The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Short-term consumption of highly processed diets varying in macronutrient content impair the sense of smell and brain metabolism in mice

OBJECTIVE

Food processing greatly contributed to increased food safety, diversity, and accessibility. However, the prevalence of highly palatable and highly processed food in our modern diet has exacerbated obesity rates and contributed to a global health crisis. While accumulating evidence suggests that chronic consumption of such foods is detrimental to sensory and neural physiology, it is unclear whether its short-term intake has adverse effects. Here, we assessed how short-term consumption (<2 months) of three diets varying in composition and macronutrient content influence olfaction and brain metabolism in mice.

METHODS

The diets tested included a grain-based standard chow diet (CHOW; 54% carbohydrate, 32% protein, 14% fat; #8604 Teklad Rodent diet , Envigo Inc.), a highly processed control diet (hpCTR; 70% carbohydrate, 20% protein, 10% fat; #D12450B, Research Diets Inc.), and a highly processed high-fat diet (hpHFD; 20% carbohydrate, 20% protein, 60% fat; #D12492, Research Diets Inc.). We performed behavioral and metabolic phenotyping, electro-olfactogram (EOG) recordings, brain glucose metabolism imaging, and mitochondrial respirometry in different brain regions. We also performed RNA-sequencing (RNA-seq) in the nose and across several brain regions, and conducted differential expression analysis, gene ontology, and network analysis.

RESULTS

We show that short-term consumption of the two highly processed diets, but not the grain-based diet, regardless of macronutrient content, adversely affects odor-guided behaviors, physiological responses to odorants, transcriptional profiles in the olfactory mucosa and brain regions, and brain glucose metabolism and mitochondrial respiration.

CONCLUSIONS

Even short periods of highly processed food consumption are sufficient to cause early olfactory and brain abnormalities, which has the potential to alter food choices and influence the risk of developing metabolic disease.

The Role of the Olfactory System in Obesity and Metabolism in Humans: A Systematic Review and Meta-Analysis

Obesity, linked to chronic diseases, poses a global health challenge. While the role of the olfactory system in energy homeostasis is well-documented in rodents, its role in metabolism regulation and obesity in humans remains understudied. This review examines the interplay between olfactory function and metabolic alterations in human obesity and the effects of bariatric surgery on olfactory capabilities in humans. Adhering to PRISMA guidelines, a systematic review and meta-analysis was conducted, focusing exclusively on original human studies. From 51 articles, 14 were selected for the meta-analysis. It was found that variations in olfactory receptor genes influence the susceptibility to odors and predisposition to weight gain and poor eating habits. Bariatric surgery, particularly sleeve gastrectomy, shows significant improvements in olfactory function (SMD 2.37, 95% CI [0.96, 3.77], I = 92%, p = 0.001), especially regarding the olfactory threshold (SMD -1.65, 95% CI [-3.03, -0.27], I = 81%, p = 0.02). There is a bidirectional relationship between olfactory function and metabolism in humans. Bariatric surgery improves olfactory perception in obese patients, but it is still unclear if impacting the olfactory system directly affects eating behavior and the energy balance. However, these findings open novel avenues for future studies addressing the olfactory system as a novel target to alter systemic metabolism in humans.

Smell, taste and food habits changes along body mass index increase: an observational study

PURPOSE

To evaluate the changes in gustatory and olfactory sensitivity and dietary habits between healthy lean subjects (LS) and participants affected by overweight (OW), stage I and II obesity and to estimate possible impact of these factors on body mass index (BMI).

METHODS

After a general and ear-nose-throat evaluation, taste and olfactory function testing by means of taste strips and sniffin' stick tests, respectively, and food habits analysis by means of food frequency questionnaire (FFQ), 221 participants (68 LS [33 female; mean age = 53.01 ± 7.54 years]; 51 OW [26 female; mean age = 51.5 ± 12.16 years]; 50 stage I obesity [24 female; mean age = 50.78 ± 13.71 years] and 52 stage II obesity [24 female; mean age = 52.21 ± 13.35 years]) were enrolled in the study.

RESULTS

Significant (p < 0.008) reductions in total and subtest taste and smell scores were found in stage I and II obesity when compared to LS and OW participants. FFQ depicted a progressive intake increase of nutrients along the BMI stages. Significant associations were found between BMI and taste/smell subtests sugar taste carbs, saturated, monounsaturated and polyunsaturated fatty acids.

CONCLUSIONS

These data demonstrated for the first time a parallel impairment in smell and taste in a large sample size of participants from lean to stage II obesity and could reinforce those previous theories claiming that the greater the ability in taste or smell qualities perception, the lower the preference for them, resulting in a lower intake of specific foods.

Diet and odor hedonic ratings: comparative study between vegetarians, flexitarians, and omnivores

OBJECTIVES

Odor hedonic perception is well known to exhibit great variability and to depend on several parameters, i.e. stimulus, context, and subject characteristics. As hedonic perception (pleasant/unpleasant character) of food odors is considered one of the most prominent dimensions in eating behavior, the question of hedonic variability in this context arises. Thus, the aim of the present study was to compare odor hedonic ratings in three populations with regard to diet (i.e. omnivore, vegetarian, and flexitarian diets).

METHODS

Four categories of odors were compared: meat, vegetable, other food, and non-food odors.

RESULTS

The results showed that vegetarian and flexitarian individuals rated meat odors as more unpleasant than omnivores, while no significant difference was found for other categories of odors.

DISCUSSION

The question of whether the diet influences the hedonic perception or/and inversely is discussed, regarding several aspects of food consumption such as eating disorders, food education, … and could further serve to manage eating behaviors.

PRACTICAL APPLICATIONS

This study evidenced that vegetarians and flexitarians specifically rated meat odors as being more unpleasant than those of omnivores. Because of the growing number of vegetarians and flexitarians in the general population, it could be suggested to take into account the odor hedonic perception (especially regarding food odors) in studies related to diets. Besides, the present results could further serve research in several aspects of food consumption such as eating disorders (anorexia, bulimia … etc.) or food education as well as the management of eating behaviors, especially in an elderly population.

Orthonasal olfactory influences on consumer food behaviour

It is often suggested in the popular press that food chains deliberately introduce enticing product aromas into (and in the immediate vicinity of) their premises in order to attract customers. However, despite the widespread use of odours in the field of sensory marketing, laboratory research suggests that their effectiveness in modulating people's food behaviours depends on a range of contextual factors. Given the evidence that has been published to date, only under a subset of conditions is there likely to be a measurable effect of the presence of ambient odours on people's food attitudes and choices. This narrative historical review summarizes the various ways in which food odours appear to bias people's food preferences (appetite) and food choices (food consumption and purchase). Emphasis is placed on those experimental studies that have been designed to investigate how the characteristics of the olfactory stimuli (e.g., the congruency between the olfactory cues and the foods, intensity and duration of exposure to odours, and taste properties of odours) modulate the effects of olfactory cues on food behaviour. The review also explores the moderating roles of individual differences, such as dietary restraint, Body Mass Index (BMI), genetic and cultural differences in odour sensitivity and perception. Ultimately, following a review of empirical studies on food-related olfaction, current approaches in scent marketing are discussed and a research agenda is proposed to help encourage further studies on the effective application of scents in promoting healthy foods.

