Accumulating evidence supports a taste component for free fatty acids in humans

Comprehensive analysis of oxidative stability and nutritional values of germinated linseed and sunflower seed oil

Germination of seeds is known to affect the nutritional composition of cold-pressed oils. This study focused on the effects of germination on the antioxidants and oxidative stability of linseed and sunflower seed oil. As hypothesized, germination led to increased antioxidant activities and tocopherol, chlorophyll and carotenoid content. Analysis revealed a 37.2 ± 3.5-fold and 11.6 ± 1.5-fold increase in polyphenol content in linseed and sunflower seed oil from germinated seeds, respectively. Using LC-HRMS/MS, profiles with up to 69 polyphenolic substances were identified in germinated seed oils for the first time. Germination promoted lipid hydrolysis, as evidenced by NMR, with overall significant decreases in triacylglycerol content leading to increased diacylglycerol and free fatty acid values. Rancimat measurements predicted a 4.10 ± 0.52-fold longer shelf-life for germinated linseed oil. This study successfully demonstrated the potential of germination to develop PUFA-rich oils with enhanced antioxidant capacity and oxidative stability.

A new perspective on obesity: perception of fat taste and its relationship with obesity

BACKGROUND

Obesity, which results from a long-term positive energy balance, is affected by many factors, especially nutrition. The sensory properties of foods are associated with increased food intake through hedonic appetite. Taste perception, a component of flavor, is also responsible for increased consumption, through reward and hedonic mechanisms. Foods with high fat and energy content are among the foods that create the reward perception. The perception of fat taste, the primary taste that has recently entered the literature, may also be associated with increased food consumption and body weight. Therefore, in this review, the relationship between fat taste and obesity is examined, using the latest literature.

RESULTS

Different hypotheses have been proposed regarding the mechanism of the relationship between fat-taste perception and obesity, such as hedonic appetite, microbiota, decreased taste perception, and increased taste threshold level. In addition, some studies examining this relationship reported significant associations between the level of fat-taste perception and obesity, whereas others did not find a significant difference.

CONCLUSION

Considering the prevalence and contribution to obesity of Western-style nutrition, characterized by high amounts of fat and sugar consumption, elucidating this relationship may be an essential solution for preventing and treating obesity.

Variability of oral/taste sensitivity to fat: An investigation of attribution from detection threshold methods with repeated measurements

Accumulating psychophysical evidence suggests substantial individual variability in oral/taste sensitivity to non-esterified, long-chain fatty acids (NEFA), which is commonly referred to as fat taste or oleogustus. Recent studies have sought to determine its associations with human factors such as body mass index (BMI) and food preferences, as it has been claimed that excessive fat consumption is related to several health conditions, including obesity. Yet, the findings are controversial. On the other hand, it has been noted that considerable variability also occurs based on the methodology used to measure the fatty acid taste. Specifically, learning effects have been observed over repeated measurements of the detection threshold of NEFA, yet there has been no methodology available to take into account these learning effects. Accordingly, in the present study, a novel methodology using a descending-block dual reminder A-Not A (DR A-Not A) method with a warm-up has been proposed to measure the NEFA detection threshold based on the signal detection theory and considering NEFA taste learning effects over repeated sessions. Homogeneous subjects (young adult Korean females within the normal BMI range, non-vegetarians) were randomized to either the novel descending-block DR A-Not A method or ascending triangle method that is commonly used for fat perception studies. Pure oleic acid emulsions were used as fat taste stimuli to be discriminated from pure mineral water. Each subject completed 14 repeated visits. For the ascending triangle method, 14 thresholds were determined using a stopping rule, while for the novel method, 7 thresholds were determined each per two consecutive days, using a criterion of a lower limit of 50% confidence interval of d' = 0.5, considering the practical aspects of taste studies in food sensory science. Based on the group median results of the last two visits, the variability of the detection thresholds was reduced using the novel descending-block DR A-Not A method due to better learning effects over repeated sessions. This shows the potential of the descending-block DR A-Not A threshold method for further studies on oral/taste sensitivity to fat.

An update on the biological characteristics and functions of tuft cells in the gut

The intestine is a powerful digestive system and one of the most sophisticated immunological organs. Evidence shows that tuft cells (TCs), a kind of epithelial cell with distinct morphological characteristics, play a significant role in various physiological processes. TCs can be broadly categorized into different subtypes depending on different molecular criteria. In this review, we discuss its biological properties and role in maintaining homeostasis in the gastrointestinal tract. We also emphasize its relevance to the immune system and highlight its powerful influence on intestinal diseases, including inflammations and tumors. In addition, we provide fresh insights into future clinical diagnostic and therapeutic strategies related to TCs.

Effect of Beta 2-Adrenergic Receptor Gly16Arg Polymorphism on Taste Preferences in Healthy Young Japanese Adults

The Gly16Arg polymorphism results in a G to C nucleotide mutation in the human beta 2-adrenergic receptor (ADRB2) gene and has a relationship with obesity; however, this substitution’s effects on food preferences are unclear. Therefore, we determined this relationship among healthy young adults (mean age, 23.4; n = 52). To evaluate food preferences, four categories of food (sweet, salty, sour, and bitter) along with high-fat foods were evaluated using a self-reporting questionnaire. Male (n = 26) and female subjects (n = 26) were genotyped for the polymorphism and further divided into three groups (two homozygous groups, GG, CC; and a heterozygous group, GC). Preference for sour foods in the GG group was higher compared with that in the CC group in females (p < 0.05). When sweet foods were classified into low- and high-fat subgroups, preference for high-fat sweet foods in the GG group was higher than that for low-fat sweet foods in all subjects (p < 0.05). The degree of preference for high-fat foods in the GG group was higher than other groups for males (p < 0.05). These results suggest that ADRB2 polymorphism is associated with food preference. Understanding the relationship of ADRB2 substitution to food preference will be valuable for designing individualized anti-obesity strategies.

