The cortical response to the oral perception of fat emulsions and the effect of taster status

Sniffing out meaning: Chemosensory and semantic neural network changes in sommeliers

Wine tasting is a very complex process that integrates a combination of sensation, language, and memory. Taste and smell provide perceptual information that, together with the semantic narrative that converts flavor into words, seem to be processed differently between sommeliers and naïve wine consumers. We investigate whether sommeliers' wine experience shapes only chemosensory processing, as has been previously demonstrated, or if it also modulates the way in which the taste and olfactory circuits interact with the semantic network. Combining diffusion-weighted images and fMRI (activation and connectivity) we investigated whether brain response to tasting wine differs between sommeliers and nonexperts (1) in the sensory neural circuits representing flavor and/or (2) in the neural circuits for language and memory. We demonstrate that training in wine tasting shapes the microstructure of the left and right superior longitudinal fasciculus. Using mediation analysis, we showed that the experience modulates the relationship between fractional anisotropy and behavior: the higher the fractional anisotropy the higher the capacity to recognize wine complexity. In addition, we found functional differences between sommeliers and naïve consumers affecting the flavor sensory circuit, but also regions involved in semantic operations. The former reflects a capacity for differential sensory processing, while the latter reflects sommeliers' ability to attend to relevant sensory inputs and translate them into complex verbal descriptions. The enhanced synchronization between these apparently independent circuits suggests that sommeliers integrated these descriptions with previous semantic knowledge to optimize their capacity to distinguish between subtle differences in the qualitative character of the wine.

A Neural Mechanism in the Human Orbitofrontal Cortex for Preferring High-Fat Foods Based on Oral Texture

Although overconsumption of high-fat foods is a major driver of weight gain, the neural mechanisms that link the oral sensory properties of dietary fat to reward valuation and eating behavior remain unclear. Here we combine novel food-engineering approaches with functional neuroimaging to show that the human orbitofrontal cortex (OFC) translates oral sensations evoked by high-fat foods into subjective economic valuations that guide eating behavior. Male and female volunteers sampled and evaluated nutrient-controlled liquid foods that varied in fat and sugar ("milkshakes"). During oral food processing, OFC activity encoded a specific oral-sensory parameter that mediated the influence of the foods' fat content on reward value: the coefficient of sliding friction. Specifically, OFC responses to foods in the mouth reflected the smooth, oily texture (i.e., mouthfeel) produced by fatty liquids on oral surfaces. Distinct activity patterns in OFC encoded the economic values associated with particular foods, which reflected the subjective integration of sliding friction with other food properties (sugar, fat, viscosity). Critically, neural sensitivity of OFC to oral texture predicted individuals' fat preferences in a naturalistic eating test: individuals whose OFC was more sensitive to fat-related oral texture consumed more fat during ad libitum eating. Our findings suggest that reward systems of the human brain sense dietary fat from oral sliding friction, a mechanical food parameter that likely governs our daily eating experiences by mediating interactions between foods and oral surfaces. These findings identify a specific role for the human OFC in evaluating oral food textures to mediate preference for high-fat foods.SIGNIFICANCE STATEMENT Fat and sugar enhance the reward value of food by imparting a sweet taste and rich mouthfeel but also contribute to overeating and obesity. Here we used a novel food-engineering approach to realistically quantify the physical-mechanical properties of high-fat liquid foods on oral surfaces and used functional neuroimaging while volunteers sampled these foods and placed monetary bids to consume them. We found that a specific area of the brain's reward system, the orbitofrontal cortex, detects the smooth texture of fatty foods in the mouth and links these sensory inputs to economic valuations that guide eating behavior. These findings can inform the design of low-calorie fat-replacement foods that mimic the impact of dietary fat on oral surfaces and neural reward systems.

Individual Differences in Chemosensory Perception Amongst Cancer Patients Undergoing Chemotherapy: A Narrative Review

Chemotherapy is an aggressive form of treatment for cancer and its toxicity directly affects the eating behavior of many patients, usually by adversely affecting their sense of smell and/or taste. These sensory alterations often lead to serious nutritional deficiencies that can jeopardize the patient's recovery, and even continue to affect their lives once treatment has terminated. Importantly, however, not all patients suffer from such alterations to their chemical senses; and those who do, do not necessarily describe the side effects in quite the same way, nor suffer from them with equal intensity. The origin of these individual differences between cancer patients undergoing chemotherapy treatment has not, as yet, been studied in detail. This review is therefore designed to encourage future research that can help to address the perceptual/sensory problems (and the consequent malnutrition) identified amongst this group of patients in a more customized/personalized manner. In particular, by providing an overview of the possible causes of these large individual differences that have been reported in the literature. For this reason, in addition to the narrative bibliographic review, several possible strategies that could help to improve the chemosensory perception of food are proposed.

