Cephalic phase insulin release: A review of its mechanistic basis and variability in humans

Artificial Sweeteners: A Double-Edged Sword for Gut Microbiome

Background and Aim: The human gut microbiome plays a crucial role in maintaining health. Artificial sweeteners, also known as non-nutritive sweeteners (NNS), have garnered attention for their potential to disrupt the balance of the gut microbiome. This review explores the complex relationship between NNS and the gut microbiome, highlighting their potential benefits and risks. By synthesizing current evidence, we aim to provide a balanced perspective on the role of AS in dietary practices and health outcomes, emphasizing the need for targeted research to guide their safe and effective use. Methods: A comprehensive literature review was conducted through searches in PubMed and Google Scholar, focusing on the effects of artificial sweeteners on gut microbiota. The search utilized key terms including "Gut Microbiome", "gut microbiota", "Eubiosis", "Dysbiosis", "Artificial Sweeteners", and "Nonnutritive Sweeteners". Results: NNS may alter the gut microbiome, but findings remain inconsistent. Animal studies often report a decrease in beneficial bacteria like Bifidobacterium and Lactobacillus, and an increase in harmful strains such as Clostridium difficile and E. coli, potentially leading to inflammation and gut imbalance. Disruptions in short-chain fatty acid (SCFA) production and gut hormone signaling have also been observed. However, human studies generally show milder or no significant changes, highlighting the limitations in translating animal model findings directly to humans. Differences in study design, dosage, exposure time, and sweetener type likely contribute to these varied outcomes. Conclusions: While NNS offer certain benefits, including reduced caloric intake and improved blood sugar regulation, their impact on gut microbiome health raises important concerns. The observed reduction in beneficial bacteria and the rise in pathogenic strains underscore the need for caution in NNS consumption. Furthermore, the disruption of SCFA production and metabolic pathways illustrates the intricate relationship between diet and gut health.

Neural and hormonal mechanisms of appetite regulation during eating

Numerous animal and clinical studies have demonstrated that the arcuate nucleus of the hypothalamus, a central regulator of appetite, plays a significant role in modulating feeding behavior. However, current research primarily focuses on long-term dietary changes and their effects on the body, with limited investigation into neuroendocrine dynamics during individual meals across diverse populations. In contrast to long-term dietary adjustments, directives for dietary behavior during a specific meal are more actionable, potentially enhancing patient adherence and achieving better outcomes in dietary behavior interventions. This review aimed to explore the neural pathways and endocrine changes activated by gastrointestinal expansion and variations in blood nutrient levels during a single meal, with the goal of informing dietary behavior guidance.

Does sweetness exposure drive 'sweet tooth'?

It is widely believed that exposure to sweetened foods and beverages stimulates the liking and desire for sweetness. Here we provide an updated review of the empirical evidence from human research examining whether exposure to sweet foods or beverages influences subsequent general liking for sweetness (‘sweet tooth’), based on the conclusions of existing systematic reviews and more recent research identified from a structured search of literature. Prior reviews have concluded that the evidence for a relationship between sweet taste exposure and measures of sweet taste liking is equivocal, and more recent primary research generally does not support the view that exposure drives increased liking for sweetness, in adults or children. In intervention trials using a range of designs, acute exposure to sweetness usually has the opposite effect (reducing subsequent liking and desire for sweet taste), while sustained exposures have no significant effects or inconsistent effects. Recent longitudinal observational studies in infants and children also report no significant associations between exposures to sweet foods and beverages with measures of sweet taste preferences. Overall, while it is widely assumed that exposure to sweetness stimulates a greater liking and desire for sweetness, this is not borne out by the balance of empirical evidence. While new research may provide a more robust evidence base, there are also a number of methodological, biological and behavioural considerations that may underpin the apparent absence of a positive relationship between sweetness exposure and liking.