Novel impacts of saliva with regard to oral health

The maintenance of balanced oral homeostasis depends on saliva. A readily available and molecularly rich source of biological fluid, saliva fulfills many functions in the oral cavity, including lubrication, pH buffering, and tooth mineralization. Saliva composition and flow can be modulated by different factors, including circadian rhythm, diet, age, drugs, and disease. Recent events have revealed that saliva plays a central role in the dissemination and detection of the SARS-CoV-2 coronavirus. A working knowledge of saliva function and physiology is essential for dental health professionals.

The tongue map and the spatial modulation of taste perception

There is undoubtedly a spatial component to our experience of gustatory stimulus qualities such as sweet, bitter, salty, sour, and umami, however its importance is currently unknown. Taste thresholds have been shown to differ at different locations within the oral cavity where gustatory receptors are found. However, the relationship between the stimulation of particular taste receptors and the subjective spatially-localized experience of taste qualities is uncertain. Although the existence of the so-called ‘tongue map’ has long been discredited, the psychophysical evidence clearly demonstrates significant (albeit small) differences in taste sensitivity across the tongue, soft palate, and pharynx (all sites where taste buds have been documented). Biases in the perceived localization of gustatory stimuli have also been reported, often resulting from tactile capture (i.e., a form of crossmodal, or multisensory, interaction). At the same time, varying responses to supratheshold tastants along the tongue's anterior-posterior axis have putatively been linked to the ingestion-ejection response. This narrative review highlights what is currently known concerning the spatial aspects of gustatory perception, considers how such findings might be explained, given the suggested balanced distribution of taste receptors for each basic taste quality where taste papillae are present, and suggests why knowing about such differences may be important.

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The existence of the tongue map has long been discredited.

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Taste receptors in the oral cavity respond to all tastes regardless of their location.

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Human psychophysical data highlights a significant spatial modulation of taste perception in the oral cavity.

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Highly-controlled studies of taste psychophysics rarely capture the full multisensory experience associated with eating and drinking.

Worldwide diversity, association potential, and natural selection in the superimposed taste genes, CD36 and GNAT3

CD36 and GNAT3 mediate taste responses, with CD36 acting as a lipid detector and GNAT3 acting as the α subunit of gustducin, a G protein governing sweet, savory, and bitter transduction. Strikingly, the genes encoding CD36 and GNAT3 are genomically superimposed, with CD36 completely encompassing GNAT3. To characterize genetic variation across the CD36-GNAT3 region, its implications for phenotypic diversity, and its recent evolution, we analyzed from ~2,500 worldwide subjects sequenced by the 1000 Genomes Project (1000GP). CD36-GNAT3 harbored extensive diversity including 8,688 single-nucleotide polymorphisms (SNPs), 414 indels, and other complex variants. Sliding window analyses revealed that nucleotide diversity and population differentiation across CD36-GNAT3 were consistent with genome-wide trends in the 1000GP (π = 0.10%, P = 0.64; FST = 9.0%, P = 0.57). In addition, functional predictions using SIFT and PolyPhen-2 identified 60 variants likely to alter protein function, and they were in weak linkage disequilibrium (r2 < 0.17), suggesting their effects are largely independent. However, the frequencies of predicted functional variants were low (P¯ = 0.0013), indicating their contributions to phenotypic variance on population scales are limited. Tests using Tajima's D statistic revealed that pressures from natural selection have been relaxed across most of CD36-GNAT3 during its recent history (0.39 < P < 0.67). However, CD36 exons showed signs of local adaptation consistent with prior reports (P < 0.035). Thus, CD36 and GNAT3 harbor numerous variants predicted to affect taste sensitivity, but most are rare and phenotypic variance on a population level is likely mediated by a small number of sites.

Common variants in the CD36 gene are associated with dietary fat intake, high-fat food consumption and serum triglycerides in a cohort of Quebec adults

BACKGROUND

The CD36 gene is a candidate for sensory detection of fatty acids and has been associated with individual differences in fat preferences and consumption. Excess adiposity may compromise sensory detection, but few studies have examined whether associations between CD36 variants and fat consumption differ between underweight/normal weight (UW/NW) and overweight/obese (OW/OB) individuals.

METHODS

Diet (assessed by food frequency questionnaire), genetic (nine variants), body mass index (BMI), lifestyle and biomarker data were obtained from the CARTaGENE biobank (n = 12,065), a Quebec cohort of middle-aged adults. Primary outcome variables included intakes (%kcal/day) of total, saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids. Secondary outcome variables included consumption (servings/day) of four food categories with high-fat content (added fats and oils, high-fat foods, desserts and MUFA- and PUFA-rich foods) and biomarkers of chronic disease. Multivariable regression models stratified by BMI category were used to assess associations between CD36 variants and outcome variables.