What is the link between personality and food behavior?

A number of personality characteristics have been linked to various aspects of taste (gustation), trigeminal, and olfactory perception. In particular, personality traits have been linked to olfactory sensory thresholds and olfactory identification abilities, as well as to the sensory-discriminative aspects of taste/flavour perception. To date, much of the research in this area has focused on Sensation Seeking (including Experience Seeking, and Openness to Novel Experiences), with the latter being linked to a preference for spicy, and possibly also crunchy, sour, and bitter foods/drinks. Novelty-seeking has also been linked to a preference for salty foods, while anxious individuals appear to enjoy a much narrower range of foods. A bidirectional link has also been documented between taste and mood. Certain of the personality-based differences in taste/flavour perception and food behaviour have been linked to differences in circulating levels of neurotransmitters and hormones in both normal and clinical populations. Taken together, therefore, the evidence that has been published to date supports a number of intriguing connections between personality traits and taste perception/food behaviour.

Basal ganglia lateralization in different types of reward

Reward processing is a fundamental human activity. The basal ganglia are recognized for their role in reward processes; however, specific roles of the different nuclei (e.g., nucleus accumbens, caudate, putamen and globus pallidus) remain unclear. Using quantitative meta-analyses we assessed whole-brain and basal ganglia specific contributions to money, erotic, and food reward processing. We analyzed data from 190 fMRI studies which reported stereotaxic coordinates of whole-brain, within-group results from healthy adult participants. Results showed concordance in overlapping and distinct cortical and sub-cortical brain regions as a function of reward type. Common to all reward types was concordance in basal ganglia nuclei, with distinct differences in hemispheric dominance and spatial extent in response to the different reward types. Food reward processing favored the right hemisphere; erotic rewards favored the right lateral globus pallidus and left caudate body. Money rewards engaged the basal ganglia bilaterally including its most anterior part, nucleus accumbens. We conclude by proposing a model of common reward processing in the basal ganglia and separate models for money, erotic, and food rewards.

Thermal taster status: Temperature modulation of cortical response to sweetness perception

Temperature is known to impact taste perception, but its reported effect on sweet taste perception in humans is inconsistent. Here, we assess whether thermal taste phenotype alters the temperature modulation of the brains' response to sweet samples and sweet taste perception. Participants (n = 24 balanced for thermal tasters (TT) and thermal non-tasters (TnT), 25 ± 7 years (mean ± SD), 10 males) underwent a thermal taste phenotyping session to study responses to cooling and warming of the tongue using a thermode. In a separate session, functional Magnetic Resonance Images (fMRI) were collected during sweet samples (87 mM sucrose) delivery at two temperatures ('cold' (5 ± 2 °C) and 'ambient' (20 ± 2 °C)) and the perceived sweetness intensity rated.In the phenotyping session, TTs had heightened perceptual temperature sensitivity to cooling and warming of the tongue using a thermode compared to TnTs. Although there was no significant effect during the fMRI session, the fMRI response to the 'cold sweet' sample across all participants was significantly increased in anterior insula/frontal operculum and mid-insula compared to the 'ambient sweet' sample, likely to reflect the perceptual difference to temperature rather than taste perception. TTs showed significantly increased fMRI activation patterns compared with TnTs and an interaction effect between thermal taster status and sample temperature, with TTs showing selectively greater cortical responses to 'cold sweet' samples compared to TnTs in somatosensory regions (SI and SII).The increase in cortical activation in somatosensory cortices to the 'cold sweet' stimulus correlated with perceptual ratings of temperature sensitivity to the thermode. The results highlight the importance of investigating the effects of thermal taster phenotype across a range of temperatures representing the reality of consumer consumption to beverages.

Flavor Chemistry of Virgin Olive Oil: An Overview

Virgin olive oil (VOO) has unique chemical characteristics among all other vegetable oils which are of paramount importance for human health. VOO constituents are also responsible of its peculiar flavor, a complex sensation due to a combination of aroma, taste, texture, and mouthfeel or trigeminal sensations. VOO flavor depends primarily on the concentration and nature of volatile and phenolic compounds present in olive oil which can change dramatically depending on agronomical and technological factors. Another aspect that can change the flavor perception is linked to the oral process during olive oil tasting. In fact, in this case, some human physiological and matrix effects modulate the flavor release in the mouth. The present review aims to give an overview on VOO flavor, with particular emphasis on the mechanisms affecting its production and release during a tasting.

