The relationship between olfactory dysfunction and cognitive decline is a subject of increasing interest in neuroscience, particularly as demographic trends suggest a rise in age-related cognitive disorders, notably dementia. An enlightening article published in Translational Psychiatry on April 28, 2025, authored by Huiling Song and colleagues, presents novel findings exploring how the nasal microbiome may bridge the connection between diminished olfactory function and cognitive health.

Introduction: The Aging Brain and Cognitive Health

As the global population ages, cognitive decline has emerged as a pressing public health issue. The World Health Organization estimates that the prevalence of dementia could rise from 55 million individuals in 2019 to approximately 139 million by 2050. In parallel, research has identified a robust association between olfactory dysfunction—defined as the reduced ability to perceive odors—and cognitive impairment.

The exact nature of the relationship between olfactory function and cognitive decline remains ambiguous. While some studies suggest that reduced olfactory capacity may serve as an early indicator of cognitive impairment, others propose that both conditions might arise from common neurodegenerative processes associated with aging. This prompted a research team from Fudan University to investigate the role of the nasal microbiome in this dynamic.

Research Overview

The study involved a cohort of 510 older adults with an average age of 77.9 years. The researchers assessed olfactory function using the brief Chinese smell identification test and quantified cognitive abilities with psychometric assessments, including the mini-mental state examination and a revised version of the Hasegawa dementia scale.

Methodology

Microbiome analyses were conducted using a technique known as 16S RNA gene sequencing. This method enabled the researchers to establish detailed microbiotic profiles for each participant.

Key Findings

The study revealed significant correlations between nasal microbiome compositions and olfactory and cognitive functions:

“Emerging evidence has highlighted that olfactory dysfunction, a common feature of aging, is increasingly linked to cognitive decline in older adults.” – Huiling Song et al.

Microbial Associations

Among their findings:

  • Participants exhibiting olfactory dysfunction (hyposmia) demonstrated a higher richness of nasal bacteria.
  • A total of 18 distinct nasal bacterial genera were identified to correlate with olfactory function.
  • Notably, genera such as Acidovorax and Morganella were more abundant in individuals with hyposmia.

Impact on Cognitive Function

Further analysis showcased that certain microbiome profiles corresponded with varying degrees of cognitive impairment:

Bacterial Genus Association with Cognitive Impairment
Corynebacterium Lower prevalence of mild cognitive impairment
Dolosigranulum Higher prevalence of mild cognitive impairment
Moraxella Higher prevalence of mild cognitive impairment

This data highlights that the composition of nasal flora can significantly influence cognitive health outcomes in older populations, potentially promoting avenues for early detection of cognitive decline.

Future Implications

The researchers believe their findings could pave the way for innovative strategies aimed at monitoring cognitive health. By focusing on nasal microbiome profiles, it may be possible to develop:

  • New diagnostic tools utilizing nasal microbiome characterization to detect early signs of cognitive decline.
  • Interventions aimed at modifying the nasal microbiome to enhance cognitive resilience.

However, as the researchers indicated, further investigation into the causal mechanisms underlying these relationships is warranted.

Conclusion

This research marks an important step in uncovering the biological links between the microbiome, olfactory function, and cognitive health. By elucidating these connections, the study presents potential pathways for preventive measures and interventions in response to age-related cognitive decline.

Literature Cited

Song, H., Zou, J., et al. (2025). Nasal microbiome in relation to olfactory dysfunction and cognitive decline in older adults. Translational Psychiatry. DOI: 10.1038/s41398-025-03346-y.

World Health Organization. (2021). Dementia Fact Sheet. WHO.