RESULTS

Among UW/NW, rs1049654 and rs10499859 were associated with higher intakes of total fat, MUFA and PUFA (all P < 0.05), while rs1527483 and rs3211956 were associated with higher SFA (P = 0.0278) and lower PUFA (P = 0.0466) intake, respectively. Rs1527483 and rs3211956 were also associated with higher consumption of high-fat foods and desserts (all P < 0.05). Among OW, rs1054516 and rs3173798 were associated with higher SFA intake (both P < 0.05), and rs1054516 was also associated with higher serum triglycerides (P = 0.0065).

CONCLUSIONS

CD36 variants are associated with habitual fat consumption, which may play a role in subsequent associations with chronic-disease biomarkers. Associations differ by BMI status and dietary fat type.

The Tryptophan/Kynurenine Pathway: A Novel Cross-Talk between Nutritional Obesity, Bariatric Surgery and Taste of Fat

Diet-induced obesity (DIO) reduces the orosensory perception of lipids in rodents and in some humans. Although bariatric surgery partially corrects this alteration, underlying mechanisms remain poorly understood. To explore whether metabolic changes might explain this fat taste disturbance, plasma metabolome analyses, two-bottle choice tests and fungiform papillae (Fun) counting were performed in vertical sleeve gastrectomized (VSG) mice and sham-operated controls. An exploratory clinic study was also carried out in adult patients undergone a VSG. In mice, we found that (i) the VSG reduces both the plasma neurotoxic signature due to the tryptophan/kynurenine (Trp/Kyn) pathway overactivation and the failure of fat preference found in sham-operated DIO mice, (ii) the activity of Trp/Kyn pathway is negatively correlated to the density of Fun, and (iii) the pharmacological inhibition of the Kyn synthesis mimics in non-operated DIO mice the positive effects of VSG (i.e., decrease of Kyn synthesis, increase of Fun number, improvement of the fat taste perception). In humans, a reduction of the plasma Kyn level is only found in patients displaying a post-surgery improvement of their fat taste sensitivity. Altogether these data provide a plausible metabolic explanation to the degradation of the orosensory lipid perception observed in obesity.

Sex differences in fat taste responsiveness are modulated by estradiol

Sex as a biological variable has been the focus of increasing interest. Relatively few studies have focused, however, on differences in peripheral taste function between males and females. Nonetheless, there are reports of sex-dependent differences in chemosensitivity in the gustatory system. The involvement of endogenous changes in ovarian hormones has been suggested to account for taste discrepancies. Additionally, whether sex differences exist in taste receptor expression, activation, and subsequent signaling pathways that may contribute to different taste responsiveness is not well understood. In this study, we show the presence of both the nuclear and plasma membrane forms of estrogen receptor (ER) mRNA and protein in mouse taste cells. Furthermore, we provide evidence that estrogen increases taste cell activation during the application of fatty acids, the chemical cue for fat taste, in taste receptor cells. We found that genes important for the transduction pathway of fatty acids vary between males and females and that these differences also exist across the various taste papillae. In vivo support for the effect of estrogens in taste cells was provided by comparing the fatty acid responsiveness in male, intact female, and ovariectomized (OVX) female mice with and without hormone replacement. In general, females detected fatty acids at lower concentrations, and the presence of circulating estrogens increased this apparent fat taste sensitivity. Taken together, these data indicate that increased circulating estrogens in the taste system may play a significant role in physiology and chemosensory cellular activation and, in turn, may alter taste-driven behavior.NEW & NOTEWORTHY Using molecular, cellular, and behavioral analyses, this study shows that sex differences occur in fat taste in a mouse model. Female mice are more responsive to fatty acids, leading to an overall decrease in intake and fatty acid preference. These differences are linked to sex hormones, as estradiol enhances taste cell responsiveness to fatty acids during periods of low circulating estrogen following ovariectomy and in males. Estradiol is ineffective in altering fatty acid signaling during a high-estrogen period and in ovariectomized mice on hormone replacement. Thus, taste receptor cells are a direct target for actions of estrogen, and there are multiple receptors with differing patterns of expression in taste cells.

Lipids for Taste masking and Taste assessment in pharmaceutical formulations

Pharmaceutical products often have drawbacks of unacceptable taste and palatability which makes it quite difficult for oral administration to some special populations like pediatrics and geriatrics. To curb this issue different approaches like coating, granulation, extrusion, inclusion complexation, ion-exchange resins, etc for taste masking are employed and among them use of lipids have drawn special attention of researchers. Lipids have a lower melting point which is ideal for incorporating drugs in some of these methods like hot-melt extrusion, melt granulation, spray drying/congealing and emulsification. Lipids play a significant role as a barrier to sustain the release of drugs and biocompatible nature of lipids increases their acceptability by the human body. Further, lipids provide vast opportunities of altering pharmacokinetics of the active ingredients by modulating release profiles. In taste sensors, also known as electronic tongue or e-tongue, lipids are used in preparing taste sensing membranes which are subsequently used in preparing taste sensors. Lipid membrane taste sensors have been widely used in assessing taste and palatability of pharmaceutical and food formulations. This review explores applications of lipids in masking the bitter taste in pharmaceutical formulations and significant role of lipids in evaluation of taste and palatability.

