?

The gut is one of the main endocrine organs in our body, producing hormones acknowledged to play determinant roles in controlling appetite, energy balance and glucose homeostasis. One of the targets of such hormones is the adipose tissue, a major energetic reservoir, which governs overall metabolism through the secretion of adipokines. Disturbances either in nutrient and metabolic sensing and consequent miscommunication between these organs constitute a key driver to the metabolic complications clustered in metabolic syndrome. Thus, it is essential to understand how the disruption of this crosstalk might trigger adipose tissue dysfunction, a strong characteristic of obesity and insulin resistance. The beneficial effects of metabolic surgery in the amelioration of glucose homeostasis and body weight reduction allowed to understand the potential of gut signals modulation as a treatment for metabolic syndrome-related obesity and type 2 diabetes. In this review, we cover the effects of gut hormones in the modulation of adipose tissue metabolic and endocrine functions, as well as their impact in tissue plasticity. Furthermore, we discuss how the modulation of gut secretome, either through surgical procedures or pharmacological approaches, might improve adipose tissue function in obesity and metabolic syndrome.

The Emerging Role of Polyphenols in the Management of Type 2 Diabetes

Type 2 diabetes (T2D) is a fast-increasing health problem globally, and it results from insulin resistance and pancreatic β-cell dysfunction. The gastrointestinal (GI) tract is recognized as one of the major regulatory organs of glucose homeostasis that involves multiple gut hormones and microbiota. Notably, the incretin hormone glucagon-like peptide-1 (GLP-1) secreted from enteroendocrine L-cells plays a pivotal role in maintaining glucose homeostasis via eliciting pleiotropic effects, which are largely mediated via its receptor. Thus, targeting the GLP-1 signaling system is a highly attractive therapeutic strategy to treatment T2D. Polyphenols, the secondary metabolites from plants, have drawn considerable attention because of their numerous health benefits, including potential anti-diabetic effects. Although the major targets and locations for the polyphenolic compounds to exert the anti-diabetic action are still unclear, the first organ that is exposed to these compounds is the GI tract in which polyphenols could modulate enzymes and hormones. Indeed, emerging evidence has shown that polyphenols can stimulate GLP-1 secretion, indicating that these natural compounds might exert metabolic action at least partially mediated by GLP-1. This review provides an overview of nutritional regulation of GLP-1 secretion and summarizes recent studies on the roles of polyphenols in GLP-1 secretion and degradation as it relates to metabolic homeostasis. In addition, the effects of polyphenols on microbiota and microbial metabolites that could indirectly modulate GLP-1 secretion are also discussed.

Sweetness Perception is not Involved in the Regulation of Blood Glucose after Oral Application of Sucrose and Glucose Solutions in Healthy Male Subjects

SCOPE

This study investigates the effect of the sweetness of a sucrose versus an isocaloric glucose solution in dietary concentrations on blood glucose regulation by adjusting the sweetness level using the sweet taste inhibitor lactisole.

METHODS AND RESULTS

A total of 27 healthy males participated in this randomized, crossover study with four treatments: 10% glucose, 10% sucrose, 10% sucrose + 60 ppm lactisole, and 10% glucose + 60 ppm lactisole. Plasma glucose, insulin, glucagon-like peptide 1, and glucagon levels are measured at baseline and 15, 30, 60, 90, and 120 min after beverage consumption. Test subjects rated the sucrose solution to be sweeter than the isocaloric glucose solution, whereas no difference in sweetness is reported after addition of lactisole to the sucrose solution. Administration of the less sweet glucose solution versus sucrose led to higher blood glucose levels after 30 min, as reflected by a lower ΔAUC for sucrose (1072 ± 136) than for glucose (1567 ± 231). Application of lactisole leads to no differences in glucose, insulin, or glucagon responses induced by sucrose or glucose.

CONCLUSION

The results indicate that the structure of the carbohydrate has a stronger impact on the regulation of blood glucose levels than the perceived sweetness.

