Metabolomic profiling of biofluids, e.g., urine, plasma, has generated vast and ever-increasing amounts of knowledge over the last few decades. Paradoxically, metabolomic analysis of saliva, the most readily-available human biofluid, has lagged. This review explores the history of saliva-based metabolomics and summarizes current knowledge of salivary metabolomics. Current applications of salivary metabolomics have largely focused on diagnostic biomarker discovery and the diagnostic value of the current literature base is explored. There is also a small, albeit promising, literature base concerning the use of salivary metabolomics in monitoring athletic performance. Functional roles of salivary metabolites remain largely unexplored. Areas of emerging knowledge include the role of oral host–microbiome interactions in shaping the salivary metabolite profile and the potential roles of salivary metabolites in oral physiology, e.g., in taste perception. Discussion of future research directions describes the need to begin acquiring a greater knowledge of the function of salivary metabolites, a current research direction in the field of the gut metabolome. The role of saliva as an easily obtainable, information-rich fluid that could complement other gastrointestinal fluids in the exploration of the gut metabolome is emphasized.

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Salivary Metabolomics: From Diagnostic Biomarker Discovery to Investigating Biological Function.

Taste hedonics is a well-documented driver of food consumption. The role of sweetness in directing ingestive behavior is largely rooted in biology. One can then intuit that individual differences in sweet-liking may constitute an indicator of variations in the susceptibility to diet-related health outcomes. Despite half a century of research on sweet-liking, the best method to identify the distinct responses to sweet taste is still debated. To help resolve this issue, liking and intensity ratings for eight sucrose solutions ranging from 0 to 1 M were collected from 148 young adults (29% men). Hierarchical cluster analysis (HCA) revealed three response patterns: a sweet-liker (SL) phenotype characterized by a rise in liking as concentration increased, an inverted U-shaped phenotype with maximum liking at 0.25 M, and a sweet-disliker (SD) phenotype characterized by a decline in liking as a function of concentration. Based on sensitivity and specificity analyses, present data suggest the clearest discrimination between phenotypes is seen with 1.0 M sucrose, where a liking rating between −15 and +15 on a −50/+50 scale reliably distinguished individuals with an inverted U-shaped response from the SLs and the SDs. If the efficacy of this approach is confirmed in other populations, the discrimination criteria identified here can serve as the basis for a standard method for classifying sweet taste liker phenotypes in adults.

https://www.mdpi.com/2072-6643/11/1/129/pdf/1

Quantifying Sweet Taste Liker Phenotypes: Time for Some Consistency in the Classification Criteria

Whole saliva is a mixture composed by the secretions of the major and minor salivary glands and the crevicular fluid, bacteria, cells and food debris. Its properties (flow and composition) are highly intra- and inter-individually dependent and reflect the health status of individuals. Saliva plays a key role in the eating process and on the perception of flavour. Flavour corresponds to the combined effect of taste sensations, aromatics and chemical feeling factors evoked by food in the oral cavity. It is a key determinant of food consumption and intake. This review summarises the evidence about the role of saliva in flavour perception and its potential contribution to food intake. All in all, evidence on the relationships between salivary parameters and both food perception and feeding behaviour is presented. This review emphasises that new studies accounting for the effect of salivary constituents on flavour alterations due to diseases (i.e. cancer, obesity and diabetes) are lacking and are expected in the incoming years.

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Main effects of human saliva on flavour perception and the potential contribution to food consumption.

The objective of this work was to investigate whether the biological film lining the tongue may play a role in taste perception. For that purpose, the tongue film and saliva of 21 healthy subjects were characterized, focusing on microorganisms and their main metabolic substrates and products. In parallel, taste sensitivity was evaluated using a test recently developed by our group, and the links between biological and sensory data were explored by a correlative approach. Saliva and tongue film differed significantly in biochemical composition (proportions of glucose, fructose, sucrose, and lactic, butyric, and acetic acids) and in microbiological profiles: compared to saliva, tongue film was characterized by significantly lower proportions of Bacteroidetes (p<0.001) and its main genus Prevotella (p<0.01) and significantly higher proportions of Firmicutes (p<0.01), Actinobacteria (p<0.001), and the genus Streptococcus (p<0.05). Generic taste sensitivity was linked to biological variables in the two compartments, but variables that appeared influent in saliva (flow, organic acids, proportion of Actinobacteria and Firmicutes) and in tongue film (sugars and proportions of Bacteroidetes) were not the same. This study points to two interesting areas in taste research: the oral microbiome and the specific characterization of the film lining the tongue.

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The Associations between Biochemical and Microbiological Variables and Taste Differ in Whole Saliva and in the Film Lining the Tongue.