Chemosensory perception and food preferences in colorectal cancer patients undergoing adjuvant chemotherapy

BACKGROUND AND AIM

Cancer is one of the major public health problems, with colorectal cancer being one of the most occurring types of cancer. During treatment, patients may experience changes in their dietary intake due to side-effects of treatment, like changes in chemosensory perception, i.e. smell and taste function. This study investigated alterations in chemosensory perception and food preferences in colorectal cancer patients during and after adjuvant chemotherapy.

METHODS

Objective olfactory and gustatory function were measured by the Sniffin' Sticks and the Taste Strips test. Subjective smell and taste perception were determined with a questionnaire, while food preferences were assessed with a computer-based ranking task. To investigate changes during chemotherapy, patients undergoing adjuvant chemotherapy were measured before the start, halfway through (approximately 3 months after the start of adjuvant chemotherapy), and within one month after finishing chemotherapy (longitudinal measurements, n = 15 patients). As a comparison group, colorectal cancer patients not undergoing chemotherapy (n = 20), underwent the same measurements at similar time points. To measure changes after treatment, chemosensory perception and food preferences of patients who had undergone chemotherapy treatment were measured once, either at 6, 12 or 24 months after diagnosis (cross-sectional measurements; n = 20 for all time points). Changes during treatment were assessed using linear mixed model analyses, and changes after treatment were assessed with a one-way ANOVA or a Kruskal Wallis test.

RESULTS

Objective olfactory and gustatory function did not differ statistically significantly between any of the groups and at any time point during or after treatment (all p > 0.05). In contrast, subjective smell (F(1,84) = 8.17, p = 0.005) and taste (F(1,99) = 4.08, p = 0.046) perception were rated statistically significantly lower by patients undergoing chemotherapy than the comparison group during treatment. At 6 months after diagnosis, patients who underwent chemotherapy rated their subjective taste perception significantly lower than patients at 12 and 24 months after treatment (F(2,57) = 12.05, p = 0.002). Food preferences did not change during treatment, or thereafter (all p > 0.05). Preference for protein-rich foods was positively correlated with objective gustatory function (r = 0.36, p < 0.001), while the preference for low-energy foods showed a negative correlation with objective gustatory function (r = -0.28, p = 0.004).

CONCLUSIONS

Similar to other cancer patient populations, mainly subjective smell and taste perception are affected in colorectal cancer patients undergoing adjuvant chemotherapy. Changes in objective olfactory and gustatory function in relation to chemotherapy were not detected by the tests used in our study nor did food preferences change. However, it should be noted that subjective changes in smell and taste perception can affect subsequent flavor perception and food enjoyment, which might negatively impact eating behavior and nutritional intake.

Salivary proteome of a Neotropical primate: potential roles in host defense and oral food perception

BACKGROUND

Saliva contains a very complex mixture of proteins for defense against microbiological pathogens and for oral food perception. Howler monkeys are Neotropical primates that can consume a mostly leaf diet. They are well known to thrive in highly disturbed habitats where they may cope with a diversity of dietary challenges and infection risks. We aimed to describe the salivary proteome of howlers to contribute to better understanding of their physiology.

METHODS

We analyzed the salivary proteins of wild black howler monkeys (Alouatta pigra), by SDS-PAGE-1-D and Nano LC-MS/MS and categorized them by their function involved in host defense and oral food perception.

RESULTS

Our proteomic analysis identified 156 proteins in howler saliva including a number of host defense peptides that are the first line of defense in mammals, such as defensin, cathelicidin, dermcidin, and lactotransferrin, and proteins with anti-bacterial, anti-fungal, and anti-viral capacity, such as IgA, IgG, IgM, BPI, salivary heat shock 70 kDa protein, beta-2-microbulin, and protein S-100. We also identified key proteins necessary for taste perception, including salivary carbonic anhydrase VI, cystatin D, IgA, and fatty acid-binding protein. Proteins to detect astringent foods were identifying, including four members of cystatins (A, B, C and D), lactoperoxidase, and histidine-rich proteins. No chitinase and amylase were identified as would be expected because howlers do not eat insects and little starch. These findings provide basic information to future studies in oral biology, ingestive physiology, and physiological ecology of mammals and non-human primates.

CD36 gene polymorphism -31118 G > A (rs1761667) is associated with overweight and obesity but not with fat preferences in Mexican children

CD36 glycoprotein is a candidate receptor involved in the gustatory detection of lipids and emerging evidence has suggested that genetic variations in CD36 may modulate the oral perception threshold to fatty acids. Here, we analyzed the association of -31118 G > A polymorphism in CD36 gene with nutritional status and preferences for fatty foods in Mexican children. Genotyping of SNP rs1761667 was performed in school-age children (n = 63) in addition to sensory tests evaluating the preference and satisfaction score assigned to oil-based sauces of different fatty acid composition. The G allele was associated with high BMI z-score in children (OR = 2.43, 95% (CI 1.02-5.99); p = 0.02) but CD36 genotypes (AA, GA, and GG) did not show significant association with the preference and satisfaction scores assigned to oil-based sauces. The BMI z-score showed no association with the preference to oil-based sauces; however, children with normal weight gave higher satisfaction scores to sauces with a high content of unsaturated fatty acids than to sauces rich in saturated fatty acids (0.56 ± 1.26 vs. 0.06 ± 1.22; p = 0.02). Therefore, the G allele of -31118 G > A SNP in CD36 gene is associated with overweight and obesity in Mexican children but do not appear to modulate the preferences and satisfaction scores to fat.