Could Insulin Be a Better Regulator of Appetite/Satiety Balance and Body Weight Maintenance in Response to Glucose Exposure Compared to Sucrose Substitutes? Unraveling Current Knowledge and Searching for More Appropriate Choices

BACKGROUND

Insulin exerts a crucial impact on glucose control, cellular growing, function, and metabolism. It is partially modulated by nutrients, especially as a response to the intake of foods, including carbohydrates. Moreover, insulin can exert an anorexigenic effect when inserted into the hypothalamus of the brain, in which a complex network of an appetite/hunger control system occurs. The current literature review aims at thoroughly summarizing and scrutinizing whether insulin release in response to glucose exposure may be a better choice to control body weight gain and related diseases compared to the use of sucrose substitutes (SSs) in combination with a long-term, well-balanced diet.

METHODS

This is a comprehensive literature review, which was performed through searching in-depth for the most accurate scientific databases and applying effective and relevant keywords.

RESULTS

The insulin action can be inserted into the hypothalamic orexigenic/anorexigenic complex system, activating several anorexigenic peptides, increasing the hedonic aspect of food intake, and effectively controlling the human body weight. In contrast, SSs appear not to affect the orexigenic/anorexigenic complex system, resulting in more cases of uncontrolled body weight maintenance while also increasing the risk of developing related diseases.

CONCLUSIONS

Most evidence, mainly derived from in vitro and in vivo animal studies, has reinforced the insulin anorexigenic action in the hypothalamus of the brain. Simultaneously, most available clinical studies showed that SSs during a well-balanced diet either maintain or even increase body weight, which may indirectly be ascribed to the fact that they cannot cover the hedonic aspect of food intake. However, there is a strong demand for long-term longitudinal surveys to effectively specify the impact of SSs on human metabolic health.

Individual differences in cephalic-phase insulin response are stable over time and predict glucose tolerance in mice

Oral stimulation by glucose triggers a rapid insulin response, which enhances glucose tolerance. This so-called cephalic-phase insulin response (CPIR) has been documented in many mammal species, but its functional properties are poorly characterized. Here, we studied CPIR in lean C57BL/6 mice. Experiment 1 asked whether the large individual differences in CPIR magnitude were real or reflected experimental noise. We measured CPIR magnitude four times across a period of 30 days in the same mice. The individual differences in CPIR magnitude were remarkably stable across the repeated trials, indicating that they were real. Experiment 2 examined the functional consequences of individual differences in CPIR magnitude. We found that higher CPIR magnitudes contributed to larger postprandial insulin responses and greater glucose tolerance. Experiment 3 examined the observation that the CPIRs in Experiments 1 and 2 were associated with a rapid rise in blood glucose. To determine whether the rapid rise in blood glucose caused the CPIRs, we asked whether mice would generate a CPIR if we prevented cephalic-phase stimulation of beta cells by either delivering the glucose intragastrically or blocking parasympathetic input to the pancreatic beta cells with atropine. The mice subjected to these treatments experienced a rapid rise in blood glucose, but they did not exhibit a CPIR. This indicates that it was the oral glucose stimulation, and not the rise in blood glucose, that triggered the CPIRs in Experiments 1 and 2. We conclude that (i) individual differences in CPIR magnitude are stable over time; (ii) CPIR magnitudes predicted postprandial insulin responses and glucose tolerance; and (iii) a rapid rise in blood glucose is not sufficient to trigger a CPIR in mice.

Molecular cell types as functional units of the efferent vagus nerve

The vagus nerve vitally connects the brain and body to coordinate digestive, cardiorespiratory, and immune functions. Its efferent neurons, which project their axons from the brainstem to the viscera, are thought to comprise "functional units" - neuron populations dedicated to the control of specific vagal reflexes or organ functions. Previous research indicates that these functional units differ from one another anatomically, neurochemically, and physiologically but have yet to define their identity in an experimentally tractable way. However, recent work with genetic technology and single-cell genomics suggests that genetically distinct subtypes of neurons may be the functional units of the efferent vagus. Here we review how these approaches are revealing the organizational principles of the efferent vagus in unprecedented detail.

