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Why Did Mice That Couldn't Smell Lose More Weight Than Mice That Could?

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Everyone knows that when you've got a stuffy nose, food doesn't taste as good. That's because your sense of smell is intimately linked with your sense of taste. But your sense of smell might have more to do with diet than we previously thought: instead of just making food taste better or worse, it may directly influence how your body gains and loses weight.

A Burger By Any Other Name Would Smell As Sweet

A study published in July 2017 in the journal Cell Metabolism details how researchers used gene therapy to make it so certain mice lost their sense of smell for a few weeks. They split all of the mice into two groups, fed one group a diet full of the kinds of fatty foods that normally lead to obesity, and fed the other group a normal diet.

After three months of this, something very weird happened. Even though all of the mice in the normal-diet group ate the same food and got the same amount of exercise, the mice that couldn't smell ended up weighing less than the ones that could. These bizarre results were even more pronounced in the high-fat diet group: the smell-impaired mice weighed a full 16 percent less than their smelling counterparts. Weirder still, when the scientists gave some mice a hypersensitive sense of smell, those animals became obese — on the exact same diet.

Upon closer inspection, researchers discovered what was going on: the smell-impaired mice ended up burning more calories in their brown fat. As opposed to the mostly inert, energy-storing white fat we're all familiar with, brown fat is specialized to produce heat by burning calories. Humans are born with plenty of brown fat to keep themselves warm until they develop the ability to shiver, but lose most of it as they get older. Mice, however, have plenty, since it's their main way of maintaining body temperature. Not only did the smell-impaired animals' calorie-burning brown fat jump into overdrive, but some of their white fat started behaving more like brown fat.

Real-World Possibilities

Animals are more sensitive to smells when they're hungry than when they're full, coauthor Celine Riera notes in a press release. That could be why losing a sense of smell rewired these mice to use more energy: it may have made their brains think they had eaten more than they did. Co-author Andrew Dillin went even further. "Sensory systems play a role in metabolism," he said. "Weight gain isn't purely a measure of the calories taken in; it's also related to how those calories are perceived."

So could this work in humans? It's true that humans who lose their sense of smell — usually as a side effect of aging or brain disease — often become anorexic, but it's unclear whether that's because of that fact on its own or because losing your sense of smell makes eating less pleasurable. That can lead to depression, which can reduce your appetite even more. It's also important to note that the smell-impaired mice also had a big spike in the stress hormone noradrenaline — a spike big enough to cause heart attack in humans.

Still, it could be a viable option for people who are already considering drastic alternatives like bariatric surgery. "For that small group of people, you could wipe out their smell for maybe six months and then let the olfactory neurons grow back, after they've got their metabolic program rewired," Dillin said.

For more on the superpowers of smell and how you can use your nose to improve your life, check out this episode of the Curiosity Podcast.

Why Smell is the Most Powerful Sense

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