Perception of creaminess in foods

Food texture and mouthfeel play a crucial role in product and consumer acceptability. Creaminess, enjoyed by consumers, is a complex, multimodal sensory perception involving olfactory, gustatory and tactile cues. Oral viscosity and lubrication are the key underlying physical properties that define the mechanism of creaminess perception. Thickness, smoothness, mouth-coating, and dairy flavor can together play roles in the sensation of creaminess. The aim of this review is to present an understanding of the term" creaminess" along with different modalities involved in its perception, and to explore the oral physiological parameters and key physical properties that may be involved in the different oral modalities. An analysis of the previously examined links between food structure and composition and oral physiological parameters is presented. The review also presents a brief summary of previous models describing contributions of taste, aroma, and textural sensations. It emphasizes on the role of oral processing in testing proposed models with experimental evidence supporting those models and the future trends to enhance creaminess.

Basic Taste: A Perceptual Concept

That the perceptual world of human taste is made up of four or five basic taste qualities is commonly accepted in the field of taste perception. Nevertheless, critics identify two issues that challenge this view. First, some argue that the term "basic tastes" cannot be precisely defined and, thus, is scientifically meaningless. Others accept the concept of basic tastes but believe there are many more. I argue here that it is most parsimonious to employ a perceptual definition of basic taste. I conclude that there are indeed four basic tastes (with a potential fifth) that constitute the building blocks of the human taste experience. Evidence cited includes historical writings from Chinese, Indian, and Greek cultures, ethnopharmacological research, and modern biological and psychological investigations. These perceptual "data" provide strong and convincing evidence, collected over thousands of years and from many different cultures, that the human perceptual world consists of the same basic taste qualities.

Therapeutic potential of ectopic olfactory and taste receptors

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

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

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

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.

Umami as an 'Alimentary' Taste. A New Perspective on Taste Classification

Applied taste research is increasingly focusing on the relationship with diet and health, and understanding the role the sense of taste plays in encouraging or discouraging consumption. The concept of basic tastes dates as far back 3000 years, where perception dominated classification with sweet, sour, salty, and bitter consistently featuring on basic taste lists throughout history. Advances in molecular biology and the recent discovery of taste receptors and ligands has increased the basic taste list to include umami and fat taste. There is potential for a plethora of other new basic tastes pending the discovery of taste receptors and ligands. Due to the possibility for an ever-growing list of basic tastes it is pertinent to critically evaluate whether new tastes, including umami, are suitably positioned with the four classic basic tastes (sweet, sour, salty, and bitter). The review critically examines the evidence that umami, and by inference other new tastes, fulfils the criteria for a basic taste, and proposes a subclass named ‘alimentary’ for tastes not meeting basic criteria.

Cerebral gustatory activation in response to free fatty acids using gustatory evoked potentials in humans

There is some evidence of specific oro-detection of FFAs in rodents and humans. The aim of this study was to record gustatory evoked potentials (GEPs) in response to FFA solutions and to compare GEPs in response to linoleic acid solution with GEPs obtained after stimulation with sweet and salty tastants. Eighteen healthy men were randomly stimulated with fatty (linoleic acid), sweet (sucrose), and salty (NaCl) solutions at two concentrations in the first experiment. Control recordings (n = 14) were obtained during stimulation by a paraffin oil mixture without FFA or by water. In the second experiment, 28 men were randomly stimulated with five FFA solutions and a paraffin emulsion. GEPs were recorded with electroencephalographic electrodes at Cz, Fz, and Pz. GEPs were observed in response to FFA in all participants. GEP characteristics did not differ according to the quality and the concentration of the solutions in the first experiment and according to the FFA in the second experiment. This study describes for the first time GEPs in response to FFA and demonstrates that the presence of FFA in the mouth triggers an activation of the gustatory cortex. These data reinforce the concept that fat taste could be the sixth primary taste.

Oral sensations and secretions

Sensations experienced in the mouth influence food choices, both immediately and in the long term. Such sensations are themselves influenced by experience with flavors, the chemical environment of the mouth, genetics of receptors for flavors, and individual behavior in the chewing of food. Gustation, the sense of taste, yields information about nutrients, influences palatability, and feeds into the human body's preparation to receive those nutrients. Olfaction, the sense of smell, contributes enormously to defining and identifying food flavors (and is experienced even after placing food inside the mouth). Another vital component of food flavor is texture, which contributes to palatability, especially if a food's texture violates a person's expectations. Next, chemesthesis is the sense of chemically induced irritancy and temperature, for example spiciness and stinging. All of these sensations are potentially modified by saliva, the chemical and physical media of the mouth. As a person experiences the culmination of these oral sensations, modified through an individual's own unique saliva, the flavors in turn influence both what and how a person eats.

Taste sensitivity in anorexia nervosa: A systematic review

OBJECTIVE

There is evidence for altered processing of taste in anorexia nervosa, particularly in the areas of reward processing and hedonic sensitivity. However, research on whether people with anorexia nervosa identify taste stimuli accurately, known as taste sensitivity, has yielded mixed findings. The objective of this study was to synthesize the literature on taste sensitivity in this disorder to provide a basis for future discussion on whether altered taste sensitivity may be also implicated in wider atypical taste processing in anorexia.

METHOD

Electronic databases were searched systematically to identify published research examining taste sensitivity in anorexia. Search terms were "anorexia nervosa", or "eating disorder", combined with "taste". 18 studies met inclusion criteria.