Preference for dietary fat: From detection to disease

Recent advances in the field of taste physiology have clarified the role of different basic taste modalities and their implications in health and disease and proposed emphatically that there might be a distinct cue for oro-sensory detection of dietary long-chain fatty acids (LCFAs). Hence, fat taste can be categorized as a taste modality. During mastication, LCFAs activate tongue lipid sensors like CD36 and GPR120 triggering identical signaling pathways as the basic taste qualities do; however, the physico-chemical perception of fat is not as distinct as sweet or bitter or other taste sensations. The question arises whether "fat taste" is a basic or "alimentary" taste. There is compelling evidence that fat-rich dietary intervention modulates fat taste perception where an increase or a decrease in lipid contents in the diet results, respectively, in downregulation or upregulation of fat taste sensitivity. Evidently, a decrease in oro-sensory detection of LCFAs leads to high fat intake and, consequently, to obesity. In this article, we discuss recent relevant advances made in the field of fat taste physiology with regard to dietary fat preference and lipid sensors that can be the target of anti-obesity strategies.

Genetic Taster Status as a Mediator of Neural Activity and Swallowing Mechanics in Healthy Adults

As part of a larger study examining relationships between taste properties and swallowing, we assessed the influence of genetic taster status (GTS) on measures of brain activity and swallowing physiology during taste stimulation in healthy men and women. Twenty-one participants underwent videofluoroscopic swallowing study (VFSS) and functional magnetic resonance imaging (fMRI) during trials of high-intensity taste stimuli. The precisely formulated mixtures included sour, sweet-sour, lemon, and orange taste profiles and unflavored controls. Swallowing physiology was characterized via computational analysis of swallowing mechanics plus other kinematic and temporal measures, all extracted from VFSS recordings. Whole-brain analysis of fMRI data assessed blood oxygen responses to neural activity associated with taste stimulation. Swallowing morphometry, kinematics, temporal measures, and neuroimaging analysis revealed differential responses by GTS. Supertasters exhibited increased amplitude of most pharyngeal movements, and decreased activity in the primary somatosensory cortex compared to nontasters and midtasters. These preliminary findings suggest baseline differences in swallowing physiology and the associated neural underpinnings associated with GTS. Given the potential implications for dysphagia risk and recovery patterns, GTS should be included as a relevant variable in future research regarding swallowing function and dysfunction.

The Role of Intrinsic and Extrinsic Sensory Factors in Sweetness Perception of Food and Beverages: A Review

When it comes to eating and drinking, multiple factors from diverse sensory modalities have been shown to influence multisensory flavour perception and liking. These factors have heretofore been strictly divided into either those that are intrinsic to the food itself (e.g., food colour, aroma, texture), or those that are extrinsic to it (e.g., related to the packaging, receptacle or external environment). Given the obvious public health need for sugar reduction, the present review aims to compare the relative influences of product-intrinsic and product-extrinsic factors on the perception of sweetness. Evidence of intrinsic and extrinsic sensory influences on sweetness are reviewed. Thereafter, we take a cognitive neuroscience perspective and evaluate how differences may occur in the way that food-intrinsic and extrinsic information become integrated with sweetness perception. Based on recent neuroscientific evidence, we propose a new framework of multisensory flavour integration focusing not on the food-intrinsic/extrinsic divide, but rather on whether the sensory information is perceived to originate from within or outside the body. This framework leads to a discussion on the combinability of intrinsic and extrinsic influences, where we refer to some existing examples and address potential theoretical limitations. To conclude, we provide recommendations to those in the food industry and propose directions for future research relating to the need for long-term studies and understanding of individual differences.