Timing and Nutrient Type of Isocaloric Snacks Impacted Postprandial Glycemic and Insulinemic Responses of the Subsequent Meal in Healthy Subjects

The aim of the study was to explore the impact of both the macronutrient composition and snacking timing on the postprandial glycemic insulinemic responses and food intake. Seventeen healthy female volunteers completed the randomized crossover trials. The volunteers were provided a standard breakfast and lunch at 8:00 and 13:00, respectively, and an ad libitum dinner at 18:00. Provided at either 10:30 (midmorning) or 12:30 (preload), the glycemic effects of the three types of 70 kcal snacks, including chicken breast (mid-C and pre-C), apple (mid-A and pre-A), and macadamia nut (mid-M and pre-M), were compared with the non-snack control (CON), evaluated by continuous glucose monitoring (CGM). The mid-M showed increased insulin resistance after lunch compared with CON, while the pre-M did not. The pre-A stabilized the glycemic response in terms of all variability parameters after lunch, while the mid-A had no significant effect on postprandial glucose control. Both the mid-C and pre-C improved the total area under the glucose curve, all glycemic variability parameters, and the insulin resistance within 2 h after lunch compared with CON. The pre-C attained the lowest energy intake at dinner, while the mid-A and the mid-M resulted in the highest. In conclusion, the chicken breast snack effectively stabilized postprandial glycemic excursion and reduced insulin resistance while the macadamia snack did not, regardless of ingestion time. Only as a preload could the apple snack mitigate the glucose response after the subsequent meal.

Oral glucose sensing in cephalic phase insulin release

Oral stimulation with foods or food components elicits cephalic phase insulin release (CPIR), which limits postprandial hyperglycemia. Despite its physiological importance, the specific gustatory mechanisms that elicit CPIR have not been clearly defined. While most studies point to glucose and glucose-containing saccharides (e.g., sucrose, maltodextrins) as being the most consistent elicitors, it is not apparent whether this is due to the detection of glucose per se, or to the perceived taste cues associated with these stimuli (e.g., sweetness, starchiness). This study investigated potential sensory mechanisms involved with eliciting CPIR in humans, focusing on the role of oral glucose detection and associated taste. Four stimulus conditions possessing different carbohydrate and taste profiles were designed: 1) glucose alone; 2) glucose mixed with lactisole, a sweet taste inhibitor; 3) maltodextrin, which is digested to starchy- and sweet-tasting products during oral processing; and 4) maltodextrin mixed with lactisole and acarbose, an oral digestion inhibitor. Healthy adults (N = 22) attended four sessions where blood samples were drawn before and after oral stimulation with one of the target stimuli. Plasma c-peptide, insulin, and glucose concentrations were then analyzed. Whereas glucose alone elicited CPIR (one-sample t-test, p < 0.05), it did not stimulate the response in the presence of lactisole. Likewise, maltodextrin alone stimulated CPIR (p < 0.05), but maltodextrin with lactisole and acarbose did not. Together, these findings indicate that glucose is an effective CPIR stimulus, but that an associated taste sensation also serves as an important cue for triggering this response in humans.

Sugar signals from oral glucose transporters elicit cephalic-phase insulin release in mice

Cephalic-phase insulin release (CPIR) occurs before blood glucose increases after a meal. Although glucose is the most plausible cue to induce CPIR, peripheral sensory systems involved are not fully elucidated. We therefore examined roles of sweet sensing by a T1R3-dependent taste receptor and sugar sensing by oral glucose transporters in the oropharyngeal region in inducing CPIR. Spontaneous oral ingestion of glucose significantly increased plasma insulin 5 min later in wild-type (C57BL/6) and T1R3-knockout mice, but intragastric infusion did not. Oral treatment of glucose transporter inhibitors phlorizin and phloretin significantly reduced CPIR after spontaneous oral ingestion. In addition, a rapid increase in plasma insulin was significantly smaller in WT mice with spontaneous oral ingestion of nonmetabolizable glucose analog than in WT mice with spontaneous oral ingestion of glucose. Taken together, the T1R3-dependent receptor is not required for CPIR, but oral glucose transporters greatly contribute to induction of CPIR by sugars.

