There may be more to the saying ‘stop to smell the roses' — it may help to keep your brain sharp
Many of us have experienced the sensation of a smell immediately returning us to our childhoods. Freshly-cut grass reminds me of athletics in the back field at school. Enticing aromas from sizzling sausages create mental images of delicious smells wafting up the stairs on Sunday mornings. A turf fire brings me back to summer evenings with cousins in the country.
Recent research, led by the National Institute on Aging and published in Neurology (2023), suggests that a declining ability to detect scents as we age could be linked to conditions such as Parkinson's and Alzheimer's. However, the good news is that other studies have found that training our sense of smell may delay cognitive decline and might even help reverse some of its effects.
Our olfactory ability works like a muscle — the more we use it, the stronger it gets. It never occurred to me that exercising my sense of smell was important to keeping it in tip-top condition, and I am not the only one who underrates its importance.
In 2015, a survey of some 7,000 young people worldwide found that they would 'rather give up their sense of smell than their smartphone'.
Human olfaction is less well developed than it is in animals. Dogs are renowned for their tendency to sniff the air, vegetation, other humans, or anything that crosses their path.
In the late 19th century, the neuroanatomist Paul Broca defined all mammals as osmatiques, such as dogs, or anosmatiques — animals less guided by their snouts, including dolphins, whales, monkeys, and humans. His categorisation of mammals into two groups was based on variations in the size of the olfactory bulb. A dog's olfactory bulb is about 40 times as big as that of humans.
However, more recent evidence suggests that humans' sense of smell is more refined than previously thought. In 2014, research at Rockefeller University in New York used 128 odorous molecules to test participants' ability to notice any change in composition. The researchers found that participants could distinguish subtle changes in scents to a remarkable degree. A further study found that humans can distinguish up to 5,000 odours.
Links to mood
Brain injury, viral infections, and chronic sinusitis are common causes of anosmia, or a lack of the sense of smell.
A study in Nature (2022) investigated just how deeply olfactory dysfunction can affect people. Lead author Thomas Hummel, at the Dresden University of Technology in Germany, tracked 171 people with a damaged sense of smell over 11 months. The researchers found a clear link between patients' ability to smell and their depressive symptoms over this period. Even more interesting, as their olfactory function improved during recovery, so did their mood.
There is mounting evidence of a link between olfactory ability and cognitive function.
A study published in Alzheimer's Research and Therapy (2019) assessed the sensitivity of 7,000 participants and found a clear correlation between the ability to discern odours and mental abilities.
The weaker the sense of smell, the worse participants scored in verbal fluency, attention, memory, and learning. The researchers suggested that olfactory testing could be a useful screening test for cognitive impairment.
A study published in Alzheimer's Research and Therapy (2019) assessed the sensitivity of 7,000 participants and found a clear correlation between the ability to discern odours and mental abilities.
Other studies have reached similar conclusions, with some researchers suggesting that lost smell sensitivity could contribute to the brain's deterioration.
Brain scans support this theory. A study in Frontiers in Allergy (2023) reported widespread loss of brain grey matter accompanying olfactory dysfunction, with the most pronounced changes seen in the olfactory bulb itself.
Research has also linked olfactory dysfunction with impaired immunity. The Karolinska Institute in Sweden found a clear link between participants' ability to distinguish between smells such as rotten eggs, urine, vomit, and fermented fish, and markers of inflammation in their saliva. The fouler the stench, the higher the levels of inflammation.
This finding suggests that the foul stench primed the body to produce an inflammatory response to protect against pathogens. Meanwhile, pleasant odours such as eucalyptus, lavender, ginger, citrus, and peppermint have been shown to suppress inflammation.
Could a healthy sense of smell help to keep our immune system in check, raising inflammation when it perceives a potential threat to our health and lowering it when we are in a safe environment? Scientists at the Karolinska Institute suggested that olfactory dysfunction may throw this balance out of kilter, leading to chronic inflammation that damages the brain.
Smelling trouble
Paying more attention to our sense of smell could help improve our olfactory sensitivity and support cognitive function.
A study published in Laryngoscope (2009) recruited 56 people with olfactory dysfunction. Forty were assigned to smell training over 12 weeks, during which time they sniffed four odours for at least 10 seconds twice a day and kept a diary of their experiences. The study group demonstrated increased odour sensitivity, compared to no change in the control group. Crucially, the training was shown to boost brain function.
A study in Neuropsychology Review (2023) found promising evidence that smell training can slow or even reverse certain signs of cognitive decline. This included some evidence of neurological changes in the brain, in regions such as the hippocampus, that are important for cognitive ability.
There may be more to the saying 'stop to smell the roses' than taking the time to enjoy life. It might also be helping to keep your brain sharp as you age.
Pure essential oils, such as lemon, clove, eucalyptus, and rose, are available in health shops or online and can help make regular 'smell training' part of your daily routine.
Catherine Conlon is a public health doctor
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