As humans age, particularly after middle age, their brain functions, cognitive abilities and memory can deteriorate to varying degrees. Ageing-related disorders marked by cognitive decline, particularly dementia, have become increasingly widespread over the past decades.

Estimates suggest that the number of individuals diagnosed with dementia could increase from 55 million in 2019 to around 139 million by 2050. Understanding the factors contributing to cognitive decline and devising methods to detect the first signs of dementia is thus of the utmost importance, as it could help to reliably pick up its emergence and plan therapeutic interventions accordingly.

In recent years, some studies have found a link between people's ability to perceive and identify odours (i.e., olfactory function) and their cognitive abilities as older adults. While the relationship between olfactory dysfunction and cognitive decline is now well-documented, whether one causes the other or they are the result of similar ageing-related or neurodegenerative mechanisms remains unclear.

Researchers at Fudan University in China recently carried out a study to further explore the link between olfactory dysfunction and cognitive decline in older adults. Their findings, published in Translational Psychiatry, suggest that the nasal microbiome (i.e., the set of microorganisms residing in each individual's nasal passages) could contribute to the relationship between olfactory function and cognitive health.

"Emerging evidence has highlighted that olfactory dysfunction, a common feature of ageing, is increasingly linked to cognitive decline in older adults," wrote Huiling Song, Jiaojiao Zou and their colleagues.

"However, research on the underlying mechanism, particularly the role of the nasal microbiome, remains limited. In this study, we investigated the associations between olfactory function, the nasal microbiome, and cognition among 510 older adults with an average age of 77.9 years."

As part of their study, the researchers recruited 510 older adults and looked at their olfactory function, nasal microbiomes and cognitive abilities. Their olfactory function was assessed using a procedure known as the brief Chinese smell identification test.

To test the cognitive abilities of the study participants, the researchers asked them to complete tests based on two psychometric measures, known as the mini-mental state examination and a revised Hasegawa dementia scale. Finally, they generated microbiome profiles for each participant using a method known as 16s RNA gene sequencing.

"We observed that olfactory dysfunction (i.e., hyposmia) was associated with a higher richness of nasal bacteria, and such observation was replicated in an external dataset," wrote Song, Zou and their colleagues. "A total of 18 nasal bacterial genera were identified to be associated with olfactory function, with eight genera, such as Acidovorax and Morganella, being enriched in the hyposmic group."

When they analysed the data they collected, the researchers unveiled correlations between some microbial profiles and different levels of cognitive impairments. In fact, they found that people with microbiome profiles marked by a prevalence of some genus of bacteria were found to have milder cognitive impairments than those that predominantly contained another genus of bacteria.

"A composite microbial index of nasal olfactory function significantly improved the reclassification accuracy of the traditional risk model in distinguishing hyposmic from normosmic participants (P = 0.008)," wrote Song, Zou and their colleagues. "Furthermore, participants with a nasal biotype dominated by Corynebacterium had a lower prevalence of mild cognitive impairment compared to those dominated by Dolosigranulum or Moraxella."

Overall, the results suggest that the types of bacteria populating the noses of individuals could contribute to the previously reported link between older people's olfactory function and cognition.

In the future, their study could pave the way for further research focusing on the nasal microbiome profiles they identified, which may help to devise new strategies to detect cognitive decline early and plan targeted treatments.