Unbalanced Occlusion Modifies the Pattern of Brain Activity During Execution of a Finger to Thumb Motor Task

In order to assess possible influences of occlusion on motor performance, we studied by functional magnetic resonance imaging (fMRI) the changes in the blood oxygenation level dependent (BOLD) signal induced at brain level by a finger to thumb motor task in a population of subjects characterized by an asymmetric activation of jaw muscles during clenching (malocclusion). In these subjects, appropriate occlusal correction by an oral orthotic (bite) reduced the masticatory asymmetry. The finger to thumb task was performed while the subject's dental arches were touching, in two conditions: (a) with the teeth in direct contact (Bite OFF) and (b) with the bite interposed between the arches (Bite ON). Both conditions required only a very slight activation of masticatory muscles. Maps of the BOLD signal recorded during the movement were contrasted with the resting condition (activation maps). Between conditions comparison of the activation maps (Bite OFF/Bite ON) showed that, in Bite OFF, the BOLD signal was significantly higher in the trigeminal sensorimotor region, the premotor cortex, the cerebellum, the inferior temporal and occipital cortex, the calcarine cortex, the precuneus on both sides, as well as in the right posterior cingulate cortex. These data are consistent with the hypothesis that malocclusion makes movement performance more difficult, leading to a stronger activation of (a) sensorimotor areas not dealing with the control of the involved body part, (b) regions planning the motor sequence, and (c) the cerebellum, which is essential in motor coordination. Moreover, the findings of a higher activation of temporo-occipital cortex and precuneus/cingulus, respectively, suggest that, during malocclusion, the movement occurs with an increased visual imagery activity, and requires a stronger attentive effort.

Fatty Acid Lingual Application Activates Gustatory and Reward Brain Circuits in the Mouse

The origin of spontaneous preference for dietary lipids in humans and rodents is debated, though recent compelling evidence has shown the existence of fat taste that might be considered a sixth taste quality. We investigated the implication of gustatory and reward brain circuits, triggered by linoleic acid (LA), a long-chain fatty acid. The LA was applied onto the circumvallate papillae for 30 min in conscious C57BL/6J mice, and neuronal activation was assessed using c-Fos immunohistochemistry. By using real-time reverse transcription polymerase chain reaction (RT-qPCR), we also studied the expression of mRNA encoding brain-derived neurotrophic factor (BDNF), Zif-268, and Glut-1 in some brain areas of these animals. LA induced a significant increase in c-Fos expression in the nucleus of solitary tract (NST), parabrachial nucleus (PBN), and ventroposterior medialis parvocellularis (VPMPC) of the thalamus, which are the regions known to be activated by gustatory signals. LA also triggered c-Fos expression in the central amygdala and ventral tegmental area (VTA), involved in food reward, in conjunction with emotional traits. Interestingly, we noticed a high expression of BDNF, Zif-268, and Glut-1 mRNA in the arcuate nucleus (Arc) and hippocampus (Hipp), where neuronal activation leads to memory formation. Our study demonstrates that oral lipid taste perception might trigger the activation of canonical gustatory and reward pathways.

Fat perception in the human frontal operculum, insular and somatosensory cortex

Here, we combined magnetic resonance imaging with lesion-symptom mapping in patients with chronic brain lesions to investigate brain representations of sugar and fat perception. Patients and healthy controls rated chocolate milkshakes that only differed in sugar or fat content. As compared to controls, patients showed an impaired fat, but not sugar perception. Impairments in fat perception overlapped with the anterior insula and frontal operculum, together assumed to underpin gustatory processing. We also identified the mid-dorsal insula as well as the primary and secondary somatosensory cortex - regions previously assumed to integrate oral-sensory inputs. These findings suggest that fat perception involves a specific set of brain regions that were previously reported to underpin gustatory processing and oral-sensory integration processes.

Sour taste increases swallowing and prolongs hemodynamic responses in the cortical swallowing network

Sour stimuli have been shown to upregulate swallowing in patients and in healthy volunteers. However, such changes may be dependent on taste-induced increases in salivary flow. Other mechanisms include genetic taster status (Bartoshuk LM, Duffy VB, Green BG, Hoffman HJ, Ko CW, Lucchina LA, Weiffenbach JM. Physiol Behav 82: 109-114, 2004) and differences between sour and other tastes. We investigated the effects of taste on swallowing frequency and cortical activation in the swallowing network and whether taster status affected responses. Three-milliliter boluses of sour, sour with slow infusion, sweet, water, and water with infusion were compared on swallowing frequency and hemodynamic responses. The sour conditions increased swallowing frequency, whereas sweet and water did not. Changes in cortical oxygenated hemoglobin (hemodynamic responses) measured by functional near-infrared spectroscopy were averaged over 30 trials for each condition per participant in the right and left motor cortex, S1 and supplementary motor area for 30 s following bolus onset. Motion artifact in the hemodynamic response occurred 0-2 s after bolus onset, when the majority of swallows occurred. The peak hemodynamic response 2-7 s after bolus onset did not differ by taste, hemisphere, or cortical location. The mean hemodynamic response 17-22 s after bolus onset was highest in the motor regions of both hemispheres, and greater in the sour and infusion condition than in the water condition. Genetic taster status did not alter changes in swallowing frequency or hemodynamic response. As sour taste significantly increased swallowing and cortical activation equally with and without slow infusion, increases in the cortical swallowing were due to sour taste.

