Localization of brain activation by umami taste in humans

Synergistic effect of combining umami substances enhances perceived saltiness

Umami substances have the potential to enhance the perception of saltiness and thus reduce sodium intake. Two sensory evaluation experiments were conducted, involving participants tasting salt solutions, and solutions with added umami substances at equal sodium concentrations. Umami substances included sodium glutamate (MSG), disodium inosinate (IMP), and the combination of them which has a synergistic effect and is a closer match to commonly-consumed foods. In Experiment 1, using the two-alternative forced-choice (2-AFC) method by 330 consumers, paired comparisons were conducted at three different sodium concentrations. The combination of MSG and IMP enhanced the perception of saltiness (p < .001 in the difference test), whereas presenting either umami substance in isolation failed to do so (p > .05 in the similarity test). Significant order effects occurred in paired comparisons. In Experiment 2, a two-sip time-intensity (TI) analysis with trained panellists verified these results and found that tasting MSG and IMP either simultaneously or successively enhanced saltiness perception at equal sodium concentrations. These findings indicate that the synergistic effect of umami substances may be the cause of saltiness enhancement, and represents a potential strategy for sodium reduction while satisfying the consumer demand for saltiness perception. Considering the application in food processing and in food pairing, umami substances can potentially be used to help to reduce salt intake in food consumption.

The Effect of Korean Native Chicken Breed Information on Consumer Sensory Evaluation and Purchase Behavior

This study examined how chicken breed affects consumer behavior. The breeds evaluated were a Korean native chicken (KNC) and a commercial broiler (CB). Consumer behavior was measured by evaluating sensory attributes (taste, odor, and texture) and purchase behaviors [satisfaction, purchase intention, and willingness to pay (WTP)]. The sensory evaluation was conducted using healthy Korean consumers (n=100). The chicken was cooked in baeksuk (Korean traditional chicken soup), which is a seasonal summer health food in Korea. The participants evaluated sensory attributes and purchase behaviors between blinded samples of baeksuk (CB) and unblinded samples of baeksuk (KNC). The sensory evaluation involved chicken breasts and legs. The participants considered KNC as having a more umami taste, a chewier and juicer texture, and a less metallic odor than CB. Moreover, when participants were given KNC breed information, they expressed higher satisfaction, purchase intention, and WTP. The results of this study contribute to consumer behavior literature by revealing the effects of breed information on consumer sensory perceptions and purchase behaviors. Furthermore, the findings provide evidence that branding KNC is one way to increase its demand and low market share while contributing to sustainable consumption.

Effective Connectivity in the Human Brain for Sour Taste, Retronasal Smell, and Combined Flavour

The anterior insula and rolandic operculum are key regions for flavour perception in the human brain; however, it is unclear how taste and congruent retronasal smell are perceived as flavours. The multisensory integration required for sour flavour perception has rarely been studied; therefore, we investigated the brain responses to taste and smell in the sour flavour-processing network in 35 young healthy adults. We aimed to characterise the brain response to three stimulations applied in the oral cavity—sour taste, retronasal smell of mango, and combined flavour of both—using functional magnetic resonance imaging. Effective connectivity of the flavour-processing network and modulatory effect from taste and smell were analysed. Flavour stimulation activated middle insula and olfactory tubercle (primary taste and olfactory cortices, respectively); anterior insula and rolandic operculum, which are associated with multisensory integration; and ventrolateral prefrontal cortex, a secondary cortex for flavour perception. Dynamic causal modelling demonstrated that neural taste and smell signals were integrated at anterior insula and rolandic operculum. These findings elucidated how neural signals triggered by sour taste and smell presented in liquid form interact in the brain, which may underpin the neurobiology of food appreciation. Our study thus demonstrated the integration and synergy of taste and smell.

