First objective evaluation of taste sensitivity to 6-n-propylthiouracil (PROP), a paradigm gustatory stimulus in humans

Gene Methylation Affects Salivary Levels of the Taste Buds' Trophic Factor, Gustin Protein

The salivary protein, Gustin/carbonic anhydrase VI, has been described as a trophic factor responsible for the growth of taste buds. We found, in a genetically homogeneous population, that the polymorphism rs2274333 (A/G) of the Gustin gene is crucial for the full functionality of the protein and is associated with taste sensitivity. However, other studies have failed to find this evidence. Here, we verified if Gustin gene methylation can affect the salivary levels of the protein, also concerning the polymorphism rs2274333 and PROP bitter responsiveness. The Gustin gene methylation profiling and the quantification of the Gustin salivary levels were determined in sixty-six volunteers genotyped for the polymorphism rs2274333 (A/G) (Ser90Gly in the protein sequence). The fungiform papillae density was also determined. The results confirm our earlier observations by showing that AA genotypes had a greater density of fungiform taste papillae, whereas the GG genotypes showed a lower density. We also found variations in the protein levels in the three genotype groups and an inverse relationship between Gustin gene methylation and the salivary levels of the protein, mostly evident in AA and ST volunteers, i.e., in volunteers who would be carriers of the functional isoform of the protein. These findings could justify the conflicting data in the literature.

Recent advancements in the taste transduction mechanism, identification, and characterization of taste components

In the realm of human nutrition, the phenomenon known as taste refers to a distinctive sensation elicited by the consumption of food and various compounds within the oral cavity and on the tongue. Moreover, taste affects the overall comfort in the oral cavity, and is a fundamental attribute for the assessment of food items. Accordingly, clarifying the material basis of taste would be conducive to deepening the cognition of taste, investigating the mechanism of taste presentation, and accurately covering up unpleasant taste. In this paper, the basic biology and physiology of transduction of bitter, umami, sweet, sour, salty, astringent, as well as spicy tastes are reviewed. Furthermore, the detection process of taste components is summarized. Particularly, the applications, advantages, and distinctions of various isolation, identification, and evaluation methods are discussed in depth. In conclusion, the future of taste component detection is discussed.

Time-frequency analysis of gustatory event related potentials (gERP) in taste disorders

In taste disorders, the key to a correct diagnosis and an adequate treatment is an objective assessment. Compared to psychophysical tests, EEG-derived gustatory event-related potentials (gERP) could be used as a less biased measure. However, the responses identified using conventional time-domain averaging show a low signal-to-noise ratio. This study included 44 patients with dysgeusia and 59 healthy participants, who underwent a comprehensive clinical examination of gustatory function. gERPs were recorded in response to stimulation with two concentrations of salty solutions, which were applied with a high precision gustometer. Group differences were examined using gERP analyzed in the canonical time domain and with Time-Frequency Analyses (TFA). Dysgeusic patients showed significantly lower scores for gustatory chemical and electrical stimuli. gERPs failed to show significant differences in amplitudes or latencies between groups. However, TFA showed that gustatory activations were characterized by a stronger power in controls than in patients in the low frequencies (0.1-4 Hz), and a higher desynchronization in the alpha-band (8-12 Hz). Hence, gERPs reflect the altered taste sensation in patients with dysgeusia. TFA appears to enhance the signal-to-noise ratio commonly present when using conventional time-domain averaging, and might be of assistance for the diagnosis of dysgeusia.

Human T2R38 Bitter Taste Receptor Expression and COVID-19: From Immunity to Prognosis

Background

Bitter taste-sensing type 2 receptor (T2Rs or TAS2Rs) found on ciliated epithelial cells and solitary chemosensory cells have a role in respiratory tract immunity. T2Rs have shown protection against SARS-CoV-2 by enhancing the innate immune response. The purpose of this review is to outline the current sphere of knowledge regarding this association.

Methods

A narrative review of the literature was done by searching (T2R38 OR bitter taste receptor) AND (COVID-19 OR SARS-CoV-2) keywords in PubMed and google scholar.

Results

T2R38, an isoform of T2Rs encoded by the TAS2R38 gene, may have a potential association between phenotypic expression of T2R38 and prognosis of COVID-19. Current studies suggest that due to different genotypes and widespread distributions of T2Rs within the respiratory tract and their role in innate immunity, treatment protocols for COVID-19 and other respiratory diseases may change accordingly. Based on the phenotypic expression of T2R38, it varies in innate immunity and host response to respiratory infection, systemic symptoms and hospitalization.

Conclusion

This review reveals that patients' innate immune response to SARS-COV-2 could be influenced by T2R38 receptor allelic variations.

Automated identification of the genetic variants of TAS2R38 bitter taste receptor with supervised learning

Several studies were focused on the genetic ability to taste the bitter compound 6-n-propylthiouracil (PROP) to assess the inter-individual taste variability in humans, and its effect on food predilections, nutrition, and health. PROP taste sensitivity and that of other chemical molecules throughout the body are mediated by the bitter receptor TAS2R38, and their variability is significantly associated with TAS2R38 genetic variants. We recently automatically identified PROP phenotypes with high precision using Machine Learning (mL). Here we have used Supervised Learning (SL) algorithms to automatically identify TAS2R38 genotypes by using the biological features of eighty-four participants. The catBoost algorithm was the best-suited model for the automatic discrimination of the genotypes. It allowed us to automatically predict the identification of genotypes and precisely define the effectiveness and impact of each feature. The ratings of perceived intensity for PROP solutions (0.32 and 0.032 mM) and medium taster (MT) category were the most important features in training the model and understanding the difference between genotypes. Our findings suggest that SL may represent a trustworthy and objective tool for identifying TAS2R38 variants which, reducing the costs and times of molecular analysis, can find wide application in taste physiology and medicine studies.

A Simple Taste Test for Clinical Assessment of Taste and Oral Somatosensory Function-The "Seven-iTT"

Taste dysfunctions may occur, for example, after viral infection, surgery, medications, or with age. In clinical practice, it is important to assess patients' taste function with rapidity and reliability. This study aimed to develop a test that assesses human gustatory sensitivity together with somatosensory functions of astringency and spiciness. A total of 154 healthy subjects and 51 patients with chemosensory dysfunction rated their gustatory sensitivity. They underwent a whole-mouth identification test of 12 filter-paper strips impregnated with low and high concentrations of sweet, sour, salty, bitter (sucrose, citric acid, NaCl, quinine), astringency (tannin), and spiciness (capsaicin). The percentage of correct identifications for high-concentrated sweet and sour, and for low-concentrated salty, bitter and spicy was lower in patients as compared with healthy participants. Interestingly, a lower identification in patients for both astringent concentrations was found. Based on the results, we proposed the Seven-iTT to assess chemo/somatosensory function, with a cut-off of 6 out of 7. The test score discriminated patients from healthy controls and showed gender differences among healthy controls. This quantitative test seems to be suitable for routine clinical assessment of gustatory and trigeminal function. It also provides new evidence on the mutual interaction between the two sensory systems.

