All about our mysterious sense of smell

Lisa Fields

The world’s leading researchers are unlocking facts about the least understood of our senses. From the intricate link between taste and smell to the idea of "scent training", there's so much more than we think to our sense of smell 

Your sense of smell is largely responsible for your ability to taste food.

“Flavour is really an integrated experience that combines what happens on your tongue—that’s sweet, sour, salty, bitter, spicy—with smell,” says Asifa Majid, psychology professor at the University of York, UK. “When you put something in your mouth, the molecules go into your nasal cavity. Your tongue might be able to tell that there’s fat in that chocolate, and that’s something the brain finds very rewarding, but the formal chocolate experience comes from all the molecules going into your nose.”

Women out-perform men when both groups are asked to identify a certain number of scents.

Woman smelling a rose

That holds true across all age groups. “This could be due both to women being better able to perceive the smell and women being better at verbalizing the odour—that is, providing the odour with a verbal label,” says Erika Jonsson Laukka, senior researcher at the Karolinska Institute’s Ageing Research Centre in Stockholm. Her research shows that when people were asked to memorize eight scents (including garlic, fish, turpentine, and lemon) and were then given a scent test, which included some of the original scents and some new scents, the women were better able to identify whether or not a scent was one of the ones that had been memorized. Women were also better at identifying the scents by name.

After age 50 or so, our sense of smell starts to decline.

Older woman with foods

“This loss accelerates as people get older,” says Dr. Thomas Hummel, director of the Smell and Taste Centre at the Technische Universität, Dresden. “Among people over 50, a quarter have a loss. In people over 80, about a third will have no olfactory function at all. But half of those over 80 still have a good sense of smell.”

People often think that those who are blind, deaf, or have lost their ability to smell will have another sense heightened, but according to research, this is merely an old wives’ tale. For example, Dr. Hummel has studied the sense of smell in blind people and has not found the sense to be heightened.

“That’s a little bit of a myth,” Dr. Hummel says. “We’ve been looking at this in very large groups—up to 40 people—with congenital blindness, and people with acquired blindness, and they are not better in their sense of smell. There may be individuals who are really good. But when you look at larger groups, you don’t see it. It’s not there.”

"Half of those over 80 still have a good sense of smell"

“Every time we have a cold, a toll is taken,” says Richard Doty, director of the University of Pennsylvania’s Smell and Taste Centre in Philadelphia. Cumulative damage from a lifetime of illnesses suffered by the average person contributes to smell loss in many older adults. In the case of a cold, “The virus damages little elements of the epithelium—the lining of the olfactory region where the receptors are located, at the top of the nose—pockmarking it. By the time we get into our 60s, 70s and 80s, it looks like cheesecloth.”

Smell loss can be an early symptom of Parkinson’s or Alzheimer’s.

It could indicate illness or chronic disease, so see your doctor if you notice your sense of smell fades, says York’s Professor Majid. “But for most of us, it’s not a concern. It’s just part of getting older.”

Recently, the Covid-19 virus has caused temporary smell loss among people of any age.

Man with surgical mask around neck trying to smell a lemon

“There’s 10 per cent or so—that’s the current estimate—where the smell loss lingers,” Dr. Hummel says. (See sidebar, “Fresh Insights, Thanks to Covid-19.”) And, he says, anyone may also lose the sense of smell, temporarily or permanently, after having a concussion or other trauma to the head. “This does not mean anything, then, in terms of these future cognition functions or future health.” If your doctor can’t find a cause for your smell loss, it’s likely age-related.

Your eating habits may be compromised by loss of smell.

“You think, ‘I remember how this tasted in the past. I guess they don’t have as good tomatoes or meat any more,’” says Dr. Johannes Frasnelli, psychology professor at the University of Montreal. “But in fact, it’s your sense of smell that may be working less.”

“Some people will find food is no longer enjoyable, get depressed over it, or don’t want to eat with their friends,” the University of Pennsylvania’s Doty says. “Others will start eating lots of junk food, spices, salt, to try to make foods more enjoyable.”

