Olfactory dysfunction in type 2 diabetes mellitus: an additional manifestation of microvascular disease?

Olfactory function in diabetes mellitus

Diabetes mellitus (DM) is an increasingly common disease in both children and adults. In addition to neuronal and/or vascular disorders, it can cause chemosensory abnormalities including olfactory deterioration. The purpose of this article is to summarize current knowledge on olfactory function in DM, highlighting the impact of co-morbidities, especially obesity, thyroid dysfunction, chronic kidney disease and COVID-19 on olfactory outcomes. Research to date mostly shows that olfactory impairment is more common in people with diabetes than in the general population. In addition, the presence of concomitant diseases is a factor increasing olfactory impairment. Such a correlation was shown for type 1 diabetes, type 2 diabetes and gestational diabetes. At the same time, not only chronic diseases, but also DM in acute conditions such as COVID-19 leads to a higher prevalence of olfactory disorders during infection. Analyzing the existing literature, it is important to be aware of the limitations of published studies. These include the small number of patients studied, the lack of uniformity in the methods used to assess the sense of smell, frequently relying on rated olfactory function only, and the simultaneous analysis of patients with different types of diabetes, often without a clear indication of diabetes type. In addition, the number of available publications is small. Certainly, further research in this area is needed. From a practical point of view decreased olfactory performance may be an indicator for central neuropathy and an indication for assessing the patient's nutritional status, examining cognitive function, especially in older patients and performing additional diagnostic tests, such as checking thyroid function, because all those changes were correlated with smell deterioration.

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.

Reduction in olfactory dysfunction prevalence among patients with diabetes taking metformin

KEY POINTS

Metformin treatment is associated with reduced olfactory dysfunction (OD) in diabetic patients Metformin may possess potential protective effects on olfaction beyond glycemic control.

Incidence of Olfactory Dysfunction and Associated Factors: A Nationwide Cohort Study From South Korea

OBJECTIVES

Olfactory dysfunction, a reduced or complete loss of the ability to smell, is gaining attention because of its substantial impact on an individual's quality of life and the possibility that it is an important sign of underlying disease. However, olfactory dysfunction is underdiagnosed in the general population due to diagnostic difficulty and unpredictable prognosis. This study aimed to evaluate the prevalence of clinically diagnosed olfactory dysfunction in South Korea by using well-organized, nationwide, population-based cohort data, and the associations between olfactory dysfunction and risk of neurodegenerative disorders.

METHODS

We investigated the Korean National Health Insurance Service-National Sample Cohort for patients diagnosed with olfactory dysfunction according to the International Classification of Diseases. Annual and overall incidence and prevalence of olfactory dysfunction during 2003 to 2013 and patient characteristics were analyzed. Based on those identified patients who were later diagnosed with neurodegenerative disorder, hazard ratios (HRs) of sociodemographic factors and comorbidities associated with neurodegenerative disorder were evaluated using a Cox proportional hazard regression model.

RESULTS

In total, 6296 patients were clinically diagnosed with olfactory dysfunction during the study period (524.67 patients/year). The prevalence increased annually and was higher in female patients. The incidence of neurodegenerative disorders among patients with olfactory dysfunction was 4.2% within the study period. Multivariate cox regression analysis of the patients (n = 249) revealed that diabetes mellitus (HR = 1.976) and depression (HR = 2.758) were significant risk factors.

CONCLUSIONS

Olfactory dysfunction is underdiagnosed in South Korea, but it is clinically important considering the possibility of presymptom of neurodegenerative disorders. In clinical practice, we should consider its association with neurodegenerative disorders and possibly other systemic conditions.

Analysis of risk factors affecting olfactory dysfunction in patients with chronic rhinosinusitis: Highlighting the role of metabolic syndrome

Objective

This study aims to evaluate the relationship between chronic sinusitis (CRS) and metabolic syndrome (MS) in a Chinese population and to explore the risk factors for olfactory dysfunction in patients with CRS.

Methods

A total of 387 CRS patients were enrolled. Olfactory function was assessed by the Sniffin' Sticks 12-item test and MS was diagnosed according to the guidelines. Logistic regression analysis was performed on CRS patients to screen independent risk factors of olfactory dysfunction, adjusted for confounding factors.

Results

Among 387 patients, average age of visit and duration of onset were 48.7 years and 1.8 years, respectively. The prevalence of MS was 15.0%. CRS patients with MS were more likely to be older (51.2 vs. 46.8, p = .004), predominantly male (p < .001) and have a higher proportion of olfactory dysfunction (62.1% vs. 44.1%, p = .018) than those without MS. In multivariate logistic regression analysis, MS was associated with olfactory dysfunction in CRS patients (OR: 2.06, 95% CI: 1.14-3.72, p = .016). This association remained significant after controlling for confounding factors. In addition, nasal polyps (OR: 13.41, 95% CI: 8.11-22.17, p < .001) and allergic rhinitis (OR: 3.16, 95% CI: 1.67-5.99, p < .001) were also risk factors for olfactory dysfunction after adjusting for confounding factors.

Conclusions

MS is associated with olfactory dysfunction in patients with CRS. MS, nasal polyps, and allergic rhinitis are risk factors for olfactory dysfunction in CRS patients.

Level of evidence

IV.

Olfactory Function and Olfactory Disorders

The sense of smell is important. This became especially clear to patients with infection-related olfactory loss during the SARS-CoV-2 pandemic. We react, for example, to the body odors of other humans. The sense of smell warns us of danger, and it allows us to perceive flavors when eating and drinking. In essence, this means quality of life. Therefore, anosmia must be taken seriously. Although olfactory receptor neurons are characterized by regenerative capacity, anosmia is relatively common with about 5 % of anosmic people in the general population. Olfactory disorders are classified according to their causes (e. g., infections of the upper respiratory tract, traumatic brain injury, chronic rhinosinusitis, age) with the resulting different therapeutic options and prognoses. Thorough history taking is therefore important. A wide variety of tools are available for diagnosis, ranging from short screening tests and detailed multidimensional test procedures to electrophysiological and imaging methods. Thus, quantitative olfactory disorders are easily assessable and traceable. For qualitative olfactory disorders such as parosmia, however, no objectifying diagnostic procedures are currently available. Therapeutic options for olfactory disorders are limited. Nevertheless, there are effective options consisting of olfactory training as well as various additive drug therapies. The consultation and the competent discussion with the patients are of major importance.

Synergistic effects of hIAPP and Aβ1-42 impaired the olfactory function associated with the decline of adult neurogenesis in SVZ

According to many in the field，the prevalence of Alzheimer's disease (AD) in type II diabetes (T2DM) populations is considerably higher than that in the normal population. Human islet amyloid polypeptide (hIAPP) is considered to be a common risk factor for T2DM and AD. Preliminary observations around T2DM animal model show that the decrease of adult neural stem cells (NSCs) in the subventricular zone (SVZ) is accompanied by olfactory dysfunction. Furthermore, impaired olfactory function could serve as to an early predictor of neurodegeneration，which is associated with cognitive impairment. However, the synergistic effects between hIAPP and amyloid-beta (Aβ) _1-42 in the brain and the neurodegeneration remains to be further clarified. In this study, olfactory capacity, synaptic density, status of NSC in SVZ, and status of newborn neurons in olfactory bulb (OB) were assessed 6 months after stereotactic injection of oligomer Aβ_1-42 into the dens gyrus (DG) of hIAPP-/+ mice or wild-type homogenous mice. Our results set out that Aβ₄₂ and amylin co-localized into OB and raised Aβ₄₂ deposition in hIAPP^-/+ mice compared with wild-type brood mice. In addition, 6 months after injection of Aβ_1-42 in hIAPP^-/+ mice, these mice showed increased olfactory dysfunction, significant loss of synapses, depletion of NSC in SVZ, and impaired cell renewal in OB. Our present study suggested that the synergistic effects between hIAPP and Aβ_1-42 impairs olfactory function and was associated with decreased neurogenesis in adults with SVZ.

