Can olfactory loss predict disease? Study ties loss of smell to 139 health conditions

By Priyanjana Pramanik, MSc.Reviewed by Lily Ramsey, LLMNov 1 2024

Preventing memory loss with smell training? Olfactory enrichment found effective in new study.

Study: Inflammation and olfactory loss are associated with at least 139 medical conditions. Image Credit: fizkes/Shutterstock.com

In a recent review article published in Frontiers in Molecular Science, researchers discussed the links between genetic, physical, and neurological conditions and olfactory loss and inflammation.

They concluded that inflammation, environment, and neuroanatomy can connect loss of smell to several conditions and that stimulating the olfactory senses may be useful in treating and preventing these health issues by reducing inflammation in the brain and body.

Health conditions and olfactory loss

Olfactory dysfunction is linked with 139 medical conditions, including genetic or hereditary, physical, and neurological conditions. Extensive research supports connections with conditions such as rhinitis, depression, Parkinson’s, Alzheimer’s, and coronavirus disease-2019 (COVID-19).

In many cases, olfactory loss may appear before symptoms of the conditions are experienced. This indicates that loss of smell could predict future cognitive declines and even mortality.

Olfactory senses are correlated with cognitive capabilities such as decision-making, verbal fluency, and memory. They are also a strong indicator of mortality and outperform heart disease as a predictor.

There are gender differences, with olfactory thresholds in men correlating strongly with executive function and language, but in women, olfactory discrimination is linked to visuospatial abilities.

Causal mechanisms and underlying processes

The olfactory system may be impacted by conditions affecting both the brain and body. Neurological and somatic conditions can harm the olfactory system, while olfactory dysfunction may increase the risk of developing some diseases. Olfactory dysfunction is often linked to inflammation, seen in conditions like hereditary and neurological disorders.

Inflammation can damage the olfactory system through inhaled agents (such as odors or pollution) or blood.

Conditions like Parkinson’s show sniffing issues linked to respiratory inflammation, while loss of sense of smell in people with COVID-19 is also thought to result from inflammation. However, some conditions like Kallmann syndrome cause olfactory loss without inflammation.

COVID-19 patients benefit variably from treatments, and nasal steroids effectively reduce inflammation in young adults. Olfactory training shows age-dependent efficacy, especially for younger individuals with smell impairments. Certain scents (ginger, lavender, and eucalyptus) have anti-inflammatory effects in animal studies, suggesting potential therapeutic roles.

Diet may also be a risk factor, with diets long in healthy fats linked to increased risk of cognitive and olfactory loss. In mice, supplementation with omega-3 prevented memory and olfactory loss.

Olfactory loss is associated with memory decline, particularly in dementia, due to direct brain pathways between smell and memory centers.

Brain regions involved in memory (the amygdala and hippocampus) deteriorate with olfactory loss. Factors like infections, stress, pollution, and smoking can impair both olfaction and memory.

Longitudinal studies show COVID-19-related olfactory loss predicts damage in memory-related brain regions, linked COVID-19 to cognitive decline and increased risk of Alzheimer’s disease.

Benefits of olfactory enrichment

Olfactory enrichment, or the process of stimulating the sense of smell, improves cognitive symptoms. Exposure to essential oils can improve brain health by balancing neurotransmitters, reducing oxidative stress and inflammation, and boosting cognition, memory, and neuroprotection.

There are also benefits to memory in older adults, as olfactory stimulation enhances memory in healthy adults, especially older adults, through daily exposure to essential oils over months. Minimal nighttime olfactory enrichment (2 hours/night for 6 months) significantly improved memory scores in older adults.

In dementia patients, studies have found that frequent olfactory exposure helps improve memory, depression symptoms, attention, and language skills. A rich olfactory environment, combined with education and social activities, may foster cognitive reserve, potentially protecting against dementia symptoms, even if the disease itself is present.

Researchers hypothesize that these benefits could be due to three mechanisms.

First, olfactory dysfunction and cognitive decline are linked to inflammation, and pleasant smells can reduce inflammation, possibly helping with memory and cognitive symptoms in neurological conditions.

Second, daily olfactory exposure can increase the brain's olfactory-processing regions, potentially aiding memory and cognitive function. The third could be due to electrical stimulation, as stimulating the olfactory bulb in animal models has shown promise in reducing beta-amyloid plaque (associated with Alzheimer’s) and improving working memory.

Conclusions

Olfactory dysfunction is linked to at least 139 medical conditions. Often, olfactory loss appears before the medical condition’s symptoms, suggesting it may increase vulnerability to these conditions and can predict memory decline and increased risk of mortality.

Inflammation may play a key role in linking olfactory dysfunction to memory issues and other medical symptoms, but environmental and neuroanatomical influences also contribute, potentially creating a two-way relationship.

Olfactory enrichment can enhance memory in both healthy and dementia-affected adults, possibly stemming from its anti-inflammatory effects. Pleasant scents might help reduce inflammation, potentially alleviating symptoms in various medical conditions.