You've heard the joke from people who politely decline indulgent foods at social gatherings: "Just smelling pizza makes me gain weight," they say, and everyone chuckles.
Turns out—to everyone's dismay—there might actually be some truth to that.
Just the ability to smell your food might lead to weight gain, according to a new study published in the journal Cell Metabolism. In the study, researchers found that mice with an intact sense of smell grow obese on a high-fat diet, while their littermates (who've had their sense of smell removed—poor little guys) can eat the same food and the same amount of food and stay at a low weight.
Mice researchers say whattt? It's no secret that smell plays a big part in taste—or "food appreciation and selection," as the researchers explain. But this isn't a matter of tasting the food and deciding to eat more; the fact that the mice were eating the same type and amount of food—and getting the same amount of physical activity—means that the stuff going in your nose is affecting a lot more than anyone thought.
The researchers found that mice without a sense of smell showed "increased energy expenditure and enhanced fat burning capacity as a consequence of enhanced sympathetic nerve activity," according to the report. Translation: The lack of the sense of smell triggered a more active sympathetic nervous system (the one responsible for the "fight or flight" response), which in turn increased calorie burn and fat burn. And even when eating a high-fat diet, the non-smelling mice seemed less likely to develop visceral fat (dangerous fat deposits around your organs) than the mice who could smell their food.
That ramped-up sympathetic nervous system response in the non-smelling mice was expressed as a "drastic increase" in circulating levels of adrenaline, a stress-response hormone that's great at burning fat. This hormone appeared to activate the mice's stores of brown fat (a type of fat that actually burns calories) to burn white fat as fuel, and to convert some white fat stores to brown fat. (Read more about brown vs. white fat here.)
Now, the kicker: When the researchers removed the obese mice's sense of smell, the mice quickly lost fat and reduced their insulin resistance. "The data presented here show that even relatively short-term loss of smell improves metabolic health and weight loss, despite the negative consequences of being on a [high-fat diet]," the researchers wrote.
But before you decide to nix your nose in the name of weight loss, know that your sense of smell is super important for a lot of other things. Just a few months ago, a study linked a better sense of smell to a longer and more social life. (Plus, losing your sense of smell might be a total libido killer.) Not to mention, this study was done on mice—which, obv, are quite different from humans.
While the researchers acknowledge the potential future use of this information to help regulate metabolism and hormones in humans, for now, it's best to keep sniffing and leave the rest to science.