Investigations and Outcomes for Olfactory Disorders

Purpose of Review

To provide a detailed overview of the investigations and core outcome measures for olfactory disorders.

Recent Findings

Olfactory disorders can have a detrimental impact to the quality of life of patients. There are a wide range of causes of olfactory loss including sinonasal conditions, idiopathic, post-head trauma or infection. This review highlights the key investigations and reasoning for their use to clinically assess and research patients with olfactory disorders. In addition, this review outlines the core outcome measures for olfaction that will help inform future research in olfactory disorders.

Summary

A systematic approach with history taking and examination particularly with nasal endoscopy can determine the cause of the olfactory disorder in most cases. Specific olfactory disorder questionnaires can demonstrate the impact on quality of life, while psychophysical testing can objectively assess and monitor olfaction over time. Olfactory-evoked potentials and functional MRI are reserved for research, whereas CT and MRI imaging are used depending on history and examination. A core outcome set for olfaction has been developed that will help standardise the outcome measures used in olfaction and olfactory disorders research.

Diabetes Induces Permanent Deleterious Effects in the Olfactory Bulb Associated with Increased Tyrosine Hydroxylase Expression and ERK1/2 Phosphorylation

Diabetes mellitus type 2 (T2D) complications include brain damage which increases the risk of neurodegenerative diseases and dementia. An early manifestation of neurodegeneration is olfactory dysfunction (OD), which is also presented in diabetic patients. Previously, we demonstrated that OD correlates with IL-1β and miR-146a overexpression in the olfactory bulb (OB) on a T2D rodent model, suggesting the participation of inflammation on OD. Here, we found that OD persists on a long-term T2D condition after the downregulation of IL-1β. Remarkably, OD was associated with the increased expression of the dopaminergic neuronal marker tyrosine hydroxylase, ERK1/2 phosphorylation, and reduced neuronal activation on the OB of diabetic rats, suggesting the participation of the dopaminergic tone on the OD derived from T2D. Dopaminergic neurons are susceptible in neurodegenerative diseases such as Parkinson's disease; therefore further studies must be performed to completely elucidate the participation of these neurons and ERK1/2 signaling on olfactory impairment.

Bloody olfaction? Confounding associations of sex and age on the influence of blood parameters and body weight on odor identification performance in healthy adults

Olfactory function and nutrition are closely related and may influence each other via metabolic parameters. However, the relationship between nutritional blood parameters and olfactory performance is still unclear. Inconclusive findings exist for specific blood parameters. In this extensive analysis, we examined the relationship between olfactory performance, measured with MONEX-40, as well as intensity and pleasantness ratings with 38 metabolic blood parameters, age, sex, and the anthropometric measurements body mass index (BMI) and body fat percentage (BFP). Therefore, we included data of 418 healthy, well-phenotyped Caucasians of the Enable cohort. We replicated age-dependent olfactory identification scores (p < 0.001) and found slight evidence for a body fat dependence measured with BFP (BF₁₀ = 10.466). We further identified a sex difference only in middle-aged adults (p < 0.001) that could be explained by environmental factors. Several blood parameters correlated significantly with the MONEX-40 score (p < 0.05 - p < 0.001). However, these effects diminished after adjusting for sex and age (p > 0.9) that were identified as confounders. The same applies for BFP. In addition, no parameters were identified to correlate significantly with perceived olfactory intensity or pleasantness score if controlled for sex and age (p > 0.08). Our results suggest that metabolic blood parameters are not related to olfactory identification performance in a relevant manner and highlight the importance of controlling for sex and age in chemosensory research.

Olfactory dysfunction in type II diabetes: Therapeutic options and lessons learned from other etiologies - A scoping review

INTRODUCTION

Olfactory dysfunction (OD) is highly prevalent amongst type 2 diabetes mellitus (DM2) patients and has many associated health risks. For example, OD can lead to poor nutrition, safety issues related to diminished hazard detection, and increased mortality rates. While limited research exists about therapeutics for DM2-associated OD, recovery of olfactory function is better studied in other pathologic states. The objectives of this scoping review are to synthesize the existing data on interventions for DM2-associated OD and present the evidence for therapies that have been utilized for non-DM2-associated causes of OD. Additionally, the potential therapeutic opportunities for patients with DM2 are explored.

METHODS

A scoping review was conducted with a medical librarian to identify studies investigating treatments of DM2-related OD. 6 databases were searched (Embase, CINAHL, the Cochrane Library, Google Scholar, OVID Medline, and Web of Science). Studies were eligible if the primary discussion involved treatment of olfactory deficits in the context of DM2. All publication dates were included, and studies published in languages other than English were excluded.

RESULTS

3631 articles were identified; 3 articles met inclusion criteria and underwent full text review. Hyperbaric oxygen (HBO), the DPP-4 inhibitor Linagliptin and the GLP-1 agonists Exenatide and Liraglutide are the only therapeutics that have been used in the context of DM2. Only HBO and GLP-1 agonists produced statistically significant improvements in olfactory identification. The literature regarding non-DM2-associated OD supports interventions such as olfactory training, dietary supplements, and intranasal insulin. Specifically, olfactory training was very effective in many contexts such as post-viral and traumatic OD while being affordable and non-invasive.

CONCLUSION

This scoping review of olfactory rehabilitation options for DM2-induced OD demonstrates a paucity of prospective investigations of plausible therapeutics. Additionally, treatments for OD related to non-DM2-associated etiologies, such as olfactory training, are well-studied, efficacious, and should be investigated in the context of DM2. Future investigation has the potential to enhance the quality of clinical intervention for OD and improve short- and long-term outcomes for DM2 patients.

Impact of Systemic Diseases on Olfactory Function in COVID-19 Infected Patients

Background

COVID-19 (SARS-CoV-2/2019-nCoV) is now a major public health threat to the world. Olfactory dysfunctions (ODs) are considered potential indicating symptoms and early case identification triaging for coronavirus disease 2019 (COVID-19). The most common reported comorbidities are diabetes mellitus, chronic lung disease, and cardiovascular disease. The objective of this study was to evaluate prevalence of different types of smell disorders in patients with laboratory-confirmed COVID-19 infection and impact of involved systemic diseases.

Methodology

A cross-sectional retrospective study has been done for patients with laboratory-confirmed COVID-19 infection (mild-to-moderate). The data collected from patient's files and developed online electronic questionnaire (WhatsApp) based on the patients most common and recurrent reported data including: a) symptoms of olfactory dysfunction and associated covid19 symptoms fever and headache, cough, sore throat, pneumonia, nausea, vomiting and diarrhea, arthralgia and myalgia and taste dysfunction. b) Associated systemic diseases including: diabetes, hypertension, asthma, chronic renal disease, chorionic liver disease and hypothyroidism.

Results

Of 308 patients confirmed with Covid-19 infection, (72.4%) developed OD distributed as follows; complete anosmia (57.8%), troposmia (8.4%), hyposmia (2.9%), partial anosmia (2.6%) and euosmia (0.6%). Significantly increased prevalence of diabetes, hypertension asthma in the group with olfactory dysfunction (p < 0.001), chronic liver disease (p = 0.005), and hypothyroidism (p = 0.03).

Conclusion

The development of ODs after Covid-19 infection was associated with mild disease form and lower hospitalization. In addition, it showed significant relationship with preexisting systemic diseases. Anosmia is the common modality of ODs.

COVID-19: Are We Facing Secondary Pellagra Which Cannot Simply Be Cured by Vitamin B3?