Odour-imagery ability is linked to food craving, intake, and adiposity change in humans

It is well-known that food-cue reactivity (FCR) is positively associated with body mass index (BMI)1 and weight change2, but the mechanisms underlying these relationships are incompletely understood. One prominent theory of craving posits that the elaboration of a desired substance through sensory imagery intensifies cravings, thereby promoting consumption3. Olfaction is integral to food perception, yet the ability to imagine odours varies widely4. Here we test in a basic observational study whether this large variation in olfactory imagery drives FCR strength to promote adiposity in 45 adults (23 male). We define odour-imagery ability as the extent to which imagining an odour interferes with the detection of a weak incongruent odour (the 'interference effect'5). As predicted in our preregistration, the interference effect correlates with the neural decoding of imagined, but not real, odours. These perceptual and neural measures of odour imagery are in turn associated with FCR, defined by the rated craving intensity of liked foods and cue-potentiated intake. Finally, odour imagery exerts positive indirect effects on changes in BMI and body-fat percentage over one year via its influences on FCR. These findings establish odour imagery as a driver of FCR that in turn confers risk for weight gain.

Remote psychophysical testing of smell in patients with persistent olfactory dysfunction after COVID-19

Olfactory dysfunction associated with coronavirus 2 (SARS-CoV-2) infection is in most cases transient, recovering spontaneously within a few days. However, in some patients it persists for a long time, affects their everyday life and endangers their health. Hence, we focused on patients with persistent loss of smell. The aim of this study was to evaluate olfactory dysfunction using a standardized test. Due to the pandemic, olfactory testing was performed online. Smell tests (Odorized Markers Test, OMT) were sent home to the patients. Together with the smell self-testing, participants reported and assessed several parameters (age, sex, subjective assessment of smell and taste, nasal patency, etc.) in an online questionnaire. Based on the questionnaire outcomes, the results were sent to the patients along with a list of participating otolaryngologists who provided them with professional care. From March to June 2021, 1025 patients requested smell testing, of these, 824 met the inclusion criteria of this study. The duration of the olfactory dysfunction at the time of testing ranged from 1 month to 1 year. Using the OMT, impaired smell ability-anosmia or hyposmia-was confirmed in 82.6% of participants. A total of 17.4% of participants were determined to be normosmic however, more than 50% of them complained of parosmia and/or phantosmia. Our study demonstrates the relevance of psychophysical smell testing and its suitability for remote use during the pandemic. This study also revealed several correlations between prolonged olfactory dysfunction and the monitored parameters.

Effects of weight change on taste function; a systematic review

BACKGROUND

The aim of this review is to evaluate the relationship between weight status and taste perception and preference of sweet, salt, fat, bitter, and sour through reviewing observational and interventional studies with objective methods.

METHODS

A comprehensive literature search was performed in 6 online databases of PubMed, Scopus, Web of Science, Cochrane, Embase, and Google Scholar up to October 2021. The following keywords were used in the search strategy: (Taste OR "Taste Perception" OR "Taste Threshold" OR "Taste preference" OR "Taste sensitivity" OR "Taste changes") AND (weight OR "Weight gain" OR "weight loss" OR "weight change").

RESULTS

Most observational studies indicate that four taste sensitivities or perceptions (especially sweet and salt taste perception) are lower in subjects with overweight and obesity. The longitudinal studies reported that sweet and fat preference is increased along with weight gain in adults. It is concluded that taste perceptions are decreased in individuals with overweight and obesity, especially in men. Also, taste perception and preference change after weight loss but not significantly.

CONCLUSION

It is suggested that the results of the interventional studies are not conclusive and need further studies with the same and standard design adjusting cofounding variables including genetic, gender, age and food condition of subjects.

Odor imagery but not perception drives risk for food cue reactivity and increased adiposity

Mental imagery has been proposed to play a critical role in the amplification of cravings. Here we tested whether olfactory imagery drives food cue reactivity strength to promote adiposity in 45 healthy individuals. We measured odor perception, odor imagery ability, and food cue reactivity using self-report, perceptual testing, and neuroimaging. Adiposity was assessed at baseline and one year later. Brain responses to real and imagined odors were analyzed with univariate and multivariate decoding methods to identify pattern-based olfactory codes. We found that the accuracy of decoding imagined, but not real, odor quality correlated with a perceptual measure of odor imagery ability and with greater adiposity changes. This latter relationship was mediated by cue-potentiated craving and intake. Collectively, these findings establish odor imagery ability as a risk factor for weight gain and more specifically as a mechanism by which exposure to food cues promotes craving and overeating.

Taste but not smell sensitivities are linked to dietary macronutrient composition

Macronutrient intake and composition of diets (i.e., carbohydrate, protein, and fat) can vary substantially across individuals. Chemosensory functions are hypothesised to play a key role in modulating nutrient choices and intake. The present study tests links between individual gustatory or olfactory supra-threshold sensitivities and dietary macronutrient intake. A total of 98 European males (N = 81 for final analyses; age: 20-40 yo; BMI: 18.9-48.1 kg∙m^-2) were tested for supra-threshold sensitivities (d') to 3-gustatory (i.e., Sucrose, MSG, Dairy fat) and 3-olfactory stimuli (i.e., Vanillin, Methional, Maltol/Furaneol), followed by a 4-day weighed Food Record to give measures of macronutrient intake (kJ) and composition (%). With multivariate analyses (i.e., K-mean clustering, PCA, and Hierarchical Regression), gustatory and olfactory d' were compared across groups of individuals with distinct macronutrient composition or intake. Significant differences in gustatory d' were found across the clusters based on macronutrient composition (p < 0.05), but not for clusters based on intake. Hierarchical regressions suggested that gustatory d' played a significant role in predicting dietary carbohydrate composition and intake, with one-unit d' increase predicting reduction of 3%-4.66% (R² = 0.21, F_(5,75) = 5.38, p = 0.001). Moreover, every one-unit increase in d' to MSG increased protein composition by 3.45% (R² = 0.10, F_(5,75) = 2.83, p = 0.022) and intake by 392 kJ (R² = 0.08, F_(5,75) = 2.41, p = 0.044). By contrast, olfactory d' showed little association to macronutrient composition or intake (p > 0.05). Overall, we present intriguing new evidence that gustatory, but not olfactory, sensitivities are linked to dietary macronutrient composition, with relatively little effect on actual intake. These findings highlight possible action of a sensory-mediated mechanism guiding food choices.

