As you walk past a corner bakery, you may be drawn in by the fresh smell of candy wafting from the front door. You’re not alone: knowing that people make decisions based on their nose has led big brands like Cinnabon and Panera Bread to pump the scent of baked goods into their restaurants, resulting in huge spikes in sales.
But according to a new study, the food you ate at the bakery just before your walk can affect your chances of having a sweet treat – and not just because you are full.
Northwestern University scientists found that people were less sensitive to food odors because of the meal they had recently eaten. So if, for example, you are nibbling on baked goods from a colleague before your walk, you are less likely to stop by this sweet smelling bakery.
The study “Olfactory Perceptual Decision Making is biased by motivational state” will be published on August 26th in the journal PLOS Biology.
The smell regulates what we eat and vice versa
The study found that participants who had just eaten a meal with cinnamon rolls or pizza were less likely to experience “meal-related” smells, but no unmatching smells. The results were then confirmed by brain scans, which showed that brain activity was similarly altered in parts of the brain that process odors.
These results show that just like smell regulates our food, what we eat, in turn, regulates our sense of smell.
Feedback between food intake and the olfactory system can have an evolutionary advantage, said Thorsten Kahnt, assistant professor of neurology and psychiatry and behavioral science at Northwestern University Feinberg School of Medicine, the corresponding study author.
“When you think of our ancestors who roamed the forest in search of food, they find and eat berries and then no longer react as sensitive to the smell of berries,” said Kahnt. “But maybe they are still sensitive to the smell of mushrooms, so this could theoretically help facilitate the variety in food and nutrient intake.”
Kahnt said that while we don’t see the hunter-gatherer adjustment come into play in day-to-day decision-making, the connection between our nose, what we are looking for, and what we can see with our nose can still be very important. For example, when the nose is not working properly, the feedback loop can break, leading to problems with eating disorders and obesity. There may even be links to sleep disorders, another link to the olfactory system that the Kahnt lab is researching.
Using brain imaging, behavioral tests and non-invasive brain stimulation, the Kahnt laboratory is investigating how the sense of smell controls learning and appetite behavior, particularly in relation to psychiatric diseases such as obesity, addiction and dementia. In a previous study, the team found that the brain’s response to smells is altered in sleep-deprived participants, and they next wanted to know if and how food intake changes our ability to perceive food smells.
According to Laura Shanahan, postdoctoral fellow at the Kahnt laboratory and first and co-author of the study, there is very little work on how olfactory perception changes due to various factors. “There is some research on odor comfort,” Shanahan said, “but our work focuses on how sensitive you are to these smells in various states.”
Pizza and pine; Cinnamon and cedar
To conduct the study, the team developed a novel task in which participants were presented with a smell that was a mixture of a food and a non-food smell (either “pizza and pine” or “cinnamon bun and cedar” smells, which “pair well” and differ from each other). The ratio of food and non-food odor varied in every mix, from pure food to pure non-food. After a mixture was presented, participants were asked whether the food or non-food odor was dominant.
Participants did the task twice in an MRI scanner: first when they were hungry, then after they had eaten a meal that matched one of the two smells.
“In parallel to the first part of the experiment in the MRI scanner, I was preparing the meal in another room,” said Shanahan. “We wanted everything fresh and ready and warm because we wanted the participants to eat as much as they can until they are very full.”
The team then calculated how much food odor was required in the mixture at each session for the participant to perceive the food odor as dominant. The team found that when participants were hungry, they needed a lower percentage of food odor in a mixture to perceive it as dominant – for example, a hungry participant may need a 50% mixture of cinnamon rolls and cedarwood when they were hungry, but 80% when full, cinnamon buns.
By imaging the brain, the team provided further evidence for the hypothesis. Brain scans from the MRI showed a parallel change in the part of the brain that processes odors after a meal. The brain’s response to an odor adapted to a meal was less “food-like” than the response to an unadapted food odor.
Apply the results to future sleep deprivation research
The findings from this study will enable the Kahnt laboratory to take on more complex projects. Kahnt said with a better understanding of the feedback loop between smell and food intake, he hopes to bring the project’s circle back to sleep deprivation to see if lack of sleep could affect the loop in any way. He added that with brain imaging, there are more questions about how adaptation can affect sensory and decision-making circles in the brain.
“After the meal, the olfactory cortex no longer represented as much as food smells, so the adaptation seems to take place relatively early in processing,” said Kahnt. “We are tracking how this information is changed and how the changed information is used by the rest of the brain to make food intake decisions.”
Reference: Shanahan LK, Bhutani S, Kahnt T. The olfactory perception decision is influenced by the state of motivation. PLOS biology. 2021; 19 (8): e3001374. doi: 10.1371 / journal.pbio.3001374
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