Sweet Taste Antagonist Lactisole Administered in Combination with Sucrose, But Not Glucose, Increases Energy Intake and Decreases Peripheral Serotonin in Male Subjects

Knowledge regarding the involvement of sweetness perception on energy intake is scarce. Here, the impact of glucose and sucrose sweetness, beyond their caloric load, on subsequent food intake and biomarkers of satiation was evaluated by co-administration of the sweet taste receptor inhibitor lactisole. A total of 27 healthy, male subjects received solutions of either 10% glucose w/o 60 ppm lactisole or 10% sucrose w/o 60 ppm lactisole. Subsequent food intake from a standardized breakfast was evaluated 2 h after receiving the respective test solution. Changes in postprandial plasma concentrations of cholecystokinin, ghrelin, and serotonin were determined over a period of 120 min, as was the body temperature. Administration of lactisole to the sucrose solution increased the energy intake from the subsequent standardized breakfast by 12.9 ± 5.8% (p = 0.04), led to a decreased Δ AUC of the body core temperature by 46 ± 20% (p = 0.01), and time-dependently reduced Δ serotonin plasma concentrations (−16.9 ± 6.06 ng/mL vs. −0.56 ± 3.7 ng/mL after sucrose administration, p = 0.03). The present study shows that lactisole increases energy intake and decreases plasma serotonin concentrations as well as body core temperature induced by sucrose, but not glucose. This finding may be associated with the different binding affinities of sucrose and glucose to the sweet taste receptor.

Sugar Reduction in Dairy Food: An Overview with Flavoured Milk as an Example

Owing to the public health concern associated with the consumption of added sugar, the World Health Organization recommends cutting down sugar in processed foods. Furthermore, due to the growing concern of increased calorie intake from added sugar in sweetened dairy foods, the present review provides an overview of different types and functions of sugar, various sugar reduction strategies, and current trends in the use of sweeteners for sugar reduction in dairy food, taking flavoured milk as a central theme where possible to explore the aforementioned aspects. The strength and uniqueness of this review are that it brings together all the information on the available types of sugar and sugar reduction strategies and explores the current trends that could be applied for reducing sugar in dairy foods without much impact on consumer acceptance. Among different strategies for sugar reduction, the use of natural non-nutritive sweeteners (NNSs), has received much attention due to consumer demand for natural ingredients. Sweetness imparted by sugar can be replaced by natural NNSs, however, sugar provides more than just sweetness to flavoured milk. Sugar reduction involves multiple technical challenges to maintain the sensory properties of the product, as well as to maintain consumer acceptance. Because no single sugar has a sensory profile that matches sucrose, the use of two or more natural NNSs could be an option for food industries to reduce sugar using a holistic approach rather than a single sugar reduction strategy. Therefore, achieving even a small sugar reduction can significantly improve the diet and health of an individual.

Genetic variants of TAS2R38 bitter taste receptor associate with distinct gut microbiota traits in Parkinson's disease: A pilot study

The non-tasting form of the bitter taste receptor, TAS2R38, has been shown as a genetic risk factor associated with the development of Parkinson's disease (PD). Specific taste receptors that are expressed in the lower gastrointestinal tract may respond to alteration in gut microbiota composition, detecting bacterial molecules, and regulate immune responses. Given the importance of brain-gut-microbiota axis and gene-environment interactions in PD, we investigate the associations between the genetic variants of TAS2R38 and gut microbiota composition in 39 PD patients. The results confirm that the majority of PD patients have reduced sensitivity to 6-n-propylthiouracil (PROP) and are carriers of at least one non-functional TAS2R38 AVI haplotype. Moreover, we found this correlation to be associated with a reduction in bacteria alpha-diversity with a predominant reduction of Clostridium genus. We hypothesised that the high frequency of the non-taster form of TAS2R38 associated with a diminuition of Clostridium bacteria in PD might determine a reduction in the activation of protective signalling-molecules useful in preserving gut homeostasis. This pilot study, by identifying a decrease in specific bacteria associated with a reduced sensitivity to PROP, adds essential information that opens new avenues of research into the association of PD microbiota composition and sensory modification.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Nutrient sensing of gut luminal environment

