Sucrose activates human taste pathways differently from artificial sweetener

Differences in brain responses between lean and obese women to a sweetened drink

BACKGROUND

Ingestion of sweet food is driven by central reward circuits and restrained by endocrine and neurocrine satiety signals. The specific influence of sucrose intake on central affective and reward circuitry and alterations of these mechanisms in the obese are incompletely understood. For this, we hypothesized that (i) similar brain regions are engaged by the stimulation of sweet taste receptors by sucrose and by non-nutrient sweeteners and (ii) during visual food-related cues, obese subjects show greater brain responses to sucrose compared with lean controls.

METHODS

In a double-blind, crossover design, 10 obese and 10 lean healthy females received a sucrose or a non-nutrient sweetened beverage prior to viewing food or neutral images. BOLD signal was measured using a 1.5 Tesla MRI scanner.

KEY RESULTS

Viewing food images after ingestion of either drink was associated with engagement of similar brain regions (amygdala, hippocampus, thalamus, anterior insula). Obese differed from lean subjects in behavioral and brain responses rating both beverages as less tasteful and satisfying, yet demonstrating greater brain responses. Obese subjects also showed engagement of an additional brain network (including anterior insula, anterior cingulate, hippocampus, and amygdala) only after sucrose ingestion.

CONCLUSIONS & INFERENCES

Obese subjects had a reduced behavioral hedonic response, yet a greater engagement of affective brain networks, particularly after sucrose ingestion, suggesting that in obese subjects, lingual and gut-derived signaling generate less central hedonic effects than food-related memories in response to visual cues, analogous to response patterns implicated in food addiction.

The addictive dimensionality of obesity

Our brains are hardwired to respond and seek immediate rewards. Thus, it is not surprising that many people overeat, which in some can result in obesity, whereas others take drugs, which in some can result in addiction. Though food intake and body weight are under homeostatic regulation, when highly palatable food is available, the ability to resist the urge to eat hinges on self-control. There is no homeostatic regulator to check the intake of drugs (including alcohol); thus, regulation of drug consumption is mostly driven by self-control or unwanted effects (i.e., sedation for alcohol). Disruption in both the neurobiological processes that underlie sensitivity to reward and those that underlie inhibitory control can lead to compulsive food intake in some individuals and compulsive drug intake in others. There is increasing evidence that disruption of energy homeostasis can affect the reward circuitry and that overconsumption of rewarding food can lead to changes in the reward circuitry that result in compulsive food intake akin to the phenotype seen with addiction. Addiction research has produced new evidence that hints at significant commonalities between the neural substrates underlying the disease of addiction and at least some forms of obesity. This recognition has spurred a healthy debate to try and ascertain the extent to which these complex and dimensional disorders overlap and whether or not a deeper understanding of the crosstalk between the homeostatic and reward systems will usher in unique opportunities for prevention and treatment of both obesity and drug addiction.

Localization of the primary taste cortex by contrasting passive and attentive conditions

The primary taste cortex is located in the insula. However, exactly where in the insula the human primary taste cortex is located remains a controversial issue. Human neuroimaging studies have shown prominent variation concerning the location of taste-responsive activation within the insula. A standard protocol for gustatory testing in neuroimaging studies has not been developed, which might underlie such variations. In order to localize the primary taste cortex in an fMRI experiment, we used a taste delivery system to suppress non-taste stimuli and psychological effects. Then, we compared brain response to taste solution during a passive tasting task condition and a taste quality identification task condition to verify whether this cognitive task affected the location of taste-responsive activation within the insula. To examine which part of insula is the primary taste area, we performed dynamic causal modeling (DCM) to verify the neural network of the taste coding-related region and random-effects Bayesian model selection (BMS) at the family level to reveal the optimal input region. Passive tasting resulted in activation of the right middle insula (MI), and the most favorable model selected by DCM analysis showed that taste effect directly influenced the MI. Additionally, BMS results at the family level suggested that the taste inputs entered into the MI. Taken together, our results suggest that the human primary taste cortex is located in the MI.