Early-life influences of low-calorie sweetener consumption on sugar taste

Children and adolescents are the highest consumers of added sugars, particularly from sugar-sweetened beverages (SSB). Regular consumption of SSB early in life induces a variety of negative consequences on health that can last into adulthood. Low-calorie sweeteners (LCS) are increasingly used as an alternative to added sugars because they provide a sweet sensation without adding calories to the diet. However, the long-term effects of early-life consumption of LCS are not well understood. Considering LCS engage at least one of the same taste receptors as sugars and potentially modulate cellular mechanisms of glucose transport and metabolism, it is especially important to understand how early-life LCS consumption impacts intake of and regulatory responses to caloric sugars. In our recent study, we found that habitual intake of LCS during the juvenile-adolescence period significantly changed how rats responded to sugar later in life. Here, we review evidence that LCS and sugars are sensed via common and distinct gustatory pathways, and then discuss the implications this has for shaping sugar-associated appetitive, consummatory, and physiological responses. Ultimately, the review highlights the diverse gaps in knowledge that will be necessary to fill to understand the consequences of regular LCS consumption during important phases of development.

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Oral stimulation with maltodextrin: Effect on cephalic phase insulin release

Cephalic phase insulin release (CPIR) occurs following sensory stimulation with food-related stimuli, and has been shown to limit postabsorptive hyperglycemia. While the specific stimuli that elicit CPIR in humans have not been clearly defined, previous research points to sugars as having potential importance. Maltodextrins are a starch-derived food ingredient commonly found in a variety of processed food products. When consumed, salivary α-amylase rapidly cleaves its component saccharides into smaller units, leading to the production of sugars in the mouth. Here, we investigated whether humans elicit CPIR after tasting but not swallowing maltodextrin, and whether the degree of CPIR exhibited is affected by individuals' salivary α-amylase activity. We found that a gelatin-based stimulus containing 22% w/v maltodextrin elicited CPIR in healthy individuals (N = 22) following a modified sham-feeding protocol using both insulin and c-peptide as indices of the response. However, the degree of CPIR measured did not differ across three groupings (low, medium, or high) of effective α-amylase activity by either index. In a follow-up experiment, a subset of participants (N = 14) underwent the same protocol using a gelatin stimulus without maltodextrin, and no observable CPIR ensued. These findings suggest that oral stimulation with maltodextrin elicits CPIR in humans, but that individual differences in effective salivary α-amylase activity may not necessarily be predictive of the degree of CPIR.

Use of c-peptide as a measure of cephalic phase insulin release in humans

Cephalic phase insulin release (CPIR) is a rapid pulse of insulin secreted within minutes of food-related sensory stimulation. Understanding the mechanisms underlying CPIR in humans has been hindered by its small observed effect size and high variability within and between studies. One contributing factor to these limitations may be the use of peripherally measured insulin as an indicator of secreted insulin, since a substantial portion of insulin is metabolized by the liver before delivery to peripheral circulation. Here, we investigated the use of c-peptide, which is co-secreted in equimolar amounts to insulin from pancreatic beta cells, as a proxy for insulin secretion during the cephalic phase period. Changes in insulin and c-peptide were monitored in 18 adults over two repeated sessions following oral stimulation with a sucrose-containing gelatin stimulus. We found that, on average, insulin and c-peptide release followed a similar time course over the cephalic phase period, but that c-peptide showed a greater effect size. Importantly, when insulin and c-peptide concentrations were compared across sessions, we found that changes in c-peptide were significantly correlated at the 2 min (r = 0.50, p = 0.03) and 4 min (r = 0.65, p = 0.003) time points, as well as when participants' highest c-peptide concentrations were considered (r = 0.64, p = 0.004). In contrast, no significant correlations were observed for changes in insulin measured from the sessions (r = -0.06-0.35, p > 0.05). Herein, we detail the individual variability of insulin and c-peptide concentrations measured during the cephalic phase period, and identify c-peptide as a valuable metric for insulin secretion alongside insulin concentrations when investigating CPIR.