Olfactory decoding is positively associated with ad libitum food intake in sated humans

The role of olfaction in eating behavior and body weight regulation is controversial. Here we reanalyzed data from a previous functional magnetic resonance imaging study to test whether central olfactory coding is associated with hunger/satiety state, food intake, and change in body weight over one year in healthy human adults. Since odor quality and category are coded across distributed neural patterns that are not discernible with traditional univariate analyses, we used multi-voxel pattern analyses to decode patterns of brain activation to food versus nonfood odors. We found that decoding accuracies in the piriform cortex and amygdala were greater in the sated compared to hungry state. Sated decoding accuracies in these and other regions were also associated with post-scan ad libitum food intake, but not with weight change. These findings demonstrate that the fidelity of olfactory decoding is influenced by meal consumption and is associated with immediate food intake, but not longer-term body weight regulation.

Smell Impairment in Stage I-II Obesity: Correlation with Biochemical Regulators and Clinical Aspects

OBJECTIVE

To evaluate the differences in olfactory sensitivity, nutritional habits, levels of modulators of feeding and smell, bioelectrical impedance analysis (BIA) measures and metabolic assays between two groups of participants with stage I and II obesity and reciprocal relationships between these parameters.

METHODS

Eighteen participants with stage I (11 female; mean age = 54.3 ± 13.1 years) and 20 participants with stage II (10 female; mean age = 54.5 ± 11.9) obesity underwent a food frequency questionnaire and Sniffin' Sticks® test battery, anthropometric parameters, and BIA measurements as well as metabolic assays (including plasma levels of leptin, insulin, ghrelin, glucose, insulin-like growth factor-1 [IGF-1] and usual laboratory parameters).

RESULTS

The stage II obesity participants demonstrated significant higher levels of insulin and leptin and lower levels of ghrelin and IGF-1, a reduction in odor identification (OI) and in total olfactory score, and an increase in visceral and total fat percentage. Among a mosaic of multiple correlations, ghrelin was found to positively correlate with OI and leptin negatively with odor discrimination.

CONCLUSION

The present study expands the notions positing the olfactory perception - and its connections with metabolic cues, foods habits and BIA measures - changes across the two most important obesity stages. This could ameliorate clinical and research deepening of obesity-related olfactory behavior with possible consequences on diagnosis, treatment and prevention of onset and development of obesity, thus opening possible future strategies involving multidisciplinary contributions.

LEVEL OF EVIDENCE

Level 3 Laryngoscope, 2022.

Olfactory system and energy metabolism: a two-way street

Olfactory perception guides daily decisions regarding food consumption, social interactions, and predator avoidance in all mammalian species. Volatile inputs, comprising odorants and pheromones, are relayed to the olfactory bulb (OB) from nasal sensory neurons cells and transferred to secondary processing regions within the brain. Olfaction has recently been shown to shape homeostatic and maladaptive processes of energy intake and expenditure through neuronal circuits involving the medial basal hypothalamus. Reciprocally, gastrointestinal hormones, such as ghrelin and leptin, the secretion of which depends on satiety and adiposity levels, might also influence olfactory sensitivity to alter food-seeking behaviors. Here, in addition to reviewing recent updates on identifying these neuronal networks, we also discuss how bidirectional neurocircuits existing between olfactory and energy processing centers can become dysregulated during obesity.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Relationship between Olfactory Function and BMI in Normal Weight Healthy Subjects and Patients with Overweight or Obesity

Smell plays a critical role in food choice and intake by influencing energy balance and body weight. Malnutrition problems or modified eating behaviors have been associated with olfactory impairment or loss. The obesity epidemic is a serious health problem associated with an increased risk of mortality and major physical comorbidities. The etiopathogenesis of obesity is complex and multifactorial, and one of the main factors contributing to the rapid increase in its incidence is the environment in which we live, which encourages the overconsumption of foods rich in energy, such as saturated fats and sugars. By means of the “Sniffin’ Sticks” test, we measured the olfactory threshold, discrimination and identification score (TDI score) in patients of the Obesity Center of the University Hospital (OC; n = 70) and we compared them with that of healthy normal weight controls (HC; n = 65). OC patients demonstrated a significantly lower olfactory function than HC subjects both general and specific for the ability to discriminate and identify odors, even when they were considered separately as females and males. For OC patients, a negative correlation was found between body mass index (BMI) and olfactory scores obtained by each subject, both when they were divided according to gender and when they were considered all together. Besides, normosmic OC patients showed a significantly lower BMI than hyposmic ones. A reduced sense of smell may contribute to obesity involving the responses of the cephalic phase, with a delay in the achievement of satiety and an excessive intake of high-energy foods and drinks.

Olfactory and Gustatory Supra-Threshold Sensitivities Are Linked to Ad Libitum Snack Choice

Snacking is a common eating habit in the modern food environment. Individual snack choices vary substantially, with sweet versus savoury snacks linked to differential health outcomes. The role of olfactory and gustatory sensitivities in snack choices and consumption is yet to be tested. A total of 70 Caucasian young males (age: 21−39 years; BMI: 20.5−40.5 kg∙m−2) were tested for their supra-threshold sensitivities to sweet and savoury associated odours and tastants (vanillin, methional; sucrose, NaCl). The participants also attended an ad libitum task in which their intakes of sweet and savoury snacks were recorded and analysed. Univariate and multivariate analyses were used to test for relationships between odour/taste sensitivities and sweet versus savoury snack intake. Results indicated that individual sensitivities to sweet-associated stimuli (e.g., vanillin, sucrose) were negatively linked with intake of the congruent (e.g., sweet) snacks and positively linked with incongruent (e.g., savoury) snacks (p < 0.05). These differences were reflected by energy intake rather than consumption weight (p > 0.05). This study outlines the fundamental roles of olfactory and gustatory sensitivities in snack choices and offers novel insights into inter-individual variability in snack consumption.

Food Neophobia, Odor and Taste Sensitivity, and Overall Flavor Perception in Food

Smell, which allows us to gather information about the hedonic value of an odor, is affected by many factors. This study aimed to assess the relationship among individual factors, odor sensitivity, and enjoyment, and to evaluate how overall flavor perception and liking in actual food samples are affected by odor sensitivity. A total of 749 subjects, from four different Italian regions, participated in the study. The olfactory capabilities test on four odors (anise, banana, mint, and pine), as well as PROP (6-n-prpyl-2-thiouracil) status and food neophobia were assessed. The subjects were clustered into three groups of odor sensitivity, based on the perceived intensity of anise. The liking and intensity of the overall flavor were evaluated for four chocolate puddings with increasing sweetness (C1, C2, C3, and C4). The individual variables significantly affected the perceived intensity and liking of the odors. Even if all of the odor sensitivity groups perceived the more intensely flavored samples as the C1 and C4 chocolate puddings, the high-sensitivity group scored the global flavor of all of the samples as more intense than the low-sensitivity group. The low-sensitive subjects evaluated the liking of the sweeter samples with higher scores than the moderate-sensitive subjects, whereas the high-sensitive subjects gave intermediate scores. In conclusion, odor sensitivity plays a pivotal role in the perception and liking of real food products; this has to be taken into account in the formulation of new products, suitable for particular categories with reduced olfactory abilities.