Sensing of nutrients by chemosensory cells in the gastrointestinal tract plays a key role in transmitting food-related signals, linking information about the composition of ingested foods to digestive processes. In recent years, a number of G protein-coupled receptors (GPCR) responsive to a range of nutrients have been identified. Many are localised to intestinal enteroendocrine (chemosensory) cells, promoting hormonal and neuronal signalling locally, centrally and to the periphery. The field of gut sensory systems is relatively new and still evolving. Despite huge interest in these nutrient-sensing GPCR, both as sensors for nutritional status and targets for preventing the development of metabolic diseases, major challenges remain to be resolved. However, the gut expressed sweet taste receptor, resident in L-enteroendocrine cells and responsive to dietary sweetener additives, has already been successfully explored and utilised as a therapeutic target, treating weaning-related disorders in young animals. In addition to sensing nutrients, many GPCR are targets for drugs used in clinical practice. As such these receptors, in particular those expressed in L-cells, are currently being assessed as potential new pathways for treating diabetes and obesity. Furthermore, growing recognition of gut chemosensing of microbial-produced SCFA acids has led further attention to the association between nutrition and development of chronic disorders focusing on the relationship between nutrients, gut microbiota and health. The central importance of gut nutrient sensing in the control of gastrointestinal physiology, health promotion and gut-brain communication offers promise that further therapeutic successes and nutritional recommendations will arise from research in this area.

Polygonatum cyrtonema Hua Polysaccharide Promotes GLP-1 Secretion from Enteroendocrine L-Cells through Sweet Taste Receptor-Mediated cAMP Signaling

Glucagon-like peptide-1 (GLP-1) secreted from enteroendocrine L-cells is a pleiotropic hormone with beneficial potential related to islet function, diet control, glucose homeostasis, inflammation relief, and cardiovascular protection. The present study aimed at investigating the effect of Polygonatum cyrtonema polysaccharide (PCP) after structural identification on GLP-1 secretion and the possible mechanism involved in the PCP-stimulated secretion of GLP-1. It was found that GLP-1 secretion was effectively promoted (p < 0.01) by PCP both in rats with oral administration for 5 weeks (13.9 ± 0.3-35.8 ± 0.3 pmol/L) and ileal administration within 2 h (13.6 ± 0.4-34.1 ± 1.1 pmol/L) and in enteroendocrine NCI-H716 cells with direct stimulation within 24 h (2.05 ± 0.3-20.7 ± 0.2 pmol/L). The sweet taste receptor T1R2/T1R3 was identified to be essential for NCI-H716 cells to directly recognize PCP. The intervention experiments showed that PCP-stimulated GLP-1 secretion was significantly depressed (p < 0.01) not only by antibodies, siRNA, and the inhibitor of T1R2/T1R3 but also by an adenylate cyclase inhibitor. These results suggest that PCP stimulates GLP-1 secretion from enteroendocrine cells possibly through activation of the T1R2/T1R3-mediated cAMP signaling pathway.

Intense Sweeteners, Taste Receptors and the Gut Microbiome: A Metabolic Health Perspective

Intense sweeteners (IS) are often marketed as a healthier alternative to sugars, with the potential to aid in combating the worldwide rise of diabetes and obesity. However, their use has been counterintuitively associated with impaired glucose homeostasis, weight gain and altered gut microbiota. The nature of these associations, and the mechanisms responsible, are yet to be fully elucidated. Differences in their interaction with taste receptors may be a potential explanatory factor. Like sugars, IS stimulate sweet taste receptors, but due to their diverse structures, some are also able to stimulate bitter taste receptors. These receptors are expressed in the oral cavity and extra-orally, including throughout the gastrointestinal tract. They are involved in the modulation of appetite, glucose homeostasis and gut motility. Therefore, taste genotypes resulting in functional receptor changes and altered receptor expression levels may be associated with metabolic conditions. IS and taste receptors may both interact with the gastrointestinal microbiome, and their interactions may potentially explain the relationship between IS use, obesity and metabolic outcomes. While these elements are often studied in isolation, the potential interactions remain unexplored. Here, the current evidence of the relationship between IS use, obesity and metabolic outcomes is presented, and the potential roles for interactions with taste receptors and the gastrointestinal microbiota in modulating these relationships are explored.