Dietary fat induces sustained reward response in the human brain without primary taste cortex discrimination

To disentangle taste from reward responses in the human gustatory cortex, we combined high density electro-encephalography with a gustometer delivering tastant puffs to the tip of the tongue. Stimuli were pure tastants (salt solutions at two concentrations), caloric emulsions (two milk preparations identical in composition except for fat content) and a mixture of high fat milk with the lowest salt concentration. Early event-related potentials (ERPs) showed a dose-response effect for increased taste intensity, with higher amplitude and shorter latency for high compared to low salt concentration, but not for increased fat content. However, the amplitude and distribution of late potentials were modulated by fat content independently of reported intensity and discrimination. Neural source estimation revealed a sustained activation of reward areas to the two high-fat stimuli. The results suggest calorie detection through specific sensors on the tongue independent of perceived taste. Finally, amplitude variation of the first peak in the event-related potential to the different stimuli correlated with papilla density, suggesting a higher discrimination power for subjects with more fungiform papillae.

Post-prandial carbohydrate ingestion during 1-h of moderate-intensity, intermittent cycling does not improve mood, perceived exertion, or subsequent power output in recreationally-active exercisers

BACKGROUND

This study compared the effects of ingesting water (W), a flavored carbohydrate-electrolyte (CE) or a flavored non-caloric electrolyte (NCE) beverage on mood, ratings of perceived exertion (RPE), and sprint power during cycling in recreational exercisers.

METHODS

Men (n = 23) and women (n = 13) consumed a 24-h standardized diet and reported 2-4 h post-prandial for all test sessions. After a familiarization session, participants completed 50 min of stationary cycling in a warm environment (wet bulb globe temperature = 25.0°C) at ~ 60-65% of heart rate reserve (146 ± 4 bpm) interspersed with 5 rest periods of 2 min each. During exercise, participants consumed W, CE, or NCE, served in a counterbalanced cross-over design. Beverage volume was served in 3 aliquots equaling each individual's sweat losses (mean 847 ± 368 mL) during the familiarization session. Profiles of Mood States questionnaires (POMS) were administered and blood glucose levels were determined pre- and post- sub-maximal cycling. Following sub-maximal exercise, participants completed 3 30-s Wingate anaerobic tests (WAnT) with 2.5 min rest between tests to assess performance.

RESULTS

Blood glucose was higher (p <  0.05) after 50 min of submaximal cycling just prior to the WAnT for CE (6.1 ± 1.7 mmol/L) compared to W (4.9 ± 1.5 mmol/L) and NCE (4.6 ± 1.2 mmol/L). Nonetheless, there were no differences among treatments in peak (642 ± 153, 635 ± 143, 650 ± 141 watts for W, NCE, and CE, respectively; p  =  0.44) or mean (455 ± 100, 458 ± 95, 454 ± 95 watts for W, NCE, and CE, respectively; p = 0.62) power for the first WAnT or mean (414 ± 92, 425 ± 85, 423 ± 82 watts, respectively; p = 0.13) power output averaged across all 3 WAnT. Likewise, RPE during submaximal exercise, session RPE, and fatigue and vigor assessed by POMS did not differ among beverage treatments (p > 0.05).

CONCLUSIONS

Carbohydrate ingestion consumed by recreational exercisers during a 1-h, moderate-intensity aerobic workout did not alter mood or perceived exertion, nor did it affect subsequent anaerobic performance under the conditions of this study. Drinking caloric sport beverages does not benefit recreational exercisers in a non-fasted state.

Sucralose, a synthetic organochlorine sweetener: overview of biological issues

Sucralose is a synthetic organochlorine sweetener (OC) that is a common ingredient in the world's food supply. Sucralose interacts with chemosensors in the alimentary tract that play a role in sweet taste sensation and hormone secretion. In rats, sucralose ingestion was shown to increase the expression of the efflux transporter P-glycoprotein (P-gp) and two cytochrome P-450 (CYP) isozymes in the intestine. P-gp and CYP are key components of the presystemic detoxification system involved in first-pass drug metabolism. The effect of sucralose on first-pass drug metabolism in humans, however, has not yet been determined. In rats, sucralose alters the microbial composition in the gastrointestinal tract (GIT), with relatively greater reduction in beneficial bacteria. Although early studies asserted that sucralose passes through the GIT unchanged, subsequent analysis suggested that some of the ingested sweetener is metabolized in the GIT, as indicated by multiple peaks found in thin-layer radiochromatographic profiles of methanolic fecal extracts after oral sucralose administration. The identity and safety profile of these putative sucralose metabolites are not known at this time. Sucralose and one of its hydrolysis products were found to be mutagenic at elevated concentrations in several testing methods. Cooking with sucralose at high temperatures was reported to generate chloropropanols, a potentially toxic class of compounds. Both human and rodent studies demonstrated that sucralose may alter glucose, insulin, and glucagon-like peptide 1 (GLP-1) levels. Taken together, these findings indicate that sucralose is not a biologically inert compound.