Associations between Sweet Taste Sensitivity and Polymorphisms (SNPs) in the TAS1R2 and TAS1R3 Genes, Gender, PROP Taster Status, and Density of Fungiform Papillae in a Genetically Homogeneous Sardinian Cohort

Individual differences in sweet taste sensitivity can affect dietary preferences as well as nutritional status. Despite the lack of consensus, it is believed that sweet taste is impacted by genetic and environmental variables. Here we determined the effect of well-established factors influencing the general taste variability, such as gender and fungiform papillae density, specific genetic variants (SNPs of TAS1R2 and TAS1R3 receptors genes), and non-specific genetic factors (PROP phenotype and genotype), on the threshold and suprathreshold sweet taste sensitivity. Suprathreshold measurements showed that the sweet taste response increased in a dose-dependent manner, and this was related to PROP phenotype, gender, rs35874116 SNP in the TAS1R2 gene, and rs307355 SNP in the TAS1R3 gene. The threshold values and density of fungiform papillae exhibited a strong correlation, and both varied according to PROP phenotype. Our data confirm the role of PROP taste status in the sweet perception related to fungiform papilla density, show a higher sweet sensitivity in females who had lower BMI than males, and demonstrate for the first time the involvement of the rs35874116 SNP of TAS1R2 in the sweet taste sensitivity of normal weight subjects with body mass index (BMI) ranging from 20.2 to 24.8 kg/m2. These results may have an important impact on nutrition and health mostly in subjects with low taste ability for sweets and thus with high vulnerability to developing obesity or metabolic disease.

Emotional responses to taste and smell stimuli: Self-reports, physiological measures, and a potential role for individual and genetic factors

Taste and olfaction elicit conscious feelings by direct connection with the neural circuits of emotions that affects physiological responses in the body (e.g., heart rate and skin conductance). While sensory attributes are strong determinants of food liking, other factors such as emotional reactions to foods may be better predictors of consumer choices even for products that are equally-liked. Thus, important insights can be gained for understanding the full spectrum of emotional reactions to foods that inform the activities of product developers and marketers, eating psychologist and nutritionists, and policy makers. Today, self-reported questionnaires and physiological measures are the most common tools applied to study variations in emotional perception. The present review discusses these methodological approaches, underlining their different strengths and weaknesses. We also discuss a small, emerging literature suggesting that individual differences and genetic variations in taste and smell perception, like the genetic ability to perceive the bitter compound PROP, may also play a role in emotional reactions to aromas and foods.

The Effects of Cranberry Polyphenol Extract (CPE) Supplementation on Astringency and Flavor Perception as a Function of PROP Taster Status and Other Individual Factors

This study investigated whether PROP (6-n-propylthiouracil) taster status and other individual factors (gender, ethnicity, BMI, and age) are markers of variation in perceptions of astringency and other flavor attributes. Participants (n = 125) evaluated cranberry juice cocktail samples (CJC) supplemented with cranberry-derived polyphenol extract (CPE, added at 0, 0.3, 0.5 and 0.75 g/L), as well as control samples, unsweetened cranberry juice (CJ) and an aqueous solution of 0.75 g/L CPE. Subjects evaluated samples for key sensory attributes and overall liking using a 15 cm line scale. The data were analyzed using ANCOVA and machine learning tools (regression trees and random forest modeling) to examine if the latter approach would extract more meaningful insights about the roles of personal factors in sensory perceptions of cranberry-derived stimuli. ANCOVA revealed robust stimulus effects, but no effect of PROP taster status on astringency perception was observed. Several effects of PROP×gender, ethnicity, and other factors were observed on other sensory attributes and liking. ANCOVA and machine learning tools yielded similar findings, but regression trees provided a more visualized framework. These data show that PROP taster status has a limited role in astringency perception in complex samples and that other personal factors deserve greater focus in future research on astringency perception.

Automated Classification of 6-n-Propylthiouracil Taster Status with Machine Learning

Several studies have used taste sensitivity to 6-n-propylthiouracil (PROP) to evaluate interindividual taste variability and its impact on food preferences, nutrition, and health. We used a supervised learning (SL) approach for the automatic identification of the PROP taster categories (super taster (ST); medium taster (MT); and non-taster (NT)) of 84 subjects (aged 18-40 years). Biological features determined from subjects were included for the training system. Results showed that SL enables the automatic identification of objective PROP taster status, with high precision (97%). The biological features were classified in order of importance in facilitating learning and as prediction factors. The ratings of perceived taste intensity for PROP paper disks (50 mM) and PROP solution (3.2 mM), along with fungiform papilla density, were the most important features, and high estimated values pushed toward ST prediction, while low values leaned toward NT prediction. Furthermore, TAS2R38 genotypes were significant features (AVI/AVI, PAV/PAV, and PAV/AVI to classify NTs, STs, and MTs, respectively). These results, in showing that the SL approach enables an automatic, immediate, scalable, and high-precision classification of PROP taster status, suggest that it may represent an objective and reliable tool in taste physiology studies, with applications ranging from basic science and medicine to food sciences.

Association Between Bitter Taste Receptor Phenotype and Clinical Outcomes Among Patients With COVID-19

IMPORTANCE

Bitter taste receptors (T2Rs) have been implicated in sinonasal innate immunity, and genetic variation conferred by allelic variants in T2R genes is associated with variation in upper respiratory tract pathogen susceptibility, symptoms, and outcomes. Bitter taste receptor phenotype appears to be associated with the clinical course and symptom duration of SARS-CoV-2 infection.

OBJECTIVE

To evaluate the association between T2R phenotype and patient clinical course after infection with SARS-CoV-2.

DESIGN, SETTING, AND PARTICIPANTS

A prospective cohort study was performed from July 1 through September 30, 2020, at a tertiary outpatient clinical practice and inpatient hospital in the United States among 1935 participants (patients and health care workers) with occupational exposure to SARS-CoV-2.

EXPOSURE

Exposure to SARS-CoV-2.

MAIN OUTCOMES AND MEASURES

Participants underwent T2R38 phenotype taste testing to determine whether they were supertasters (those who experienced greater intensity of bitter tastes), tasters, or nontasters (those who experienced low intensity of bitter tastes or no bitter tastes) and underwent evaluation for lack of infection with SARS-CoV-2 via polymerase chain reaction (PCR) testing and IgM and IgG testing. A group of participants was randomly selected for genotype analysis to correlate phenotype. Participants were followed up until confirmation of infection with SARS-CoV-2 via PCR testing. Phenotype of T2R38 was retested after infection with SARS-CoV-2. The results were compared with clinical course.

