Richard Sima
Stopping to smell the roses is good life advice. And research suggests it may have an added benefit: it could be a good way to improve your brain health.
The loss of the sense of smell is often one of the first warning signs of neurodegenerative conditions such as Alzheimer’s disease and Parkinson’s disease even up to a decade before there are other diagnosable symptoms. Overall, 90 per cent of people with early-stage Parkinson’s and 85 per cent of those with early-stage Alzheimer’s have olfactory dysfunction, according to a 2021 paper in Ageing Research Reviews.
Experts believe losing our sense of smell, or olfaction, may be a biomarker of declining brain health and are working to make smell testing more commonplace to speed up a diagnosis.
“If someone has horrible olfaction, that seems to be like the canary in the mine where it’s a bellwether that there might be some cognitive problems that may occur later on,” says David Vance, a psychologist and professor at the University of Alabama at Birmingham.
And yet, the sense of smell has gotten short shrift. Of the five classic senses, it is the one many say they valued the least, research shows; one study found that many American students would prefer to lose their smell rather than their hair, phone or little left toe. As a research area, it is also underfunded and understudied.
“We are constantly using our sense of smell and we don’t recognise it. It is something that has been in the background since birth, and we’re just constantly using it so we don’t even know that it’s happening,” says Nicholas Rowan, an otolaryngologist and associate professor at Johns Hopkins University School of Medicine. For “people who lose their sense of smell, it’s very, very disruptive to their lives.”
But people can actually regain their ability to smell; it is something we can practice and improve with smell training – repeatedly sniffing different odours – whether or not our smell is impaired.
Intriguingly, training the nose also seems to train the brain.
Some preliminary research suggests that smell training can boost the brain’s cognitive function – even in people who have normal smelling abilities – with evidence of boosted neuroplasticity, which is the brain’s ability to reorganise or change itself.
Researchers are now working on figuring out how improving smell is linked to improved cognition.
“Where there’s smoke there’s fire, and I think there’s definitely smoke here,” says Vance, who wrote a 2024 paper about smell training in Neuropsychology Review.
So when Rowan invited me to stop by his lab to test my own smell, I couldn’t pass up the chance to see whether my nose was up to snuff.
And I’ve started sniffing scent tubes to train my own sense of smell. The fragrance of rose, the earthy freshness of eucalyptus, the warm spice of clove and the tang of lemon greet me every morning and night – stimulating my nose and, hopefully, my brain.
The smell-brain connection
There is a dynamic neural connection between our olfaction and our ability to think.
“The million-dollar question is how is it linked, and that’s what we’re trying to find out,” says Wassim Najjar, a postdoctoral research fellow working with Rowan in Johns Hopkins University’s School of Medicine.
Our olfactory nerve casts out hairlike neural fibres that dangle into the upper part of our nasal cavity. These fibres contain specialised receptors that bind to odour molecules as they waft into our nose; each nostril contains about 6 million to 10 million olfactory sensory neurons.
“It’s a piece of your brain just hanging out in your nose. It’s just sitting there. It’s exposed to the world around you,” Rowan says.
The idea that our smell is underdeveloped compared with other animals is a 19th-century myth. Studies show that we can detect anywhere between 10,000 and 1 trillion different scents – some at concentrations less than 10 parts per billion – and even outperform dogs for certain odours.
Despite this, most people don’t think about their smell until they lose it, as occurred to millions of people following bouts of COVID. But without it, food loses its flavour and we struggle to smell the warnings signalled by smoke or spoilage.
Our sense of smell may be intimately tied to our brain health because of its unique connection pattern in our brain. Information from our other senses, such as sight and hearing, takes a more indirect path by first passing through our brain’s relay hub, the thalamus, before arriving at their respective cortical areas.
For smell, information is passed from the olfactory bulb to the olfactory cortex – not needing a trip through the thalamus – making more direct connections to brain areas important for cognition, memory and mood, including the hippocampus and amygdala.
This anatomical coupling could explain why certain smells are strongly tied to emotions and memories.