Olfaction in the context of obesity and diabetes: Insights from animal models to humans

The olfactory system is at the crossroad between sensory processing and metabolic sensing. In addition to being the center of detection and identification of food odors, it is a sensor for most of the hormones and nutrients responsible for feeding behavior regulation. The consequences of modifications in body homeostasis, nutrient overload and alteration of this brain network in the pathological condition of food-induced obesity and type 2 diabetes are still not elucidated. The aim of this review was first to use both humans and animal studies to report on the current knowledge of the consequences of obesity and type 2 diabetes on odorant threshold and olfactory perception including identification discrimination and memory. We then discuss how olfactory processing can be modified by an alteration of the metabolic homeostasis of the organism and available elements on pharmacological treatments that regulate olfaction. We focus on data within the olfactory system but also on the interactions between the olfactory system and other brain networks impacted by metabolic diseases.

Behavioral and Metabolic Effects of a Calorie-Restricted Cafeteria Diet and Oleuropein Supplementation in Obese Male Rats

Diet-induced obesity models are widely used to investigate dietary interventions for treating obesity. This study was aimed to test whether a dietary intervention based on a calorie-restricted cafeteria diet (CAF-R) and a polyphenolic compound (Oleuropein, OLE) supplementation modified sucrose intake, preference, and taste reactivity in cafeteria diet (CAF)-induced obese rats. CAF diet consists of high-energy, highly palatable human foods. Male rats fed standard chow (STD) or CAF diet were compared with obese rats fed CAF-R diet, alone or supplemented with an olive tree leaves extract (25 mg/kg*day) containing a 20.1% of OLE (CAF-RO). Biometric, food consumption, and serum parameters were measured. CAF diet increased body weight, food and energy consumption and obesity-associated metabolic parameters. CAF-R and CAF-RO diets significantly attenuated body weight gain and BMI, diminished food and energy intake and improved biochemical parameters such as triacylglycerides and insulin resistance which did not differ between CAF-RO and STD groups. The three cafeteria groups diminished sucrose intake and preference compared to STD group. CAF-RO also diminished the hedonic responses for the high sucrose concentrations compared with the other groups. These results indicate that CAF-R diet may be an efficient strategy to restore obesity-associated alterations, whilst OLE supplementation seems to have an additional beneficial effect on sweet taste function.

Food olfactory cues reactivity in individuals with obesity and the contribution of alexithymia

Obesity has been associated with increased reward sensitivity to food stimuli, but a few studies have addressed this issue by using odors. This study investigated whether obesity is associated with increased liking and wanting of food odors and whether alexithymia, a psychological construct characterized by diminished affective abilities, contributes to altered responsiveness to food. Liking and wanting for food and pleasant non-food odors were measured through explicit (self-report ratings) and implicit measures (heart rate and skin conductance) in 23 women with healthy weight (HW) and 20 women with overweight/obesity (OW/OB). Differently from the HW group, the OW/OB group explicitly liked food odors less than non-food odors; but, at the implicit level, there were no differences in heart rate response for both types of odors, indicating that they were equally liked. Moreover, at variance with the HW group, the OW/OB group did not exhibit increased skin conductance response for food compared to nonfood odors. Alexithymia was associated with increased implicit liking and explicit wanting of food odors, in particular in the HW group. These findings show that obesity is characterized by high levels of implicit food liking and low levels of implicit food wanting. Moreover, both affective and motivational responses to food reward seem to be affected by alexithymia, which should be taken into account by studies evaluating the effect of cue exposure intervention for obesity treatment.

Brain response to food odors is not associated with body mass index and obesity-related metabolic health measures

Smell perception plays a role in eating behavior and might be involved in the development of obesity. In fact, olfactory function is impaired in obesity and might depend on metabolic health factors. To date, the underlying neural mechanisms remain unclear. Here, we investigate neural processing of food-related odors in normal-weight, overweight and obese individuals. Fifty-three young and healthy participants (28.8 ± 4.4 years, 27 female; 24 normal-weight, 10 overweight, and 19 obese) were presented with high- (chocolate, potato chips) and low-caloric (orange, cucumber) food odors during a functional magnetic resonance imaging (fMRI). We also assessed olfactory identification ability, body mass index (BMI), body fat percentage, insulin resistance, and leptin levels. In brief, olfactory perception of food odors was linked to brain activity in the entorhinal and piriform cortex, and the insula, hippocampus, and amygdala. Insulin resistance was negatively related to olfactory identification. Additionally, perception of sweet versus savory odors was related to a higher brain activity in the right middle/superior frontal gyrus. Finally, we found no effect of obesity status, BMI, metabolic factors, or body fat percentage on neural responses to food odors. Overall, this suggests that food odor processing might depend on factors other than body weight status or associated markers of metabolic health.

Odor-Taste-Texture Interactions as a Promising Strategy to Tackle Adolescent Overweight

The adolescence period is characterized by a considerable risk to weight gain due to the high consumption of food rich in sugar. A promising strategy to reduce sugar consumption may lie in exploiting the ability of our senses to interact to each other (cross-modal interactions). The aims were to investigate the cross-modal interactions and gustatory function in normal-weight and overweight adolescents. Fifty adolescents (25 overweight and 25 normal-weight) were involved. Subjects rated liking and attribute intensity in pudding samples obtained by adding vanilla aroma (0.1%; 0.3%), butter aroma (0.05%; 0.1%) or a thickener agent (1%; 1.5%) to a base formulation. The gustatory function was also measured through the “taste strips” methodology. Overweight adolescents were found to have a significantly (p < 0.001) worse ability to correctly identify all tastes. Cross-modal interactions occurred differently according to their body mass index, with a significant increase (p < 0.05) in sensory desirable characteristics (e.g., sweet and creaminess) due to aroma addition, especially in overweight subjects. Furthermore, butter aroma significantly increased hedonic responses only in overweight subjects. Tricking our senses in the way of perceiving sensory attributes could be a promising strategy to develop innovative food formulations with a reduced sugar amount, which will lead to a potential decrease in caloric intake and help to tackle the obesity epidemic.