What contributes to excessive diet soda intake in eating disorders: appetitive drive, weight concerns, or both?

Excessive diet soda intake is common in eating disorders. The present study examined factors contributing to excessive intake in a sample of individuals with lifetime eating disorders based on proposed DSM-5 criteria (n = 240) and non-eating disorder controls (n = 157). Individuals with eating disorders, particularly bulimia nervosa, consumed more diet soda than controls. Eating disorder symptoms that reflect increased appetitive drive or increased weight concerns were associated with increased diet soda intake. Increased weight concerns were associated with increased diet soda intake when levels of appetitive drive were high, but not when they were low. Results highlight the importance of monitoring diet soda intake in individuals with eating disorders and may have implications for the maintenance of dysregulated taste reward processing in bulimia nervosa.

Obesity and addiction: neurobiological overlaps

Drug addiction and obesity appear to share several properties. Both can be defined as disorders in which the saliency of a specific type of reward (food or drug) becomes exaggerated relative to, and at the expense of others rewards. Both drugs and food have powerful reinforcing effects, which are in part mediated by abrupt dopamine increases in the brain reward centres. The abrupt dopamine increases, in vulnerable individuals, can override the brain's homeostatic control mechanisms. These parallels have generated interest in understanding the shared vulnerabilities between addiction and obesity. Predictably, they also engendered a heated debate. Specifically, brain imaging studies are beginning to uncover common features between these two conditions and delineate some of the overlapping brain circuits whose dysfunctions may underlie the observed deficits. The combined results suggest that both obese and drug-addicted individuals suffer from impairments in dopaminergic pathways that regulate neuronal systems associated not only with reward sensitivity and incentive motivation, but also with conditioning, self-control, stress reactivity and interoceptive awareness. In parallel, studies are also delineating differences between them that centre on the key role that peripheral signals involved with homeostatic control exert on food intake. Here, we focus on the shared neurobiological substrates of obesity and addiction.

The Sweet Taste of Success

According to the resource-depletion model, self-control is a limited resource that is depleted after a period of exertion. Evidence consistent with this model indicates that self-control relies on glucose metabolism and glucose supplementation to depleted individuals replenishes self-control resources. In five experiments, we tested an alternative hypothesis that glucose in the oral cavity counteracts the deleterious effects of self-control depletion. We predicted a glucose mouth rinse, as opposed to an artificially sweetened placebo rinse, would lead to better self-control after depletion. In Studies 1 to 3, participants engaging in a depleting task performed significantly better on a subsequent self-control task after receiving a glucose mouth rinse, as opposed to participants rinsing with a placebo. Studies 4 and 5 replicated these findings and demonstrated that the glucose mouth rinse had no effect on self-control in nondepleted participants. Results are consistent with a neural rather than metabolic mechanism for the effect of glucose supplementation on self-control.

Current status of functional imaging in eating disorders

Eating Disorders are complex psychiatric problems that involve biologic and psychological factors. Brain imaging studies provide insights about how functionally connected brain networks may contribute to disturbed eating behavior, resulting in food refusal and altered body weight, but also body preoccupations and heightened anxiety. In this article, we review the current state of brain imaging in eating disorders, and how such techniques may help identify pathways that could be important in the treatment of those often detrimental disorders.

The black agonist-receptor model of high potency sweeteners, and its implication to sweetness taste and sweetener design

The dose responses of the most commonly used high potency sweeteners (HPSs) have been measured by a more precise sensory procedure. The data were analyzed by Black's pharmacological model that takes into account not only agonist binding affinity but transduction efficiency as well. HPSs are clearly segregated into 2 groups depending on whether they bind to T1R2 or T1R3 of the receptor heterodimer. Surprisingly, the more potent sweeteners have lower transduction efficiencies. The implications of these on consumer product development and HPS design are discussed.