Saccharin Supplementation Inhibits Bacterial Growth and Reduces Experimental Colitis in Mice

Non-caloric artificial sweeteners are frequently discussed as components of the “Western diet”, negatively modulating intestinal homeostasis. Since the artificial sweetener saccharin is known to depict bacteriostatic and microbiome-modulating properties, we hypothesized oral saccharin intake to influence intestinal inflammation and aimed at delineating its effect on acute and chronic colitis activity in mice. In vitro, different bacterial strains were grown in the presence or absence of saccharin. Mice were supplemented with saccharin before or after induction of acute or chronic colitis using dextran sodium sulfate (DSS) and the extent of colitis was assessed. Ex vivo, intestinal inflammation, fecal bacterial load and composition were studied by immunohistochemistry analyses, quantitative PCR, 16 S RNA PCR or next generation sequencing in samples collected from analyzed mice. In vitro, saccharin inhibited bacterial growth in a species-dependent manner. In vivo, oral saccharin intake reduced fecal bacterial load and altered microbiome composition, while the intestinal barrier was not obviously affected. Of note, DSS-induced colitis activity was significantly improved in mice after therapeutic or prophylactic treatment with saccharin. Together, this study demonstrates that oral saccharin intake decreases intestinal bacteria count and hence encompasses the capacity to reduce acute and chronic colitis activity in mice.

First-Phase Insulin and Amylin after Bariatric Surgery: A Prospective Randomized Trial on Patients with Insulin Resistance or Diabetes after Gastric Bypass or Sleeve Gastrectomy

BACKGROUND

Most patients with severe obesity show glucose intolerance. Early after sleeve gastrectomy (LSG) or gastric bypass (LRYGB), a marked amelioration in glycemic control occurs. The underlying mechanism is not yet clear.

OBJECTIVE

To determine whether the improvement in glycemic control on the level of endocrine pancreatic function is due to an increased first-phase insulin secretion comparing LRYGB to LSG.

SETTING

University of Basel Hospital and St. Clara Research Ltd., Basel, Switzerland.

METHODS

Sixteen morbidly obese patients with severe obesity and different degrees of insulin resistance were randomized to LSG or LRYGB, and islet cell functions were tested by intravenous glucose and intravenous arginine administration before and 4 weeks after surgery.

RESULTS

Fasting insulin and glucose levels and homeostasis model assessment insulin resistance were significantly lower in both groups after surgery compared to baseline, while no change was seen in fasting C-peptide, amylin, and glucagon. After intravenous glucose stimulation, no statistically significant pre- to postoperative change in area under the curve (AUC 0-60 min) was seen for insulin, glucagon, amylin, and C-peptide. No statistically significant pre- to postoperative change in incremental AUC for first-phase insulin release (AUC 0-10 min), second-phase insulin secretion (AUC 10-60 min), and insulin/glucose ratio could be shown in either group. Arginine-stimulated insulin and glucagon release showed no pre- to postoperative change.

CONCLUSION

Intravenous glucose and arginine administrations show no pre- to postoperative changes of insulin release, amylin, glucagon, or C-peptide concentrations, and no differences between LRYGB and LSG were found. The postoperative improvement in glycemic control is not caused by changes in endocrine pancreatic hormone secretion.

Effects of Sucralose Ingestion versus Sucralose Taste on Metabolic Responses to an Oral Glucose Tolerance Test in Participants with Normal Weight and Obesity: A Randomized Crossover Trial