How smell affects brain health
The 2024 Lancet Commission on dementia listed hearing loss and vision loss as risk factors for dementia that we can mitigate. But one longitudinal study of 408 adults reported that smell was more predictive of cognitive decline than vision and hearing.
Smell loss is linked to reduced volume in brain regions in our olfactory system but also in brain areas important for cognition, memory and emotion. The olfactory nerve is the only cranial nerve able to regenerate, a biological feat that smell therapy tries to take advantage of. Conversely, neuroimaging studies find that smell training can increase brain volume and neural activity in some of these areas.
Converging research suggests smell training can boost overall cognition, including in domains of verbal learning and memory, and attention in dementia patients and healthy older adults, potentially even slowing cognitive decline.
These studies are still preliminary, and many have small sample sizes, experts say. But the effects of smell training seems “compelling and interesting,” Rowan says. You won’t “become Einstein, but maybe you can stave off the untoward effects of unhealthy ageing.”
Passing the sniff test
Monitoring smell abilities could improve early detection of cognitive decline.
“I think that every older adult as part of their wellness exam through Medicare should have a smell test,” Rowan says. A smell test can be relatively inexpensive, he says, a mere fraction of the tens of thousands needed annually for dementia care. But, because they are not typically covered by insurance, physicians won’t often run them.
“I want to test everybody. But if I’m going to just lose financially on it, there’s no incentive to do it,” said Rowan, who is getting an MBA at the Wharton School to tackle the economics of making smell tests more routine.
As for me, I undertook a longer, more comprehensive battery in Rowan’s office at the Johns Hopkins School of Medicine that tested my olfactory threshold (measuring the lowest concentration of odour that someone can detect); olfactory discrimination (telling the difference between different scents); and olfactory identification (figuring out what the scent actually is).
It was a humbling experience.
After an hour of sniffing, I ended up with a score of 32.25 out of a possible 48, just below average for men in my age group. Though my ego was bruised, Najjar, who patiently administered my exam, was quick to reassure me that my smell was still within the normal range (between 30.75 and 41.25), and now we have a baseline to compare against.
Besides, poor smell in and of itself is not cause for alarm, Rowan says.
There is a wide range in olfactory abilities across the population. Women tend to perform better than men, and younger people tend to perform better than older adults. (I can comfort myself that I was a bit better than an average 65-year-old man.)
An acute loss of smell is a reason for concern, and slow declines in smell paired with other difficulties of memory or mobility is a sign to talk with your doctor, Rowan says.
Getting up to snuff
So what should you do if you don’t think your smell is quite up to snuff or you want to improve it?
First, protect your nose. “Don’t be exposed to things that you’re breathing in that are going to potentially influence or hurt your nose,” especially if it’s a regular exposure, Rowan says.
Second, you can practice smelling with smell training kits or ones you can make yourself with essential oils. There is no set protocol, but many studies have participants sniff different scents for 10 to 20 seconds at a time once or twice a day. The most common scents used are rose, eucalyptus, clove and lemon. (Smell training differs from aromatherapy because the smelling is intentional, Vance says.)
Though more research is required for smell training, “if you want to try it, I don’t think it’s going to hurt anything. It could only help,” Vance says. “And it could be fun. It might be something, especially older adults, they might want to do with their grandkids.”
Consistency is also key.
Just as you wouldn’t expect to pick up a new language with haphazard study once in a while, keeping up smell training over a long period of weeks is essential. “There’s a language of smell,” Vance says.
Smell training kits need not be the only way we exercise our olfaction, says Najjar, who is working with Rowan on a clinical trial testing the cognitive effects of a smell training device they developed.
“It’s really an easy kind of sense to train in a way that everything is accessible around us,” Najjar says.
We are bathed in odours all day long, even if we aren’t consciously aware of them. As such, we can be smell training all the time – if we pay attention.
The fruits and vegetables in the produce aisle. The foliage after a summer rain. A cup of coffee in the morning.
So go ahead: stop and smell the roses. It is good for you.
Washington Post
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