The Influence of Common Noncommunicable Diseases on Chemosensory Perception and Clinical Implications in Children and Adolescents

An increased incidence of noninfectious chronic diseases, such as obesity, diabetes, and allergies, has been noted in the last century, especially in the last 2 to 3 generations. Evidence suggested that the interrelation among these chronic conditions in pediatric age (e.g., children and adolescents aged 4-16 y) is complex and still unknown, reinforcing the interest of pediatricians in these diseases. Of interest is the need to better understand the link between these pathologies and sensory perception, since the chemical senses of taste and smell, together with chemesthesis, are reported to have a role in food choices and may provide a novel target for intervention in the treatment of these pathologies. This review aims to explore the current evidence on the link between these chronic conditions and chemosensory perception (i.e., taste and smell). In addition, the putative role that chemosensory perception may have on food choices and eating behavior of children and adolescents affected by these diseases are highlighted. Furthermore, the review addresses the unexplored issues that need to be investigated in this area. The literature data search suggested that no clear relation between taste and smell perception and the aforementioned diseases in young population yet exists. However, some possible trends have been highlighted in the adult population, in whom the duration of disease might have affected the relation. There is a need for further, high-quality, hypothesis-led research, with robust measures of taste and smell functions as the primary outcomes, to strengthen or deny this evidence.

The role of an individual's olfactory discriminability in influencing snacking and habitual energy intake

Recent studies have revealed close links between human olfaction, appetite, and food choice. However, it remains unclear whether olfactory sensitivity plays a direct role in determining food and energy intake. The present study addresses this question by assessing relationships between individual olfactory discriminability (at a suprathreshold level), snacking, and habitual energy intake. A total of 92 healthy Caucasian males (mean age = 26.1, SD = 5.8) were tested for their olfactory discriminability (measured by d') to three food-related odorants (O1 - Vanillin, O2 - Methional, and O3 - Maltol/Furaneol mixture) with a 2-AFC method of constant stimuli. These sensory data were then analysed with two separate measures of food consumption - (1) snack energy intake within an ad libitum buffet setting; (2) habitual energy intake using four-day weighed food records. Univariate analyses of variance revealed significant results with regards to O1. Specifically, individuals with higher discriminability consumed significantly less energy from snacking as opposed to their less sensitive counterparts (p = 0.05). However, no significant relationship was found between individual olfactory discriminability and habitual energy intake. While recent years have seen increasing research focus on how external olfactory cues affect food consumption, our study offers particularly novel insights regarding the role of individual olfactory sensitivity in shaping eating behaviour.

Increased Brain Reward Responsivity to Food-Related Odors in Obesity

OBJECTIVE

Food odors serve as powerful stimuli signaling the food quality and energy density and direct food-specific appetite and consumption. This study explored obesity-related brain activation in response to odors related to high- or low-energy-dense foods.

METHODS

Seventeen participants with obesity (BMI > 30 kg/m² ; 4 males and 13 females) and twenty-one with normal weight (BMI < 25 kg/m² ; 9 males and 12 females) underwent a functional magnetic resonance imaging scan in which they received chocolate (high-energy-dense food) and cucumber (low-energy-dense food) odor stimuli. Participants' olfactory and gustatory functions were assessed by the "Sniffin' Sticks" and "Taste Strips" tests, respectively.

RESULTS

Compared with normal-weight controls, participants with obesity had lower odor sensitivity (phenylethyl alcohol) and decreased odor discrimination ability. However, participants with obesity demonstrated greater brain activation in response to chocolate compared with cucumber odors in the bilateral inferior frontal operculum and cerebellar vermis, right ventral anterior insula extending to putamen, right middle temporal gyrus, and right supramarginal areas.

CONCLUSIONS

The present study provides preliminary evidence that obesity is associated with heightened brain activation of the reward and flavor processing areas in response to chocolate versus cucumber odors, possibly because of the higher energy density and reinforcing value of chocolate compared with cucumber.

Regulation of Metabolic Health by an "Olfactory-Hypothalamic Axis" and Its Possible Implications for the Development of Therapeutic Approaches for Obesity and T2D

The olfactory system is responsible for the reception, integration and interpretation of odors. However, in the last years, it has been discovered that the olfactory perception of food can rapidly modulate the activity of hypothalamic neurons involved in the regulation of energy balance. Conversely, the hormonal signals derived from changes in the metabolic status of the body can also change the sensitivity of the olfactory system, suggesting that the bidirectional relationship established between the olfactory and the hypothalamic systems is key for the maintenance of metabolic homeostasis. In the first part of this review, we describe the possible mechanisms and anatomical pathways involved in the modulation of energy balance regulated by the olfactory system. Hence, we propose a model to explain its implication in the maintenance of the metabolic homeostasis of the organism. In the second part, we discuss how the olfactory system could be involved in the development of metabolic diseases such as obesity and type two diabetes and, finally, we propose the use of intranasal therapies aimed to regulate and improve the activity of the olfactory system that in turn will be able to control the neuronal activity of hypothalamic centers to prevent or ameliorate metabolic diseases.

Weight Status and Effects of Non-Tobacco Flavors on E-Cigarette Product Appeal

BACKGROUND

Flavors in tobacco products may be salient drivers of tobacco product use among people with overweight or obesity. Yet, whether perceived appeal of e-cigarettes with different flavors varies as a function of weight status is unknown. Purpose: To conduct secondary data analyses of a laboratory experiment to examine whether weight moderates differences in perceived appeal of fruit, menthol, and tobacco flavored e-cigarettes in young adults who vape. Methods: Using a within-subjects experimental design, young adults in Los Angeles, CA, USA. (Mage = 25.36 ± 4.42 Range: 18-35) with normal weight (n = 48) or overweight/obesity (n = 51) were administered standardized doses of e-cigarette solutions varying in flavor (fruit, menthol, tobacco). Following each administration, participants rated the appeal of the solution (range: 0-100). Results: The extent to which menthol (vs. tobacco)-flavored e-cigarettes were rated more appealing was amplified among participants with overweight or obesity versus normal weight participants (flavor × weight interaction Estimate = 7.54, p = .01 95% CI = [2.30, 12.80]). There were no differences in the effects of fruit (vs. tobacco) flavored e-cigarettes on appeal as a function of weight status. Conclusions: Menthol flavors in e-cigarettes may be disproportionately appealing to young adults with overweight and obesity.

Supplemental data for this article is available online at https://doi.org/10.1080/10826084.2021.1899229.

Radiological evidence to changes in the olfactory bulb volume depending on body mass index in the childhood

OBJECTIVE

Energy balance is preserved through the exchange between body weight and adipose tissue across the multi-faceted complex network that is composed of the sensorial, metabolic, and neuro-endocrine circuits. The olfactory control of energy homeostasis is maintained through the interplay between the olfactory bulb (OB) and adipose tissue. While extremely studied, most researches still report controversial results and sensorial regulation of obesity is not fully understood. This study aims to investigate the interplay between olfactory bulb volume (OBV) as a radiological clue of sensorial control and obesity in children.