RESULTS

A total of 1935 individuals (1101 women [56.9%]; mean [SD] age, 45.5 [13.9] years) participated in the study. Results of phenotype taste testing showed that 508 (26.3%) were supertasters, 917 (47.4%) were tasters, and 510 (26.4%) were nontasters. A total of 266 participants (13.7%) had positive PCR test results for SARS-CoV-2. Of these, 55 (20.7%) required hospitalization. Symptom duration among patients with positive results ranged from 0 to 48 days. Nontasters were significantly more likely than tasters and supertasters to test positive for SARS-CoV-2 (odds ratio, 10.1 [95% CI, 5.8-17.8]; P < .001), to be hospitalized once infected (odds ratio, 3.9 [1.5-10.2]; P = .006), and to be symptomatic for a longer duration (mean [SE] duration, 23.7 [0.5] days vs 13.5 [0.4] days vs 5.0 [0.6] days; P < .001). A total of 47 of 55 patients (85.5%) with COVID-19 who required inpatient admission were nontasters. Conversely, 15 of 266 patients (5.6%) with positive PCR test results were supertasters.

CONCLUSIONS AND RELEVANCE

This cohort study suggests that T2R38 receptor allelic variants were associated with participants' innate immune response toward SARS-CoV-2. The T2R phenotype was associated with patients' clinical course after SARS-CoV-2 infection. Nontasters were more likely to be infected with SARS-CoV-2 than the other 2 groups, suggesting enhanced innate immune protection against SARS-CoV-2.

Allyl Isothiocyanate: A TAS2R38 Receptor-Dependent Immune Modulator at the Interface Between Personalized Medicine and Nutrition

Understanding individual responses to nutrition and medicine is of growing interest and importance. There is evidence that differences in bitter taste receptor (TAS2R) genes which give rise to two frequent haplotypes, TAS2R38-PAV (functional) and TAS2R38-AVI (non-functional), may impact inter-individual differences in health status. We here analyzed the relevance of the TAS2R38 receptor in the regulation of the human immune response using the TAS2R38 agonist allyl isothiocyanate (AITC) from Brassica plants. A differential response in calcium mobilization upon AITC treatment in leucocytes from healthy humans confirmed a relevance of TAS2R38 functionality, independent from cation channel TRPV1 or TRPA1 activation. We further identified a TAS2R38-dependence of MAPK and AKT signaling activity, bactericidal (toxicity against E. coli) and anti-inflammatory activity (TNF-alpha inhibition upon cell stimulation). These in vitro results were derived at relevant human plasma levels in the low micro molar range as shown here in a human intervention trial with AITC-containing food.

Differences in Salivary Proteins as a Function of PROP Taster Status and Gender in Normal Weight and Obese Subjects

Taste plays an important role in processes such as food choices, nutrition status and health. Salivary proteins contribute to taste sensitivity. Taste reduction has been associated with obesity. Gender influences the obesity predisposition and the genetic ability to perceive the bitterness of 6-n-propylthiouracil (PROP), oral marker for food preferences and consumption. We investigated variations in the profile of salivary proteome, analyzed by HPLC-ESI-MS, between sixty-one normal weight subjects (NW) and fifty-seven subjects with obesity (OB), based on gender and PROP sensitivity. Results showed variations of taste-related salivary proteins between NW and OB, which were differently associated with gender and PROP sensitivity. High levels of Ps-1, II-2 and IB-1 proteins belonging to basic proline rich proteins (bPRPs) and PRP-1 protein belonging to acid proline rich proteins (aPRPs) were found in OB males, who showed a lower body mass index (BMI) than OB females. High levels of Ps-1 protein and Cystatin SN (Cyst SN) were found in OB non-tasters, who had lower BMI than OB super-tasters. These new insights on the role of salivary proteins as a factor driving the specific weight gain of OB females and super-tasters, suggest the use of specific proteins as a strategic tool modifying taste responses related to eating behavior.

Treatment Protocol for COVID-19 Based on T2R Phenotype

COVID-19 has become a global pandemic of the highest priority. Multiple treatment protocols have been proposed worldwide with no definitive answer for acure. A prior retrospective study showed association between bitter taste receptor 38 (T2R38) phenotypes and the severity of COVID-19. Based on this, we proposed assessing the different T2R38 phenotypes response towards a targeted treatment protocol. Starting July 2020 till December 2020, we tested subjects for T2R38 phenotypic expression (supertasters, tasters, and nontasters). Subjects who were subsequently infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (diagnosed via PCR) were included. Based on their taster status, supertasters were given dexamethasone for 4 days; tasters were given azithromycin and dexamethasone +/− hydroxychloroquine for 7 days; and nontasters were given azithromycin and dexamethasone for 12 days. Subjects were followed prospectively and their outcomes were documented. Seven hundred forty-seven COVID-19 patients were included, with 184 (24.7%) supertasters, 371 (49.6%) tasters, and192 (25.7%) nontasters. The average duration of symptoms with the treatment protocol was 5 days for supertasters, 8.1 days for tasters, and 16.2 days for nontasters. Only three subjects (0.4%) required hospitalization (3/3 nontasters). Targeted treatment protocol showed significant correlation (p < 0.05) based on patients’ T2R38 phenotypic expression. Assessing treatment protocols for COVID-19 patients according to their T2R38 phenotype could provide insight into the inconsistent results obtained from the different studies worldwide. Further study is warranted on the categorization of patients based on their T2R38 phenotype.

COVID-19 as a worldwide selective event and bitter taste receptor polymorphisms: An ecological correlational study

Background

Given the observed olfactory and gustatory dysfunctions in patients with COVID-19 and recent findings on taste receptors possible important activities in the immune system, we elected to estimate the correlation between COVID-19 mortality and polymorphism of a particular type of bitter taste receptor gene called TAS2R38, in a worldwide epidemiological point of view.

Methods

Pooled rate of each of the rs713598, rs1726866, rs10246939, and PAV/AVI polymorphisms of the TAS2R38 gene was obtained in different countries using a systematic review methodology and its relationship with the mortality of COVID-19. Data were analyzed by the comprehensive meta-analysis software and SPSS.

Results

There was only a significant reverse Pearson correlation in death counts and PAV/AVI ratio, p = 0.047, r = −0.503. Also, a significant reverse correlation of PAV/AVI ratio and death rate was seen, r = −0.572 p = 0.021. rs10246939 ratio had a significant positive correlation with death rate, r = 0.851 p = 0.031. Further analysis was not significant. Our results showed that the higher presence of PAV allele than AVI, and a higher rate of G allele than A in rs10246939 polymorphism in a country, could be associated with lower COVID-19 mortality. While assessing all three polymorphisms showed a huge diversity worldwide.

Conclusion

Due to extraoral activities of bitter taste receptor genes, especially in mucosal immunity, this gene seems to be a good candidate for future studies on COVID-19 pathophysiology. Also, the high worldwide diversity of TAS2R38 genes polymorphism and its possible assassination with mortality raises concerns about the efficiency of vaccine projects in different ethnicities.