Mechanisms of umami taste perception: From molecular level to brain imaging

Due to unique characteristics, umami substances have gained much attention in the food industry during the past decade as potential replacers to sodium or fat to increase food palatability. Umami is not only known to increase appetite, but also to increase satiety, and hence could be used to control food intake. Therefore, it is important to understand the mechanism(s) involved in umami taste perception. This review discusses current knowledge of the mechanism(s) of umami perception from receptor level to human brain imaging. New findings regarding the molecular mechanisms for detecting umami tastes and their pathway(s), and the peripheral and central coding to umami taste are reviewed. The representation of umami in the human brain and the individual variation in detecting umami taste and associations with genotype are discussed. The presence of umami taste receptors in the gastrointestinal tract, and the interactions between the brain and gut are highlighted. The review concludes that more research is required into umami taste perception to include not only oral umami taste perception, but also the wider "whole body" signaling mechanisms, to explore the interaction between the brain and gut in response to umami perception and ingestion.

Association of Disinhibited Eating and Trait of Impulsivity With Insula and Amygdala Responses to Palatable Liquid Consumption

Eating behavior is not only influenced by the current energy balance, but also by the behavioral characteristics of eating. One of the recognized eating behavior constructs is ‘disinhibited eating,’ which refers to the tendency to overeat in response to negative emotional states or the presence of highly palatable foods. Food-related disinhibition is involved in binge eating, weight gain, and obesity and is also associated with the trait of impulsivity, which in turn, is linked to weight gain or maladaptive eating. However, the relationships among food-related disinhibition, the trait of impulsivity, and the neural substrates of eating behaviors in adolescence remain unclear. Therefore, we designed a functional magnetic resonance imaging (fMRI) study to examine the associations between brain responses to palatable liquid consumption and disinhibited eating behavior or impulsivity in healthy adolescents. Thirty-four adolescents (mean age ± standard deviation = 17.12 ± 1.91 years, age range = 14–19 years, boys = 15, girls = 19) participated in this study. Disinhibited eating was assessed with the disinhibition subscale of the Three-Factor Eating Questionnaire, while impulsivity was assessed using the Barratt impulsiveness scale. Participants received two fMRI sessions−a palatable liquid consumption fMRI and a resting-state fMRI. The fMRI experiment showed that increased disinhibited eating was positively associated with a greater insular response to palatable liquid consumption, while increased impulsivity was positively correlated with a greater amygdala response. The resting-state fMRI experiment showed that increased disinhibited eating was positively correlated with strengthened intrinsic functional connectivity between the insula and the amygdala, adjusting for sex (estimates of the beta coefficients = 0.146, standard error = 0.068, p = 0.040). Given that the amygdala and insular cortex are structurally and functionally connected and involved in trait impulsivity and ingestive behavior, our findings suggest that increased disinhibited eating would be associated with impulsivity via strengthened intrinsic functional connectivity between the insula and amygdala and linked to maladaptive eating.

The use of 'artificial saliva' as a neutral control condition in gustatory research: Artificial saliva is not a neutral gustatory stimulus

Distilled water with NaHCO₃ and KCl is a solution often referred to as 'artificial saliva' because its chemical composition mimics human saliva. It is often used as a control stimulus in gustatory research, especially in neuroimaging, owing to the claim that it does not produce a response in primary gustatory cortex Yet evidence that human research volunteers perceive this liquid as affectively neutral is lacking. Unpublished data from our lab suggested that this solution might be perceived as aversive. This study set out to systematically test the parameters influencing taste neutrality. We used two different concentrations of distilled water with NaHCO₃ and KCl, as well as bottled water as a control stimulus. Healthy adults rated all stimuli on two separate scales to rule out an interpretation based on the specifics of a single scale. Our participants rated artificial saliva as aversive on both scales. The bottled water was rated as neutral in valence on both scales, and as significantly less intense in sensation than both concentrations of the artificial saliva. This is the first study to have directly tested the subjective feelings that accompany the ingestion of these oft-used solutions on a trial-by-trial basis. We found that these stimuli, which were previously assumed to be neutral, may not be perceived as such by research participants. Therefore, future gustatory studies should take care when using this solution as a neutral baseline. It is advised that trial-by-trial ratings are collected. Also, depending on the nature of future studies, bottled water may be considered as a preferable neutral baseline.