SUBJECTS AND METHOD

Children (n = 195) were classified into four groups based on body mass index (BMI) percentiles: normal weight (n = 89), overweight (n = 31), obese (n = 32) and morbidly obese (n = 43). OBV were calculated using MRI.

RESULTS

Mean OBV was higher in children with obesity than in those of normal weights. The means of OBV are found higher in the overweight and obese children (43.76 ± 9.50-49.29 ± 8.61 mm³) than in those of morbidly obese (38.23 ± 11.52 mm³) (p < 0.001). In overweight and obese children, a positive correlation were found between the BMI and OBV (r_overweigh = 0.275-r_obese = 0.377), while in the morbidly obese group, there was a negative correlation (r_{severelyobese} = -0.445).

CONCLUSION

This study reveals that OBV is higher in obese children. Also, it shows that there is a positive correlation between OBV and BMI in overweight and obese children and a negative correlation in the morbidly obese group. These radiological bimodal changes in OBV indicate that olfactory control acts to provide energy balance, mediated by positive in the overweight and obese children, negative feedback in the morbidly obese group.

Reduced Olfactory Bulb Volume in Obesity and Its Relation to Metabolic Health Status

Smell perception plays an important role in eating behavior and might be involved in body weight gain. Since a body of literature implies that olfactory perception and function is hampered in obesity, we here investigate neuroanatomical correlates of this phenomenon. We assessed olfactory bulb (OB) volume with magnetic resonance imaging in 67 healthy participants with a body mass index (BMI) from 18.9 to 45.4 kg/m2 (mean = 28.58 ± 6.64). Moreover, we obtained psychophysiological data on olfactory ability (Sniffin' Sticks, Food associated odor test) and self-report measurements on eating behavior. Additionally, we collected parameters associated with metabolic health in obesity (waist-hip ratio, waist-height ratio, leptin levels, body fat percentage, fat mass index, insulin resistance) to investigate recently proposed mechanistic explanatory models of why olfaction may be altered in obesity. We showed that OB volume was significantly lower in participants with obesity when compared to those of normal weight. Moreover, we found weak to moderate negative correlations between OB volume and BMI and related measures of metabolic health, especially leptin, body fat percentage, waist-height ratio and insulin resistance. However, neither OB volume nor BMI were related to olfactory function in our young and healthy sample. Nevertheless, our results provide first indications that obesity is associated with brain anatomical changes in the OBs.

Western Diet Accelerates the Impairment of Odor-Related Learning and Olfactory Memory in the Mouse

Olfactory dysfunction could be an early indicator of cognitive decline in type 2 diabetes (T2D). However, whether obesity affects olfaction in people with T2D is unclear. This question needs to be addressed, because most people with T2D are obese. Importantly, whether different contributing factors leading to obesity (e.g., different components of diet or gain in weight) affect specific olfactory functions and underlying mechanisms is unknown. We examined whether two T2D-inducing obesogenic diets, one containing a high proportion of fat (HFD) and one with moderate fat and high sugar (Western diet, WD), affect odor detection/discrimination, odor-related learning, and olfactory memory in the mouse. We also investigated whether the diets impair adult neurogenesis, GABAergic interneurons, and neuroblasts in the olfactory system. Here, we further assessed olfactory cortex volume and cFos expression-based neuronal activity. The WD-fed mice showed declined odor-related learning and olfactory memory already after 3 months of diet intake (p = 0.046), although both diets induced similar hyperglycemia and weight gain compared to those of standard diet-fed mice (p = 0.0001 and p < 0.0001, respectively) at this time point. Eight months of HFD and WD diminished odor detection (p = 0.016 and p = 0.045, respectively), odor-related learning (p = 0.015 and p = 0.049, respectively), and olfactory memory. We observed no changes in the investigated cellular mechanisms. We show that the early deterioration of olfactory parameters related to learning and memory is associated with a high content of sugar in the diet rather than with hyperglycemia or weight gain. This finding could be exploited for understanding, and potentially preventing, cognitive decline/dementia in people with T2D. The mechanisms behind this finding remain to be elucidated.

Alteraciones del olfato en la obesidad

Aun cuando el sentido del olfato es fundamental para la percepción de los alimentos, su relación con la obesidad ha sido poco investigada. Este artículo es una revisión de la literatura sobre esa relación. La evidencia actual tanto en animales como en humanos apoya la hipótesis de que el olfato está alterado en esta condición. Estudios recientes señalan que los individuos obesos podrían tener una mayor sensibilidad a los olores de alimentos altos en calorías, y menor para los de bajo valor calórico incluidos los no asociados con alimentos. El estudio de la percepción quimiosensorial es un área prometedora para avanzar en el entendimiento de los mecanismos involucrados en la obesidad.

Consequences of Obesity on the Sense of Taste: Taste Buds as Treatment Targets?

Premature obesity-related mortality is caused by cardiovascular and pulmonary diseases, type 2 diabetes mellitus, physical disabilities, osteoarthritis, and certain types of cancer. Obesity is caused by a positive energy balance due to hyper-caloric nutrition, low physical activity, and energy expenditure. Overeating is partially driven by impaired homeostatic feedback of the peripheral energy status in obesity. However, food with its different qualities is a key driver for the reward driven hedonic feeding with tremendous consequences on calorie consumption. In addition to visual and olfactory cues, taste buds of the oral cavity process the earliest signals which affect the regulation of food intake, appetite and satiety. Therefore, taste buds may play a crucial role how food related signals are transmitted to the brain, particularly in priming the body for digestion during the cephalic phase. Indeed, obesity development is associated with a significant reduction in taste buds. Impaired taste bud sensitivity may play a causal role in the pathophysiology of obesity in children and adolescents. In addition, genetic variation in taste receptors has been linked to body weight regulation. This review discusses the importance of taste buds as contributing factors in the development of obesity and how obesity may affect the sense of taste, alterations in food preferences and eating behavior.

Insulin Resistance Is Associated with Reduced Food Odor Sensitivity across a Wide Range of Body Weights

The worldwide obesity epidemic is a major health problem driven by the modern food environment. Recently, it has been shown that smell perception plays a key role in eating behavior and is altered in obesity. However, the underlying mechanisms of this phenomenon are not well understood yet. Since the olfactory system is closely linked to the endocrine system, we hypothesized that hormonal shifts in obesity might explain this relationship. In a within-subject, repeated-measures design, we investigated sensitivity to a food and a non-food odor in the hungry and sated state in 75 young healthy (26 normal weight, 25 overweight, and 24 obese) participants (37 women). To determine metabolic health status and hormonal reactivity in response to food intake, we assessed pre- and postprandial levels of insulin, leptin, glucose, and ghrelin. Odor sensitivity did not directly depend on body weight status/body mass index (BMI) or hunger state. However, we could establish a strong negative mediating effect of insulin resistance on the relationship between BMI/waist-hip ratio and olfactory sensitivity for the food odor. These findings indicate an impact of metabolic health status on sensitivity to food odors. Our results contribute to a better understanding of the mechanisms behind altered smell perception in obesity.