Changes of Taste, Smell and Eating Behavior in Patients Undergoing Bariatric Surgery: Associations with PROP Phenotypes and Polymorphisms in the Odorant-Binding Protein OBPIIa and CD36 Receptor Genes

Bariatric surgery is the most effective long-term treatment for severe obesity and related comorbidities. Although patients who underwent bariatric surgery report changes of taste and smell perception, results from sensory studies are discrepant and limited. Here, we assessed taste and smell functions in 51 patients before, one month, and six months after undergoing bariatric surgery. We used taste strip tests to assess gustatory function (including sweetness, saltiness, sourness, umaminess, bitterness and oleic acid, a fatty stimulus), the “Sniffin’ Sticks” test to assess olfactory identification and the 3-Factor Eating Questionnaire to assess eating behavior. We also explored associations between these phenotypes and flavor-related genes. Results showed an overall improvement in taste function (including increased sensitivity to oleic acid and the bitterness of 6-n-propylthiouracil (PROP)) and in olfactory function (which could be related to the increase in PROP and oleic acid sensitivity), an increase in cognitive restraint, and a decrease in disinhibition and hunger after bariatric surgery. These findings indicate that bariatric surgery can have a positive impact on olfactory and gustatory functions and eating behavior (with an important role of genetic factors, such PROP tasting), which in turn might contribute to the success of the intervention.

Bioelectrical signal associated with sweet taste transduction in humans is a hyperpolarizing potential on the lingual epithelium

The lingual surface potential (LSP), which hyperpolarizes in response to salt and bitter stimuli, is thought to be a bioelectrical signal associated with taste transduction in humans. In contrast, a recent study reported sweet and sour stimuli to evoke a depolarization of the LSP. We questioned the origin of such a depolarization because liquid junction potentials (JPs), which arise at the interfaces of recording electrode and taste solutions, are neglected in the report. We recorded the LSPs to sucrose and NaCl solutions on the human tongue using an Ag/AgCl electrode. To estimate JPs generated by each taste solution, we made an agar model to simulate the human tongue. The lingual surface was rinsed with a 10 mM NaCl solution that mimics the sodium content of the lingual fluid. In the human tongue, sucrose dissolved in distilled water evoked a depolarizing LSP that could be attributed to JPs, resulting from the change in electrolyte concentration of the taste solution. Sucrose dissolved in 10 mM NaCl solution evoked a hyperpolarizing LSP which became more negative in a concentration-dependent manner (300-1500 mM). Lactisole (3.75 mM), an inhibitor of sweet taste, significantly reduced the LSPs and decreased perceived intensity of sweetness by human subjects. The negative JPs generated by 100 mM NaCl in the agar model were not different from the LSPs to 100 mM NaCl. When the electrolyte environment on the lingual surface is controlled for JPs, the bioelectrical signal associated with sweet taste transduction is a hyperpolarizing potential.

Time Course of Salivary Protein Responses to Cranberry-Derived Polyphenol Exposure as a Function of PROP Taster Status

Astringency is a complex oral sensation, commonly experienced when dietary polyphenols interact with salivary proteins. Most astringent stimuli alter protein levels, which then require time to be replenished. Although it is standard practice in astringency research to provide breaks in between stimuli, there is limited consensus over the amount of time needed to restore the oral environment to baseline levels. Here we examined salivary protein levels after exposure to 20 mL of a model stimulus (cranberry polyphenol extract, 0.75 g/L CPE) or unsweetened cranberry juice (CJ), over a 10 min period. Whole saliva from healthy subjects (n = 60) was collected at baseline and after 5 and 10 min following either stimulus. Five families of proteins: basic proline-rich proteins (bPRPs); acidic proline-rich proteins (aPRPs); histatins; statherin; and S-type cystatins, were analyzed in whole saliva via HPLC-low resolution-ESI-IT-MS, using the area of the extracted ion current (XIC) peaks. Amylase was quantified via immunoblotting. In comparison to baseline (resting), both stimuli led to a rise in levels of aPRPs (p < 0.000) at 5 min which remained elevated at 10 min after stimulation. Additionally, an interaction of PROP taster status and time was observed, wherein super-tasters had higher levels of amylase in comparison to non-tasters after stimulation with CJ at both timepoints (p = 0.014–0.000). Further, male super-tasters had higher levels of bPRPs at 5 min after stimulation with both CJ and CPE (p = 0.015–0.007) in comparison to baseline. These data provide novel findings of interindividual differences in the salivary proteome that may influence the development of astringency and that help inform the design of sensory experiments of astringency.

Genetic variants of TAS2R38 bitter taste receptor associate with distinct gut microbiota traits in Parkinson's disease: A pilot study

The non-tasting form of the bitter taste receptor, TAS2R38, has been shown as a genetic risk factor associated with the development of Parkinson's disease (PD). Specific taste receptors that are expressed in the lower gastrointestinal tract may respond to alteration in gut microbiota composition, detecting bacterial molecules, and regulate immune responses. Given the importance of brain-gut-microbiota axis and gene-environment interactions in PD, we investigate the associations between the genetic variants of TAS2R38 and gut microbiota composition in 39 PD patients. The results confirm that the majority of PD patients have reduced sensitivity to 6-n-propylthiouracil (PROP) and are carriers of at least one non-functional TAS2R38 AVI haplotype. Moreover, we found this correlation to be associated with a reduction in bacteria alpha-diversity with a predominant reduction of Clostridium genus. We hypothesised that the high frequency of the non-taster form of TAS2R38 associated with a diminuition of Clostridium bacteria in PD might determine a reduction in the activation of protective signalling-molecules useful in preserving gut homeostasis. This pilot study, by identifying a decrease in specific bacteria associated with a reduced sensitivity to PROP, adds essential information that opens new avenues of research into the association of PD microbiota composition and sensory modification.

Disgust Propensity and the Bitter Aftertaste Response

Introduction

A high level of disgust propensity (the general tendency to respond with the emotion of disgust to any given situation) is associated with an increased sensitivity to bitter taste. The present study examined the relationship between disgust propensity and the sensitivity to bitter aftertaste.

Methods

A total of 200 women rinsed their mouth with concentrated wormwood tea (Artemisia absinthium). The resulting aftertaste was evaluated (intensity of bitterness and disgust) every 15 s for 10 min. A multiple linear regression analysis was calculated to capture the association between aftertaste ratings and affective variables (e.g., disgust propensity, depression symptoms).

Results

Higher disgust propensity was associated with higher initial disgust ratings and faster reduction of disgust over time. Higher depression scores were associated with a slower disgust reduction.

Conclusion

We demonstrated that affective variables predict the temporal course of the wormwood aftertaste response. Having a higher disgust propensity was associated with a shortened disgust recovery.

Implications

A shortened disgust recovery may be adaptive because it enables faster processing of new disgust stimuli.

