You don't have to eat food to get fat — just smell it

The smell of food is one of life’s pleasures. It is well known that those who lose their sense of smell often lose weight — traditionally it was thought that those unable to smell their food because of age, injury or disease lost weight due to loss of pleasure in eating and depression, which itself can cause loss of appetite.

However, in an interesting experiment at the University of California, Berkeley, more information came to light. Scientists used gene therapy to destroy olfactory neurons in the noses of adult mice but spared the stem cells, so that the animals lost their sense of smell only temporarily — for about three weeks — before the olfactory neurons regrew.

Mice with impaired ability to smell, and their littermates with normal smelling capabilities, were fed identical high-fat diets. Those without a sense of smell stayed slim, while those who could smell became obese. The smell-deficient mice ate the same amount of fatty food as mice that retained their sense of smell but simply did not put on weight. In addition, mice with a boosted sense of smell — super smellers — got even fatter on a high-fat diet than the mice with normal smell.

These results suggest that the odour of what we eat may play an important role in how the body deals with calories. If you can’t smell your food, you may burn it rather than store it.

This points to a connection between the olfactory or smell system and regions of the brain that regulate metabolism — in particular, the hypothalamus — though the neural circuits are still unknown.

“This paper is one of the first studies that really shows if we manipulate olfactory inputs we can actually alter how the brain perceives energy balance and how the brain regulates energy balance,¨ said Céline Riera, a former UC Berkeley postdoctoral fellow now at Cedars-Sinai Medical Center in Los Angeles.

Image caption: After UC Berkeley researchers temporarily eliminated the sense of smell in the mouse on the right, it remained a normal weight while eating a high-fat diet. The mouse on the left, which retained its sense of smell, ballooned up on the same high-fat diet. Image credit: Céline Riera & Andrew Dillin, UC Berkeley.