RESULTS

The review of the findings suggest that individuals with AN may experience reduced taste sensitivity that may improve following recovery. However, there was a significant variability in results across studies, potentially reflecting methodological problems including low sample sizes, experimental designs, and uncontrolled confounding variables.

DISCUSSION

This review suggests that altered taste sensitivity could represent a component in the wider altered taste processing observed in anorexia nervosa. However, the heterogeneity of findings highlight the need for future research to consider methodological issues raised by this review.

Obese Subjects With Specific Gustatory Papillae Microbiota and Salivary Cues Display an Impairment to Sense Lipids

Some obese subjects overeat lipid-rich foods. The origin of this eating behavior is unknown. We have here tested the hypothesis that these subjects could be characterized by an impaired fatty taste sensitivity linked to a change in the gustatory papillae microbial and salivary environment. The composition of microbiota and saliva surrounding the circumvallate papillae was analyzed in combination with the orosensory lipid detection threshold in normal weight (NW) and obese (O) adults. Microbial architecture was similar to what was known in feces, but with an increased frequency of Proteobacteria. No difference in the orosensory sensitivity to lipids and composition of oral microbiota and saliva was observed between NW and O subjects. By contrast, specific bacterial and salivary signatures were found in lipid non-tasters, irrespectively of BMI. A multivariate approach highlighted that the salivary flow, lysozyme activity, total antioxidant capacity and TM7 bacterial family discriminated between tasters and non-tasters. Subgroup analysis of obese tasters (OT) versus obese non-tasters (ONT) identified specific bacterial metabolic pathways (i.e. phosphotransferase and simple sugar transport systems) as being higher in ONT. Altogether with the identification of a set of significant salivary variables, our study suggests that an "obese tongue" phenotype is associated with decreased orosensory sensitivity to lipids in some obese subjects.

Nutrition and taste and smell dysfunction

Food selection plays a pivotal role in maintaining adequate nutrient intake, thus elucidating drivers of food choice is a meaningful strategy to maintain health and manage disease. Taste and smell are key determinants of food choice and warrant careful consideration. In this review, we first discuss how sensory stimulation influences food selection and metabolism. We then review the evidence regarding the relationship between taste and smell dysfunction and food preferences and selection, with attention given to contexts of certain chronic diseases. We conclude with brief recommendations for the management of chemosensory disorders. While sensory abilities influence food selection, the effect of taste and smell dysfunction on long-term consumption patterns and health status must be considered in light of environment, exposure, and culture.

Nutrient-Sensing Biology in Mammals and Birds

Nutrient-sensing mechanisms have emerged as the fringe articulating nutritional needs with dietary choices. Carbohydrate, amino acid, fatty acid, mineral, and water-sensing receptors are highly conserved across mammals and birds, consisting of a repertoire of 22 genes known to date. In contrast, bitter receptors are highly divergent and have a high incidence of polymorphisms within and between mammals and birds and are involved in the adaptation of species to specific environments. In addition, the expression of nutrient-sensing genes outside the oral cavity seems to mediate the required decision-making dialogue between the gut and the brain by translating exogenous chemical stimuli into neuronal inputs, and vice versa, to translate the endogenous signals relevant to the nutritional status into specific appetites and the control of feed intake. The relevance of these sensors in nondigestive systems has uncovered fascinating potential as pharmacological targets relevant to respiratory and cardiovascular diseases.

Evidence supporting oral sensitivity to complex carbohydrates independent of sweet taste sensitivity in humans

Compared to simple sugars, complex carbohydrates have been assumed invisible to taste. However, two recent studies proposed that there may be a perceivable taste quality elicited by complex carbohydrates independent of sweet taste. There is precedent with behavioural studies demonstrating that rats are very attracted to complex carbohydrates, and that complex carbohydrates are preferred to simple sugars at low concentrations. This suggests that rats may have independent taste sensors for simple sugars and complex carbohydrates. The aim of this paper is to investigate oral sensitivities of two different classes of complex carbohydrates (a soluble digestible and a soluble non-digestible complex carbohydrate), and to compare these to other caloric and non-nutritive sweeteners in addition to the prototypical tastes using two commonly used psychophysical measures. There were strong correlations between the detection thresholds and mean intensity ratings for complex carbohydrates (maltodextrin, oligofructose) (r = 0.94, P < 0.001). There were no significant correlations between the detection thresholds of the complex carbohydrates (maltodextrin, oligofructose) and the sweeteners (glucose, fructose, sucralose, Rebaudioside A, erythritol) (all P > 0.05). However, moderate correlations were observed between perceived intensities of complex carbohydrates and sweeteners (r = 0.48–0.61, P < 0.05). These data provide evidence that complex carbohydrates can be sensed in the oral cavity over a range of concentrations independent of sweet taste sensitivity at low concentrations, but with partial overlap with sweet taste intensity at higher concentrations.