Electrophysiological Responses from the Human Tongue to the Six Taste Qualities and Their Relationships with PROP Taster Status

Taste buds containing receptor cells that primarily detect one taste quality provide the basis for discrimination across taste qualities. The molecular receptor multiplicity and the interactions occurring between bud cells encode information about the chemical identity, nutritional value, and potential toxicity of stimuli before transmitting signals to the hindbrain. PROP (6-n-propylthiouracil) tasting is widely considered a marker for individual variations of taste perception, dietary preferences, and health. However, controversial data have been reported. We present measures of the peripheral gustatory system activation in response to taste qualities by electrophysiological recordings from the tongue of 39 subjects classified for PROP taster status. The waveform of the potential variation evoked depended on the taste quality of the stimulus. Direct relationships between PROP sensitivity and electrophysiological responses to taste qualities were found. The largest and fastest responses were recorded in PROP super-tasters, who had the highest papilla density, whilst smaller and slower responses were found in medium tasters and non-tasters with lower papilla densities. The intensities perceived by subjects of the three taster groups correspond to their electrophysiological responses for all stimuli except NaCl. Our results show that each taste quality can generate its own electrophysiological fingerprint on the tongue and provide direct evidence of the relationship between general taste perception and PROP phenotype.

Insights into smell and taste sensitivity in normal weight and overweight-obese adolescents

Research examining connections between BMI and smell and taste sensitivity in adolescents has been minimal, methodologically inconsistent, and inconclusive. We sought to address this issue with an exploratory study of smell and taste sensitivity in overweight-obese (high BMI) and normal BMI male and female adolescents (ages 12-16 years), using previously validated chemosensory testing measures (Sniffin' Sticks, Taste Strips, 6-n-propylthiouracil: PROP), and taking pubertal stage into account. Puberty was evaluated with the validated Pubertal Development Scale and participants were then classified as either "early" or "late" pubertal stage. We used the phenylethyl alcohol (PEA) version of the Sniffin' Sticks olfactory threshold test and found that high BMI adolescents had significantly greater olfactory sensitivity than normal BMI adolescents. This observation contradicts previous results in overweight adults tested with the n-butanol version of Sniffin' Sticks. We also found that participants in early puberty had significantly higher olfactory sensitivity than participants in late puberty. No significant findings for taste sensitivity were obtained, though there is a suggestion that puberty may affect salty taste thresholds. Our results illuminate a potentially important difference in sensitivity to pure olfactory versus olfactory-trigeminal stimuli as a function of BMI, which the PEA and n-butanol versions of the Sniffin' Sticks respectively assess; and for the first time demonstrate variation in chemosensory acuity in relation to pubertal stage. These findings have implications for eating behavior during adolescence.

Electrophysiologic assessment of olfactory and gustatory function

This chapter reviews approaches for assessing human and gustatory function using electrophysiologic methods. Its focus is on changes in electrical signals, including summated generator potentials that occur after nasal or oral stimulation. In the first part of the review, we describe tools available to the clinician for assessing olfactory and nasotrigeminal function, including modern electroencephalography (EEG) analysis of brain responses both in the time domain and in the time-frequency (TF) domain. Particular attention is paid to chemosensory event-related potentials (CSERPs) and their potential use in medical-legal cases. Additionally, we focus on the changes of summated generator potentials from the olfactory and respiratory nasal epithelium that could provide new diagnostic insights. In the second part, we describe gustatory event-related potentials (gCSERPs) obtained using a relatively new computer controlled gustometer. A device for presenting different pulses of electrical current to the tongue is also described, with weaker pulses likely reflecting gCSERPs and stronger ones trigeminal CSERPs. Finally, summated generator potentials from the surface of the tongue during gustatory stimulation are described that may prove useful for examining peripheral taste function.

Taste Changes in Patients with Inflammatory Bowel Disease: Associations with PROP Phenotypes and polymorphisms in the salivary protein, Gustin and CD36 Receptor Genes

Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract resulting from interactions among various factors with diet being one of the most significant. IBD-related dietary behaviors are not clearly related to taste dysfunctions. We analyzed body mass index (BMI) and perception of six taste qualities and assessed effects of specific taste genes in IBD patients and healthy subjects (HC). BMI in IBD patients was higher than in HC subjects. Taste sensitivity to taste qualities was reduced in IBD patients, except for sour taste, which was higher than in HC subjects. Genetic variations were related to some taste responses in HC subjects, but not in IBD patients. Frequencies of genotype AA and allele A in CD36 polymorphism (rs1761667) were significantly higher in IBD patients than in HC subjects. The taste changes observed could be explained by the oral pathologies and microbiome variations known for IBD patients and can justify their typical dietary behaviors. The lack of genetic effects on taste in IBD patients indicates that IBD might compromise taste so severely that gene effects cannot be observed. However, the high frequency of the non-tasting form of CD36 substantiates the fact that IBD-associated fat taste impairment may represent a risk factor for IBD.

TAS2R38 bitter taste receptor and attainment of exceptional longevity

Bitter taste receptors play crucial roles in detecting bitter compounds not only in the oral cavity, but also in extraoral tissues where they are involved in a variety of non‒tasting physiological processes. On the other hand, disorders or modifications in the sensitivity or expression of these extraoral receptors can affect physiological functions. Here we evaluated the role of the bitter receptor TAS2R38 in attainment of longevity, since it has been widely associated with individual differences in taste perception, food preferences, diet, nutrition, immune responses and pathophysiological mechanisms. Differences in genotype distribution and haplotype frequency at the TAS2R38 gene between a cohort of centenarian and near-centenarian subjects and two control cohorts were determined. Results show in the centenarian cohort an increased frequency of subjects carrying the homozygous genotype for the functional variant of TAS2R38 (PAV/PAV) and a decreased frequency of those having homozygous genotype for the non-functional form (AVI/AVI), as compared to those determined in the two control cohorts. In conclusion, our data providing evidence of an association between genetic variants of TAS2R38 gene and human longevity, suggest that TAS2R38 bitter receptor can be involved in the molecular physiological mechanisms implied in the biological process of aging.

