It’s often said that we eat with our eyes first, but it’s actually our sense of smell that’s running the show, a new study says. And with the exception of misleading scents like the falsely inviting citrus sweetness of an otherwise awful cough syrup, it’s usually a good thing that smell comes before taste—right?
Not so, a surprising new study from the University of California, Berkeley, suggests. In fact, a good sense of smell might be the reason that certain people have trouble losing weight.
In a series of experiments, Berkeley researchers found that smell-impaired mice—whose olfactory senses were temporarily inhibited during the study—ate the same high-fat diet as mice with unimpaired senses of smell, but only gained 10 percent more weight, at most. Meanwhile, the mice that retained their senses of smell gained “about 100 percent of their normal weight, ballooning up to 60 grams” from 25 to 30 grams. As the second group of mice became obese, they also develop a sensitivity to insulin and glucose intolerance, while the former group did not suffer the same consequences.
Later, when the Berkeley team tested the same diet on a group of mice with an even more acute sense of smell, known as super-smellers, these subjects gained even more weight.
As it turns out, the very act of smelling food before you eat it informs your body of what to do with those calories, according to the study, published this week in the journal Cell Metabolism. After eating a fatty diet, the smell-impaired mice easily burned those calories, efficiently turning their beige fat cells—stored fat cells that accumulate around the thighs and midriff—to brown fat cells, which burn fatty acids, producing heat. Some of these smell-impaired mice were able to do this with nearly all their beige fat cells, such that the high-fat diet had almost no impact on their weight. For these mice, even their white fat cells, which surround our internal organs, shrank in size.
“I was shocked—the effect was so robust,” the study’s senior author, biologist Andrew Dillin told the LA Times. “I was convinced they were just eating less. When it became clear they weren’t, I thought, ‘Wow, this is incredibly interesting.’”
In the latter half of the study, researchers inhibited the olfactory senses of the mice that had grown obese, and found that they, too, finally lost weight on the same fatty diet and even regained a healthy glucose response. The next step in this research is to consider how humans might respond to olfactory intervention as a means of controlling weight.
“Sensory systems play a role in metabolism. Weight gain isn’t purely a measure of the calories taken in; it’s also related to how those calories are perceived,” Dillin said. “If we can validate this in humans, perhaps we can actually make a drug that doesn’t interfere with smell but still blocks that metabolic circuitry. That would be amazing.” One important negative side effect associated with loss of smell, however, was an increase in noradrenaline, a stress hormone which, in humans, can lead to a heart attack at high levels.
Still, the considerable results from this particular study have made its researchers incredibly hopeful about the future of obesity and weight research in human subjects. Perhaps, in place of complicated and dangerous procedures, such as gastric bypass, doctors might be able to temporarily interrupt a patient’s sense of smell until their weight becomes more manageable.
“People with eating disorders sometimes have a hard time controlling how much food they are eating and they have a lot of cravings,” said Céline Riera, another author of the paper and a former UC Berkeley postdoctoral fellow. “We think olfactory neurons are very important for controlling pleasure of food and if we have a way to modulate this pathway, we might be able to block cravings in these people and help them with managing their food intake.”
Will it work for humans? Only time will tell. It's certainly an interesting finding, but it won't stop us from taking a deep whiff of that aromatic stew or popping into every bakery we pass just for the smell.