Here, we tested the hypothesis that sucralose differentially affects metabolic responses to labeled oral glucose tolerance tests (OGTTs) in participants with normal weight and obesity. Participants (10 with normal weight and 11 with obesity) without diabetes underwent three dual-tracer OGTTs preceded, in a randomized order, by consuming sucralose or water, or by tasting and expectorating sucralose (e.g., sham-fed; sweetness control). Indices of β-cell function and insulin sensitivity (S_I) were estimated using oral minimal models of glucose, insulin, and C-peptide kinetics. Compared with water, sucralose ingested (but not sham-fed) resulted in a 30 ± 10% increased glucose area under the curve in both weight groups. In contrast, the insulin response to sucralose ingestion differed depending on the presence of obesity: decreased within 20–40 min of the OGTT in normal-weight participants but increased within 90–120 min in participants with obesity. Sham-fed sucralose similarly decreased insulin concentrations within 60 min of the OGTT in both weight groups. Sucralose ingested (but not sham-fed) increased S_I in normal-weight participants by 52 ± 20% but did not affect S_I in participants with obesity. Sucralose did not affect glucose rates of appearance or β-cell function in either weight group. Our data underscore a physiological role for taste perception in postprandial glucose responses, suggesting sweeteners should be consumed in moderation.

Mechanisms controlling hormone secretion in human gut and its relevance to metabolism

The homoeostatic regulation of metabolism is highly complex and involves multiple inputs from both the nervous and endocrine systems. The gut is the largest endocrine organ in our body and synthesises and secretes over 20 different hormones from enteroendocrine cells that are dispersed throughout the gut epithelium. These hormones include GLP-1, PYY, GIP, serotonin, and CCK, each of whom play pivotal roles in maintaining energy balance and glucose homeostasis. Some are now the basis of several clinically used glucose-lowering and weight loss therapies. The environment in which these enteroendocrine cells exist is also complex, as they are exposed to numerous physiological inputs including ingested nutrients, circulating factors and metabolites produced from neighbouring gut microbiome. In this review, we examine the diverse means by which gut-derived hormones carry out their metabolic functions through their interactions with different metabolically important organs including the liver, pancreas, adipose tissue and brain. Furthermore, we discuss how nutrients and microbial metabolites affect gut hormone secretion and the mechanisms underlying these interactions.

Therapeutic potential of ectopic olfactory and taste receptors

Olfactory and taste receptors are expressed primarily in the nasal olfactory epithelium and gustatory taste bud cells, where they transmit real-time sensory signals to the brain. However, they are also expressed in multiple extra-nasal and extra-oral tissues, being implicated in diverse biological processes including sperm chemotaxis, muscle regeneration, bronchoconstriction and bronchodilatation, inflammation, appetite regulation and energy metabolism. Elucidation of the physiological roles of these ectopic receptors is revealing potential therapeutic and diagnostic applications in conditions including wounds, hair loss, asthma, obesity and cancers. This Review outlines current understanding of the diverse functions of ectopic olfactory and taste receptors and assesses their potential to be therapeutically exploited.

Review: Chemosensing of nutrients and non-nutrients in the human and porcine gastrointestinal tract

The gastrointestinal tract (GIT) is an interface between the external and internal milieus that requires continuous monitoring for nutrients or pathogens and toxic chemicals. The study of the physiological/molecular mechanisms, mediating the responses to the monitoring of the GIT contents, has been referred to as chemosensory science. While most of the progress in this area of research has been obtained in laboratory rodents and humans, significant steps forward have also been reported in pigs. The objective of this review was to update the current knowledge on nutrient chemosensing in pigs in light of recent advances in humans and laboratory rodents. A second objective relates to informing the existence of nutrient sensors with their functionality, particularly linked to the gut peptides relevant to the onset/offset of appetite. Several cell types of the intestinal epithelium such as Paneth, goblet, tuft and enteroendocrine cells (EECs) contain subsets of chemosensory receptors also found on the tongue as part of the taste system. In particular, EECs show specific co-expression patterns between nutrient sensors and/or transceptors (transport proteins with sensing functions) and anorexigenic hormones such as cholecystokinin (CCK), peptide tyrosine tyrosine (PYY) or glucagon-like peptide-1 (GLP-1), amongst others. In addition, the administration of bitter compounds has an inhibitory effect on GIT motility and on appetite through GLP-1-, CCK-, ghrelin- and PYY-labelled EECs in the human small intestine and colon. Furthermore, the mammalian chemosensory system is the target of some bacterial metabolites. Recent studies on the human microbiome have discovered that commensal bacteria have developed strategies to stimulate chemosensory receptors and trigger host cellular functions. Finally, the study of gene polymorphisms related to nutrient sensors explains differences in food choices, food intake and appetite between individuals.

