When Andrew Dillon, a cell biologist at the University of California Berkeley, set out to study the effect of the sense of smell on weight in mice, he assumed that he’d find that mice without a sense of smell would enjoy food less, eat less, and therefore weigh less. Scientists have known that smell shapes the way we tastes things—usually for the better—so without it, mice eating even a scrumptious, high-fat diet would enjoy it less.
He was right—kind of. The results of work he and his team published on July 5 in the journal Cell Metabolism show that adult mice without a sense of smell did weigh about 16% less than those with their sniffers intact. But the reason came out of left field: It was because somehow, without a sense of smell the rodents burned more energy from fat.
“I was convinced they were just eating less,” he told the LA Times. “When it became clear they weren’t, I thought, ‘Wow, this is incredibly interesting.’”
The team of researchers studied normal adult mice, and then a group of adult mice who had been genetically modified to lose their smelling abilities after a whiff of a chemical in a lab. They fed both groups of mice either regular diets or high fat diets for three months. On the regular diet, mice without a sense of smell weighed only slightly less, but on the high fat diet, the difference was much more pronounced. Furthermore, when overweight mice had their own sense of smell removed, they started losing weight, too.
When the researchers examined the amount of food each group was eating, they were surprised to find that it was more or less the same. So was their exercise. However, the mice without a sense of smell seemed to be burning a different kind of fat tissue that uses more calories.
Mammals like mice (and also us) have two kinds of fat tissue, both of which we need. There’s brown fat, which helps keep us warm by making heat, and then there’s white fat, which is used as energy reserves (and is typically associated with weight gain in humans). Infants tend to have a lot more brown fat than adults, and mice tend to have more brown fat than humans.
When the researchers examined the fat content of mice after the experiment was over, they found that mice without a sense of smell seemed to be using more of their brown fat tissue than the others.
There’s a long road of research ahead before scientists can say whether losing a sense of smell can bolster weight loss efforts in humans. But Dillin thinks this work opens the door for more research into the relationship between smell and brown fat. “There’s more to gaining weight than just eating food—it’s how you are perceiving the food,” he told Science.