Individual Variation in PROP Status, Fungiform Papillae Density, and Responsiveness to Taste Stimuli in a Large Population Sample

Despite considerable research investigating the role of 6-n-propylthiouracil bitterness perception and variation of fungiform papillae density in food perception, this relationship remains controversial as well as the association between the 2 phenotypes. Data from 1119 subjects (38.6% male; 18-60 years) enrolled in the Italian Taste project were analyzed. Responsiveness to the bitterness of 6-n-propylthiouracil was assessed on the general Labeled Magnitude Scale. Fungiform papillae density was determined from manual counting on digital images of the tongue. Solutions of tastes, astringent, and pungent sensations were prepared to be moderate/strong on a general Labeled Magnitude Scale. Four foods had tastants added to produce 4 variations in target sensations from weak to strong (pear juice: citric acid, sourness; chocolate pudding: sucrose, sweetness; bean purée: sodium chloride, saltiness; and tomato juice: capsaicin, pungency). Women gave ratings to 6-n-propylthiouracil and showed fungiform papillae density that was significantly higher than men. Both phenotype markers significantly decreased with age. No significant correlations were found between 6-n-propylthiouracil ratings and fungiform papillae density. Fungiform papillae density variation does not affect perceived intensity of solutions. Responsiveness to 6-n-propylthiouracil positively correlated to perceived intensity of most stimuli in solution. A significant effect of fungiform papillae density on perceived intensity of target sensation in foods was found in a few cases. Responsiveness to 6-n-propylthiouracil positively affected all taste intensities in subjects with low fungiform papillae density whereas there were no significant effects of 6-n-propylthiouracil in those with high fungiform papillae density. These data highlight a complex interplay between 6-n-propylthiouracil status and fungiform papillae density and the need of a critical reconsideration of their role in food perception and acceptability.

Association between the rs2590498 polymorphism of Odorant Binding Protein (OBPIIa) gene and olfactory performance in healthy subjects

Olfactory function varies by several orders of magnitude among healthy individuals, who may exhibit a reduced sensitivity (hyposmia), a high sensitivity (hyperosmia), or an olfactory blindness (anosmia). Environmental and genetic factors seem to account for this variability. Most of odorant molecules are hydrophobic and it has been suggested that odorants are transported to the olfactory receptors by means of odorant binding proteins (OBPs). Aim of this study was to evaluate the presence of a relationship between the olfactory performance of healthy subjects and the polymorphism in the odor binding-protein (OBPIIa) gene, the only OBP found in the olfactory epithelium of humans. Using the "Sniffin' Sticks" Extended Test we assessed the olfactory performance in 69 subjects, who were genotyped for the rs2590498 polymorphism of the OBPIIa gene, whose major allele A has been associated with a higher retronasal perception as compared to the minor allele G. We found that subjects homozygous for the A-allele exhibited threshold scores higher than subjects homozous for the G-allele or heterozygous. In addition, subjects classified as normosmic and hyposmic differed on the basis of genotype distribution and allelic frequencies. In fact, a normosmic condition was associated with genotype AA and allele A and a hyposmic condition was associated with genotype GG and allele G. In conclusion, our results show that a relationship exists between the physiological variations of olfactory performance and the OBPIIa gene polymorphism.

Human Tongue Electrophysiological Response to Oleic Acid and Its Associations with PROP Taster Status and the CD36 Polymorphism (rs1761667)

The perception of fat varies among individuals and has also been associated with CD36 rs1761667 polymorphism and genetic ability to perceive oral marker 6-n-propylthiouracil (PROP). Nevertheless, data in the literature are controversial. We present direct measures for the activation of the peripheral taste system in response to oleic acid by electrophysiological recordings from the tongue of 35 volunteers classified for PROP taster status and genotyped for CD36. The waveform of biopotentials was analyzed and values of amplitude and rate of potential variation were measured. Oleic acid stimulations evoked positive monophasic potentials, which represent the summated voltage change consequent to the response of the stimulated taste cells. Bio-electrical measurements were fully consistent with the perceived intensity during stimulation, which was verbally reported by the volunteers. ANOVA revealed that the amplitude of signals was directly associated, mostly in the last part of the response, with the CD36 genotypes and PROP taster status (which was directly associated with the density of papillae). The rate of potential variation was associated only with CD36, primarily in the first part of the response. In conclusion, our results provide direct evidence of the relationship between fat perception and rs1761667 polymorphism of the CD36 gene and PROP phenotype.

The contribution of gustatory input to larval acceptance and female oviposition choice of potential host plants in Papilio hospiton (Géné)

The Lepidopteran Papilio hospiton uses only plants belonging to the Apiaceae and the Rutaceae families as hosts. Both adult females and larvae are equipped with gustatory receptor neurons (GRNs) capable of detecting sugars, bitters and salts, thus providing information for evaluating the chemical composition of the plant. Since the activation of these neurons may affect insect behavior, the aim of this study were: (a) to study the gustatory sensitivity of both females and larvae to the sap of two Apiaceae, Foeniculum vulgare (fennel) and Daucus carota (carrot), that are not used as host plants; (b) to cross-compare the spike activity evoked from these two plants with that evoked by Ferula communis (ferula), the host plant preferred by ovipositing females of P. hospiton and where the larvae perform best; (c) finally, to confirm that the gustatory system can provide the central nervous system with the necessary information to evaluate differences between plant saps. The results show that: (a) fennel and carrot both evoke a higher neural activity from the bitter-sensitive neurons and lower from the sugar-sensitive neurons with respect to ferula, in both adult females and larvae; (b) on the basis of the different patterns of neural activity generated in tarsal, lateral and medial sensilla by fennel and carrot versus ferula, both adult and larvae possess enough information to discriminate among these plants; (c) adult females of P. hospiton lay eggs where the larvae have the greatest growth success and this confirms the importance of taste sensitivity in host plants selection.

The evolution of taste and perinatal programming of taste preferences

Taste is important for food intake. The fetus first experiences taste through amniotic fluid, and later via mother's milk. Early human experience with taste has a key importance for later acceptance of food. Dietary behavior is determined by the interaction of many different factors. The development of the olfactory and taste receptors begins at 7-8 weeks of gestation. An early sensitive period probably exists when flavor preference is established. Sweet taste is preferred in early childhood; this is the reason why children are at increased risk of over-consuming saccharides. Gustatory sensitivity declines with age. The threshold for the perception of each basic taste differs, and is established genetically. In this review, we summarize published data on taste preferences and its development and changes during life.

Human T2R38 Bitter Taste Receptor Expression in Resting and Activated Lymphocytes

The human G-protein-coupled bitter taste receptor T2R38 has recently been demonstrated to be expressed on peripheral blood neutrophils, monocytes and lymphocytes. To further define a potential contribution of the T2R38 receptor in adaptive immune response, the objective of this study was to analyze its expression in resting and activated lymphocytes and T cell subpopulations. Freshly isolated PBMC from healthy donors were used for expression analysis by flow cytometry. Quantum™ MESF beads were applied for quantification in absolute fluorescence units. Activation methods of T cells were anti-CD3/CD28, phytohaemagglutinin (PHA) or phorbol 12-myristate 13-acetate (PMA) together with ionomycin. Lymphocytes from young donors expressed higher levels of T2R38 compared to the elderly. CD3+ T cells expressed higher levels that CD19+ B cells. Receptor expression followed T cell activation with an upregulation within 24 h and a peak at 72 h. Higher levels of T2R38 were produced in lymphocytes by stimulation with anti-CD3/CD28 compared to PHA or PMA/ionomycin. Both subpopulations of CD4+ as well as CD8+ T cells were found to express the T2R38 receptor; this was higher in CD4+ than CD8+ cells; the amount of T2R38 in central and effector memory cells was higher as compared to naïve cells, although this was not statistically significant for CD8+ cells without prior activation by anti-CD3/CD28. Upon treatment of PBMC with the natural T2R38 agonist goitrin Calcium flux was activated in the lymphocyte population with functional T2R38 receptor at >20 μM which was completely blocked by phospholipase Cβ-2 inhibitor U73211. Further, goitrin selectively inhibited TNF-alpha secretion in PBMC with functional T2R38. This quantitative analysis of T2R38 expression in distinct PBMC subsets may provide a basis for understanding the significance of bitter compounds in immune modulation. Whether these findings can have implications for the treatment of inflammatory and immunologic disorders by bitter tasting pharmaceuticals or foods needs further investigation.