Prior Consumption of a Fat Meal in Healthy Adults Modulates the Brain's Response to Fat

BACKGROUND

The consumption of fat is regulated by reward and homeostatic pathways, but no studies to our knowledge have examined the role of high-fat meal (HFM) intake on subsequent brain activation to oral stimuli.

OBJECTIVE

We evaluated how prior consumption of an HFM or water load (WL) modulates reward, homeostatic, and taste brain responses to the subsequent delivery of oral fat.

METHODS

A randomized 2-way crossover design spaced 1 wk apart was used to compare the prior consumption of a 250-mL HFM (520 kcal) [rapeseed oil (440 kcal), emulsifier, sucrose, flavor cocktail] or noncaloric WL on brain activation to the delivery of repeated trials of a flavored no-fat control stimulus (CS) or flavored fat stimulus (FS) in 17 healthy adults (11 men) aged 25 ± 2 y and with a body mass index (in kg/m²) of 22.4 ± 0.8. We tested differences in brain activation to the CS and FS and baseline cerebral blood flow (CBF) after the HFM and WL. We also tested correlations between an individual's plasma cholecystokinin (CCK) concentration after the HFM and blood oxygenation level-dependent (BOLD) activation of brain regions.

RESULTS

Compared to the WL, consuming the HFM led to decreased anterior insula taste activation in response to both the CS (36.3%; P < 0.05) and FS (26.5%; P < 0.05). The HFM caused reduced amygdala activation (25.1%; P < 0.01) in response to the FS compared to the CS (fat-related satiety). Baseline CBF significantly reduced in taste (insula: 5.7%; P < 0.01), homeostatic (hypothalamus: 9.2%, P < 0.01; thalamus: 5.1%, P < 0.05), and reward areas (striatum: 9.2%; P < 0.01) after the HFM. An individual's plasma CCK concentration correlated negatively with brain activation in taste and oral somatosensory (ρ = -0.39; P < 0.05) and reward areas (ρ = -0.36; P < 0.05).

CONCLUSIONS

Our results in healthy adults show that an HFM suppresses BOLD activation in taste and reward areas compared to a WL. This understanding will help inform the reformulation of reduced-fat foods that mimic the brain's response to high-fat counterparts and guide future interventions to reduce obesity.

Changes in liking for sweet and fatty foods following weight loss in women are related to prop phenotype but not to diet

OBJECTIVE

Changes in perceived intensity and liking of tasted foods have not been studied during weight loss from dieting. These outcomes were examined during a 6-month lifestyle intervention in women who had been classified by sensitivity to the bitter taste marker, 6-n-propylthiouracil (PROP), and then randomized to a low-fat or low-carbohydrate diet.

METHODS

Sixty-nine women (BMI = 34.4 kg/m(2) ; age = 44.2 years) followed the low-fat diet (n = 31) or low-carbohydrate diet (n = 38). At baseline and at 3, and 6 months, they rated overall liking and intensity of attributes in strawberry milk and salad dressing varying in sucrose (0%, 15%, and 30% wt/vol) or fat (10%, 30%, 50% wt/vol) content, respectively.

RESULTS

Perceived intensity of the attributes did not change. For all participants, the 15% and 30% sucrose milk samples were equally liked at baseline and 3 months, but by 6 months, the 15% sucrose sample was highest liked (P < 0.007). Also, the 50% fat sample was most liked at baseline and least liked by 6 months (P = 0.04), and this effect was most pronounced in the nontasters (P < 0.02). There were no effects of diet prescription on liking.

CONCLUSIONS

Weight loss from dieting resulted in a hedonic shift for foods with lower sucrose and fat content.