Taste is influenced by several factors. However, whether habitual exercise level is associated with differences in taste perception has received little investigation. The aim of this study was to determine if habitual exercise is associated with differences in taste perception in men. Active (n = 16) and inactive (n = 14) males, between ages 18–55, underwent two days of sensory testing, using prototypical taste stimuli of high and low concentrations for sweet, salt, bitter, sour, umami, and carbohydrate (maltodextrin). Mean perceived intensity and hedonic ratings were recorded. Eating behaviour was assessed by the three factor eating questionnaire and food intake by EPIC food frequency questionnaire (FFQ). There were moderate to large differences between the two groups in perceived intensity for sweet taste at the high concentration and umami taste at both high and low concentrations, with active males recording a higher perceived intensity (p < 0.05 for all). The active group also recorded a greater dislike for umami low and carbohydrate low concentration (p < 0.01). Salt, bitter and sour perception did not significantly differ between the two groups. FFQ analysis showed no difference in % energy from macronutrients between the groups. Eating behaviour traits correlated with sweet taste intensity and umami taste liking, independent of activity status. Results indicated that sweet and umami taste perception differ in active compared to inactive males. Habitual exercise level should be considered in taste perception research and in product development. Whether differences in taste perception could be one factor influencing food intake and thus energy balance with habitual exercise warrants further investigation.

https://www.mdpi.com/2072-6643/11/1/155/pdf/1

Sweet and Umami Taste Perception Differs with Habitual Exercise in Males.

Sweet taste plays a critical role in determining food preferences and choices. Similar to what happens for other oral sensations, individuals differ in their sensitivity for sweet taste and these inter-individual differences may be responsible for variations in food acceptance. Despite evidence that saliva plays a role in taste perception, this fluid has been mainly studied in the context of bitterness or astringency. We investigated the possible relationship between sweet taste sensitivity and salivary composition in subjects with different sucrose detection thresholds. Saliva collected from 159 young adults was evaluated for pH, total protein concentration and glucose. One- and bi-dimensional electrophoresis (2-DE) were performed and protein profiles compared between sweet sensitivity groups, with proteins that were differently expressed being identified by MALDI-FTICR-MS. Moreover, Western blotting was performed for salivary carbonic anhydrase VI (CA-VI) and cystatins and salivary amylase enzymatic activity was assessed in order to compare groups. Females with low sensitivity to sweet taste had higher salivary concentrations of glucose compared to those with sensitivity. For protein profiles, some differences were sex-dependent, with higher levels of α-amylase and CA-VI in low-sensitivity individuals and higher levels of cystatins in sensitive ones for both sexes. Body mass index was not observed to affect the association between salivary proteome and taste sensitivity. To our knowledge, these are the first data showing an association between sweet taste and saliva proteome.

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Salivary proteome and glucose levels are related with sweet taste sensitivity in young adults.

6-n-propylthiouracil (PROP) is widely used in nutritional and health studies. However, only a limited number of studies have compared saliva composition between PROP taster phenotypes. The main aim of this work was to assess the relationship between saliva proteome and PROP responsiveness in young adults (N = 152) taking into account body mass index (BMI) and sex. SDS-PAGE and 2-DE protein profiles of saliva collected before and after PROP stimulation were compared between super-tasters (ST) and non-tasters (NT) and proteins of interest identified by mass spectrometry. These groups were also compared for their salivary levels of type-S cystatins and carbonic anhydrase VI (CA-VI). The relationship between salivary proteome and PROP responsiveness was affected by sex and BMI. Even so, immunoglobulin chains increased and prolactin-inducible protein and type-S cystatins decreased in ST group, both in men and women. Only when the subject's weight was considered in the analysis was it possible to observe differences between ST and NT in the levels of type-S cystatins, suggesting that BMI has an effect on the relationship between these salivary proteins and bitter taste response. PROP stimulation induced changes in the amounts of salivary amylase, cystatins, and CA-VI, which differed between ST and NT and may be important in differentiating PROP responsiveness groups. Among the proteins identified, most of the isoforms associated with PROP responsiveness had low pI.

Practical applications
Inter-individual differences in 6-n-propylthiouracil (PROP) responsiveness correspond to the most studied variations in bitterness perception and have been accepted as a general marker of oral chemosensory sensitivity. Differences in PROP phenotype appears to be related to food preferences and dietary behavior. The mechanisms involved in inter-individual variation in bitter taste perception are complex and are not completely explained by genetics. Identifying salivary constituents potentially involved in PROP responsiveness can increase the understanding of taste function and consequently food acceptance. Moreover, it is important to know whether factors such as weight status and/or sex affect the way in which saliva influences taste perception.

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C. C. Pinheiro

Papers

Salivary proteome and glucose levels are related with sweet taste sensitivity in young adults.

Relationship between saliva protein composition and 6‐n‐Propylthiouracil bitter taste responsiveness in young adults