the sensory system : smell

 Smell, or olfaction, is mediated by the olfactory system, which includes      the olfactory pithelium in the nasal cavity and the olfactory bulb in the  brain. Odor molecules enter the nasal cavity, where they bind to olfactory  receptor neurons in the olfactory epithelium, triggering electrical  signals transmitted to the olfactory bulb and then to higher brain regions  for processing.

  Disorders affecting smell include:

• Anosmia (loss of smell)
• Hyposmia (reduced sense of smell)
• Parosmia (distorted sense of smell)
• Phantosmia (perceiving smells that aren't present)
• Age-related decline in olfactory function

Research suggests potential links between olfactory dysfunction and  metabolic disorders like insulin resistance and diabetes. Chronic hyperglycemia and microvascular complications associated with diabetes may contribute to olfactory nerve damage and impaired olfactory function.

Obesity-related conditions like obstructive sleep apnea and nasal  congestion can impair the sense of smell. Additionally, obesity-related  inflammation may affect olfactory receptors and neural pathways involved  in smell perception. 

Diabetes-related neuropathy can affect the olfactory nerve, leading to  anosmia (loss of smell) or hyposmia (reduced ability to smell). Changes  in smell perception may impact appetite and food enjoyment in  individuals with diabetes. 

Anosmia refers to the complete loss of the sense of smell. It can result from various causes, including nasal congestion, head trauma, neurological disorders, or viral infections such as the common old. While metabolic factors are not typically directly implicated in anosmia, conditions associated with hyperinsulinemia, insulin resistance, or metabolic syndrome, such as diabetes mellitus, may increase the risk of nerve damage or vascular changes affecting olfactory function. Chronic hyperglycemia and oxidative stress associated with diabetes can impair nerve function, potentially contributing to anosmia.

" Over 24% of older American adults (approximately 14 million) are  estimated to have reduced olfactory sensitivity. Previous studies have  provided evidence that patients with diabetes mellitus (DM) or its  complications are at increased risk of olfactory dysfunction. We  therefore investigated whether smell dysfunction was associated with  DM-related biomarkers, including fasting blood glucose, glycohemoglobin,  serum insulin, and homeostasis model assessment of insulin sensitivity  (HOMA-IR), in older US adults."

"We found a significant association between smell dysfunction and  severe insulin resistance in older US adults. Our data suggests that  insulin resistance may be mechanistically linked to loss of smell  function"

 Hyposmia refers to a reduced sense of smell, where individuals may have difficulty detecting or distinguishing odors. It can be caused by various factors, including nasal congestion, nasal polyps, aging, or neurological conditions.

Metabolic syndrome-associated conditions, such as obesity or diabetes mellitus, may indirectly affect olfactory function through alterations in nasal airflow or inflammatory responses in the nasal passages. Insulin resistance and hyperinsulinemia may exacerbate inflammation or vascular changes in the nasal mucosa, potentially contributing to hyposmia.

" The prevalence of olfactory dysfunction was significantly higher in  older people with MetS than in those without MetS in both sexes  "

"  Results of this pilot study shed light on the link between cerebral  insulin level and an impaired sense of smell. This research line might  provide a better understanding of olfactory loss in relation to eating  and dietary behavior, and could offer opportunities to develop faster  therapeutic intervention for patients with olfactory dysfunction   "

 " We have provided evidence of an association between lowered insulin  sensitivity ..... and deteriorated olfactory function. " 

 "Nowadays, it is well accepted that obesity and metabolic syndrome are  diseases that constitute a global public health issue. In consequence,  the interest in the study of the effects these pathologies produce in  the central nervous system has greatly increased in the last decades.  One of the most overlooked topics in the literature is the impact they  exert in sensory systems, among which is olfaction. The olfactory system  is related to a number of vital functions, like the activation of  defense mechanisms, contribution to appetitive and digestive reflexes,  recognition of conspecifics, and even has socio-sexual implications. It  has been discovered that the olfactory system also plays a crucial role  in food intake, the choice of foods, appetite and satiety mechanisms;  therefore, it is involved in obesity development. Clinical studies have  proven that obese patients exhibit hyposmia more frequently than  aged-matched healthy controls. Olfactory alterations have also been  found in obese rodents or in animals with similar features of human  metabolic syndrome. The causes of this association are still being  investigated. This work reviews the studies that have tried to  understand this association from a preclinical and clinical approach as  well as those biological mechanisms that could be involved. The  evidences here presented suggest that obesity and metabolic syndrome  affect the adequate function of olfactory sensory system. " 

Parosmia is a condition where individuals perceive smells differently from how they actually are. Common triggers include viral infections, head trauma, or nasal irritation.

While the exact mechanisms of parosmia are not fully understood, metabolic factors such as insulin resistance and hyperinsulinemia may contribute to alterations in olfactory processing or neuronal signaling, potentially leading to distorted perceptions of smell. Metabolic syndrome-associated conditions, such as diabetes mellitus, may exacerbate nerve damage or inflammation in the olfactory pathways, further influencing parosmia symptoms.