Metabolic control via nutrient-sensing mechanisms: role of taste receptors and the gut-brain neuroendocrine axis

Nutrient sensing plays an important role in ensuring that appropriate digestive or hormonal responses are elicited following the ingestion of fuel substrates. Mechanisms of nutrient sensing in the oral cavity have been fairly well characterized and involve lingual taste receptors. These include heterodimers of G protein-coupled receptors (GPCRs) of the taste receptor type 1 (T1R) family for sensing sweet (T1R2-T1R3) and umami (T1R1-T1R3) stimuli, the T2R family for sensing bitter stimuli, and ion channels for conferring sour and salty tastes. In recent years, several studies have revealed the existence of additional nutrient-sensing mechanisms along the gastrointestinal tract. Glucose sensing is achieved by the T1R2-T1R3 heterodimer on enteroendocrine cells, which plays a role in triggering the secretion of incretin hormones for improved glycemic and lipemic control. Protein hydrolysates are detected by Ca²⁺-sensing receptor, the T1R1-T1R3 heterodimer, and G protein-coupled receptor 92/93 (GPR92/93), which leads to the release of the gut-derived satiety factor cholecystokinin. Furthermore, several GPCRs have been implicated in fatty acid sensing: GPR40 and GPR120 respond to medium- and long-chain fatty acids, GPR41 and GPR43 to short-chain fatty acids, and GPR119 to endogenous lipid derivatives. Aside from the recognition of fuel substrates, both the oral cavity and the gastrointestinal tract also possess T2R-mediated mechanisms of recognizing nonnutrients such as environmental contaminants, bacterial toxins, and secondary plant metabolites that evoke a bitter taste. These gastrointestinal sensing mechanisms result in the transmission of neuronal signals to the brain through the release of gastrointestinal hormones that act on vagal and enteric afferents to modulate the physiological response to nutrients, particularly satiety and energy homeostasis. Modulating these orally accessible nutrient-sensing pathways using particular foods, dietary supplements, or pharmaceutical compounds may have therapeutic potential for treating obesity and metabolic diseases.

Taste Receptors in Upper Airway Innate Immunity

Taste receptors, first identified on the tongue, are best known for their role in guiding our dietary preferences. The expression of taste receptors for umami, sweet, and bitter have been demonstrated in tissues outside of the oral cavity, including in the airway, brain, gastrointestinal tract, and reproductive organs. The extra-oral taste receptor chemosensory pathways and the endogenous taste receptor ligands are generally unknown, but there is increasing data suggesting that taste receptors are involved in regulating some aspects of innate immunity, and may potentially control the composition of the nasal microbiome in healthy individuals or patients with upper respiratory diseases like chronic rhinosinusitis (CRS). For this reason, taste receptors may serve as potential therapeutic targets, providing alternatives to conventional antibiotics. This review focuses on the physiology of sweet (T1R) and bitter (T2R) taste receptors in the airway and their activation by secreted bacterial products. There is particular focus on T2R38 in sinonasal ciliated cells, as well as the sweet and bitter receptors found on specialized sinonasal solitary chemosensory cells. Additionally, this review explores the impact of genetic variations in these receptors on the differential susceptibility of patients to upper airway infections, such as CRS.

Taste and the Gastrointestinal tract: from physiology to potential therapeutic target for obesity

Flavor is the combination of gustatory, olfactory and trigeminal sensations, representing the three main sensory pathways that allow detecting environmental chemical substances. Taste, in particular, is a complex chemosensory path that allows identification of substances present in ingested foods and beverages. In this manuscript, we propose a conceptual roadmap from aspects related to the evolution and the physiological role of taste, up to the current knowledge about its implication in the modulation of a healthy state, or obesity. More specifically, we focused on the role of stimulation of taste receptors in releasing gut hormones (also known as enterohormones), and their effects on the regulation of food intake, by inducing satiety, either by locally acting (in the gastrointestinal tract), or centrally (in the brain). Recent evidence demonstrated that some enterohormones are able to modulate gastrointestinal motility, thus affecting an orexigenic responses in the central nervous system. In keeping with this, we discuss the ability of the gustatory system to be a final checkpoint control for food intake regulation, and we speculate about taste perception manipulation in the management of obesity.

