HONG KONG — Of our five senses, it’s often the one that we think about the least: our sense of smell.
Our olfactory sense delivers critical information, from sniffing out potential dangers like smoke to picking up the aroma of freshly baked bread.
New studies have uncovered more reasons to appreciate this sense. Boosting its ability has been linked to better memory and cognition.
During the coronavirus pandemic, everyone got better at registering how well their noses were working: losing that sense was often an early symptom of infection. That’s because a viral illness can damage the nerve receptors in your nose and can change your perception of smell.
You can test this by pinching your nose shut when you eat something — say chocolate — and you’ll only pick up the sweet taste.
Our sense of smell is also crucial for memory.
The olfactory system has the only direct input into the memory centres of the brain and therefore has much more impact on them than the other senses, says Michael Leon, professor of neurobiology and behaviour at the University of California Irvine.
Prof Leon, with his team, Dr Cynthia Woo and Professor Michael Yassa, recently published research that showed how boosting sense of smell could significantly improve cognition — by more than double — in people aged 60 to 85.
“People in the modern world are chronically deprived of olfactory stimulation and they need regular multi-odour stimulation to be able to maintain their memory,” Prof Leon says.
Loss of sense of smell accompanies as many as 70 neurological and psychiatric diseases. Prof Leon says it’s the first symptom of Alzheimer’s, Parkinson’s disease — and depression.
Rachel Pacyna, a University of Chicago medical student whose goal is to use neuroscience to investigate issues in women’s health, including ageing and memory, explains the link.
The sense of smell is affected in diseases such as Alzheimer’s because both smell and memory are processed in similar regions of the brain.
In particular, disease in the entorhinal cortex — described as “the gateway” for information entering and leaving the hippocampus — and in the hippocampus itself could lead to cell death in the brain that causes both impaired sense of smell and memory problems.
This happens, she says, because the build-up of amyloid plaques and tangles that are linked to Alzheimer’s leads to cell loss in brain regions associated with memory, particularly the hippocampus.
Ms Pacyna’s study showed that people with a faster decline in their sense of smell were more likely to develop dementia.
Olfactory dysfunction — reduction in the ability to smell — could predict cognitive decline up to 15 years before it manifests. It could be used as a promising — easy, accessible, affordable — early biomarker of brain health, for Alzheimer’s disease detection in particular.
It could even predict imminent death: another study of adults aged 60 to 85 found that those who had lost the ability to identify particular smells — including rose and peppermint — were more than three times as likely to die in the next five years.
Vision and hearing loss have already been linked to an elevated dementia risk.
Now it seems losing your sense of smell also poses a risk. As Prof Michael Yassa, who took part in the University of California study says, if you think about our other senses — sight, hearing — we do something about them as we get older.
But unlike vision changes which we manage with glasses, and hearing impairment with hearing aids, “there has been no intervention for the loss of smell”.
HOW TO SAFEGUARD YOUR SENSE OF SMELL
AVOID HEAD TRAUMA
The thin plate in the nose that connects to the olfactory bulb is fragile and sensitive to injury. Head trauma can pose a risk; people have lost their sense of smell after a sports injury or car accident. So wear a helmet while cycling or doing contact or extreme sports.
FOLLOW A GOOD DIET
Eating well benefits us in many ways — including safeguarding our sense of smell. A lack of certain nutrients, including zinc and vitamin B12, is linked to the loss of this sense.
STOP SMOKING
Tobacco use has been shown to kill the brain cells that help interpret scent information and it impairs the ability to smell.