Immune response to SARS-CoV-2 and ensuing inflammation pose a huge challenge to the host’s nicotinamide adenine dinucleotide (NAD⁺) metabolism. Humans depend on vitamin B3 for biosynthesis of NAD⁺, indispensable for many metabolic and NAD⁺-consuming signaling reactions. The balance between its utilization and resynthesis is vitally important. Many extra-pulmonary symptoms of COVID-19 strikingly resemble those of pellagra, vitamin B3 deficiency (e.g., diarrhoea, dermatitis, oral cavity and tongue manifestations, loss of smell and taste, mental confusion). In most developed countries, pellagra is successfully eradicated by vitamin B3 fortification programs. Thus, conceivably, it has not been suspected as a cause of COVID-19 symptoms. Here, the deregulation of the NAD⁺ metabolism in response to the SARS-CoV-2 infection is reviewed, with special emphasis on the differences in the NAD⁺ biosynthetic pathway’s efficiency in conditions predisposing for the development of serious COVID-19. SARS-CoV-2 infection-induced NAD⁺ depletion and the elevated levels of its metabolites contribute to the development of a systemic disease. Acute liberation of nicotinamide (NAM) in antiviral NAD⁺-consuming reactions potentiates “NAM drain”, cooperatively mediated by nicotinamide N-methyltransferase and aldehyde oxidase. “NAM drain” compromises the NAD⁺ salvage pathway’s fail-safe function. The robustness of the host’s NAD⁺ salvage pathway, prior to the SARS-CoV-2 infection, is an important determinant of COVID-19 severity and persistence of certain symptoms upon resolution of infection.

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.

Olfactory disfunction and diabetic complications in type 2 diabetic patients: a pilot study

PURPOSE

Scarce information on the prevalence and characteristics of olfactory disfunction (OD) in type 2 diabetic (T2D) patients are available. The aims of this study were (1) to assess the olfactory function in T2D patients and to compare it with a control group of individuals without T2D, and (2) to evaluate the differences in OD within T2D patients according to the presence of diabetic complications.

METHODS

A group of 39 T2D patients and a control group of 39 healthy individuals were enrolled. Each subject underwent an evaluation of the olfactory performance using the Sniffing Olfactory Screening Test (SOST) and completed a questionnaire assessing the subjective perception of olfaction. According to the presence of diabetic complications, the group of T2D patients was divided into two subgroups. Non-parametric tests and regression analysis were used for statistical analysis.

RESULTS

No differences in the subjective perception of olfaction were demonstrated among T2D patients (with and without complications) and controls. A significant difference for the SOST score was demonstrated among the different groups. In particular, OD was more frequent in T2D patients than in controls. In addition, OD was far more frequent in T2D patients with complications. Regression analysis did not demonstrate any significant association between OD and clinical/demographic characteristics of T2D patients.

CONCLUSION

T2D patients were more frequently affected by OD. The subgroup analysis suggested a possible relationship between OD and diabetic complications since patients with T2D diabetic complications demonstrated lower olfactory abilities than controls subjects and T2D patients without diabetic complications.

Brain Structure as a Correlate of Odor Identification and Cognition in Type 2 Diabetes

Background: It has been reported that type 2 diabetes (T2DM) is associated with olfactory identification (OI) impairments and cognitive decline. However, the relationship between OI impairments and cognitive decline is largely unknown in T2DM patients.

Methods: Sixty-eight T2DM patients and 68 healthy controls underwent 3D-T1 MRI scans, olfactory and cognitive assessments. The cortical thickness of olfaction-related brain regions, olfactory and cognitive scores were compared between groups. Correlation analyses were carried out among cognition, olfaction, and cortical thickness of olfaction-related brain regions.

Results: First, the cognitive and olfactory test scores of T2DM patients were lower than healthy subjects. Second, higher olfactory scores were associated with increased cortical thickness in the left parahippocampal gyrus and bilateral insula in T2DM. Third, higher olfactory scores were associated with higher cognitive performance in T2DM. Fourth, some cognitive performances were related to cortical thickness in the left parahippocampal gyrus and left insula in T2DM.

Conclusion: These findings indicated that olfactory dysfunction may be useful for future applications that attempt to predict cognitive decline or develop tailored therapies in T2DM patients.

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.

Olfactory Dysfunction: A Complication of Diabetes or a Factor That Complicates Glucose Metabolism? A Narrative Review

The present narrative review presents emerging data regarding the association between diabetes mellitus and olfactory dysfunction and discusses the role of olfactory dysfunction in glucose metabolism. We searched relevant published articles in PubMed and Google Scholar until October 2021. Main key words included “olfactory dysfunction”, “diabetes mellitus”, and “glucose metabolism”. Olfactory dysfunction has been associated with diabetes mellitus. Furthermore, it has been proposed to be a diabetic complication, given that it has been linked with microvascular complications, such as diabetic peripheral neuropathy. Interestingly, it has been suggested that olfactory dysfunction is a manifestation of central neuropathy in diabetes, a hypothesis based on the observation that diabetes, olfactory dysfunction, and cognitive decline often coexist. However, evidence is limited and inconsistent. More importantly, olfactory and endocrine systems are closely linked, and olfactory dysfunction plays a significant role in glucose metabolism and obesity. Indeed, food behaviour and energy balance are influenced by olfaction status.

Machine Learning Refutes Loss of Smell as a Risk Indicator of Diabetes Mellitus

Because it is associated with central nervous changes, and olfactory dysfunction has been reported with increased prevalence among persons with diabetes, this study addressed the question of whether the risk of developing diabetes in the next 10 years is reflected in olfactory symptoms. In a cross-sectional study, in 164 individuals seeking medical consulting for possible diabetes, olfactory function was evaluated using a standardized clinical test assessing olfactory threshold, odor discrimination, and odor identification. Metabolomics parameters were assessed via blood concentrations. The individual diabetes risk was quantified according to the validated German version of the “FINDRISK” diabetes risk score. Machine learning algorithms trained with metabolomics patterns predicted low or high diabetes risk with a balanced accuracy of 63–75%. Similarly, olfactory subtest results predicted the olfactory dysfunction category with a balanced accuracy of 85–94%, occasionally reaching 100%. However, olfactory subtest results failed to improve the prediction of diabetes risk based on metabolomics data, and metabolomics data did not improve the prediction of the olfactory dysfunction category based on olfactory subtest results. Results of the present study suggest that olfactory function is not a useful predictor of diabetes.

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.

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.

Differences in taste and smell perception between type 2 diabetes mellitus patients and healthy controls

BACKGROUND AND AIMS

The senses of taste and smell are essential determinants of food choice, which in turn may contribute to the development of chronic diseases, including diabetes. Although past studies have evaluated the relationship between type 2 diabetes mellitus (DM2) and senses disorders, this relationship remains controversial. In this study, we evaluated taste and smell perception in DM2 patients and healthy controls (HC). Moreover, we analyzed the association of chemosensory impairments with anthropometric and clinical outcomes (e.g. Body Mass Index (BMI), Fasting blood glucose (FBG), drugs, cardiovascular diseases (CVD), and hypertension) in DM2 patients.

METHODS AND RESULTS

The study included 94 DM2 patients and 244 HC. Taste recognition for 6-n-propylthiouracil (PROP), quinine, citric acid, sucrose, and sodium chloride (NaCl) compounds was assessed using a filter paper method, while smell recognition of 12 odorants was performed using a Sniffin' sticks test. We found that a higher percentage of DM2 patients showed identification impairment in salt taste (22% vs. 5%, p-value<0.0009) and smell recognition (55% vs. 27%, p-value = 0.03) compared to HC. We also observed that 65% of hypertensive DM2 subjects presented smell identification impairment compared to 18% of non-hypertensive patients (p-value = 0.019). Finally, patients with impairments in both taste and smell showed elevated FBG compared to patients without impairment (149.6 vs.124.3 mg/dL, p-value = 0.04).

CONCLUSION

The prevalence of taste and smell identification impairments was higher in DM2 patients compared to HC, and a possible relationship with glycemic levels emerged.