Carbohydrate Taste Sensitivity Is Associated with Starch Intake and Waist Circumference in Adults

Background: Recent studies have proposed that humans may perceive complex carbohydrates and that sensitivity to simple carbohydrates is independent of sensitivity to complex carbohydrates. Variation in oral complex carbohydrate sensitivity may influence food consumption.Objective: This study aimed to investigate the associations between oral complex carbohydrate sensitivity, anthropometry, and dietary intake in adults.Methods: We assessed oral sensitivity to complex carbohydrates (maltodextrin and oligofructose) by measuring detection thresholds (DTs) and suprathreshold intensity perceptions (STs) for 34 participants, including 16 men (mean ± SEM age : 26.2 ± 0.4 y; range: 24-30 y) and 18 women (age: 29.4 ± 2.1 y; range: 24-55 y). We also measured height, weight, and waist circumference (WC) and participants completed a 4-d food diary and a food-frequency questionnaire.Results: Measurements of oral sensitivity to complex carbohydrates were significantly correlated with WC and dietary energy and starch intakes (DT: r = -0.38, P < 0.05; ST: r = 0.36-0.48, P < 0.05). When participants were grouped into tertiles, there were significant differences in WC and total energy or starch intakes for those who were more sensitive or experienced high intensity compared with those who were less sensitive or experienced low intensity. Being more sensitive or experiencing high intensity was associated with greater energy (7968-8954 kJ/d) and starch (29.1-29.8% of energy) intakes and a greater WC (88.2-91.4 cm) than was being less sensitive or experiencing low intensity (6693-7747 kJ/d, 20.9-22.2% of energy, and 75.5-80.5 cm, respectively).Conclusion: Complex carbohydrate sensing is associated with WC and consumption of complex carbohydrates and energy in adults. This trial was registered at anzctr.org.au as ACTRN12616001356459.

Oil Perception-Detection Thresholds for Varying Fatty Stimuli and Inter-individual Differences

Multiple lines of research have demonstrated that humans can perceive fat in the form of free fatty acids (FFAs). However, the dietary concentration of FFAs is generally very low and fat is mainly consumed as triacylglycerol (TAG). The aim of this study was to examine the perception of different fatty stimuli and possible associations between them. Therefore, detection thresholds for 4 fatty stimuli (oleic acid [FFA], paraffin oil [mixture of hydrocarbon molecules], canola oil [TAG-rich], and canola oil spiked with oleic acid [rich in TAGs and FFAs]) were determined in 30 healthy participants. Additionally, inter-individual differences in fat perception were examined. It was observed that oleic acid was perceivable at significantly lower concentrations than all other stimuli (P < 0.001). Similarly, canola oil with oleic acid was detectable at lower concentrations than canola oil alone (P < 0.001). Moreover, canola oil detection thresholds were significantly lower than paraffin oil detection thresholds (P = 0.017). Participants who were sensitive for low concentrations for oleic acid showed lower detection thresholds for canola oil with and without oleic acid, compared with participants that were less sensitive for oleic acid. The results of this study demonstrate that the higher the concentrations of FFAs in the stimuli, the lower the individual fat detection threshold. Moreover, participants being sensitive for lower concentrations of FFAs are also more likely to detect low concentrations of TAG-rich fats as it is found in the human diet.

Fat Taste Sensitivity Is Associated with Short-Term and Habitual Fat Intake

Evidence suggests individuals less sensitive to fat taste (high fat taste thresholds (FTT)) may be overweight or obese and consume greater amounts of dietary fat than more sensitive individuals. The aims of this study were to assess associations between FTT, anthropometric measurements, fat intake, and liking of fatty foods. FTT was assessed in 69 Australian females (mean age 41.3 (15.6) (SD) years and mean body mass index 26.3 (5.7) kg/m²) by a 3-alternate forced choice methodology and transformed to an ordinal scale (FT rank). Food liking was assessed by hedonic ratings of high-fat and reduced-fat foods, and a 24-h food recall and food frequency questionnaire was completed. Linear mixed regression models were fitted. FT rank was associated with dietary % energy from fat ( β ^ = 0.110 [95% CI: 0.003, 0.216]), % energy from carbohydrate ( β ^ = -0.112 [-0.188, -0.035]), and frequency of consumption of foods per day from food groups: high-fat dairy ( β ^ = 1.091 [0.106, 2.242]), meat & meat alternatives ( β ^ = 0.669 [0.168, 1.170]), and grain & cereals ( β ^ = 0.771 [0.212, 1.329]) (adjusted for energy and age). There were no associations between FT rank and anthropometric measurements or hedonic ratings. Therefore, fat taste sensitivity appears to be associated with short-term fat intake, but not body size in this group of females.

Genetics of fat intake in the determination of body mass

Body mass and fat intake are multifactorial traits that have genetic and environmental components. The gene with the greatest effect on body mass is FTO (fat mass and obesity-associated), but several studies have shown that the effect of FTO (and of other genes) on body mass can be modified by the intake of nutrients. The so-called gene–environment interactions may also be important for the effectiveness of weight-loss strategies. Food choices, and thus fat intake, depend to some extent on individual preferences. The most important biological component of food preference is taste, and the role of fat sensitivity in fat intake has recently been pointed out. Relatively few studies have analysed the genetic components of fat intake or fatty acid sensitivity in terms of their relation to obesity. It has been proposed that decreased oral fatty acid sensitivity leads to increased fat intake and thus increased body mass. One of the genes that affect fatty acid sensitivity is CD36 (cluster of differentiation 36). However, little is known so far about the genetic component of fat sensing. We performed a literature review to identify the state of knowledge regarding the genetics of fat intake and its relation to body-mass determination, and to identify the priorities for further investigations.