Understanding Neuronal Architecture in Obesity through Analysis of White Matter Connection Strength

Despite the prevalence of obesity, our understanding of its neurobiological underpinnings is insufficient. Diffusion weighted imaging and calculation of white matter connection strength are methods to describe the architecture of anatomical white matter tracts. This study is aimed to characterize white matter architecture within taste-reward circuitry in a population of obese individuals. Obese (n = 18, age = 28.7 ± 8.3 years) and healthy control (n = 24, age = 27.4 ± 6.3 years) women underwent diffusion weighted imaging. Using probabilistic fiber tractography (FSL PROBTRACKX2 toolbox) we calculated connection strength within 138 anatomical white matter tracts. Obese women (OB) displayed lower and greater connectivity within taste-reward circuitry compared to controls (Wilks' λ < 0.001; p < 0.001). Connectivity was lower in white matter tracts connecting insula, amygdala, prefrontal cortex (PFC), orbitofrontal cortex (OFC) and striatum. Connectivity was greater between the amygdala and anterior cingulate cortex (ACC). This study indicates that lower white matter connectivity within white matter tracts of insula-fronto-striatal taste-reward circuitry are associated with obesity as well as greater connectivity within white matter tracts connecting the amygdala and ACC. The specificity of regions suggests sensory integration and reward processing are key associations that are altered in and might contribute to obesity.

Lipids and obesity: Also a matter of taste?

Obesity is undoubtedly one of the major public health challenges worldwide because of its rapid progression and deleterious effects of associated diseases. The easier access to tasty and energy-dense foods is thought to greatly contribute to this epidemic. Studies also report that obese subjects and animals (rats and mice) preferentially consume foods rich in fat when they can choose. The origin of this eating behavior remains elusive. Over the last decade, the existence of a taste of fat, besides textural and olfactory cues, was supported by a growing number of studies. The existence of a sixth taste modality devoted to the detection/perception of dietary lipids might offer additive information on the quality of food. While the sense of taste is recognized to be a driving-force guiding food choice, interest in the putative relationships between lipids, gustation and obesity is only now emerging. This mini-review will attempt to summarize our current knowledge on this new field of research.

Thermal taster status: Evidence of cross-modal integration

Thermal taster status refers to the finding that, in some individuals, thermal stimulation of the tongue elicits a phantom taste. Little is known regarding the mechanism for this, it is hypothesised to be a result of cross-wiring between gustatory and trigeminal nerves whose receptors co-innervate papillae on the tongue. To address this, we use functional magnetic resonance imaging to perform the first study of whether the cortical response to gustatory-trigeminal samples is altered with thermal taster status. We study the response to cold (6°C) gustatory (sweet) samples at varying levels of trigeminal stimulation elicited by CO2 (no CO2 , low CO2 , high CO2 ) in thermal taster (TT) and thermal non-taster (TnT) groups, and evaluate associated behavioural measures. Behaviourally, the TT group perceived gustatory and trigeminal stimuli significantly more intense than TnTs, and were significantly more discriminating of CO2 level. fMRI data revealed elevated cortical activation to the no CO2 sample for the TT group compared to TnT group in taste, oral somatosensory and reward areas. In TnTs, a significant positive modulation in cortical response with increasing level of CO2 was found across taste, somatosensory and reward areas. In contrast, in TTs, a reduced positive modulation with increasing level of CO2 was found in somatosensory areas (SI, SII), whilst a significant negative modulation was found in taste (anterior insula) and reward (ACC) areas. This difference in cortical response to trigeminal stimuli supports cross-modal integration in TTs, with gustatory and trigeminal nerves highly stimulated by cold gustatory samples due to their intertwined nature. Hum Brain Mapp 37:2263-2275, 2016. © 2016 Wiley Periodicals, Inc.

Controversies in fat perception

Nutritional fat is one of the most controversial topics in nutritional research, particularly against the background of obesity. Studies investigating fat taste perception have revealed several associations with sensory, genetic, and personal factors (e.g. BMI). However, neuronal activation patterns, which are known to be highly sensitive to different tastes as well as to BMI differences, have not yet been included in the scheme of fat taste perception. We will therefore provide a comprehensive survey of the sensory, genetic, and personal factors associated with fat taste perception and highlight the benefits of applying neuroimaging research. We will also give a critical overview of studies investigating sensory fat perception and the challenges resulting from multifaceted methodological approaches. In conclusion, we will discuss a multifactorial approach to fat perception to gain a better understanding of the underlying mechanisms that cause varying fat sensitivity which could be responsible for overeating. Such knowledge might be beneficial in new treatment strategies for obesity and overweight.

Recent advances in fatty acid perception and genetics

This article summarizes new knowledge about the contribution of genetic variation to person-to-person differences underlying some sensory aspects of dietary fatty acids. Receptors on the taste cells of the human tongue arise from genes that have marked variation in DNA sequence, which, in some cases, is associated with differences in how these lipids in foods are perceived. These perceptual differences may affect food selection.