Relationships between Cognitive Function and Odor Identification, Balance Capability, and Muscle Strength in Middle-Aged Persons with and without Type 2 Diabetes

AIM

We investigated the relationship between cognitive function and olfactory and physical functions in middle-aged persons with and without type 2 diabetes (T2D) to examine the potential of olfactory and physical functions as biomarkers for early cognitive impairment.

METHODS

Enrolled were 70 T2D patients (age 40 to <65 y) and 81 age-matched control participants without diabetes. Cognitive function was assessed by the Montreal Cognitive Assessment (MoCA), Trail Making Test parts A and B (TMT-A/-B), Wisconsin Card Sorting Test (WCST), Quick Inventory of Depressive Symptomatology Self-Report (QIDS), and Starkstein Apathy Scale (SAS). Multiple linear regression analyses were performed.

RESULTS

Odor identification was an independent determinant shown in the results of the TMT-A in the entire participant group and was independently associated with the MoCA and TMT-B in the T2D group. Balance capability assessed with a stabilometer was independently associated with all cognitive function tests except for QISD and SAS in the entire participant group and the T2D group and was independently associated with TMT-A in the control group. Knee extension strength was independently associated with the SAS in the entire participant group and the T2D group.

CONCLUSIONS

Odor identification, balance capability, and knee extension strength were potential markers for cognitive decline in middle-aged persons with T2D.

Connecting Peripheral to Central Neuropathy: Examination of Nerve Conduction Combined with Olfactory Tests in Patients with Type 2 Diabetes

AIM

Few studies have investigated the associations between diabetic peripheral neuropathy (DPN) and cognitive decline. Olfactory impairment is related to neurodegenerative diseases and type 2 diabetes mellitus (T2DM); however, the cognitive alterations of patients with DPN and the role of olfactory function in DPN are not known. We explored alterations in cognition with DPN and the associations of neuropathy parameters with cognition and olfaction.

METHODS

Healthy controls (HCs) and patients with T2DM underwent nerve-conduction tests, detailed cognitive assessment, olfactory-behavior tests, and odor-induced functional magnetic resonance imaging (fMRI). T2DM patients were divided into two groups (non-DPN [NDPN] and DPN). Olfactory brain regions showing different activation between the two groups were selected for functional connectivity (FC) analyses. A structural equation model (SEM) was also generated to demonstrate the association among cognition, olfactory, and neuropathy parameters.

RESULTS

One hundred individuals (36 HCs, 36 NDPN, and 28 DPN) were matched for age, sex, and educational level. Compared with the NDPN group, the DPN group had significantly lower scores for memory and processing speed, as well as lower olfactory identification and memory scores, decreased activation of the left frontal lobe, and reduced seed-based functional connectivity in the right insula. The nerve conduction velocity in patients with T2DM was associated with cognitive functions. The association between nerve conduction and executive function was mediated by olfactory behavior.

CONCLUSION

Patients with DPN had worse cognition than the NDPN patients in the domains of memory and processing speed. Cognitive dysfunction could be predicted by olfactory-behavior tests and electrophysiological examination.

Altered Functional Connectivity Strength of Primary Visual Cortex in Subjects with Diabetic Retinopathy

OBJECTIVE

The purpose of the study was to find the differences in intrinsic functional connectivity (FC) patterns of the primary visual area (V1) among diabetic retinopathy (DR), diabetes mellitus (DM), and healthy controls (HCs) applying resting-state functional magnetic resonance imaging (rs-fMRI).

PATIENTS AND METHODS

Thirty-five subjects with DR (18 males and 17 females), 22 DM (10 males and 12 females) and 38 HCs (16 males and 22 females) matched for sex, age, and education underwent rs-fMRI scanning. Seed-based FC analysis was performed to find the alterations in the intrinsic FC patterns of V1 in DR compared with DM and HCs.

RESULTS

The study found that DR patients had a significant lower FC between the bilateral calcarine (CAL)/left lingual gyrus (LING) (BA 17/18) and the left V1, and between the bilateral CAL/left LING (BA 17/18) and the right V1 compared with the HCs. Meanwhile, patients with DR exhibited higher FC strength between the left V1 and the bilateral Caudate/Olfactory/Orbital superior frontal gyrus (OSFG), and between the bilateral Caudate/Olfactory/OSFG (BA 3/4/6) and the right V1. Compared with DM group, patients with DR showed increased FC strength between the right CAL (BA 17/18) and the right V1. DM group exhibited lower FC strength between the left fusiform and the left V1, and between the bilateral CAL and the right V1 when compared with HCs. Moreover, DM group was observed to have higher FC strength between the left superior frontal gyrus and the left V1.

CONCLUSION

Our findings indicated that DR patients exhibited FC disruptions between V1 and higher visual regions at rest, which may reflect the aberrant information communication in the V1 area of DR individuals. The findings offer important insights into the neuromechanism of vision disorder in DR patients.

Alterations of 5-hydroxymethylcytosines in circulating cell-free DNA reflect retinopathy in type 2 diabetes

Diabetic retinopathy (DR) is a common microvascular complication that may cause severe visual impairment and blindness in patients with type 2 diabetes mellitus (T2DM). Early detection of DR will expand the range of potential treatment options and enable better control of disease progression. Epigenetic dysregulation has been implicated in the pathogenesis of microvascular complications in patients with T2DM. We sought to explore the diagnostic value of 5-hydroxymethylcytosines (5hmC) in circulating cell-free DNA (cfDNA) for DR, taking advantage of a highly sensitive technique, the 5hmC-Seal. The genome-wide 5hmC profiles in cfDNA samples from 35 patients diagnosed with DR and 35 age-, gender-, diabetic duration-matched T2DM controls were obtained using the 5hmC-Seal, followed by a case-control analysis and external validation. The genomic distribution of 5hmC in cfDNA from patients with DR reflected potential gene regulatory relevance, showing co-localization with histone modification marks for active expression (e.g., H3K4me1). A three-gene signature (MESP1, LY6G6D, LINC01556) associated with DR was detected using the elastic net regularization on the multivariable logistic regression model, showing high accuracy to distinguish patients with DR from T2DM controls (AUC [area under curve] = 91.4%; 95% CI [confidence interval], 84.3- 98.5%), achieving a sensitivity of 88.6% and a specificity of 91.4%. In an external testing set, the 5hmC model detected 5 out of 6 DR patients and predicted 7 out of 8 non-DR patients with other microvascular complications. Circulating cfDNA from patients with DR contained 5hmC information that could be exploited for DR detection. As a novel non-invasive approach, the 5hmC-Seal holds the promise to be an integrated part of patient care and surveillance tool for T2DM patients.

Olfactory Dysfunction in a Mexican Population Outside of COVID-19 Pandemic: Prevalence and Associated Factors (the OLFAMEX Study)

PURPOSE OF REVIEW

To study the prevalence of olfactory loss and its associated factors in a Mexican population a cross-sectional analytical study based on a population interviewed about health, epidemiologic aspects, and sense of smell (tested with four scents: rose, banana, perfume, and gas) was conducted to evaluate olfactory detection, memory, and identification. Levels of sense of smell perception were determined when the participants detected, recognized, or identified all (normosmia), 1-3 (hyposmia), or none (anosmia) of the odorants. Associated factors of olfactory dysfunction were identified by multivariate analysis (odds ratio, 95%CI).

RECENT FINDINGS

Olfactory dysfunction is a prevalent disorder affecting up to 20% of the general population. In addition to viral infection, including COVID-19, a number of other causes and factors may also be involved. 1,956 surveys were conducted and 1,921 were analyzed. Most of the participants (62.1%) were women. The general prevalence of olfactory dysfunction, regarding detection, was 7.2% (7.1% hyposmia, 0.1% anosmia). Age-related olfactory deterioration was observed in both sexes from the 5th decade of life (OR 2.74, p = 0.0050). Women showed better olfactory identification (OR 0.73, p = 0.0010). Obesity (OR 1.97, p = 0.0070), low educational level, bad/very bad self-perceived olfactory function (OR 2.74, p = 0.0050), olfactory loss for less than one week (OR 1.35, p = 0.0030), exposure to toxics/irritants (OR 1.31, p = 0.0030), active smoking (OR 1.58, p < 0.0010), and type 2 diabetes mellitus (OR 2.68, 95%CI 1.74-4.10, p < 0.0001) were identified as factors associated with olfactory dysfunction. These results in a Mexican population suggest better olfactory identification (verbalization) in females. Age was a determining factor in the olfactory deterioration process and obesity and diabetes mellitus were also associated with olfactory disorders. Finally, these findings reinforce the differential diagnosis with other potential causes of sense of smell loss, during the COVID-19 outbreak.