Taste uncoupled from nutrition fails to sustain the reinforcing properties of food

Recent findings suggest the reward system encodes metabolic value independent of taste, provoking speculation that the hedonic value of taste could be derived from nutritional value as a secondary appetitive property. We therefore dissociated and compared the impact of nutrition and taste on appetitive behavior in several paradigms. Though taste alone induces preference and increased consumption, in the absence of nutritional value its reinforcing properties are greatly diminished and it does not, like sucrose, induce increased responding over time. In agreement with behavioral data, saccharin-evoked (but not sucrose-evoked) dopamine release is greatly attenuated following pre-exposure, suggesting that nutritional value is critical for dopamine-mediated reward and reinforcement. Further supporting the primacy of nutrition over taste, genetically increased dopaminergic tone enhances incentive associated with nutritional value with minimal impact on taste-based, hedonic incentive. Overall, we suggest that the sensory-hedonic incentive value associated with taste functions as a conditioned stimulus that requires nutritional value to sustainably organize appetitive behavior.

Altered processing of sweet taste in the brain of diet soda drinkers

Artificially sweetened beverage consumption has been linked to obesity, and it has been hypothesized that considerable exposure to nonnutritive sweeteners may be associated with impaired energy regulation. The reward system plays an integral role in modulating energy intake, but little is known about whether habitual use of artificial sweetener (i.e., diet soda consumption) may be related to altered reward processing of sweet taste in the brain. To investigate this, we examined fMRI response after a 12-hour fast to sucrose (a nutritive sweetener) and saccharin (a nonnutritive sweetener) during hedonic evaluation in young adult diet soda drinkers and non-diet soda drinkers. Diet soda drinkers demonstrated greater activation to sweet taste in the dopaminergic midbrain (including ventral tegmental area) and right amygdala. Saccharin elicited a greater response in the right orbitofrontal cortex (Brodmann Area 47) relative to sucrose in non-diet soda drinkers. There was no difference in fMRI response to the nutritive or nonnutritive sweetener for diet soda drinkers. Within the diet soda drinkers, fMRI activation of the right caudate head in response to saccharin was negatively associated with the amount of diet sodas consumed per week; individuals who consumed a greater number of diet sodas had reduced caudate head activation. These findings suggest that there are alterations in reward processing of sweet taste in individuals who regularly consume diet soda, and this is associated with the degree of consumption. These findings may provide some insight into the link between diet soda consumption and obesity.

Food and drug reward: overlapping circuits in human obesity and addiction

Both drug addiction and obesity can be defined as disorders in which the saliency value of one type of reward (drugs and food, respectively) becomes abnormally enhanced relative to, and at the expense of others. This model is consistent with the fact that both drugs and food have powerful reinforcing effects-partly mediated by dopamine increases in the limbic system-that, under certain circumstances or in vulnerable individuals, could overwhelm the brain's homeostatic control mechanisms. Such parallels have generated significant interest in understanding the shared vulnerabilities and trajectories between addiction and obesity. Now, brain imaging discoveries have started to uncover common features between these two conditions and to delineate some of the overlapping brain circuits whose dysfunctions may explain stereotypic and related behavioral deficits in human subjects. These results suggest that both obese and drug-addicted individuals suffer from impairments in dopaminergic pathways that regulate neuronal systems associated not only with reward sensitivity and incentive motivation, but also with conditioning (memory/learning), impulse control (behavioural inhibition), stress reactivity, and interoceptive awareness. Here, we integrate findings predominantly derived from positron emission tomography that shed light on the role of dopamine in drug addiction and in obesity, and propose an updated working model to help identify treatment strategies that may benefit both of these conditions.