Altered salience network connectivity predicts macronutrient intake after sleep deprivation

Although insufficient sleep is a well-recognized risk factor for overeating and weight gain, the neural mechanisms underlying increased caloric (particularly fat) intake after sleep deprivation remain unclear. Here we used resting-state functional magnetic resonance imaging and examined brain connectivity changes associated with macronutrient intake after one night of total sleep deprivation (TSD). Compared to the day following baseline sleep, healthy adults consumed a greater percentage of calories from fat and a lower percentage of calories from carbohydrates during the day following TSD. Subjects also exhibited increased brain connectivity in the salience network from the dorsal anterior cingulate cortex (dACC) to bilateral putamen and bilateral anterior insula (aINS) after TSD. Moreover, dACC-putamen and dACC-aINS connectivity correlated with increased fat and decreased carbohydrate intake during the day following TSD, but not during the day following baseline sleep. These findings provide a potential neural mechanism by which sleep loss leads to increased fat intake.

Imaging methodologies and applications for nutrition research: what can functional MRI offer?

Food intake is influenced by a complex regulatory system involving the integration of a wide variety of sensory inputs across multiple brain areas. Over the past decade, advances in neuroimaging using functional MRI (fMRI) have provided valuable insight into these pathways in the human brain. This review provides an outline of the methodology of fMRI, introducing the widely used blood oxygenation level-dependent contrast for fMRI and direct measures of cerebral blood flow using arterial spin labelling. A review of fMRI studies of the brain's response to taste, aroma and oral somatosensation, and how fat is sensed and mapped in the brain in relation to the pleasantness of food, and appetite control is given. The influence of phenotype on individual variability in cortical responses is addressed, and an overview of fMRI studies investigating hormonal influences (e.g. peptide YY, cholecystokinin and ghrelin) on appetite-related brain processes provided. Finally, recent developments in MR technology at ultra-high field (7 T) are introduced, highlighting the advances this can provide for fMRI studies to investigate the neural underpinnings in nutrition research. In conclusion, neuroimaging methods provide valuable insight into the mechanisms of flavour perception and appetite behaviour.

Neural responses to visual food cues according to weight status: a systematic review of functional magnetic resonance imaging studies

Emerging evidence from recent neuroimaging studies suggests that specific food-related behaviors contribute to the development of obesity. The aim of this review was to report the neural responses to visual food cues, as assessed by functional magnetic resonance imaging (fMRI), in humans of differing weight status. Published studies to 2014 were retrieved and included if they used visual food cues, studied humans >18 years old, reported weight status, and included fMRI outcomes. Sixty studies were identified that investigated the neural responses of healthy weight participants (n = 26), healthy weight compared to obese participants (n = 17), and weight-loss interventions (n = 12). High-calorie food images were used in the majority of studies (n = 36), however, image selection justification was only provided in 19 studies. Obese individuals had increased activation of reward-related brain areas including the insula and orbitofrontal cortex in response to visual food cues compared to healthy weight individuals, and this was particularly evident in response to energy dense cues. Additionally, obese individuals were more responsive to food images when satiated. Meta-analysis of changes in neural activation post-weight loss revealed small areas of convergence across studies in brain areas related to emotion, memory, and learning, including the cingulate gyrus, lentiform nucleus, and precuneus. Differential activation patterns to visual food cues were observed between obese, healthy weight, and weight-loss populations. Future studies require standardization of nutrition variables and fMRI outcomes to enable more direct comparisons between studies.

Olive oil aroma extract modulates cerebral blood flow in gustatory brain areas in humans

BACKGROUND

Low- and high-fat meals affect homeostatic and gustatory brain areas differentially. In a previous study, we showed that a high-fat meal decreased cerebral blood flow (CBF) in homeostatic brain areas (hypothalamus), whereas a low-fat meal increased CBF in gustatory regions (anterior insula).

OBJECTIVE

The aim of this study was to investigate the long-lasting effect of fat-free flavor-active compounds of olive oil on the brain and whether those aroma components can trigger fat-associated brain responses in homeostatic and gustatory regions.

DESIGN

Eleven healthy male subjects participated in a functional magnetic resonance imaging study. On 2 measurement days, subjects consumed single-blinded a plain low-fat yogurt or low-fat yogurt mixed with a fat-free aroma extract of olive oil. Resting CBF was measured before and 30 and 120 min after yogurt intake. Hunger was rated before each measurement. Blood samples were collected at 6 time points.