Dietary Branched-Chain Amino Acids Regulate Food Intake Partly through Intestinal and Hypothalamic Amino Acid Receptors in Piglets

Strategies to increase feed intake are of great importance for producing more meat in swine production. Intestinal and hypothalamic amino acid receptors are found to largely participate in feed intake regulation. The purpose of the current research is to study the function of branched-chain amino acid (BCAA) supplementation in the regulation of feed intake through sensors that can detect amino acids in piglets. Twenty-four piglets were assigned one of four treatments and fed one of the experimental diets for either a short period (Expt. 1) or a long period (Expt. 2): a normal protein diet (NP, 20.04% CP), a reduced-protein diet (RP, 17.05% CP), or a reduced-protein test diet supplemented with one of two doses of BCAAs (BCAA1, supplemented with 0.13% l-isoleucine, 0.09% l-leucine, and 0.23% l-valine; BCAA2, supplemented with the 150% standardized ileal digestibility BCAA requirement, as recommended by the National Research Council (2012)). In Expt. 1, no differences were observed in the feed intake among piglets fed different diets ( P > 0.05). In Expt. 2, compared with the RP group, the feed intake of piglets was significantly increased after sufficient BCAAs were supplemented in the BCAA1 group, which was associated with decreased cholecystokinin secretion ( P < 0.05), down-regulated expression of type-1 taste receptors 1/3 (T1R1/T1R3) in the intestine, as well as increased expression of pro-opiomelanocortin, activated general control nonderepressible 2 (GCN2), and eukaryotic initiation factor 2α (eIF2α) in the hypothalamus ( P < 0.05). However, the feed intake was decreased for unknown reasons when the piglets were fed a BCAA over-supplemented diet. Our study confirmed that a BCAA-deficient diet inhibited feed intake through two potential ways: regulating the amino acid T1R1/T1R3 receptor in the intestine or activating GCN2/eIF2α pathways in the hypothalamus.

Non-Nutritive Sweeteners and Their Implications on the Development of Metabolic Syndrome

Individuals widely use non-nutritive sweeteners (NNS) in attempts to lower their overall daily caloric intake, lose weight, and sustain a healthy diet. There are insufficient scientific data that support the safety of consuming NNS. However, recent studies have suggested that NNS consumption can induce gut microbiota dysbiosis and promote glucose intolerance in healthy individuals that may result in the development of type 2 diabetes mellitus (T2DM). This sequence of events may result in changes in the gut microbiota composition through microRNA (miRNA)-mediated changes. The mechanism(s) by which miRNAs alter gene expression of different bacterial species provides a link between the consumption of NNS and the development of metabolic changes. Another potential mechanism that connects NNS to metabolic changes is the molecular crosstalk between the insulin receptor (IR) and G protein-coupled receptors (GPCRs). Here, we aim to highlight the role of NNS in obesity and discuss IR-GPCR crosstalk and miRNA-mediated changes, in the manipulation of the gut microbiota composition and T2DM pathogenesis.

Immunoglobulin G modulation of the melanocortin 4 receptor signaling in obesity and eating disorders