Males and females show differential brain activation to taste when hungry and sated in gustatory and reward areas

Although males and females differ in eating behavior and prevalence rates for eating disorders and obesity, little is known about gender differences in cortical activation to pleasant and unpleasant pure tastes during the physiological states of hunger and satiety. Twenty-one healthy young adults (12 females and 9 males) underwent two functional magnetic resonance imaging scans. Using four pure tastants of differing qualities (i.e., salty, sour, bitter, sweet), the present study examined gender differences in fMRI activation during two motivational states (hunger and satiety). There was greater change in fMRI activation from hunger to satiety in males than females in response to all tastes within the middle frontal gyrus (BA 10), insula, and cerebellum. Males also had greater change in activation from hunger to satiety, relative to females, in limbic regions including dorsal striatum, amygdala, parahippocampal gyrus, and posterior and anterior cingulate; however, activation was stimulus dependent, despite equivalent ratings in perceived pleasantness and intensity. Interestingly, males and females showed significant change from hunger to satiety in response to citric acid, suggesting that in addition to gender and physiological condition, stimulus quality is an important factor in taste fMRI activation. These gender differences may have implications for the pathophysiology of eating disorders and obesity.

Intravascular food reward

Consumption of calorie-containing sugars elicits appetitive behavioral responses and dopamine release in the ventral striatum, even in the absence of sweet-taste transduction machinery. However, it is unclear if such reward-related postingestive effects reflect preabsorptive or postabsorptive events. In support of the importance of postabsorptive glucose detection, we found that, in rat behavioral tests, high concentration glucose solutions administered in the jugular vein were sufficient to condition a side-bias. Additionally, a lower concentration glucose solution conditioned robust behavioral responses when administered in the hepatic-portal, but not the jugular vein. Furthermore, enteric administration of glucose at a concentration that is sufficient to elicit behavioral conditioning resulted in a glycemic profile similar to that observed after administration of the low concentration glucose solution in the hepatic-portal, but not jugular vein. Finally using fast-scan cyclic voltammetry we found that, in accordance with behavioral findings, a low concentration glucose solution caused an increase in spontaneous dopamine release events in the nucleus accumbens shell when administered in the hepatic-portal, but not the jugular vein. These findings demonstrate that the postabsorptive effects of glucose are sufficient for the postingestive behavioral and dopaminergic reward-related responses that result from sugar consumption. Furthermore, glycemia levels in the hepatic-portal venous system contribute more significantly for this effect than systemic glycemia, arguing for the participation of an intra-abdominal visceral sensor for glucose.

Similarities in hypothalamic and mesocorticolimbic circuits regulating the overconsumption of food and alcohol

Historically, studies of food intake regulation started with the hypothalamus and gradually expanded to mesocorticolimbic regions, while studies of drug use began with mesocorticolimbic regions and now include the hypothalamus. As research on ingestive behavior has progressed, it has uncovered more and more similarities between the regulation of palatable food and drug intake. It has also identified specific neurochemicals involved in palatable food and drug intake. Hypothalamic orexigenic neurochemicals specifically involved in controlling fat ingestion, including galanin, enkephalin, orexin and melanin-concentrating hormone, show positive feedback with this macronutrient, with these peptides both increasing fat intake and being further stimulated by its intake. This positive relationship offers some explanation for why foods high in fat are so often overconsumed. Research in Bart Hoebel's laboratory in conjunction with our own has shown that consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by these neurochemical systems involved in fat intake, consistent with evidence closely relating fat and ethanol consumption. Both fat and ethanol intake are also regulated by dopamine and acetylcholine acting in mesocorticolimbic nuclei. This close relationship of fat and ethanol is likely driven in part by circulating lipids, which are increased by fat and ethanol intake, known to increase expression and levels of the neurochemicals, and found to promote further intake of fat and ethanol. Compellingly, recent studies suggest that these systems may already be dysregulated in animals prone to consuming excess fat or ethanol, even before they have ever been exposed to these substances. Further understanding of these systems involved in consummatory behavior will allow researchers to develop effective therapies for the treatment of overeating as well as drug abuse.

Sensing via intestinal sweet taste pathways

The detection of nutrients in the gastrointestinal (GI) tract is of fundamental significance to the control of motility, glycemia and energy intake, and yet we barely know the most fundamental aspects of this process. This is in stark contrast to the mechanisms underlying the detection of lingual taste, which have been increasingly well characterized in recent years, and which provide an excellent starting point for characterizing nutrient detection in the intestine. This review focuses on the form and function of sweet taste transduction mechanisms identified in the intestinal tract; it does not focus on sensors for fatty acids or proteins. It examines the intestinal cell types equipped with sweet taste transduction molecules in animals and humans, their location, and potential signals that transduce the presence of nutrients to neural pathways involved in reflex control of GI motility.