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.

Factors driving olfactory loss in patients with chronic rhinosinusitis: a case control study

BACKGROUND

Olfactory dysfunction (OD) in chronic rhinosinusitis (CRS) is common. It is likely that numerous factors such as sex, race, age, allergies, asthma, smoking, and other comorbidities play a role in CRS-related OD. In order to determine which aspects of OD are due solely to CRS and which are associated with other confounders, control populations are needed to allow appropriate risk assessments.

METHODS

Prospective, multi-institutional enrollment of patients with CRS and control subjects without CRS was performed. Demographic information, comorbidities, and olfactory testing (Sniffin' Sticks) of threshold (T), discrimination (D), and identification (I) scores (TDI) was collected.

RESULTS

A total of 224 patients with CRS and 164 control subjects were enrolled. Olfaction was worse in CRS patients compared to controls (mean ± standard deviation (SD) TDI = 22.4 ± 9.5 vs 28.8 ± 7.0, respectively, p < 0.001). Only 27% of CRS patients were normosmic compared to 49% of controls (p < 0.001). When stratifying by nasal polyp (NP) status, CRSwNP patients had significant impairments in TDI, T, D, and I compared to controls with mean differences of 11.2, 3.3, 3.5, and 4.4 points, respectively (all p < 0.001). In contrast, CRSsNP patients only had impaired T when compared to controls with a mean difference of 2.2 points (p < 0.001). Multivariate modeling of TDI scoring showed that OD was driven by polyps, asthma, diabetes, and age. CRSsNP was not independently associated with worse TDI scores.

CONCLUSION

OD in CRS patients is multifactorial. Independent drivers appear to be polyp status, asthma, diabetes, and age. OD in patients with CRSsNP is similar to controls with the exception of impaired thresholds.

Insulin resistance is associated with impaired olfactory function in adult patients with type 1 diabetes: A cross-sectional study

AIM

To investigate whether insulin resistance is a predictor for decreased olfactory function in adult type 1 diabetes patients (T1DM).

MATERIALS AND METHODS

The following parameters were examined in the group of 113 T1DM participants: body mass index (BMI), waist-hip ratio (WHR), TG/HDL ratio, glycated hemoglobin (HbA_1c ), visceral fat (VF) in body bioimpedance, specific calculators (eGDR, VAI). Bilateral olfactory test score (BOTS) was performed using 12-odour-tests from Sniffin' Sticks. Then participants were allocated to one of two groups: normosmia (10-12 odours identified) or hyposmia/anosmia (0-9 odours). The association between BOTS and insulin resistance indicators was analyzed using: Spearman's rank correlation, multivariate linear regression analysis, and receiver operating characteristic (ROC) curve.

RESULTS

49.6% participants were diagnosed with hyposmia/anosmia, median BOTS was 10. BOTS correlated significantly with: WHR, TG, VF index, TG/HDL ratio, VAI, and eGDR. In multivariate linear regression analysis higher WHR turned out to be statistically significant independent predictor of lower BOTS (β = -0.36; P = .005) after adjustment for age, sex, TG and peripheral neuropathy (R² = 0.19; P = .0005). The ROC analysis indicated a WHR cut-off of 0.92 [area under the ROC curve (AUC): 0.737; 95% confidence interval (CI): 0.647-0.828, P < .0001] as the best among evaluated factors significantly affecting hyposmia/anosmia occurrence (sensitivity of this cut-off 0.50 and specificity 0.86).

CONCLUSIONS

We have provided evidence of an association between lowered insulin sensitivity expressed in bioelectrical impedance analysis, anthropometrical (WHR), laboratory (TG/HDL ratio) measurements, specific calculators (eGDR, VAI) and deteriorated olfactory function.

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.

Olfactory Memory in Depression: State and Trait Differences between Bipolar and Unipolar Disorders

Background: Changes in olfactory recognition memory may constitute sensory markers in depression. Significant differences may exist between unipolar and bipolar depression. Our study compares olfactory memory between control, unipolar, and bipolar patients in depressed and euthymic states in order to identify potential markers of depression. Methods: 176 participants were recruited in 5 groups: depressed bipolar (DB), euthymic bipolar (EB), depressed unipolar (DU), euthymic unipolar (EU), and controls (HC). The participants had a standardized clinical and olfactory assessment (olfactory memory, evaluation of pleasantness, intensity, familiarity, and emotional aspect of smells). Results: DU, DB, and EU patients had a deficit in olfactory memory compared to HC. DB patients had lower capacity to recognize new odors. DB and DU patients had more limited detection of unfamiliar odors than HC. DB patients rated odors as less pleasant compared to the other groups. All groups had lower hedonic ratings than HC. DB patients had lower emotional ratings than EU patients. Conclusions: Olfactory memory is impaired in depressive states, thus constituting a state marker of depression. Impairments in olfactory memory persist after remission of bipolar depression, thus constituting a possible trait marker of bipolarity. Hedonic rating differentiates unipolar from bipolar depression. This is the first study that identifies a sensory marker differentiating between unipolar and bipolar depression.

Relationship between progression of type 2 diabetes mellitus and olfactory function

OBJECTIVES

In the literature, diabetes mellitus was mentioned as one of the etiologic factors of olfactory disorder. However, association between olfactory dysfunction and complications of type 2 diabetes mellitus is unclear. The aim of this study was to determine if there is any correlation between olfactory dysfunction and complications of diabetes mellitus.

METHODS

The study population included eighty-five (85) patients with type 2 diabetes mellitus (56 females and 29 males, mean age 55.4 ± 9.4 years). The routine laboratory and ophthalmoscope examinations were used in the study. The Connecticut Chemosensory Clinical Research Center odor test was performed to all patients. Patients were grouped (normal, anosmia, mild hyposmia, moderate hyposmia, severe hyposmia) in respect to olfactory function.

RESULT

Distribution of the patients was 34.1% male (29) and 65.9% female (56). Mean Hemoglobin A1c value was 9.0 ± 2.7. The distribution of complications was 38.8% nephropathy, 25.9% retinopathy, 24.7% microalbuminuria. In Odor Test classification, statistically significant difference was not detected in nephropathy, retinopathy and microalbuminuria ratios (p = 0.523, p = 0.057, p = 0.993).

CONCLUSIONS

This study revealed that in odor test classification, statistically significant difference was not detected between the patients with complications (nephropathy, retinopathy, and microalbuminuria) and the patients without complications.

Mechanisms Linking Olfactory Impairment and Risk of Mortality

Olfaction is a sense involved in a complex set of tasks, influencing eating behavior, increasing awareness of environmental hazards and affecting social communication. Surprisingly, smell disorders are very frequent, especially in the elderly population. Several recent studies conducted mostly in older subjects have demonstrated a strong association between olfactory impairment and overall mortality risk, with anosmia being even more predictive of 5 years mortality risk than cardiovascular disease. Presently, the underlying pathophysiology linking olfactory impairment to mortality remains unknown and only putative mechanisms are suggested. This review aims to examine the link between olfactory impairment and mortality and to discuss existing ideas on underlying existing mechanisms including, (1) the effect of olfactory loss on nutrition, life-threatening situations and social interactions, (2) associated neurodegenerative diseases, (3) accelerated brain aging, and (4) reflection of general health status being reflected in olfactory function.

