How can the nose possibly predict your brain health?

When most people think about protecting their brain, they think about crossword puzzles, exercise or eating more blueberries. Few of us spare much thought for our sense of smell. Yet, it may be one of the most revealing windows into brain health.

Most of us have experienced how powerful smell can be. Certain scents may transport us back decades, evoking vivid memories of someone we knew or places we visit. Unlike our other senses, smell has a direct connection to parts of the brain responsible for memory and emotion – particularly the hippocampus and amygdala. These regions are deeply involved in forming memories.

Research over many years has shown that a reduced sense of smell, known as anosmia, is associated with an increased risk of developing Alzheimer’s disease and several other neurodegenerative disorders. In fact, changes in smell can occur years before memory symptoms become obvious.

Why does this happen? The answer lies in the anatomy of the brain. Alzheimer’s disease often begins in areas involved in processing smell before spreading to regions responsible for memory, reasoning and language. As these early changes occur, people may gradually lose the ability to identify familiar odours without realising.

Many years ago, smell testing formed part of the assessment of people with suspected dementia. Patients were asked to identify familiar scents such as coffee, peppermint or cinnamon in standardised smell tests. Although these tests are no longer used routinely to diagnose dementia, they remain valuable research tools.

It should be noted that a poor sense of smell does not mean you have dementia. There are many far-more common causes, including chronic sinus disease, nasal polyps, smoking, head injury and, as many of us discovered during the COVID-19 pandemic, viral infections. However, an unexplained and persistent loss of smell — particularly if accompanied by subtle changes in memory or thinking — should prompt a discussion with your doctor.

Over the past decade, researchers have uncovered a consistent pattern linking several other senses with cognitive decline.

Hearing loss is now recognised as one of the most important potentially modifiable risk factors for dementia. Untreated hearing impairment appears to increase the brain’s workload, reduce social engagement and may accelerate cognitive decline.

Vision tells a similar story. Older adults with untreated cataracts or significant visual impairment have also been shown to have a higher risk of developing dementia. Encouragingly, studies suggest that restoring vision through cataract surgery may even reduce that risk.

Hearing, vision and smell are part of the same picture. Our brains depend on a constant stream of rich sensory information. When that input is diminished, the brain may become less stimulated and less connected with the world around us.

The good news is that we can do something about many of these problems. Regular hearing tests, eye examinations and prompt treatment of hearing or vision impairment are simple but important steps towards maintaining brain health. While we cannot yet reverse smell loss caused by neurodegenerative disease, recognising changes early may identify people who would benefit from further assessment.

Looking after our senses may seem like a small thing, but together with exercise, good nutrition, quality sleep and staying socially and mentally active, it forms another piece of the puzzle. Sometimes the road to a healthier brain begins simply by stopping to smell the roses.

 Kailas Roberts is a psychogeriatrician and author of Mind Your Brain: The Essential Australian Guide to Dementia, available at bookstores and online. Visit yourbraininmind.com or uqp.com.au