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How Our Sense of Smell Could Be Nudging Us to a More Balanced Diet

28 AUGUST 2021

Your nose might be guiding you to make more balanced food decisions, according to a small new study.

When 30 participants ate either cinnamon buns or pizza, researchers found they became less sensitive to those smells than when they were still hungry.

 

Anyone who has pinched their nose and eaten a forkful of food knows their sense of smell can influence how something tastes, but this new research suggests how something tastes can also influence our sense of smell, drawing us toward some foods and away from others.

Researchers suspect this is our brain's way of making sure we don't eat too much of the same thing – a possible adaptive strategy to optimize our search for food.

"If you think about our ancestors roaming the forest trying to find food, they find and eat berries and then aren't as sensitive to the smell of berries anymore," explains neurologist Thorsten Kahnt from Northwestern University.

"But maybe they're still sensitive to the smell of mushrooms, so it could theoretically help facilitate diversity in food and nutrient intake."

Previous studies have found our subjective opinions on whether certain foods smell pleasant depends on what we've eaten, but this study sought to determine whether hunger and satiation can change our sensitivity to smells.

Only a few animal studies and one human study to date have explored this idea, which means there are a lot of unanswered questions.

 

Research on rodents, for instance, has found fasting can influence the level of activity in the olfactory bulb, which processes smell in the brain.

What's more, in 2019, two of the same authors of the new research found sleep deprivation in humans can sharpen the smells of high-energy foods within the insular cortex, a part of the olfactory pathway that responds to food-related stimuli.

These results were preliminary, but they suggested our appetite could influence our sensitivity to certain smells.

In the new study, researchers had 30 participants show up hungry to the lab, having fasted for at least six hours. Inside an MRI scanner, these volunteers were then presented with a series of 10 smells, all of which contained a mix of two odors: One scent was pizza or cinnamon buns, and the other was either a pine or cedar scent.

The ratio of food to non-food in the odor mixes was different for each of the 10 samples, and participants determined which smell they thought was more dominant for each one: pizza or pine, or cinnamon or cedar.

 

Volunteers were then fed a meal of either pizza or cinnamon buns before completing the task again from within the MRI machine.

"In parallel with the first part of the experiment running in the MRI scanner, I was preparing the meal in another room," explains Northwestern neurologist and the study's first author Laura Shanahan.

"We wanted everything fresh and ready and warm because we wanted the participant to eat as much as they could until they were very full."

Participants were able to quickly identify odors with more purity (when one smell was clearly dominant). But when the mix of aromas became more even, what participants had eaten at the lab seemed to have an impact on what they smelled.

Those who were given a meal of pizza, for instance, were less likely to smell pizza when it was mixed in with pine. Meanwhile, those who had their fill of cinnamon buns were less likely to smell the baked goods when they were mixed in with cedar.

Yet when these same participants were hungry earlier in the day, they were much better at making out the dominant odor.

 

For instance, one hungry participant earlier in the day might have needed only half of the odor to smell like pizza to perceive it as dominant to pine. But later, when this same participant had eaten their fill of pizza, they might have needed 80 percent of the odor to smell like pizza to perceive it as dominant.

When examining results from the MRI machines, researchers noticed a similar pattern occurring in the brain.

The scans revealed different olfactory pathways are activated after a meal than before a meal. For instance, after feasting on cinnamon buns, participants showed less "food-like" responses to that very same sweet smell.

Unfortunately, MRI brain scans are limited in that they are not able to measure neural activity directly in the olfactory bulb – so we still don't know where these changes to our odor perception are really happening in the brain.

"We're following up on how that information is changed and how the altered information is used by the rest of the brain to make decisions about food intake," says Kahnt.

The study was published in PLOS Biology