A review of the alleged health hazards of monosodium glutamate

Monosodium glutamate (MSG) is an umami substance widely used as flavor enhancer. Although it is generally recognized as being safe by food safety regulatory agencies, several studies have questioned its long-term safety. The purpose of this review was to survey the available literature on preclinical studies and clinical trials regarding the alleged adverse effects of MSG. Here, we aim to provide a comprehensive overview of the reported possible risks that may potentially arise following chronic exposure. Furthermore, we intend to critically evaluate the relevance of this data for dietary human intake. Preclinical studies have associated MSG administration with cardiotoxicity, hepatotoxicity, neurotoxicity, low-grade inflammation, metabolic disarray and premalignant alterations, along with behavioral changes. Moreover, links between MSG consumption and tumorigenesis, increased oxidative stress and apoptosis in thymocytes, as well as genotoxic effects in lymphocytes have been reported. However, in reviewing the available literature, we detected several methodological flaws, which led us to conclude that these studies have limited relevance for extrapolation to dietary human intakes of MSG risk exposure. Clinical trials have focused mainly on the effects of MSG on food intake and energy expenditure. Besides its well-known impact on food palatability, MSG enhances salivary secretion and interferes with carbohydrate metabolism, while the impact on satiety and post-meal recovery of hunger varied in relation to meal composition. Reports on MSG hypersensitivity, also known as 'Chinese restaurant syndrome', or links of its use to increased pain sensitivity and atopic dermatitis were found to have little supporting evidence. Based on the available literature, we conclude that further clinical and epidemiological studies are needed, with an appropriate design, accounting for both added and naturally occurring dietary MSG. Critical analysis of existing literature, establishes that many of the reported negative health effects of MSG have little relevance for chronic human exposure and are poorly informative as they are based on excessive dosing that does not meet with levels normally consumed in food products.

Brain Imaging of Taste Perception in Obesity: a Review

PURPOSE OF REVIEW

We summarize neuroimaging findings related to processing of taste (fat, salt, umami, bitter, and sour) in the brain and how they influence hedonic responses and eating behaviors and their role in obesity.

RECENT FINDINGS

Neuroimaging studies in obese individuals have revealed alterations in reward/motivation, executive control/self-regulation, and limbic/affective circuits that are implicated in food and drug addiction. Psychophysical studies show that sensory properties of food ingredients may be associated with anthropometric and neurocognitive outcomes in obesity. However, few studies have examined the neural correlates of taste and processing of calories and nutrient content in obesity. The literature of neural correlated of bitter, sour, and salty tastes remains sparse in obesity. Most published studies have focused on sweet, followed by fat and umami taste. Studies on calorie processing and its conditioning by preceding taste sensations have started to delineate a dynamic pattern of brain activation associated with appetition. Our expanded understanding of taste processing in the brain from neuroimaging studies is poised to reveal novel prevention and treatment targets to help address overeating and obesity.

Good practice in food-related neuroimaging

The use of neuroimaging tools, especially functional magnetic resonance imaging, in nutritional research has increased substantially over the past 2 decades. Neuroimaging is a research tool with great potential impact on the field of nutrition, but to achieve that potential, appropriate use of techniques and interpretation of neuroimaging results is necessary. In this article, we present guidelines for good methodological practice in functional magnetic resonance imaging studies and flag specific limitations in the hope of helping researchers to make the most of neuroimaging tools and avoid potential pitfalls. We highlight specific considerations for food-related studies, such as how to adjust statistically for common confounders, like, for example, hunger state, menstrual phase, and BMI, as well as how to optimally match different types of food stimuli. Finally, we summarize current research needs and future directions, such as the use of prospective designs and more realistic paradigms for studying eating behavior.

Affective value, intensity and quality of liquid tastants/food discernment in the human brain: An activation likelihood estimation meta-analysis