RESULTS

The extract-containing yogurt elicited higher CBF in the frontal operculum 30 and 120 min after a meal. Furthermore, the activity change in the anterior insula after 30 min correlated positively with the glucose change in the extract condition only. No effects were observed in the hypothalamus.

CONCLUSIONS

The anterior insula and the frontal operculum are regarded as the primary taste cortex. Modulation of the frontal operculum by the yogurt containing the olive oil extract suggests that it might be possible to simulate fat-triggered sensations in the brain on the gustatory level, possibly by ingredients the body implicitly associates with fat. This trial was registered at clinicaltrials.gov as NCT01716286.

Appetitive traits from infancy to adolescence: using behavioral and neural measures to investigate obesity risk

We come into the world with enduring predispositions towards food, which interact with environmental factors to influence our eating behaviors and weight trajectories. But our fates are not sealed - by learning more about this process we can identify ways to intervene. To advance this goal this we need to be able to assess appetitive traits such as food cue responsiveness and satiety sensitivity at different developmental stages. Assessment methods might include behavioral measures (e.g. eating behavior tests, psychometric questionnaires), but also biomarkers such as brain responses to food cues measured using fMRI. Evidence from infants, children and adolescents suggests that these indices of appetite differ not only with body weight, but also with familial obesity risk as assessed by parent weight, which reflects both genetic and environmental influences, and may provide a useful predictor of obesity development. Behavioral and neural approaches have great potential to inform each other: examining eating behavior can help us identify meaningful appetitive endophenotypes whose neural bases can be probed, while increasing knowledge of the shared neurobiology underlying appetite, obesity, and related behaviors and disorders may ultimately lead to innovative generalized interventions. Another challenge will be to combine comprehensive behavioral and neural assessments of appetitive traits with measures of relevant genetic and environmental factors within long-term prospective studies. This approach may help to identify the biobehavioral precursors of obesity, and lay the foundations for targeted neurobehavioral interventions that can interrupt the pathway to excess weight.

The representation of oral fat texture in the human somatosensory cortex

How fat is sensed in the mouth and represented in the brain is important in relation to the pleasantness of food, appetite control, and the design of foods that reproduce the mouthfeel of fat yet have low energy content. We show that the human somatosensory cortex (SSC) is involved in oral fat processing via functional coupling to the orbitofrontal cortex (OFC), where the pleasantness of fat texture is represented. Using functional MRI, we found that activity in SSC was more strongly correlated with the OFC during the consumption of a high fat food with a pleasant (vanilla) flavor compared to a low fat food with the same flavor. This effect was not found in control analyses using high fat foods with a less pleasant flavor or pleasant-flavored low fat foods. SSC activity correlated with subjective ratings of fattiness, but not of texture pleasantness or flavor pleasantness, indicating a representation that is not involved in hedonic processing per se. Across subjects, the magnitude of OFC-SSC coupling explained inter-individual variation in texture pleasantness evaluations. These findings extend known SSC functions to a specific role in the processing of pleasant-flavored oral fat, and identify a neural mechanism potentially important in appetite, overeating, and obesity.

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.

Fat intake modulates cerebral blood flow in homeostatic and gustatory brain areas in humans

BACKGROUND

The hypothalamus is the central homeostatic control region of the brain and, therefore, highly influenced by nutrients such as glucose and fat. Immediate and prolonged homeostatic effects of glucose ingestion have been well characterized. However, studies that used stimulation with fat have mainly investigated immediate perceptional processes. Besides homeostatic processes, the gustatory cortex, including parts of the insular cortex, is crucial for the processing of food items.

OBJECTIVE

The aim of this study was to investigate the effect of high- compared with low-fat meals on the hypothalamus and the insular cortex.

DESIGN

Eleven healthy men participated in a single-blinded, functional MRI study of high- and low-fat meals on 2 measurement days. Cerebral blood flow (CBF) was measured before and 30 and 120 min after intake of high- and low-fat yogurts. Hunger was rated and blood samples were taken before each CBF measurement.

RESULTS

High-fat yogurt induced a pronounced decrease in CBF in the hypothalamus, and the corresponding CBF change correlated positively with the insulin change. Furthermore, insular activity increased after 120 min in the low-fat condition only. The CBF change in both regions correlated positively in the high-fat condition.

CONCLUSIONS

The decrease in hypothalamic activity and the interaction with the insular cortex elicited by fat may contribute to an efficient energy homeostasis. Therefore, fat might be a modulator of homeostatic and gustatory brain regions and their interaction. This trial was registered at clinicaltrials.gov as NCT01516021.