This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Some people are drawn to the thick smell of bacon, sizzling and crackling in the skillet on a Saturday morning. For others, it’s the aroma of freshly baked cookies on a Friday night or the smell of McDonald’s fries creeping in through the car window. At this time of year, I find the scent of freshly baked pumpkin muffins irresistible. Of course, I’d like to think I’m not a slave to my nose, at least not when I’m nice and full from dinner.
If I were a fruit fly, my outlook might not be so good.
Already-fed fruit fly larvae exposed to certain food-related odors ate more food than larvae that didn’t experience the smells, according to research published by scientists at the University of Georgia last spring.
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“They’re not hungry, but they will get an extra kick in terms of appetite, so they will eat, for example, 30 percent extra,” said Ping Shen, lead author on the study.
The scents, which included the sweet odor of bananas or the sharper smell of balsamic vinegar, served as “cues” or triggers that the flies associated with food. The triggers motivated the fly larvae to eat, even when they’d already had dinner. That doesn’t bode so well for flies trying to watch their weight.
For the fly to feel this urge to eat, the smell has to be transported from sensory receptors in the nose to the part of the brain that regulates appetite—the brain’s “feeding center”—via a series of neurons. Part of this signal transfer involves dopamine, a neurotransmitter associated with behavior motivated by a cue or hint of something to come, like smells associated with food.
But I’m not a fly. And my brain, fortunately, is much more complex. So can the scent of freshly baked cookies paired with my brain’s neurons drive me to chow down despite the knowledge that my stomach is full?
Studies show food aromas can cause humans to overeat, although whether they will depends on a variety of other factors. In one study, researchers showed that while normal-weight children ate less food after smelling snacks like chips and pieces of cake, overweight children ate significantly more. Otherstudies show that food smells affect people on a diet more than those who aren’t trying to limit what they eat.
As in the fly brain, food-related scents appear to increase activity in areas of the human brain with neurons that use dopamine to transmit information. Researchers can use a technique called functional magnetic resonance imaging, or fMRI, to take pictures of the brain’s activity as it is exposed to different food-related stimuli. Studies imaging the human brain’s response to smell are limited, but a 2010 pilot study found that the aromas of sweet or fatty foods activated areas of the brain that are associated with the motivation to get a reward, such as a tasty piece of cake.
A much larger number of studies have considered the way the brain responds to pictures of food. However, this research may give us some insight into how the brain processes other food-related stimuli, like smell.
“The areas in the brain that regulate reward, pleasure and motivation, those regions are activated or stimulated just by the sight of food,” said Kathleen Page, who leads a lab at University of Southern California’s Keck School of Medicine that investigates brain regulation of appetite.
“Not only are those regions activated when you see pictures of kinds of palatable, appetizing, junk-type food like chocolate cake, people also have higher ratings of hunger and a desire to eat.”
In a country with restaurants lining the roads, churning out appetizing smells, and food ads on every billboard and commercial break, our brains can make eating healthy amounts challenging.
“If we are continually stimulated by our environment, that could promote overeating and, over time, weight gain and obesity,” Page said.
Ways to fight these food triggers do exist, according to Sarah Fischer, assistant professor of clinical psychology at George Mason University. If the trigger can’t be avoided, one option is to count to 10 and see if the urge to eat passes. Though a food smell or image may cause an initial reaction, data shows that the urges tend not to last.
I’ll keep this in mind next time the beautiful aroma of a pumpkin muffin calls to me from the pan on the stove.