Factors associated with odour identification in older Indonesian and white Australian adults

BACKGROUND AND AIMS

Among older adults, olfactory dysfunction is associated with cognitive impairment, lower quality of life, and increased mortality. While age is a risk factor for olfactory dysfunction, other risk factors are less well understood, and may vary between ethno-regional groups. This study investigated how associations between odour identification (OI) and various risk factors, as well as cognition and language ability, differed or were similar in two distinct ethno-regional groups of older adults.

METHODS

This cross-sectional study used data from two cohorts: 470 Indonesians (aged 67.4 ± 7.4 years) and 819 white Australians (aged 78.7 ± 4.8 years). Univariate and multivariate analyses explored whether OI test scores were associated with age, sex, education, cholesterol levels, apolipoprotein E ε4 status, smoking, diabetes, hypertension and depression scale scores, or with Mini-Mental State Examination (MMSE) and language test performance.

RESULTS

Univariate analyses identified some factors associated with OI scores in both Indonesians and white Australians, including older age and smoking with lower scores, and MMSE and language test performance with higher scores. Multivariate analyses yielded different and mutually exclusive patterns of associations in the two ethno-regional groups, with language test scores significantly associated with higher OI scores in Indonesians, and age, being male, smoking, having diabetes and higher depression scale scores significantly associated with lower OI scores in white Australians.

CONCLUSION

Ethno-regional differences may need consideration in the attempt to fully understand associations between OI and negative outcomes like dementia and mortality, and interventions for olfactory dysfunction might need to be tailored to specific ethno-regional groups. However, the difference in mean age between cohorts is a limitation of this study, and future studies should aim to compare populations with similar age distributions.

Associations of olfactory dysfunction with anthropometric and cardiometabolic measures: Findings from the 2013-2014 national health and nutrition examination survey (NHANES)

We examined associations of olfactory dysfunction with anthropometric and cardiometabolic measures in a nationally representative sample of US adults. In the 2013-2014 National Health and Nutrition Examination Survey (NHANES), 3,815 participants, 40 years and older, completed a standardized taste and smell protocol, which consisted of an 8-item odor identification test and a chemosensory questionnaire. Measured dysfunction was incorrect identification of ≥3 of 8 odors; self-reported dysfunction was affirmative response to either a recent smell problem, worse ability since age 25, or phantosmia. Survey-weighted linear regression models tested associations of olfactory dysfunction with body mass index (BMI), waist circumference (WC), blood pressure, serum total cholesterol (TC) with fractions, triglycerides, and glucose levels. Models were adjusted for age, race, education, physical activity, self-reported general health condition, smoking history, and income-to-poverty ratio, stratifying by sex and age group (middle-age 40-64 years; older ≥65 years). Relative to normal, measured olfactory dysfunction was associated with lower BMI [β=-1.6 (95% CI: -3.2, -0.01)] in older men. In middle-age women, dysfunction was associated with higher BMI and WC, whether assessed by examination [β's for BMI=3.1 (0.6, 5.5), WC=5.0 (0.3, 9.8)] or self-report [β's for BMI=2.5 (0.6, 4.3), WC=6.1 (2.2, 9.9)]. Measured dysfunction was associated with significantly higher TC [β=12.8 (7.5, 18.1)] and LDL [β=18.1 (9.1, 27.2)] among older men, but significantly lower TC [β=-15.0 (-25.0, -5.7)] and marginally lower LDL [β=-12.0 (-25.0, 1.2)] among older women. Between measured dysfunction and fasting glucose, the association was inverse [β=-7.9 (-13.0, -2.6)] among middle-age men, but positive [β=15.6 (1.5, 29.7)] among older women. No significant associations were observed with blood pressure levels. In conclusion, among US adults ≥40 years, olfactory dysfunction is associated with anthropometric and glucose and lipid levels, with associations varying by sex and age group.

Olfactory dysfunction persists after smoking cessation and signals increased cardiovascular risk

BACKGROUND

Olfaction plays a critical role in health and function in older adults, and impaired sense of smell is a strong predictor of morbidity and mortality. Smoking cigarettes causes olfactory impairment, but the mechanism of damage and ability to recover after cessation are unknown. We investigated the relationship between time since quitting and olfactory dysfunction in order to elucidate the mechanism(s) by which smoking damages the olfactory system and to inform patient counseling.

METHODS

Using longitudinal data from the National Social Life Health and Aging Project (n = 3528 older adults, including 1526 former smokers), we analyzed the association between odor identification performance and time since smoking cessation using multivariate ordinal logistic regression, adjusting for cognition and demographic variables. To test whether vascular disease plays a role, we also assessed the relationship between olfactory decline and incidence of heart attack and heart disease.

RESULTS

Former smokers who quit ≤15 years before testing had significantly impaired olfaction compared to never smokers (p = 0.04), but those who quit >15 years prior did not. Olfactory decline over 5 years showed modest evidence toward predicting increased incidence of heart attack or heart disease (p = 0.08).

CONCLUSION

Olfactory impairment in smokers persists 15 years after quitting, which is consistent with a vascular mechanism of impairment. Indeed, olfactory decline is a predictor of the development of cardiovascular disease. Taken together, these data suggest that olfactory loss may be a useful sign of underlying vascular pathology. Further investigation of olfactory loss as an early biomarker for cardiovascular disease is warranted.

The association between diabetes and olfactory impairment in adults: A systematic review and meta-analysis

Background

Evidence suggests that olfactory impairment (OI) may be a degenerative neurologic complication of diabetes; however, the association is not yet well established. The objective of this work was to systematically review existing literature on the association between diabetes and OI in adults, with meta‐analysis of evaluable studies.

Methods

A literature search encompassing 358 abstracts from the last 75 years in PubMed, EMBASE, and Cochrane was performed. English‐language articles investigating adults with diabetes and OI in comparison to control groups with original data and ≥7 subjects were included. The Newcastle‐Ottawa scale was applied for quality assessment. Two investigators independently reviewed all articles. For meta‐analysis, the odds ratio of OI in diabetes compared with control groups was calculated using the fixed effects model.

Results

The initial search yielded 358 abstracts, from which 21 articles were reviewed and 11 articles (n = 6,747) were included. Studies included were case‐control (64%) or cross‐sectional (36%) with evidence level 3b. On the Newcastle‐Ottawa scale, the mean quality assessment score for case‐control and cross‐sectional studies was 7.4 (maximum of 9) and 7.0 (maximum of 10), respectively. A statistically significant association between diabetes and olfaction compared with controls was found in 6 (55%) of the 11 articles. Four studies were eligible for meta‐analysis, which yielded an overall odds of having OI with diabetes as 1.58 times more likely than in control groups (95% CI [1.16, 2.16]; I² = 10.3%).

Conclusions

The reviewed studies support a significant association between diabetes and OI. Further studies are warranted to characterize this association.