Strangely, some individuals don’t notice an absence of aromas in their environments, or that they no longer taste their food. This may be because their sense of smell faded gradually. “Probably about a quarter of the population worldwide has a considerable loss and is not really be aware of it,” says Doty.

"Strangely, some individuals don't notice that they no longer taste their food"

For many of us, scents unlock memories that date back to childhood.

One whiff of your grandmother’s perfume on a passer-by may transport you back in time to your grandparents’ home. Researchers believe this happens because of the proximity of certain brain regions to the olfactory bulb, which sends information from the nose.

“The olfactory bulb is situated close to the amygdala, which regulates our emotions, and the hippocampus, a structure important for encoding and recollecting memories,” Stockholm’s Laukka says. “Smell-evoked memories may be more emotional, run deeper, than those cued by other senses.”

Adds Dresden’s Dr. Hummel, “When you lose the sense of smell, you lose these memories. It’s like the key is lost.” 

To improve your sense of smell, some medical treatments can work.

If your doctor suspects that inflammation has caused your smell loss, he may prescribe a course of steroids. “If after that a person’s smell comes back, then they can use judiciously a topical steroid through nasal sprays,” says Doty.

Researchers haven’t yet developed devices to improve the sense of smell. “We have glasses and hearing aids,” says Professor Majiid. “There’s nothing like that for smell.”             

But they do know that the sense of smell improves throughout childhood and into adulthood, as people are exposed to more scents and tastes. The more flavours and delicacies that you sample while growing up, the better your nose will be at detecting subtle aromas.

So expanding your diet and varying what you eat may stimulate your sense of smell. Be mindful at mealtimes, noting the aromas, flavours, and textures.

“The best thing is to have a diverse diet, a cookbook with foods from all over the world,” Dr. Frasnelli says.

Scent training might preserve our sense of smell, some research suggests. People often do this at the suggestion of an ear, nose, and throat specialist, if they go to the doctor to find out why they have lost their sense of smell. When people deliberately inhale strong odours for several weeks, they may become more sensitive to all aromas and improve their ability to smell.

Dr. Hummel believes that the technique has physiological effects. “We probably grow more olfactory receptors. There’s animal research supporting this idea.”

Dr. Frasnelli has analysed MRI brain images of people undergoing scent training and observed positive changes in brain plasticity. “They got thicker in the regions of the brain that are responsible for processing the olfactory function,” he says, “which means it’s not just a nose that is trained, it’s the whole brain.”

But don’t expect dramatic results: Most scent-training research involves younger adults, so it’s unknown how effective it may be in older adults, he warns.

Still, it’s something inexpensive that you can try on your own, even in your 70s or beyond. As Professor Majid says, “Smell does seem to be a sense that you can train throughout your lifetime.”

"Smell does seem to be a sense that you can train throughout your lifetime"

Smells aren’t universally “good” or “bad.”

Research has shown that appreciation of, or distaste for, a particular odour is often learned at an early age, based on cultural preferences or the popularity of certain foods or customs around the world.

For example, individuals who dislike the smell and taste of cheese with pungent odours—especially creamy, runny varieties—perceive the scent differently from people who enjoy such cheeses. Their negative reaction to the aroma is detectable during a functional MRI brain scan.

Intriguingly, when women were asked to judge the likeability of male body odour, they preferred the scent of men who followed a vegetarian diet to that of men who ate red meat.

Digital noses on the way?

“There have been some really interesting things happening in digital olfaction in recent years,” says Professor Majid. “For example, there is a device that can tell you if something in your fridge is going bad. There are also electronic noses that are used in urban environments, to capture if there’s something dangerous happening—for example, a gas leak.”

This has already been used in a limited capacity. When unpleasant odors pervaded in a town in southern Spain in 2017, University of Malaga researchers who had developed an e-nose were invited to identify the presence of volatile chemicals, which were believed to be emanating from an unknown point within the sewer system. The e-nose identified where the chemicals were most prevalent within the town, which coincided with the local government’s clean-up efforts.

None of these devices are yet available to consumers.

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