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.

No evidence for an association between obesity and milkshake liking

BACKGROUND

Prevailing models of obesity posit that hedonic signals override homeostatic mechanisms to promote overeating in today's food environment. What researchers mean by "hedonic" varies considerably, but most frequently refers to an aggregate of appetitive events including incentive salience, motivation, reinforcement, and perceived pleasantness. Here we define hedonic as orosensory pleasure experienced during eating and set out to test whether there is a relationship between adiposity and the perceived pleasure of a palatable and energy-dense milkshake.

METHODS

The perceived liking, wanting, and intensity of two palatable and energy-dense milkshakes were assessed using the Labeled Hedonic Scale (1), visual analog scale (VAS), and Generalized Labeled Magnitude Scale (2) in 110 individuals ranging in body mass index (BMI) from 19.3 to 52.1 kg/m². Waist circumference, waist-hip ratio, and percent body fat were also measured. Importantly, unlike the majority of prior studies, we attempted to standardize internal state by instructing participants to arrive to the laboratory neither hungry nor full and at least 1-h fasted. Data were analyzed with general linear and linear mixed effects models (GLMs). Hunger ratings were also examined prior to hedonic measurement and included as covariates in our analyses.

RESULTS

We identified a significant association between ratings of hunger and milkshake liking and wanting. By contrast, we found no evidence for a relationship between any measure of adiposity and ratings of milkshake liking, wanting, or intensity.

CONCLUSIONS

We conclude that adiposity is not associated with the pleasure experienced during consumption of our energy-dense and palatable milkshakes. Our results provide further evidence against the hypothesis that heightened hedonic signals drive weight gain.

Weight Loss, Dietary Preferences, and Reduction in the Sense of Smell with the Use of a Novel Nasal Device

INTRODUCTION

Exposure to food odors are known to increase food intake. Olfaction declines from age 50 years.

OBJECTIVE

We examined changes in the sense of smell, body weight, food preferences, and parameters of metabolic status, following the use of a specially designed nasal device.

METHODS

This is a randomized, placebo-controlled study. Participants wore a nasal device (soft silicone insert) for 5-12 h daily (device group) or inserted 2 drops of normal saline into each nostril daily (control group). Follow-up visits occurred every 2 weeks. All participants were given a 500 kcal/day reduced diet and instructed not to change their regular physical activity. Weight, food preferences, olfactory sensitivity, and blood tests were performed at baseline and after 12 weeks.

RESULTS

Of 156 participants, 65 (42%) completed the study. Sense of smell decreased in the device group (from 6.4 ± 0.9 to 4.4 ± 1.5, on a scale of 0-7, p < 0.001), and did not change in the control group. Weight loss decreased by 6.6 ± 3.7% (p = 0.001) and by 5.7 ± 3.5% (p = 0.001) in the respective groups (between-group difference, p > 0.05). Among participants aged ≤50 years, weight loss was greater in the device than in the control group (7.7 ± 4.2% vs. 4.1 ± 2.9%, p = 0.02). Insulin level and the homeostatic model assessment of insulin resistance (HOMA-IR) were significantly reduced in the device group (p = 0.02 and p = 0.01, respectively), but not in the control group. Food preferences for sugar (p < 0.02), sweet beverages (p < 0.001), and artificial sweeteners (p < 0.02) were significantly reduced in the device group compared to the control group.

CONCLUSIONS

The use of a novel self-administrated nasal device led to reduced olfactory sensitivity, improved insulin sensitivity, weight loss, and lesser preference for sweets in adults aged ≤50 years.

The Effect of a Virtual-Reality Full-Body Illusion on Body Representation in Obesity

Background. The effective illusory ownership over an artificial body in modulating body representations in healthy and eating disorders population has been repeatedly reported in recent literature. In this study, we extended this research in the field of obesity: specifically, we investigated whether ownership over a virtual body with a skinny abdomen might be successfully experienced by participants affected by obesity. Methods. Fifteen participants with obesity and fifteen healthy-weight participants took part at this study in which the VR-Full-Body Illusion was adopted. The strength of illusion was investigated through the traditional Embodiment Questionnaire, while changes in bodily experience were measured through a body size estimation task. Results. Participants with obesity as well as healthy-weight participants reported to experience the illusion. About the body size estimation task, both groups reported changes only in the estimation of the abdomen’s circumference after the experimental condition, in absence of any another difference. Discussion. Participants with obesity reported to experience the illusion over a skinny avatar, but the modulation of the bodily experience seems controversial. Future lines of research exploiting this technique for modulating body representations in obesity, specifically in terms of potential therapeutic use, were discussed.

Effect of Maternal Obesity and Preconceptional Weight Loss on Male and Female Offspring Metabolism and Olfactory Performance in Mice

According to the “developmental origins of health and disease” (DOHaD) concept, maternal obesity predisposes the offspring to non-communicable diseases in adulthood. While a preconceptional weight loss (WL) is recommended for obese women, its benefits on the offspring have been poorly addressed. We evaluated whether preconceptional WL was able to reverse the adverse effects of maternal obesity in a mouse model, exhibiting a modification of foetal growth and of the expression of genes encoding epigenetic modifiers in liver and placenta. We tracked metabolic and olfactory behavioural trajectories of offspring born to control, obese or WL mothers. After weaning, the offspring were either put on a control diet (CD) or a high-fat (HFD). After only few weeks of HFD, the offspring developed obesity, metabolic alterations and olfactory impairments, independently of maternal context. However, male offspring born to obese mother gained even more weight under HFD than their counterparts born to lean mothers. Preconceptional WL normalized the offspring metabolic phenotypes but had unexpected effects on olfactory performance: a reduction in olfactory sensitivity, along with a lack of fasting-induced, olfactory-based motivation. Our results confirm the benefits of maternal preconceptional WL for male offspring metabolic health but highlight some possible adverse outcomes on olfactory-based behaviours.