Level of Evidence

3a

Influence of chronic diseases on the olfactory function in children

The association between smell impairment and chronic diseases has been reported in some studies in adults. Such information is not available for chronic diseases in children. The aim of this study was to examine olfactory function of children with chronic diseases such as diabetes mellitus type 1, hypothyroidism, and bronchial asthma in combination with allergic rhinitis in comparison to healthy controls. The data were obtained from n = 205 participants (104 boys, 101 girls) between the age of 6 and 17 years. Seventy-eight of the participants were healthy controls, n = 43 had diabetes mellitus type 1, n = 50 suffer from allergic rhinitis or bronchial asthma, and 34 presented a reduced function of their thyroid in medical history. All participants underwent olfactory testing including olfactory threshold using "Sniffin' Sticks" and odor identification using the "U-Sniff" test. In addition, a depression inventory and cognitive testing using the Ravens Progressive Matrices was performed. No significant difference in olfactory function was observed for any of the chronic diseases in children in comparison to healthy controls. Further analysis showed a trend in significance for a subpopulation of children with bronchial asthma and comorbidities performed worse on the olfactory threshold test compared to patients with bronchial asthma without comorbidities. Pediatric patients suffering from chronic diseases scored higher on the depression inventory compared to healthy controls.Conclusion: In conclusion, this study demonstrates that the influence of chronic diseases (bronchial asthma, diabetes mellitus type 1 and hypothyroidism) on olfactory function in childhood, if any, seems to be insignificant. This is partly in contrast to adult patients. Further research should be conducted in a subgroup of patients with bronchial asthma, allergic rhinitis, and atopic dermatitis or other comorbidities to better understand the association of allergic diathesis and olfactory function and the putative pathogenesis of olfactory dysfunction. What is known: • The association between smell impairment and chronic diseases has been reported in some studies in adults. • Such information is not available for chronic diseases in children. What is new: • The influence of chronic diseases (bronchial asthma, diabetes mellitus type 1, and hypothyroidism) on olfactory function in childhood, if any, seems to be insignificant. • In patients with bronchial asthma and allergic rhinitis, only a subgroup of patients with additional comorbidity (atopic dermatitis) showed a tendency to a reduced sense of smell.

Thymol polymeric nanoparticle synthesis and its effects on the toxicity of high glucose on OEC cells: involvement of growth factors and integrin-linked kinase

Background

Nowadays, the drug delivery system is important in the treatment of diseases.

Purpose

A polymeric nanoparticle modified by oleic acid (NPMO) as a Thymol (Thy) drug release system was synthesized from Thymbra spicata and its neurotrophic and angiogenic effects on rat’s olfactory ensheathing cells (OECs) in normal (NG) and high glucose (HG) conditions were studied.

Methods

The NPMO was characterized by using different spectroscopy methods, such as infrared, HNMR, CNMR, gel permeation chromatography, dynamic light scattering, and atomic force microscopy. Load and releasing were investigated by HPLC. The toxicity against OECs diet-induced by MTT assay. ROS and generation of nitric oxide (NO) were evaluated using dichloro-dihydro-fluorescein and Griess method, respectively. The expression of protein integrin-linked kinase (ILK), vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor (BDNF), and nerve growth factor (NGF) were evaluated by Western blotting.

Results

ThyNPMO is desirable for transferring drug as a carrier. The amount of Thy and extract (E) loaded on NPMO estimated at 43±2.5% and 41±1.8%, respectively. Then, 65% and 63% of the drug load were released, respectively. Thy, ThyNPMO, E, and ENPMO prevented HG-induced OECs cell death (EC50 33±1.5, 22±0.9, 35±1.8, and 25±1.1 μM, respectively). Incubation with Thy, ThyNPMO, E ,and ENPMO at high concentrations increased cell death with LC50 105±3.5, 82±2.8, 109±4.3, and 86±3.4 μM, respectively in HG states.

Conclusion

OECs were protected by ThyNPMO and ENPMO in protective concentrations by reducing the amount of ROS and NO, maintaining ILK, reducing VEGF, and increasing BDNF and NGF. The mentioned mechanisms were totally reversed at high concentrations.

The role of insulin sensitivity and intranasally applied insulin on olfactory perception

Olfactory perception determines food selection behavior depending on energy homeostasis and nutritional status. The mechanisms, however, by which metabolic signals in turn regulate olfactory perception remain largely unclear. Given the evidence for direct insulin action on olfactory neurons, we tested olfactory performance (olfactory threshold, olfactory discrimination) in 36 subjects of normal- and overweight after administration of three different insulin doses (40 I.U., 100 I.U., 160 I.U.) or corresponding placebo volume in a within-subject design. Poor peripheral insulin sensitivity as quantified by HOMA-IR in baseline condition and increases in systemic insulin levels reactive to intranasal administration predicted poor olfactory performance. In contrast, intranasal insulin enhanced odor perception with a dose-dependent improvement of olfactory threshold. These findings indicate a new diametric impact of insulin on olfactory perception depending on peripheral or central availability.

Diabetes in the practice of otolaryngology

Diabetes mellitus is the most common endocrine disease, characterized by chronic hyperglycemia. The hyperglycemic milieu leads to endothelial injury in blood vessels of variant size, which results in microangiopathy and macroangiopathy (atherosclerosis). Consequential ischemia of nerves and hyperglycemia by itself lead to nerve degeneration and generalized neuropathy, affecting most often the sensory peripheral nerves and the autonomic nervous system. Auditory, vestibular and olfactory sensorium may be compromised by DM. People with DM have an increased susceptibility to infection, as a result of neutrophil dysfunction and impaired humoral immunity. Therefore DM predisposes to certain infectious diseases, such as fungal sinusitis or malignant otitis externa, which are rare in general population. Recovery from infections or from injuries may be compromised by coexisting DM. In this review we discuss complications of DM in the head and neck region. Otolaryngologists and general practitioners should be alert to specific conditions related to DM and be minded of the relevant complications and consequences.

The Association Between Diabetes and Olfactory Function in Adults

Diabetes is a significant chronic disease that in limited studies has been linked with olfactory dysfunction. We investigated the cross-sectional association between diabetes and olfactory dysfunction in 3151 adults aged ≥40 years who participated in US National Health and Nutrition Examination Survey 2013-2014 with information on olfactory dysfunction and diabetes. Diabetes was defined from fasting serum glucose ≥126 mg/dL, oral glucose tolerance test ≥200 mg/dL, HbA1c levels ≥6.5%, physician-diagnosed diabetes, or current use of oral hypoglycemic agents and/or insulin. Self-reported olfactory dysfunction was defined as a positive answer to any of the following questions: 1) "Have you had problem with smell in the past 12 months?"; 2) "Have you had a change in the ability to smell since age 25?", or 3) "Do you have phantom smells?". Participants were considered to have severe hyposmia or anosmia if they had <5 correct answers in the 8-item pocket smell test. Analyses were adjusted for the main confounders, including olfactory dysfunction risk factors. Compared to non-diabetics, diabetics under insulin treatment showed a higher prevalence of phantom odors [OR(95% CI): 2.42 (1.16; 5.06)] and a non-significant higher prevalence of severe hyposmia/anosmia [OR(95% CI): 1.57 (0.89; 2.78)]. Amongst diabetics, there was a significant trend to severe hyposmia/anosmia for those on more aggressive treatments [OR (95% CI) including oral and insulin treatment compared to those who reported no use of drug treatment, respectively: 1.33 (0.60; 2.96) and 2.86 (1.28; 6.40); P trend 0.01]. No association was observed between diabetes duration and prevalence of olfactory dysfunction.

Disrupted topological organization of human brain connectome in diabetic retinopathy patients

OBJECTIVE

There is increasing neuroimaging evidence that type 2 diabetes patients with retinal microvascular complications show abnormal brain functional and structural architecture and are at an increased risk of cognitive decline and dementia. However, changes in the topological properties of the functional brain connectome in diabetic retinopathy (DR) patients remain unknown. The aim of this study was to explore the topological organization of the brain connectome in DR patients using graph theory approaches.

METHODS

Thirty-five DR patients (18 males and 17 females) and 38 healthy controls (HCs) (18 males and 20 females), matched for age, sex, and education, underwent resting-state magnetic resonance imaging scans. Graph theory analysis was performed to investigate the topological properties of brain functional connectome at both global and nodal levels.

RESULTS

Both DR and HC groups showed high-efficiency small-world network in their brain functional networks. Notably, the DR group showed reduction in the clustering coefficient (P=0.0572) and local efficiency (P=0.0151). Furthermore, the DR group showed reduced nodal centralities in the default-mode network (DMN) and increased nodal centralities in the visual network (VN) (P<0.01, Bonferroni-corrected). The DR group also showed abnormal functional connections among the VN, DMN, salience network (SN), and sensorimotor network (SMN). Altered network metrics and nodal centralities were significantly correlated with visual acuity and fasting blood glucose level in DR patients.