Relative Effects of Sensory Modalities and Importance of Fatty Acid Sensitivity on Fat Perception in a Real Food Model

INTRODUCTION

Fat can be perceived through mouthfeel, odour and taste, but the influence of these modalities on fat perception remains undefined. Fatty acids are stimuli and individual sensitivity to fatty acids varies. Studies show association between fatty acid sensitivity, dietary intake and BMI, but results are conflicting. Therefore, this study examined this association, and the effect of modalities on fat perception.

METHODS

Two sub-studies were conducted. In study 1 (n = 46), fat intensity was assessed by milk/cream mixtures varying by five fat levels. Fat intensity was rated under four conditions: mouthfeel odour-masked, mouthfeel-masked, odour masked and with no masking. Mouthfeel masking was achieved using thickener and paraffin, odour masking using nose-clips. Fatty acid sensitivity was measured by 3-AFC staircase method using milk containing oleic acid (0.31-31.4 mM). In study 2 (n = 51), more fat levels were added into the intensity rating. A 2-AFC discrimination test was used to confirm whether fat levels could be distinguished. In the sensitivity test, a wider range of oleic acid was included.

RESULTS

Fat intensity was rated higher without nose clips (p < 0.0001), implying that odour increased fat perception. Mouthfeel-masked samples were rated higher, showing that increased viscosity and lubricity enhanced fat perception (p < 0.0001). Participants could distinguish fat levels based on "taste" in rating tests and 2-AFC tests. Participants were divided into high-/medium-/low-sensitivity groups. No significant difference was found in fat intensity between groups; however, the high-sensitivity group discriminated more fat levels. No association between sensitivity groups, nutrient intake or BMI was found.

CONCLUSION

Mouthfeel and odour can enhance fat perception. Fat level can be discriminated based on taste.

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.

Endocannabinoids in the Gut

Cannabis has been used medicinally for centuries to treat a variety of disorders, including those associated with the gastrointestinal tract. The discovery of our bodies' own "cannabis-like molecules" and associated receptors and metabolic machinery - collectively called the endocannabinoid system - enabled investigations into the physiological relevance for the system, and provided the field with evidence of a critical function for this endogenous signaling pathway in health and disease. Recent investigations yield insight into a significant participation for the endocannabinoid system in the normal physiology of gastrointestinal function, and its possible dysfunction in gastrointestinal pathology. Many gaps, however, remain in our understanding of the precise neural and molecular mechanisms across tissue departments that are under the regulatory control of the endocannabinoid system. This review highlights research that reveals an important - and at times surprising - role for the endocannabinoid system in the control of a variety of gastrointestinal functions, including motility, gut-brain mediated fat intake and hunger signaling, inflammation and gut permeability, and dynamic interactions with gut microbiota.

Reduced dietary intake of simple sugars alters perceived sweet taste intensity but not perceived pleasantness

BACKGROUND

Individuals who adhere to reduced-sodium diets come to prefer less salt over time, but it is unclear whether sweet taste perception is modulated by reduced sugar intake.

OBJECTIVE

The objective was to determine how a substantial reduction in dietary intake of simple sugars affects sweetness intensity and pleasantness of sweet foods and beverages.

DESIGN

Healthy men and women aged 21-54 y participated for 5 mo. After the baseline month, 2 subject groups were matched for demographic characteristics, body mass index, and intake of simple sugars. One group (n = 16; 13 of whom completed key experimental manipulations) was randomly assigned to receive a low-sugar diet during the subsequent 3 mo, with instructions to replace 40% of calories from simple sugars with fats, proteins, and complex carbohydrates. The other (control) group (n = 17; 16 of whom completed the study) did not change their sugar intake. During the final month, both groups chose any diet they wished. Each month subjects rated the sweetness intensity and pleasantness of vanilla puddings and raspberry beverages that varied in sucrose concentration.

RESULTS

ANOVA showed no systematic differences between groups in rated sweetness during the baseline or first diet month. During the second diet month, the low-sugar group rated low-sucrose pudding samples as more intense than did the control group (significant group-by-concentration interaction, P = 0.002). During the third diet month, the low-sugar subjects rated both low and high concentrations in puddings as ∼40% sweeter than did the control group (significant effect of group, P = 0.01). A weaker effect on rated sweetness was obtained for the beverages. Rated pleasantness was not affected for either of the stimuli.

CONCLUSIONS

This experiment provides empirical evidence that changes in consumption of simple sugars influence perceived sweet taste intensity. More work is needed to determine whether sugar intake ultimately shifts preferences for sweet foods and beverages. This trial was registered at clinicaltrials.gov as NCT02090478.

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.

Oleogustus: The Unique Taste of Fat

Considerable mechanistic data indicate there may be a sixth basic taste: fat. However, evidence demonstrating that the sensation of nonesterified fatty acids (NEFA, the proposed stimuli for "fat taste") differs qualitatively from other tastes is lacking. Using perceptual mapping, we demonstrate that medium and long-chain NEFA have a taste sensation that is distinct from other basic tastes (sweet, sour, salty, and bitter). Although some overlap was observed between these NEFA and umami taste, this overlap is likely due to unfamiliarity with umami sensations rather than true similarity. Shorter chain fatty acids stimulate a sensation similar to sour, but as chain length increases this sensation changes. Fat taste oral signaling, and the different signals caused by different alkyl chain lengths, may hold implications for food product development, clinical practice, and public health policy.