Melanocortin 4 receptor (MC4R) plays a key role in regulation of appetite activated by its main ligand α-melanocyte-stimulating hormone (α-MSH) in both central and peripheral targets. α-MSH also binds to circulating immunoglobulins (Igs) but the functional significance of such immune complexes (ICs) in MC4R signaling in normal and pathological conditions of altered appetite has remained unknown. To address this question, we analyzed plasma levels, affinity kinetics, and binding epitopes of α-MSH-reactive IgG extracted from plasma samples of female patients with hyperphagic obesity, anorexia nervosa, bulimia nervosa, binge-eating disorder, and healthy controls. Ability of α-MSH/IgG IC to bind and activate human MC4R were studied in vitro and to influence feeding behavior in vivo in rodents. We found that α-MSH-reactive IgG were low in obese but increased in anorectic and bulimic patients and displayed different epitope and kinetics of IC formation. Importantly, while α-MSH/IgG IC from all subjects were binding and activating MC4R, the receptor binding affinity was decreased in obesity. Additionally, α-MSH/IgG IC had lower MC4R-mediated cAMP activation threshold as compared with α-MSH alone in all but not obese subjects. Furthermore, the cellular internalization rate of α-MSH/IgG IC by MC4R-expressing cells was decreased in obese but increased in patients with anorexia nervosa. Moreover, IgG from obese patients prevented central anorexigenic effect of α-MSH. These findings reveal that MC4R is physiologically activated by IC formed by α-MSH/IgG and that different levels and molecular properties of α-MSH-reactive IgG underlie biological activity of such IC relevant to altered appetite in obesity and eating disorders.

Secretion of Gut Hormones and Expression of Sweet Taste Receptors and Glucose Transporters in a Rat Model of Obesity

OBJECTIVES

This study was undertaken to compare gut hormone secretion between high-fat-fed and control rats, and to examine the corresponding changes in the expression of sweet taste receptors and glucose transporters in the small intestine and hypothalamus.

METHODS

Four-week-old male Sprague Dawley rats were fed a standard or high-fat diet for 8 weeks (10 in each group), followed by an oral glucose tolerance test (50% glucose solution, 2 g/kg). Blood was sampled for glucose, insulin, glucagon-like peptide-1 (GLP-1) and polypeptide YY (PYY) assays. One week later, small intestinal and hypothalamic tissue were analyzed for sweet taste receptor and glucose transporter expression by real-time PCR.

RESULTS

After oral glucose, plasma GLP-1 concentrations were higher in high-fat-fed than standard-fat-fed rats (group × time interaction, p < 0.01) with significant differences at t = 15 min (p < 0.01) and 30 min (p < 0.05). Plasma PYY concentrations were lower in high-fat-fed than control rats at t = 0, 15 min (p < 0.05, respectively) and 120 min (p < 0.01). There were no differences in the expression of sweet taste receptors or glucose transporters between high-fat-fed and control rats in the duodenum, ileum, or hypothalamus.

CONCLUSIONS

Changes in GLP-1 and PYY secretion after a high-fat diet appear unrelated to any changes in the expression of sweet taste receptors or glucose transporters. Impaired PYY secretion with high-fat feeding suggests that PYY analogues may provide a potential therapy in the treatment of obesity.

The downregulation of sweet taste receptor signaling in enteroendocrine L-cells mediates 3-deoxyglucosone-induced attenuation of high glucose-stimulated GLP-1 secretion

CONTEXT

Sweet taste receptors (STRs) involve in regulating the release of glucose-stimulated glucagon-like peptide-1 (GLP-1). Our in vivo and in vitro studies found that 3-deoxyglucosone (3DG) inhibited glucose-stimulated GLP-1 secretion.

OBJECTIVE

This study investigated the role of STRs in 3DG-induced inhibition of high glucose-stimulated GLP-1 secretion.

METHODS

STC-1 cells were incubated with lactisole or 3DG for 1 h under 25 mM glucose conditions. Western blotting was used to study the expression of STRs signaling molecules and ELISA was used to analyse GLP-1 and cyclic adenosine monophosphate (cAMP) levels.

RESULTS

Lactisole inhibited GLP-1 secretion. Exposure to 25 mM glucose increased the expressions of STRs subunits when compared with 5.6 mM glucose. 3DG decreased GLP-1 secretion and STRs subunits expressions, with affecting other components of STRs pathway, including the downregulation of transient receptor potential cation channel subfamily M member 5 (TRPM5) expression and the reduction of intracellular cAMP levels.

CONCLUSION

3DG attenuates high glucose-stimulated GLP-1 secretion by reducing STR subunit expression and downstream signaling components.