CONCLUSION

DR patients showed abnormal topological organization of the human brain connectome. Specifically, the DR group showed reduction in the clustering coefficient and local efficiency, relative to HC group. Abnormal nodal centralities and functional disconnections were mainly located in the DMN, VN, SN, and SMN in DR patients. Furthermore, the disrupted topological attributes showed correlations with clinical variables. These findings offer important insight into the neural mechanism of visual loss and cognitive deficits in DR patients.

Systemic diseases and disorders

Alterations in the ability to smell or taste are of considerable consequence, impacting quality of life, safety, nutrition, and dietary activities. These primary senses are influenced by a wide range of systemic diseases and disorders that commonly involve the entire body. These include viral, bacterial, fungal, protozoal, cestode, and nematode infections that can spread throughout the gastric, lymphatic, neural, or circulatory systems as well as classic autoimmune disorders, collagen diseases, diabetes, and hypertension, and others. Although a considerable literature has evolved in which the function of both taste and smell has been assessed in a number of such disorders, quantitative chemosensory testing is still relatively rare with many disorders not receiving empirical assessment. Incongruent findings are not uncommon. This chapter reviews what is known about the influences of a wide spectrum of systemic diseases and disorders on the abilities to taste and smell.

Prevalence of taste and smell impairment in adults with diabetes: A cross-sectional analysis of data from the National Health and Nutrition Examination Survey (NHANES)

AIM

This study investigates the prevalence of smell and taste impairment in adults with diabetes and potential risk factors for sense deterioration and its influence of daily food intake.

METHODS

Data from the NHANES 2013-2014 were analyzed. Smell impairment was defined as failing to identify ≥3 of 8 odors in NHANES Pocket Smell Test. Taste impairment was defined as being unable to identify quinine or NaCl in NHANES Tongue Tip and Whole-mouth Test.

RESULTS

A total of 3204 people (428 patients with diabetes, 2776 controls) were suitable to be included. The prevalence of smell impairment in patients with diabetes was higher compared to the controls: 22% versus 15% (p<0.001). The difference prevailed after adjustment for age, BMI, alcohol misuse and smoking status. Taste was not impaired in patients with diabetes (p=0.29). Patients with diabetes and smell impairment had a lower daily calorie intake compared to patients with diabetes and normal smell function. The duration of diabetes, diabetic complications and other potential risk factors were not associated with smell dysfunction.

CONCLUSIONS

Smell dysfunction appears with a higher prevalence in patients with diabetes, and this seems to negatively affect daily food intake.

Altered Odor-Induced Brain Activity as an Early Manifestation of Cognitive Decline in Patients With Type 2 Diabetes

Type 2 diabetes is reported to be associated with olfactory dysfunction and cognitive decline. However, whether and how olfactory neural circuit abnormalities involve cognitive impairment in diabetes remains uncovered. This study thus aimed to investigate olfactory network alterations and the associations of odor-induced brain activity with cognitive and metabolic parameters in type 2 diabetes. Participants with normal cognition, including 51 patients with type 2 diabetes and 41 control subjects without diabetes, underwent detailed cognitive assessment, olfactory behavior tests, and odor-induced functional MRI measurements. Olfactory brain regions showing significantly different activation between the two groups were selected for functional connectivity analysis. Compared with the control subjects, patients with diabetes demonstrated significantly lower olfactory threshold score, decreased brain activation, and disrupted functional connectivity in the olfactory network. Positive associations of the disrupted functional connectivity with decreased neuropsychology test scores and reduced pancreatic function were observed in patients with diabetes. Notably, the association between pancreatic function and executive function was mediated by olfactory behavior and olfactory functional connectivity. Our results suggested the alteration of olfactory network is present before clinically measurable cognitive decrements in type 2 diabetes, bridging the gap between the central olfactory system and cognitive decline in diabetes.

Evaluation of olfaction and taste function in type 2 diabetic patients with and without peripheral neuropathy

AIMS

Olfaction and gustation in patients with diabetes mellitus have great significance on quality of life, and their impairment may result in possible hazards. A limited number of studies have been performed to determine the alteration of both gustatory and olfactory function in type 2 diabetic patients with diabetic peripheral neuropathy (DPN). The aim of this study was to determine whether type 2 diabetic patients, with and without DPN, exhibit major olfactory and gustatory dysfunction using validated and dependable techniques.

METHODS

An observational-analytical case-control study was conducted. Sixty patients with type 2 diabetes mellitus (T2DM) and 30 healthy control subjects with a mean age of 57.1 ± 8.4 were included in the study. Patients with T2DM were recruited from the endocrinology outpatient clinic. After clinical evaluation and electromyography examination, patients with T2DM were divided into the 2 groups, with and without DPN. After a 10-hour fasting period, blood samples were taken for the measurement of serum creatinine, lipids, and HbA1c. For the quantitative assessment of olfactory function, all participants underwent butanol threshold test and odour identification test. Gustatory function was tested administering a whole-mouth above-threshold test using sucrose solutions.

RESULTS

The control subjects showed significantly higher Sniffin' sticks and butanol threshold scores than the diabetic patients without DPN (P = .001 and P = .009). No significant difference was found in the gustatory function test between these 2 groups (P = .116). Diabetic patients with DPN had lower Sniffin' sticks scores, butanol threshold scores, and higher sucrose thresholds compared to the controls (P < .001, P < .001, and P = .002). There were no significant differences between diabetic patients with or without DPN regarding Sniffin' sticks scores, butanol threshold, and sucrose thresholds (P = .302, P = .181, and P = .118).

CONCLUSION

In conclusion, this study demonstrates that T2DM is associated with olfactory and gustatory dysfunction. The fact that there was no difference between the diabetic patients with and without DPN elicits the idea of central neuropathy. This novel finding might facilitate the addition of olfactory and gustatory tests to the methodological spectrum of afferent pathway investigations.

Type 2 diabetes impairs odour detection, olfactory memory and olfactory neuroplasticity; effects partly reversed by the DPP-4 inhibitor Linagliptin

Recent data suggest that olfactory deficits could represent an early marker and a pathogenic mechanism at the basis of cognitive decline in type 2 diabetes (T2D). However, research is needed to further characterize olfactory deficits in diabetes, their relation to cognitive decline and underlying mechanisms.

The aim of this study was to determine whether T2D impairs odour detection, olfactory memory as well as neuroplasticity in two major brain areas responsible for olfaction and odour coding: the main olfactory bulb (MOB) and the piriform cortex (PC), respectively. Dipeptidyl peptidase-4 inhibitors (DPP-4i) are clinically used T2D drugs exerting also beneficial effects in the brain. Therefore, we aimed to determine whether DPP-4i could reverse the potentially detrimental effects of T2D on the olfactory system.

Non-diabetic Wistar and T2D Goto-Kakizaki rats, untreated or treated for 16 weeks with the DPP-4i linagliptin, were employed. Odour detection and olfactory memory were assessed by using the block, the habituation-dishabituation and the buried pellet tests. We assessed neuroplasticity in the MOB by quantifying adult neurogenesis and GABAergic inhibitory interneurons positive for calbindin, parvalbumin and carletinin. In the PC, neuroplasticity was assessed by quantifying the same populations of interneurons and a newly identified form of olfactory neuroplasticity mediated by post-mitotic doublecortin (DCX) + immature neurons.

We show that T2D dramatically reduced odour detection and olfactory memory. Moreover, T2D decreased neurogenesis in the MOB, impaired the differentiation of DCX+ immature neurons in the PC and altered GABAergic interneurons protein expression in both olfactory areas. DPP-4i did not improve odour detection and olfactory memory. However, it normalized T2D-induced effects on neuroplasticity.

The results provide new knowledge on the detrimental effects of T2D on the olfactory system. This knowledge could constitute essentials for understanding the interplay between T2D and cognitive decline and for designing effective preventive therapies.