Extended Inter-Meal Interval Negatively Impacted the Glycemic and Insulinemic Responses after Both Lunch and Dinner in Healthy Subjects

This study aimed to investigate the glycemic and insulinemic effects of lunch timing based on a fixed feeding window, and the effects of apple preload on postprandial glucose and insulin responses after nutrient-balanced lunch and the subsequent high-fat dinner in healthy participants. Twenty-six participants completed four randomized, crossover experimental trials: (1) early standardized lunch at 12:00 (12S); (2) apple preload to 12S (12A+S); (3) late standardized lunch at 14:00 (14S); and (4) apple preload to 14S (14A+S); wherein twenty participants’ blood samples were collected for insulin analysis following the lunch trails. In each experimental trial, each participant equipped with a continuous glucose monitor (CGM) was provided with a standardized breakfast and a high-fat dinner to be consumed at 8:00 and 18:00, respectively. The late lunch (14S) resulted in significantly elevated glucose peak, delayed insulin peak time, decreased insulin sensitivity, and increased insulin resistance following the lunch; also decreased glycemic response following the subsequent dinner and larger blood glucose fluctuation over the 24-h period compared with the 12S. The 14A+S significantly reduced the glucose peak, the insulin peak time and the glycemic variability following the lunch, also the 24-h glycemic variability compared with the 14S. The insulin sensitivity was significantly improved in the 12A+S, compared with that of the 12S. In conclusion, the present study found that an extra 2-h inter-meal fasting before and after lunch resulted in elevated glycemic response in both macronutrient-balanced meal and high-fat meal in healthy subjects. The negative impact of a late lunch could be partly reversed by the apple preload, without a trade-off of insulin secretion.

Non-nutritive sweeteners and their impacts on the gut microbiome and host physiology

Non-nutritive sweeteners (NNS) are broadly incorporated into foods, especially those representing a growing share of the beverage market. NNS are viewed as a noncaloric and desirable alternative to sugar-based sweeteners and are thought to contribute to reducing overall caloric intake. While these compounds have been studied extensively and have long been considered inert, new research has presented a different view and raises new questions about the effects of NNS on human physiology. Namely, the influence on glucose responses, the gastrointestinal epithelium, and the gut microbiome. As the gut microbiome is now recognized as a major mediator of human health and perturbations to this community are generally associated with negative health trajectories or overt disease, interactions between NNS and the gut microbiome are of increasing interest to clinicians and researchers. Several NNS compounds are now hypothesized to affect human physiology by modulating the gut microbiome, though the mechanism for this action remains unclear. The purpose of this review is to discuss the history and current knowledge of NNS, their reported utility and effects on host physiology and the gut microbiome, and describes a model for investigating the underlying mechanism behind reported effects of NNS on the gut microbiome.

A neuroimmunometabolic view on the cephalic phase of insulin release

The cephalic phase of insulin secretion (CPIS) plays a crucial role in glucose homeostasis. However, the neural basis of CPIS and its overall relevance to metabolic health are poorly understood. Here, we preview the findings of Wiedemann et al. (2022) that address the role of IL-1β in the integration of neuro-mediated insulin release following cephalic stimulation and CPIS dysregulation in obesity.

The Effect of Artificial Sweeteners Use on Sweet Taste Perception and Weight Loss Efficacy: A Review

Excessive consumption of sugar-rich foods is currently one of the most important factors that has led to the development of the global pandemic of obesity. On the other hand, there is evidence that obesity contributes to reduced sensitivity to sweet taste and hormonal changes affecting appetite, leading to an increased craving for sweets. A high intake of sugars increases the caloric value of the diet and, consequently, leads to weight gain. Moreover, attention is drawn to the concept of the addictive properties of sugar and sugary foods. A potential method to reduce the energy value of diet while maintaining the sweet taste is using non-nutritive sweeteners (NNS). NNS are commonly used as table sugar substitutes. This wide group of chemical compounds features high sweetness almost without calories due to its high sweetening strength. NNS include aspartame, acesulfame-K, sucralose, saccharin, cyclamate, neohesperidin dihydrochalcone (neohesperidin DC), neotame, taumatin, and advantame. The available evidence suggests that replacing sugar with NNS may support weight control. However, the effect of NNS on the regulation of appetite and sweet taste perception is not clear. Therefore, the review aimed to summarize the current knowledge about the use of NNS as a potential strategy for weight loss and their impact on sweet taste perception. Most studies have demonstrated that consumption of NNS-sweetened foods does not increase sweetness preference orenergy intake. Nonetheless, further research is required to determine the long-term effects of NNS on weight management.