"  Results of this pilot study shed light on the link between cerebral  insulin level and an impaired sense of smell. This research line might  provide a better understanding of olfactory loss in relation to eating  and dietary behavior, and could offer opportunities to develop faster  therapeutic intervention for patients with olfactory dysfunction   "

Presbyosmia is a term used to describe age-related changes in the sense of smell. It refers to the gradual decline in the ability to detect and differentiate odors that occurs with advancing age. Like other sensory systems, the sense of smell can be affected by aging, leading to decreased sensitivity to odors and changes in olfactory perception. 

Several factors contribute to presbyosmia, including:

Loss of Olfactory Receptor Cells: With age, there is a natural decline in the number of olfactory receptor cells in the nasal epithelium. These cells are responsible for detecting odor molecules in the air. A reduction in the number of receptor cells can lead to decreased sensitivity to odors.

Changes in Olfactory Bulb and Brain Function:Age-related changes in the olfactory bulb, which is part of the brain responsible for processing olfactory information, can also contribute to presbyosmia. Additionally, alterations in higher brain regions involved in olfactory processing may affect the perception and interpretation of odors.

Decreased Mucous Secretion: Aging may lead to reduced mucous secretion in the nasal passages, which can affect the ability of odor molecules to dissolve and bind to olfactory receptors. This can further contribute to diminished olfactory sensitivity.

Degeneration of Olfactory Nerves: The olfactory nerves, which transmit signals from the olfactory receptor cells to the brain, may undergo degenerative changes with age. This can impair the transmission of olfactory information and reduce the ability to perceive odors.

Presbyosmia can have various effects on individuals, including:

• Difficulty detecting and identifying odors
• Reduced enjoyment of food and beverages
• Impaired ability to detect warning odors, such as those signaling      spoiled food or dangerous substances
• Changes in appetite and eating habits
• Impact on quality of life and overall well-being

Insulin resistance, hyperinsulinemia, and metabolic syndrome can potentially affect presbyosmia through several mechanisms:

Olfactory Receptor Cell Function: 

Insulin resistance and hyperinsulinemia may impair cellular function and signaling pathways in olfactory receptor cells, potentially reducing their sensitivity to odor molecules. This can contribute to diminished olfactory perception and presbyosmia.

Olfactory Bulb and Brain Function: 

Insulin resistance and metabolic syndrome-related factors such as inflammation and oxidative stress may affect the structure and function of the olfactory bulb and higher brain regions involved in olfactory processing. This can lead to alterations in olfactory perception and interpretation, contributing to presbyosmia.

Mucous Secretion: 

Insulin resistance and metabolic syndrome may influence mucous secretion in the nasal passages through various mechanisms, including chronic inflammation and changes in hormonal regulation. Reduced mucous secretion can impair the ability of odor molecules to dissolve and bind to olfactory receptors, exacerbating presbyosmia.

Olfactory Nerve Integrity: 

Insulin resistance and hyperinsulinemia can contribute to vascular dysfunction and microvascular disease, which may affect the integrity of the olfactory nerves. Degenerative changes in olfactory nerve fibers can impair the transmission of olfactory signals to the brain, further contributing to presbyosmia.

Overall, the effects of insulin resistance, hyperinsulinemia, and metabolic syndrome on presbyosmia are complex and multifactorial. These metabolic disturbances can impact various aspects of olfactory function, including receptor cell sensitivity, neural processing, mucous secretion, and nerve integrity, potentially exacerbating age-related declines in olfactory perception. However, further research is needed to fully understand the specific mechanisms underlying the relationship between metabolic disorders and presbyosmia.

" We have provided evidence of an association between lowered insulin  sensitivity ..... and deteriorated olfactory function. "

" Olfactory dysfunction and tinnitus are age-related  otorhinolaryngological disorders with a high prevalence in the elderly  population and share several common clinical features. However, there is  no study investigating the relationship between these two diseases. We  studied the prevalence of olfactory dysfunction and tinnitus among  Koreans and studied the relationship between these two diseases based on  the Korean National Health and Nutrition Examination Survey. The  subjects of this study were enrolled from the Fifth Korean National  Health and Nutrition Examination Survey (2010-2012, n = 25,534). Data of  subjects aged 40 years and older who underwent physical examination and  completed a self-reported questionnaire and other anthropometric  variables were statistically analyzed. Odds ratios were calculated to  identify the relationship between olfactory dysfunction and tinnitus,  using multiple logistic regression models. Older males, non-smokers,  non/lower alcohol drinker groups exhibited the relationship between  olfactory dysfunction and tinnitus. Metabolic syndrome and mental health  problems were associated with both olfactory dysfunction and tinnitus.  After adjusting for confounding factors, olfactory dysfunction was  significantly associated with tinnitus (OR 1.318). There was a  dose-response relationship between tinnitus severity and the odds of  olfactory dysfunction (ORs for mild, moderate and severe tinnitus were,  respectively, 1.134, 1.569 and 2.044). Additional molecular genetics and  animal studies are needed to determine the shared pathophysiology of  the two diseases. "