Oral carbohydrate sensing and exercise performance

PURPOSE OF REVIEW

Carbohydrate during exercise has been demonstrated to improve exercise performance even when the exercise is of high intensity (>75% VO2max) and relatively short duration (approximately 1 h). It has become clear that the underlying mechanisms for the ergogenic effect during this type of activity are not metabolic but may reside in the central nervous system.

RECENT FINDINGS

Carbohydrate mouth rinses have been shown to result in similar performance improvements. This would suggest that the beneficial effects of carbohydrate feeding during exercise are not confined to its conventional metabolic advantage but may also serve as a positive afferent signal capable of modifying motor output. These effects are specific to carbohydrate and are independent of taste. The receptors in the oral cavity have not (yet) been identified and the exact role of various brain areas is not clearly understood. Further research is warranted to fully understand the separate taste transduction pathways for simple and complex carbohydrates and how these differ between mammalian species, particularly in humans.

SUMMARY

Carbohydrate is detected in oral cavity by unidentified receptors and this can be linked to improvements in exercise performance.

De novo artistic activity following insular-SII ischemia

We report here the case of a female patient who developed the following behavioural changes after a brain lesion involving the left posterior insula and SII cortices. She discovered de novo artistic capabilities for painting, with an episodic and compulsive need to paint ("hyperpainting"), but also exhibited changes in her ability to feel emotions. In addition, she had a typical neuropathic pain syndrome, including provoked pain and spontaneous pain, whose intensity was worsened when she painted with cold colours. This case-report suggests some kind of synaesthesiae, which has previously been reported for other sensory modalities. These findings suggest that a cross-talk between emotional, thermosensory, pain, and motivational functions may take place during recovery, at the level of the left insular-SII cortices.

Association of body mass and brain activation during gastric distention: implications for obesity

BACKGROUND

Gastric distention (GD), as it occurs during meal ingestion, signals a full stomach and it is one of the key mechanisms controlling food intake. Previous studies on GD showed lower activation of the amygdala for subjects with higher body mass index (BMI). Since obese subjects have dopaminergic deficits that correlate negatively with BMI and the amygdala is innervated by dopamine neurons, we hypothesized that BMI would correlate negatively with activation not just in the amygdala but also in other dopaminergic brain regions (midbrain and hypothalamus).

METHODOLOGY/PRINCIPAL FINDINGS

We used functional magnetic resonance imaging (fMRI) to evaluate brain activation during GD in 24 healthy subjects with BMI range of 20-39 kg/m(2). Using multiple regression and cross-correlation analyses based on a family-wise error corrected threshold P = 0.05, we show that during slow GD to maximum volumes of 500 ml and 700 ml subjects with increased BMI had increased activation in cerebellum and left posterior insula, and decreased activation of dopaminergic (amygdala, midbrain, hypothalamus, thalamus) and serotonergic (pons) brain regions and anterior insula, regions that were functionally interconnected with one another.

CONCLUSIONS

The negative correlation between BMI and BOLD responses to gastric distention in dopaminergic (midbrain, hypothalamus, amygdala, thalamus) and serotonergic (pons) brain regions is consistent with disruption of dopaminergic and serotonergic signaling in obesity. In contrast the positive correlation between BMI and BOLD responses in posterior insula and cerebellum suggests an opposing mechanism that promotes food intake in obese subjects that may underlie their ability to consume at once large food volumes despite increasing gastric distention.

Sip size of orangeade: effects on intake and sensory-specific satiation

Sensory-specific satiation (SSS) drives food selection and contributes to meal termination. We hypothesised that smaller sips would increase SSS due to increased oro-sensory exposure, irrespective of energy content. The objective was to determine the effects of sip size and energy content on ad libitum intake of orangeade and subjective SSS for orangeade. Orangeade intake and ratings of wanting and liking were measured before and after ad libitum orangeade consumption in a 2 × 2 cross-over design (n 53). Conditions differed in energy content (no-energy v. regular-energy orangeade) and in sip size (large, 20 g/sip v. small, 5 g/sip). The mean intake of both orangeades was lower when consumed with small sips than when consumed with large sips (regular-energy, 352 v. 493 g; no-energy, 338 v. 405 g; both P < 0·001). When consumed with large sips, the mean intake of no-energy orangeade was lower than that of regular-energy orangeade (P = 0·02). When consumed with small sips, subjective SSS (based on the desire to drink) was higher for no-energy orangeade than for regular-energy orangeade (P = 0·01), while mean intake was comparable. We concluded that smaller sip size, i.e. increased oro-sensory exposure per unit of consumption, can lower intake of sweet drinks. Only with low oro-sensory exposure (large sip size) was intake higher for an energy-containing sweet drink than for a no-energy sweet drink. This suggests that intake of sweet drinks is stimulated by (metabolic) reward value and inhibited by sensory satiation. This underpins the importance of SSS for meal termination.