Taste sensitivity and divergence in host plant acceptance between adult females and larvae of Papilio hospiton

On the island of Sardinia the lepidopteran Papilio hospiton uses Ferula communis as exclusive host plant. However, on the small island of Tavolara, adult females lay eggs on Seseli tortuosum, a plant confined to the island. When raised in captivity on Seseli only few larvae grew beyond the first-second instar. Host specificity of lepidopterans is determined by female oviposition preferences, but also by larval food acceptance, and adult and larval taste sensitivity may be related to host selection in both cases. Aim of this work was: (i) to study the taste sensitivity of larvae and ovipositing females to saps of Ferula and Seseli; (ii) to cross-compare the spike activity of gustatory receptor neurons (GRNs) to both taste stimuli; (iii) to evaluate the discriminating capability between the two saps and determine which neural code/s is/are used. The results show that: (i) the spike responses of the tarsal GRNs of adult females to both plant saps are not different and therefore they cannot discriminate the two plants; (ii) larval L-lat GRN shows a higher activity in response to Seseli than Ferula, while the opposite occurs for the phagostimulant neurons, and larvae may discriminate between the two saps by means of multiple neural codes; (iii) the number of eggs laid on the two plants is the same, but the larval growth performance is better on Ferula than Seseli. Taste sensitivity differences may explain the absence of a positive relationship between oviposition preferences by adult females and plant acceptance and growth performance by larvae.

Olfactory sensitivity to major, intermediate and trace components of sex pheromone in Ceratitis capitata is related to mating and circadian rhythm

The Mediterranean fruit fly, Ceratitis capitata Wied., is a worldwide pest of several fruits given its extremely wide host range which includes more than 250 different species of fruits and vegetables. Its high biological potential is mainly due both to its ability to readily adapt to new environments and its high reproductive capacity as it completes multiple generations each year. Since sexually mature males emit a sex pheromone to call both other males for "lekking" and receptive females for mating, many studies have been directed to characterize the chemical composition of the sex pheromone. Besides, the release of sex pheromone appears to be modulated both by mating and time of day. Based on these considerations, we measured the olfactory sensitivity of antennae and palps of C. capitata to six volatiles of the male sex-pheromone: α-farnesene and geranyl acetate (major components), linalool and β-myrcene (intermediate components), β-farnesene and 2,3-butanediol (minor/trace components). The electroantennogram (EAG) and electropalpogram (EPG) responses were evaluated in both sexes, at different physiological states (virgin and mated), and at different times of the day (morning and afternoon). The results show that the EAG amplitude values in response to all stimuli are higher in the morning than in the afternoon for both sexes and in both virgin and mated insects. Furthermore, in both sexes, the olfactory sensitivity of virgin insects is higher than in mated ones. The EPG amplitude in response to all stimuli is higher in the morning in mated females than in virgin females and higher in the morning than in the afternoon in both mated sexes. By gaining knowledge on the effects of sex, physiological state and time of day on the olfactory sensitivity of C. capitata, one could better understand the medfly reproductive behavior.

Taste receptor plasticity in relation to feeding history in two congeneric species of Papilionidae (Lepidoptera)

In the peripheral taste system of insects, the responsiveness of gustatory receptor neurons (GRNs) depends on several factors, such as larval instar, feeding history, physiological state and time of day. To study the role of the feeding history, the spike activity of the maxillary taste chemosensilla in the larvae of two related species of Lepidoptera (Papilio machaon L. and Papilio hospiton Géné) raised on different host plants, was recorded with electrophysiological techniques after stimulation with simple stimuli (sugars, bitters and inorganic salt) and host plant saps, with the aim of cross-comparing their response patterns and evaluating any effects of different feeding histories. For this purpose the larvae were raised each on their preferential host plant and, in addition, P. machaon larvae was also raised on Ferula communis, the host plant preferred by P. hospiton. The GRN spike activity of the lateral and medial sensilla of each test group was measured in response to simple and complex stimuli. The taste discrimination capabilities and modalities of the two species were measured and cross-compared with the aim of studying convergence and/or divergence linked to the insect feeding history. The results show that: a) the GRN responsiveness of both sensilla in P. machaon raised on Fe. communis differs significantly from that of P. machaon on Foeniculum vulgare, but is not different from P. hospiton on Fe. communis; b) P. machaon larvae raised on Fe. communis exhibit response spectra somewhat intermediate between those of P. machaon on fennel and of P. hospiton on Fe. communis, the latter two exhibiting a wider difference from each other; c) the pattern of GRNs activity generated by each plant sap in both sensilla of P. machaon raised on Fe. communis is different from that generated when raised on Fo. vulgare, while no difference is observed with P. hospiton. The data support the hypothesis that diet-related factors may influence peripheral chemosensitivity in lepidopterous larvae.

6-n-propylthiouracil taste disruption and TAS2R38 nontasting form in Parkinson's disease

BACKGROUND

The few studies that evaluated taste function in Parkinson's disease (PD) showed inconsistent results. The inherited ability to taste the bitter compound of 6-n-propylthiouracil has been considered to be a paradigm of general taste perception. 6-n-propylthiouracil taste perception is mediated by the TAS2R38 receptor, and reduced 6-n-propylthiouracil sensitivity has been associated with several diseases not typically related to taste function.

OBJECTIVES

We evaluated the 6-n-propylthiouracil taste perception and the TAS2R38 gene as genetic risk factors for the development of idiopathic PD in PD patients and healthy controls (HC).

METHODS

The 6-n-propylthiouracil taste perception was assessed by testing the responsiveness, and the ability to recognize, 6-n-propylthiouracil and sodium chloride. The participants were classified for 6-n-propylthiouracil taster status and genotyped for the TAS2R38 gene.

RESULTS

A significant increase in the frequency of participants classified as 6-n-propylthiouracil nontasters and a reduced ability to recognize bitter taste quality of 6-n-propylthiouracil were found in PD patients when compared with healthy controls. The results also showed that only 5% of PD patients had the homozygous genotype for the dominant tasting variant of TAS2R38, whereas most of them carried the recessive nontaster form and a high number had a rare variant.