The primary dimensions of taste are affective value, intensity and quality. Numerous studies have reported the role of the insula in evaluating these dimensions of taste; however, the results were inconsistent. Therefore, in the current study, we performed meta-analyses of published data to identify locations consistently activated across studies and evaluate whether different regions of the human brain could be responsible for processing different dimensions of taste. Meta-analyses were performed on 39 experiments, with 846 total healthy subjects (without psychiatric/neurological disorders) in 34 studies reporting whole-brain results. The aim was to establish the activation likelihood estimation (ALE) of taste-mediated regional activation across the whole brain. Apart from one meta-analysis for all studies in general, three analyses were performed to reveal the clusters of activation that were attributable to processing the affective value (data from 323 foci), intensity (data from 43 foci) and quality (data from 45 foci) of taste. The ALE revealed eight clusters of activation outside the insula for processing affective value, covering the middle and posterior cingulate, pre-/post-central gyrus, caudate and thalamus. The affective value had four clusters of activation (two in each hemisphere) in the insula. The intensity and quality activated only the insula, each with one cluster on the right. The concurrence between studies was moderate; at best, 53% of the experiments contributed to the significant clusters attributable to the affective value, 60% to intensity and 50% to quality. The affective value was processed bilaterally in the anterior to middle insula, whereas intensity was processed in the right antero-middle insula, and quality was processed in the right middle insula. The right middle dorsal insula was responsible for processing both the affective value and quality of taste. The exploratory analysis on taste quality did not have a significant result if the studies using liquid food stimuli were excluded. Results from the meta-analyses on studies involving the oral delivery of liquid tastants or liquid food stimuli confirmed that the insula is involved in processing all three dimensions of taste. More experimental studies are required to investigate whether brain activations differ between liquid tastants and food. The coordinates of activated brain areas and brain maps are provided to serve as references for future taste/food studies.

Basic taste processing recruits bilateral anteroventral and middle dorsal insulae: An activation likelihood estimation meta-analysis of fMRI studies

BACKGROUND AND PURPOSE

Numerous task-based functional magnetic resonance imaging (fMRI) studies have reported the locations of basic taste representations in the human brain, but they usually employed a limited number of subjects (<20) with different methodologies and stimuli. Moreover, the reported brain regions were sometimes inconsistent. Thus, we aimed at performing a meta-analysis of the published data to identify locations consistently activated across studies, and performed a connectivity analysis to reveal how these taste processing regions connect with other brain regions.

MATERIALS AND METHODS

A meta-analysis was performed based on 34 experiments, with 238 total participants in 16 studies, to establish the activation likelihood estimation (ALE) of taste-mediated regional activation. Meta-analytic connectivity modeling (MACM) and data stored in BrainMap database were employed to reveal the functional connectivity of the regions identified by ALE with other brain regions, across all types of experiments that caused activation among healthy subjects.

RESULTS

ALE identified nine activated clusters in bilateral anteroventral and middle dorsal insulae, bilateral thalamus and caudate, bilateral pre-/postcentral gyrus, and right hippocampus. The concurrence between studies was moderate, with at best 38% of experiments contributed to the significant clusters activated by taste stimulation. Sweet taste was the predominant contributing taste. MACM revealed that at least 50% of the nine clusters coactivated with the middle cingulate cortex, medial frontal gyrus, inferior parietal lobule, and putamen.

CONCLUSION

Results suggested that fMRI studies have reported reproducible patterns of activations across studies. The basic taste stimulations resulted in activations in a mostly bilateral network. Moreover, they were connected with cognitive and emotional relevant brain regions.

Enhancement of Combined Umami and Salty Taste by Glutathione in the Human Tongue and Brain

Glutathione, a natural substance, acts on calcium receptors on the tongue and is known to enhance basic taste sensations. However, the effects of glutathione on brain activity associated with taste sensation on the tongue have not been determined under standardized taste delivery conditions. In this study, we investigated the sensory effect of glutathione on taste with no effect of the smell when glutathione added to a combined umami and salty taste stimulus. Twenty-six volunteers (12 women and 14 men; age 19-27 years) performed a sensory evaluation of taste of a solution of monosodium L-glutamate and sodium chloride, with and without glutathione. The addition of glutathione changed taste qualities and significantly increased taste intensity ratings under standardized taste delivery conditions (P < 0.001). Functional magnetic resonance imaging showed that glutathione itself elicited significant activation in the left ventral insula. These results are the first to demonstrate the enhancing effect of glutathione as reflected by brain data while tasting an umami and salty mixture.

