Indices of individual variation in taste responsiveness

A complexity of factors influences food choices and behaviours. Among these factors, the individual variability in oral responsiveness (the diversity among individuals in perceived intensity of oral sensations) plays an important role. The great difference among individuals in responsiveness to tastes and somatosensory sensations is partly due to physiological variations in chemoreceptor systems. In the present thesis, we refer to oral responsiveness, intended as the perceived intensity of the fundamental tastes and burning from capsaicin. Taste responsiveness is associated to food preferences and diet. Since, via reduced/increased sensibility, taste responsiveness modulates our response to food preferences and ultimately diet, the understanding of individual variability can determinately contribute to explaining food behaviours. Indices exist to estimate taste responsiveness. Among these indices, the Fungiform Papillae Density (FPD = papillae/cm2) and the responsiveness to 6-n-propylthiouracil (PROP) are among the most studied. The Fungiform Papillae (FP) are anatomical structures designated to the oral stimuli detection and transduction, due to their innervation with the chorda thympani nerve (taste) and trigeminal nerve (somatosensory). PROP status (being or not being responsive to bitterness of PROP, and to which degree; henceforth abbreviated PST) is correlated to responsiveness to a high number of compounds naturally found in foods responsible for a variety of taste sensations. Therefore, PST is considered a general marker for taste responsiveness. Uncertainty has recently emerged from literature on whether or not FPD can be considered a reliable indicator for taste responsiveness. While early studies found that subjects with higher number of FP had higher tactile acuity and increased responsiveness to tastes, recent large-scale studies failed to confirm the positive relationship concerning tastes. The disagreement on the relationship between FP and taste responsiveness may originate from 1. Individual variability in papillae functionality, 2. Characteristics of the population considered, and 3. Methodological issues related to the approaches used to estimate FPD and sensory response. A particularly critical source of variability is the type of stimuli used. So far, the role of FP in taste responsiveness has not been systematically studied both in standard solutions and complex food matrices. Instead, responsiveness to PROP seems a solid indicator of taste responsiveness. Also, the relationship between these two common indices is still very controversial. Finally, while for PROP status determination several approaches have been optimised, in the FP determination some critical methodological aspects must be still approached. Based on these critical issues, the aim of the present thesis was to study the role of lingual fungiform papillae in respect to tastes and PROP responsiveness. The specific objectives were: I. to explore the relationship between FPD and perceived intensity of tastes and burning from capsaicin, evaluated in water solutions and in complex food matrices; II. to investigate the relationship between FPD and PROP responsiveness; III. to improve methodological tools to measure the fungiform papillae on the tongue. In the present thesis 408 subjects (38% males, aged from 18 to 65 years) were involved, recruited in two Italian cities. The effects of FPD and PST were separately tested on the perceived intensity of tastes and burning from capsaicin, both in water solutions and in complex food matrices (Study I). The nature of the relationship between FPD and PROP was explored considering PROP responsiveness estimated by whole mouth stimulation (One-solution test) (Study II). A manual count of papillae may obscure the relationship between FPD and PROP, so to further eradicate any bias a new approach for the automated quantification of FP on the human tongue was proposed (Piochi et al., 2017) (study III). Key conclusions of the thesis are that 1. FPD variation does not affect taste responsiveness in water solutions and food stimuli; 2. PROP phenotype is confirmed to be a reliable predictor of taste responsiveness, with super-tasters individual perceiving heightened intensity both in solution and food matrices; 3. FPD and PROP phenotypes do not show a straight significant association. Several factors may account for this. These factors mainly related to the FP functionality: the simple detection of the number of FP is not informative of FP functionality (such as the taste pore density - relevant for both taste sensations and PROP – or the presence of damaged nerves), and to the combination of genetic factors (some of which are still unknown), which may additionally complicate this relationship. 4. Advancement in FPD estimation is proposed by developing a new automated FP counting methodology that overcomes noise related to manual count. The method opens interesting scenarios in studying how the variation in fungiform papillae shape and dimension (diameter size) effect their functionality.