Effect of the artificial sweetener, sucralose, on gastric emptying and incretin hormone release in healthy subjects

The incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), play an important role in glucose homeostasis in both health and diabetes. In mice, sucralose, an artificial sweetener, stimulates GLP-1 release via sweet taste receptors on enteroendocrine cells. We studied blood glucose, plasma levels of insulin, GLP-1, and GIP, and gastric emptying (by a breath test) in 7 healthy humans after intragastric infusions of 1) 50 g sucrose in water to a total volume of 500 ml (approximately 290 mosmol/l), 2) 80 mg sucralose in 500 ml normal saline (approximately 300 mosmol/l, 0.4 mM sucralose), 3) 800 mg sucralose in 500 ml normal saline (approximately 300 mosmol/l, 4 mM sucralose), and 4) 500 ml normal saline (approximately 300 mosmol/l), all labeled with 150 mg 13C-acetate. Blood glucose increased only in response to sucrose (P<0.05). GLP-1, GIP, and insulin also increased after sucrose (P=0.0001) but not after either load of sucralose or saline. Gastric emptying of sucrose was slower than that of saline (t50: 87.4+/-4.1 min vs. 74.7+/-3.2 min, P<0.005), whereas there were no differences in t50 between sucralose 0.4 mM (73.7+/-3.1 min) or 4 mM (76.7+/-3.1 min) and saline. We conclude that sucralose, delivered by intragastric infusion, does not stimulate insulin, GLP-1, or GIP release or slow gastric emptying in healthy humans.

How do you feel--now? The anterior insula and human awareness

The anterior insular cortex (AIC) is implicated in a wide range of conditions and behaviours, from bowel distension and orgasm, to cigarette craving and maternal love, to decision making and sudden insight. Its function in the re-representation of interoception offers one possible basis for its involvement in all subjective feelings. New findings suggest a fundamental role for the AIC (and the von Economo neurons it contains) in awareness, and thus it needs to be considered as a potential neural correlate of consciousness.

Imaging of brain dopamine pathways: implications for understanding obesity

Obesity is typically associated with abnormal eating behaviors. Brain imaging studies in humans implicate the involvement of dopamine (DA)-modulated circuits in pathologic eating behavior(s). Food cues increase striatal extracellular DA, providing evidence for the involvement of DA in the nonhedonic motivational properties of food. Food cues also increase metabolism in the orbitofrontal cortex indicating the association of this region with the motivation for food consumption. Similar to drug-addicted subjects, striatal DA D2 receptor availability is reduced in obese subjects, which may predispose obese subjects to seek food as a means to temporarily compensate for understimulated reward circuits. Decreased DA D2 receptors in the obese subjects are also associated with decreased metabolism in prefrontal regions involved in inhibitory control, which may underlie their inability to control food intake. Gastric stimulation in obese subjects activates cortical and limbic regions involved with self-control, motivation, and memory. These brain regions are also activated during drug craving in drug-addicted subjects. Obese subjects have increased metabolism in the somatosensory cortex, which suggests an enhanced sensitivity to the sensory properties of food. The reduction in DA D2 receptors in obese subjects coupled with the enhanced sensitivity to food palatability could make food their most salient reinforcer putting them at risk for compulsive eating and obesity. The results from these studies suggest that multiple but similar brain circuits are disrupted in obesity and drug addiction and suggest that strategies aimed at improving DA function might be beneficial in the treatment and prevention of obesity.

Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity

Exercise studies have suggested that the presence of carbohydrate in the human mouth activates regions of the brain that can enhance exercise performance but direct evidence of such a mechanism is limited. The first aim of the present study was to observe how rinsing the mouth with solutions containing glucose and maltodextrin, disguised with artificial sweetener, would affect exercise performance. The second aim was to use functional magnetic resonance imaging (fMRI) to identify the brain regions activated by these substances. In Study 1A, eight endurance-trained cyclists (VO2max 60.8 +/- 4.1 ml kg(-1) min(-1)) completed a cycle time trial (total work = 914 +/- 29 kJ) significantly faster when rinsing their mouths with a 6.4% glucose solution compared with a placebo containing saccharin (60.4 +/- 3.7 and 61.6 +/- 3.8 min, respectively, P = 0.007). The corresponding fMRI study (Study 1B) revealed that oral exposure to glucose activated reward-related brain regions, including the anterior cingulate cortex and striatum, which were unresponsive to saccharin. In Study 2A, eight endurance-trained cyclists (VO2max 57.8 +/- 3.2 ml kg(-1) min(-1)) tested the effect of rinsing with a 6.4% maltodextrin solution on exercise performance, showing it to significantly reduce the time to complete the cycle time trial (total work = 837 +/- 68 kJ) compared to an artificially sweetened placebo (62.6 +/- 4.7 and 64.6 +/- 4.9 min, respectively, P = 0.012). The second neuroimaging study (Study 2B) compared the cortical response to oral maltodextrin and glucose, revealing a similar pattern of brain activation in response to the two carbohydrate solutions, including areas of the insula/frontal operculum, orbitofrontal cortex and striatum. The results suggest that the improvement in exercise performance that is observed when carbohydrate is present in the mouth may be due to the activation of brain regions believed to be involved in reward and motor control. The findings also suggest that there may be a class of so far unidentified oral receptors that respond to carbohydrate independently of those for sweetness.

Cortical activation in response to pure taste stimuli during the physiological states of hunger and satiety

This event-related functional magnetic resonance imaging (er-fMRI) study investigated BOLD signal change in response to a series of pure gustatory stimuli that varied in stimulus quality when subjects were hungry and sated with a nutritional pre-load. Group analyses showed significant differences in activation in the hunger minus satiety condition in response to sucrose, caffeine, saccharin, and citric acid within the thalamus, hippocampus, and parahippocampus. When examining the hunger and satiety conditions, activation varied as a function of stimulus, with the majority of the stimuli exhibiting significantly greater activation in the hunger state within the insula, thalamus, and substantia nigra, in contrast to decreased activation in the satiated state within the parahippocampus, hippocampus, amygdala, and anterior cingulate. Region of interest (ROI) analysis revealed two significant interactions, ROI by physiology and ROI by physiology by stimulus. In the satiety condition, the primary (inferior and superior insulae) and secondary (OFC 11 and OFC 47) taste regions exhibited significantly greater brain activation in response to all stimuli than regions involved in processing eating behavior (hypothalamus), affect (amygdala), and memory (hippocampus, parahippocampus and entorhinal cortex). These same regions demonstrated significantly greater activation within the hunger condition than the satiety condition, with the exception of the superior insula. Furthermore, the patterns of activation differed as a function taste stimulus, with greater activation in response to sucrose than to the other stimuli. These differential patterns of activation suggest that the physiological states of hunger and satiety produce divergent activation in multiple brain areas in response to different pure gustatory stimuli.

Consumption of SC45647 and sucralose by rats selectively bred for high and low saccharin intake

Mammals' affinity for sweet tastes exists alongside dramatic variation among species and individuals in responses to sweeteners. The present paper focused on consumption by Occidental High- (HiS) and Low-Saccharin (LoS)-consuming rats in 23-h 2-bottle tests of 2 sweeteners for which few data from rats are available: SC45647 and sucralose. Every HiS and LoS rat preferred SC45647 to water at every concentration, with HiS rats consuming it more avidly. Most HiS rats preferred sucralose to water at one or more concentrations; some HiS rats and most LoS rats avoided sucralose at every concentration. However, both HiS and LoS rats preferred a sucralose-maltodextrin mixture (Splenda) to water; thus, Splenda's "bulking" ingredient maltodextrin transforms highly variable responses to sucralose into a relatively homogeneous preference for the product. Implications for the study of variation in sweet taste are discussed.