Taste intensity modulates effective connectivity from the insular cortex to the thalamus in humans

Evaluation of taste intensity is one of the most important perceptual abilities in our daily life. In contrast with extensive research findings regarding the spatial representation of taste in the insula and thalamus, little is known about how the thalamus and insula communicate and reciprocally influence their activities for processing taste intensity. To examine this neurophysiological relationship, we investigated the modulatory effect of intensity of saltiness on connections in the network processing taste signals in the human brain. These "effective connectivity" relationships refer to the neurophysiological influence (including direction and strength of influence) of one brain region on another. Healthy adults (N=34), including 17 males and 17 females (mean age=21.3years, SD=2.4; mean body mass index (BMI)=20.2kg/m(2), SD=2.1) underwent functional magnetic resonance imaging as they tasted three concentrations of sodium chloride solutions. By effective connectivity analysis with dynamic causal modeling, we show that taste intensity enhances top-down signal transmission from the insular cortex to the thalamus. These results are the first to demonstrate the modulatory effect of taste intensity on the taste network in the human brain.

The Brain Mechanisms Underlying the Perception of Pungent Taste of Capsaicin and the Subsequent Autonomic Responses

In a human fMRI study, it has been demonstrated that tasting and ingesting capsaicin activate the ventral part of the middle and posterior short gyri (M/PSG) of the insula which is known as the primary gustatory area, suggesting that capsaicin is recognized as a taste. Tasting and digesting spicy foods containing capsaicin induce various physiological responses such as perspiration from face, salivation, and facilitation of cardiovascular activity, which are thought to be caused through viscero-visceral autonomic reflexes. However, this does not necessarily exclude the possibility of the involvement of higher-order sensory-motor integration between the M/PSG and anterior short gyrus (ASG) known as the autonomic region of the insula. To reveal a possible functional coordination between the M/PSG and ASG, we here addressed whether capsaicin increases neural activity in the ASG as well as the M/PSG using fMRI and a custom-made taste delivery system. Twenty subjects participated in this study, and three tastant solutions: capsaicin, NaCl, and artificial saliva (AS) were used. Group analyses with the regions activated by capsaicin revealed significant activations in the bilateral ASG and M/PSG. The fMRI blood oxygenation level-dependent (BOLD) signals in response to capsaicin stimulation were significantly higher in ASG than in M/PSG regardless of the side. Concomitantly, capsaicin increased the fingertip temperature significantly. Although there was no significant correlation between the fingertip temperatures and BOLD signals in the ASG or M/PSG when the contrast [Capsaicin-AS] or [Capsaicin-NaCl] was computed, a significant correlation was found in the bilateral ASG when the contrast [2 × Capsaicin-NaCl-AS] was computed. In contrast, there was a significant correlation in the hypothalamus regardless of the contrasts. Furthermore, there was a significant correlation between M/PSG and ASG. These results indicate that capsaicin increases neural activity in the ASG as well as the M/PSG, suggesting that the neural coordination between the two cortical areas may be involved in autonomic responses to tasting spicy foods as reflected in fingertip temperature increases.

Functional MRI cortical activations from unilateral tactile-taste stimulations of the tongue

Functional magnetic resonance imaging (fMRI) was used for revealing activations in the human brain by lateralized tactile-gustatory stimulations of the tongue. Salt, a basic taste stimulus, and water, now recognized as an independent taste modality, were applied to either hemitongues with pads similar to the taste strips test for the clinical psychophysical evaluation of taste. With both stimuli, the observed cortical patterns of activations could be attributed to a combined somatosensory and gustatory stimulation of the tongue, with no significant differences between salt and water. Stimulation of each hemitongue evoked a bilateral activation of the anterior insula-frontal operculum, ascribable to the gustatory component of the stimulation, and a bilateral activation of the inferior part of the postcentral gyrus, ascribable to the tactile component of the stimulation. The results are in line with the notion that the representation of the tongue in the cerebral hemispheres in both the touch and the taste modalities is bilateral. Clinical and brain stimulation findings indicate that this bilaterality depends primarily on a partial crossing of the afferent pathways, perhaps with a predominance of the crossed pathway in the touch modality and the uncrossed pathway in the taste modality. Previous evidence suggests that the corpus callosum is not indispensible for this bilateral representation, but can contribute to it by interhemispheric transfer of information in both modalities.

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.