The Influence of Circadian Timing on Olfactory Sensitivity

Olfactory sensitivity has traditionally been viewed as a trait that varies according to individual differences but is not expected to change with one's momentary state. Recent research has begun to challenge this position and time of day has been shown to alter detection levels. Links between obesity and the timing of food intake further raise the issue of whether odor detection may vary as a function of circadian processes. To investigate this question, 37 (21 male) adolescents (M age = 13.7 years) took part in a 28-h forced desynchrony (FD) protocol with 17.5 h awake and 10.5 h of sleep, for 7 FD cycles. Odor threshold was measured using Sniffin' Sticks 6 times for each FD cycle (total threshold tests = 42). Circadian phase was determined by intrinsic period derived from dim light melatonin onsets. Odor threshold showed a significant effect of circadian phase, with lowest threshold occurring on average slightly after the onset of melatonin production, or about 1.5○ (approximately 21:08 h). Considerable individual variability was observed, however, peak olfactory acuity never occurred between 80.5○ and 197.5○ (~02:22-10:10 h). These data are the first to show that odor threshold is differentially and consistently influenced by circadian timing, and is not a stable trait. Potential biological relevance for connections between circadian phase and olfactory sensitivity are discussed.

Systematic review of olfactory shifts related to obesity

OBJECTIVE

The modern food environment is a key driver of rising levels of obesity. While olfaction is known to play a major role in food choice; however, its relationship to obesity is yet to be understood. This review assesses current knowledge of the interaction between obesity and olfaction.

METHODS

This review is based on observational studies comparing olfactory abilities across weight groups (N = 10) and clinical studies evaluating olfactory changes following bariatric surgery (N = 9). Meta-analyses were performed on data collected by a standard olfactory assessment tool (Sniffin΄ Sticks), to test whether olfaction has any association with body weight or bariatric surgery.

RESULTS

This review synthesizes findings derived from 38 datasets, with a total of 1432 individual olfactory assessments. The meta-analyses suggest that olfactory function is negatively correlated with body weight. In addition, Roux-en-Y gastric bypass patients frequently report olfactory changes, yet more pronounced and immediate shifts have been observed among sleeve gastrectomy recipients.

CONCLUSIONS

Our review finds strong evidence for the link between olfaction and obesity and indicates that bariatric surgery (particularly the sleeve gastrectomy) is effective in reversing olfactory decline associated with obesity. In conclusion, we present mechanistic models to underpin the observed relationship between olfaction and obesity.

Prep1 Deficiency Affects Olfactory Perception and Feeding Behavior by Impairing BDNF-TrkB Mediated Neurotrophic Signaling

Prep1 is a homeodomain transcription factor which has an important role in hindbrain development. Prep1 expression is also kept in adult mouse brain and in particular within the olfactory bulbs. Moreover, many Prep1 neurons co-localize with Calbindin-positive periglomerular interneurons in olfactory glomerular layer. However, Prep1 function in this brain region is still unknown. In this study, we show that Prep1 hypomorphic heterozygous (Prep1^i/+) mice express low levels of protein and feature a 30% reduction of olfactory bulb area, compared to WT mice. In addition, Prep1^i/+ mice olfactory bulb histological analysis indicated a 20% lower cytochrome C oxidase activity within the glomerular layer, accompanied by a reduced number of periglomerular interneurons, compared to the WT littermates. Consistently, olfactory perception test highlighted that Prep1 hypomorphic heterozygous mice display a scant ability to distinguish odors, which significantly impacts on feeding behavior, as Prep1^i/+ mice revealed a reduced preference for high-fat food. Analysis of BDNF signaling, which represents the main molecular mediator of olfactory plasticity, showed that Prep1^i/+ mouse olfactory bulbs feature a 30% reduction of TrkB receptor levels and a decreased activation of ERK1/2. Similarly, overexpression of Prep1 in mouse neuronal cells (N2A) caused an increase of TrkB expression levels, BDNF-induced ERK phosphorylation, and cell viability, compared to control cells. We conclude that Prep1 deficiency alters olfactory morpho-functional integrity and olfaction-mediated eating behavior by affecting BDNF-TrkB signaling. Prep1 could, therefore, play a crucial role in behavioral dysfunctions associated to impaired responsiveness to BDNF.

Fat Sensation: Fatty Acid Taste and Olfaction Sensitivity and the Link with Disinhibited Eating Behaviour

Perception of fat taste, aroma, and texture are proposed to influence food preferences, thus shaping dietary intake and eating behaviour and consequently long-term health. In this study, we investigated associations between fatty acid taste, olfaction, mouthfeel of fat, dietary intake, eating behaviour, and body mass index (BMI). Fifty women attended three sessions to assess oleic acid taste and olfaction thresholds, the olfactory threshold for n-butanol and subjective mouthfeel ratings of custard samples. Dietary intake and eating behaviour were evaluated using a Food Frequency and Three-Factor Eating Questionnaire, respectively. Binomial regression analysis was used to model fat taste and olfaction data. Taste and olfactory detection for oleic acid were positively correlated (r = 0.325; p < 0.02). Oleic acid taste hypersensitive women had significantly increased n-butanol olfactory sensitivity (p < 0.03). The eating behaviour disinhibition and BMI were higher in women who were hyposensitive to oleic acid taste (p < 0.05). Dietary intake of nuts, nut spreads, and seeds were significantly correlated with high olfactory sensitivity to oleic acid (p < 0.01). These findings demonstrate a clear link between fatty acid taste sensitivity and olfaction and suggest that fat taste perception is associated with specific characteristics of eating behaviour and body composition.

Non-Conscious Effect of Food Odors on Children's Food Choices Varies by Weight Status

Objective

Food cues are omnipresent in the daily environment and may influence eating behavior even non-consciously. An increased reactivity to food cues, such as food odors, has been shown to be correlated with obesity in children. The objective of this study is to investigate whether the non-conscious influence of food odors on children’s food choices varies by their weight status.

Methods

Seventy-four children, of whom 29 were obese, took part in this study. The children performed a food choice intention task presented as a computer game in which 30 pairs of food images (a fatty-sweet food picture vs. a fruit picture) successively appeared on the screen. The children had to choose the item “they most wanted to eat at the present moment” for each pair. While performing this task, the children wore a headset in which the microphone foam was odorized with a fruity odor, a fatty-sweet odor or no odor. They performed the intention task three times, one time for each olfactory condition. The odors were non-attentively perceived, i.e., none of the children were aware of the odorization of the microphone foams. The modeled probability is the probability to choose a fruit.

Results

In children with obesity, the fruity odor increased the likelihood of a fruit to be chosen compared to the no-odor condition [OR (95% CL) = 1.42 (1.13–1.78), P = 0.0028], while the fatty-sweet odor had no effect on food choice [OR (95% CL) = 1.07 (0.85–1.36), P = 0.55]. In children without obesity, both the fruity and the fatty-sweet odors decreased the likelihood to choose a fruit compared to the no-odor condition [OR (95% CL) = 0.76 (0.64–0.90), P = 0.0015, for the fruity odor and OR (95% CL) = 0.79 (0.66–0.93), P = 0.0062, for the fatty-sweet odor].

Conclusion

The different patterns of results obtained in both groups of children suggest differences in the mental representations activated by non-attentively perceived olfactory cues based on weight status.