Smell plays a much larger role when we chow down on food than most people appreciate. Without it, food becomes flavorless and bland, but it may also be having an effect on your metabolism. Researchers have found that mice with no sense of smell lost weight while those with smell became obese when fed a high-fat diet, but not for the reasons we might assume.

There have been many tests to show how smell and eating are linked, with some indicating that our sense of smell becomes heightened before a meal, and then drops once we’ve feasted. For those who have tried eating with a blocked nose, they will be fully aware of how flavorless a meal becomes.

After genetically engineering mice to lack smell, the team of researchers found that mice lost weight compared to unaltered mice fed on the same diet. At first they thought that would simply be because the enjoyment of eating has been taken down a few notches, and that the rodents were probably simply eating less food. “There is a known link between food intake and sense of smell,” said Andrew Dillin, lead author of the study published in Cell Metabolism.

But when the scientists looked in detail at how much both groups were consuming, they found something that shocked even them: Both the engineered and control group were eating exactly the same amount. While when the control group was fed the high-fat diet, they became obese, those that had their sense of smell removed lost on average a fairly substantial 16 percent of their weight.

When they compared the two strains of mice, they found dramatic differences in the type of fat that the mice had. In the rodent bodies, as in ours, there are two different types of fat, brown and white. In general, the white fat builds up around our stomachs and legs, while the brown fat is burned to produce heat via metabolism. Humans have a lot of white fat and little brown fat, while the mice have plenty of the brown variety to maintain their body temperature.

They discovered that the mice with no smell were maintaining their figure by simply burning more calories, particularly those derived from brown fat. They also deduced that the mice were converting the white fat in the bodies into the brown variety, before then burning that, too. “The mice with no sense of smell had turned on a program to burn fat,” explained Dillin.

Curiously, they also discovered that the mice also had an increase in adrenaline, which is usually produced in response to extremes, such as cold. They think that there might be a link between the olfactory system and the production of adrenaline, triggering the mice to burn more fat to maintain their temperature.

And rather beautifully, they found that if they enhance the smell in mice, rather than remove it, exactly what you think would happen, happens. The mice with "super" smell, become even more obese than the control group, as they are burning fewer calories. The researchers now plan on doing more tests, to see if they can figure out the bridge between smell and adrenaline.