CONCLUSIONS

Our results show that 6-n-propylthiouracil taster status and TAS2R38 locus are associated with PD. The 6-n-propylthiouracil test may therefore represent a novel, simple way to identify increased vulnerability to PD. Moreover, the presence of the nontasting form of TAS2R38 in PD may further substantiate that disease-associated taste disruption may represent a risk factor associated with the disease. © 2018 International Parkinson and Movement Disorder Society.

Effect of chemical interaction between oleic acid and L-Arginine on oral perception, as a function of polymorphisms of CD36 and OBPIIa and genetic ability to taste 6-n-propylthiouracil

Oral sensitivity to fats varies in individuals influencing nutritional status and health. Variations in oleic acid perception are associated with CD36 and odorant binding protein (OBPIIa) polymorphisms, and 6-n-propylthiouracil (PROP) sensitivity, which is mediated by TAS2R38 receptor. L-Arginine (L-Arg) supplementation was shown to modify the perception of the five taste qualities. Here we analyzed the effect of three concentrations (5, 10, 15 mmol/L) of L-Arg on oral perception of oleic acid in forty-six subjects classified for PROP taster status and genotyped for TAS2R38, CD36 and OBPIIa polymorphisms. L-Arg supplementation was effective in increasing the perceived intensity of oleic acid in most subjects. The lowest concentration was the most effective, especially in PROP non-tasters or medium tasters, and in subjects with at least an allele A in CD36 and OBPIIa loci. Density Functional Theory (DFT) calculations were exploited to characterize the chemical interaction between L-Arg and oleic acid, showing that a stable 1:1 oleate·ArgH⁺ adduct can be formed, stabilized by a pair of hydrogen bonds. Results indicate that L-Arg, acting as a ‘carrier’ of fatty acids in saliva, can selectively modify taste response, and suggest that it may to be used in personalized dietetic strategies to optimize eating behaviors and health.

Factors Influencing the Phenotypic Characterization of the Oral Marker, PROP

In the last several decades, the genetic ability to taste the bitter compound, 6-n-propyltiouracil (PROP) has attracted considerable attention as a model for understanding individual differences in taste perception, and as an oral marker for food preferences and eating behavior that ultimately impacts nutritional status and health. However, some studies do not support this role. This review describes common factors that can influence the characterization of this phenotype including: (1) changes in taste sensitivity with increasing age; (2) gender differences in taste perception; and (3) effects of smoking and obesity. We suggest that attention to these factors during PROP screening could strengthen the associations between this phenotype and a variety of health outcomes ranging from variation in body composition to oral health and cancer risk.

An automated system for the objective evaluation of human gustatory sensitivity using tongue biopotential recordings

The goal of this work is to develop an automatic system for the evaluation of the gustatory sensitivity of patients using an electrophysiological recording of the response of bud cells to taste stimuli. In particular, the study aims to evaluate the effectiveness and limitations of supervised classifiers in the discrimination between subjects belonging to the three 6-n-propylthiouracil (PROP) taster categories (supertasters, medium tasters, and non-tasters), exploiting features extracted from electrophysiological recordings of the tongue. Thirty-nine subjects (equally divided into the three PROP status classes by standard non-objective scaling methods) underwent a non-invasive, differential, biopotential recording of their tongues during stimulation with PROP by using a custom-made, flexible, silver electrode. Two different classifiers were trained to recognize up to seven different features extracted from the recorded depolarization signal. The classification results indicate that the identified set of features allows to distinguish between PROP tasters and non-tasters (average accuracy of 80% ± 18% and up to 94% ± 15% when only supertasters and non-tasters are considered), but medium tasters were difficult to identify. However, these apparent classification errors are related to uncertainty in the labeling procedures, which are based on non-objective tests, in which the subjects provided borderline evaluations. Thus, using the proposed method, it is possible, for the first time, to automatically achieve objective PROP taster status identification with high accuracy. The simplicity of the recording technique allows for easy reproduction of the experimental setting; thus the technique can be used in future studies to evaluate other gustatory stimuli. The proposed approach represents the first objective and automatic method to directly measure human gustatory responses and a milestone for physiological taste studies, with applications ranging from basic science to food tasting evaluations.

Taste Perception of Sweet, Sour, Salty, Bitter, and Umami and Changes Due to l-Arginine Supplementation, as a Function of Genetic Ability to Taste 6-n-Propylthiouracil

Behavioral reaction to different taste qualities affects nutritional status and health. 6-n-Propylthiouracil (PROP) tasting has been reported to be a marker of variation in taste perception, food preferences, and eating behavior, but results have been inconsistent. We showed that l-Arg can enhance the bitterness intensity of PROP, whilst others have demonstrated a suppression of the bitterness of quinine. Here, we analyze the taste perception of sweet, sour, salty, bitter, and umami and the modifications caused by l-Arg supplementation, as a function of PROP-taster status. Taste perception was assessed by testing the ability to recognize, and the responsiveness to, representative solutions of the five primary taste qualities, also when supplemented with l-Arg, in subjects classified as PROP-tasting. Super-tasters, who showed high papilla density, gave higher ratings to sucrose, citric acid, caffeine, and monosodium l-glutamate than non-tasters. l-Arg supplementation mainly modified sucrose perception, enhanced the umami taste, increased NaCl saltiness and caffeine bitterness only in tasters, and decreased citric acid sourness. Our findings confirm the role of PROP phenotype in the taste perception of sweet, sour, and bitter and show its role in umami. The results suggest that l-Arg could be used as a strategic tool to specifically modify taste responses related to eating behaviors.

Participants with Normal Weight or with Obesity Show Different Relationships of 6-n-Propylthiouracil (PROP) Taster Status with BMI and Plasma Endocannabinoids

Reduced taste sensitivity to 6-n-propylthiouracil (PROP), a genetic trait regarded as a general index for oral chemosensory perception, has been associated with a calorie-rich food preference and lower circulating endocannabinoid levels in participants with normal weight (NW), which suggests an adaptive mechanism to maintain a lean phenotype. In this study, we assessed whether participants with obesity (OB) show different patterns of plasma endocannabinoids and lipid metabolism biomarkers from those of NW, with further categorization based on their PROP sensitivity. NW and OB were classified by their PROP taster status as non-tasters (NT), medium-tasters (MT) and supertasters (ST). The blood samples were analysed for plasma endocannabinoids, nonesterified fatty acids (NEFA) and retinol, which have been associated to metabolic syndrome. In OB, we found a higher BMI and lower circulating endocannabinoids in ST vs. OB NT. However, OB ST showed lower circulating NEFA and retinol levels, which suggested a more favourable lipid metabolism and body fat distribution than those of OB NT. We confirmed lower plasma endocannabinoid levels in NW NT than in NW ST. These data suggest that PROP taste sensitivity determines metabolic changes and ultimately body mass composition differently in OB and NW.