MODULATION OF TASTE PERCEPTION AS A POSSIBLE STRATEGY IN OBESITY TREATMENT: NEW PERSPECTIVES FOR PRECISION NUTRITION

Taste and gustatory perception, defined as the ability to perceive and recognize a given taste stimulus, play a guiding role in an individual's food choices and food intake. Physiologically, the ability to taste is linked to the presence of receptors and channels on the tongue, the main organ responsible for this function. These receptors and channels bind certain molecules that stimulate basic tastes: sweet, salty, bitter sour and umami. Taste also is influenced by several factors such as age, genetic aspects, environment and pathologies. Obesity is a constantly growing multifactorial disease, due to a positive balance between caloric intake and total energy expenditure. Often, also related to metabolic disorders, obesity is also linked to behavioural dysfunctions such as binge eating disorder, food addiction, and bulimia. Data from the literature support the concept that individuals with obesity are characterized by a depletion of taste cells and thus a reduced gustatory capacity of taste stimuli. To date, management strategies for obesity are diverse and include non-pharmacological treatments such as dietary therapy, also combined with drug therapy, and bariatric surgery as first-line treatments. However, it is becoming increasingly important to find new management strategies, especially from a nutritional perspective. These strategies can improve compliance with treatment. The aim of this doctoral dissertation was to investigate the relationship between taste perception and obesity-related factors such as eating disorders and chronotype. In addition, we wanted to investigate the impact of different dietary treatments such as the Mediterranean diet and the very low calorie ketogenic diet (VLCKD) on gustatory perception of subjects with severe obesity with the goal of being able to increase management strategies for this condition. The studies and data presented in this thesis were collected and carried out in the experimental setting of the Piancavallo Hospital of the Istituto Auxologico Italiano. Piancavallo Hospital is a residential setting for individuals with severe obesity where in a multidisciplinary team a metabolic nutritional rehabilitation is set up. The method used for the assessment of taste thresholds is a gold standard method called three-alternative forced choice task (3-AFC), which is also standardized by ISO regulation. The first study aims to investigate the relationship between taste thresholds and eating behaviour disorder (ED) in the subject with severe obesity and to assess some attitudes toward food that guide food choices. We found a different response to gustatory stimuli in subjects without an eating disorder than in the presence of one or more eating disorders. Specifically, subjects with obesity and co-presence of binge eating and food addiction have higher taste thresholds (thus lower sensitivity) for salty taste than subjects without ED. In addition, subjects with ED co-presence appear to have higher emotional and external eating scores than subjects with obesity without ED. The second study presents preliminary data regarding the possible relationship between taste thresholds and chronotype in a small sample size. The assessment of chronotype was carried out through a validated questionnaire (Reduced Morningness-Eveningness Questionnaire -rMEQ). What emerges is that serotype (E-type) subjects possess higher significant alteration of taste thresholds for salty than morning types (M-type). Relationship not emerged for sweet taste. This lower sensitivity to salty taste could be a factor for the higher incidence of eating disorder and body mass index in serotype subjects compared to M-types. Other two studies aimed to evaluate the impact of two different dietary treatments such as the Mediterranean diet and Very Low Calories Ketogenic Diet (VLCKD) on the taste thresholds of subjects with obesity. Sweet taste and salty taste were always evaluated. Specifically, for the Mediterranean diet, it was found that after 4 weeks of low-calorie, moderately low-sodium dietary treatment, it was possible to detect a significant improvement in the perceptual thresholds of salty. No significant change was evaluated for sweet. The last part presents preliminary data on the possible effect of VLCKD on sweet and salty taste thresholds compared to a control group on a low-calorie Mediterranean diet. No statistically significant difference was found in either sweet or salty taste thresholds after 4 weeks of VLCKD, which was conducted entirely with formulated meals. Instead, previously published data on the beneficial effects of the Mediterranean diet on salt thresholds were confirmed. The literature is sparse on the relationship between eating disorders and taste sensitivity, but our results match those of others. The data reported on dietary treatment are also in line with those in the literature for salty taste but differ for sweet taste. This difference may be due to the shorter duration of treatment in our studies than in the literature findings. Collectively, these findings support the idea that taste thresholds may be a central element in the selection of a dietary strategy for the nutritional rehabilitation of an individual with obesity, with the goal of talored treatment. The improvement of taste thresholds should also be considered as one of the rehabilitative therapeutic goals that can improve compliance with dietary treatment and the desired weight loss.