Since our sense of smell is directly connected to our enjoyment of food, it makes a lot of sense for the first to influence the latter. But as a new experiment from Berkely University showed, things are more complex than you might think.

Smell and calories

Both of these mice are on a high-fat diet. But the right one is smell-less, while the left one has a regular sense of smell. Image credits: Cell Metabolism.

Think about your favorite food. Imagine it being right in front of you, you can feel its intense smell, and it’s beautiful. Makes you hungry, doesn’t it? Now imagine you could never smell it, ever again. Would it still be your favorite food? Probably not. Smell does a lot to influence our eating habits, and this has been established quite a while ago. So it’s really no surprise that when Berkeley researchers eliminated the sense of smell from obese mice, the mice also lost a lot of weight. But not everything is as intuitive. For instance, when these smell-less mice were given as much food as regular rats (big portions), they didn’t gain fat at all, whereas the latter doubled their weight.

If their hypothesis is correct, then the olfactory sense might be directly connected to the parts of the brain in charge of our metabolism.

“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.

Human metabolism is an incredibly complex phenomenon. When you go past the “guru” type of advice and look at the real science, you start to see just how much we’ve yet to understand about it. Let’s look at the simple calories, for instance. What is it?

A calorie is a measure of energy. In other words, when you say you’re “eating too many calories,” you’re basically consuming energy — and transforming it into a different type of energy. Just like a robot, your body requires a lot of energy to function, and we get that energy from food. When there’s too much of it, your body starts to store it, by making you fatter. But metabolism is not as simple as calories in — calories out. More and more studies are starting to show that not all calories are created equal, and when you also consider the different types of nutrients and vitamins our body needs, the picture becomes much messier.

But it’s not just the food, our bodies are also a part of the equation. Basically, different bodies treat the same food differently, and our senses might play a role in that.

“Sensory systems play a role in metabolism. Weight gain isn’t purely a measure of the calories taken in; it’s also related to how those calories are perceived,” said senior author Andrew Dillin, the Thomas and Stacey Siebel Distinguished Chair in Stem Cell Research, professor of molecular and cell biology and Howard Hughes Medical Institute Investigator. “If we can validate this in humans, perhaps we can actually make a drug that doesn’t interfere with smell but still blocks that metabolic circuitry. That would be amazing.”

Triggering the brain

Spices do a great deal to make foods smell better. Image via Max Pixel.

The key point here is that mice — just like humans — are more sensitive to smells before they’ve eaten, rather than after. So the smell of your favorite dish? You feel it more when you’re hungry. If you don’t feel any smell at all, your body might simply interpret that as you not being hungry. In “brain language,” this translates as “we already have enough food, feel free to burn the rest” — so it simply starts burning more food!

This idea seemed to stand in the mice trials. The smell-deficient mice rapidly burned calories by up-regulating their sympathetic nervous system, which is known to increase fat burning. Basically, smell-less mice became a lean mean fat burning machine.

When the mice lost their smell, they also regained normal glucose tolerance. Manifested abnormally, glucose tolerance can lead to diabetes.

But it’s not like we should all just give up our sense of smell. Smell-less mice also exhibited a large increase of noradrenaline, a hormone that mobilizes the brain and body for action. In humans, high levels of noradrenaline (also called norepinephrine) are associated with heart attacks.

But eliminating a person’s sense of smell (especially temporarily) doesn’t seem that invasive when you consider that people are willingly carrying out stomach stapling or bariatric surgery. Even with the increased noradrenaline, the benefits seem to greatly outweigh the risks, especially if the smell could be regained.

“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.

The team already developed several ways to knock out a person’s sense of smell. A particularly interesting method is through a virus, with effects lasting up to three weeks. Ideally, people would pay at least some attention to how much they eat and this wouldn’t be necessary. Unfortunately, experience has shown us that at least sometimes, drastic measures are necessary.

“People with eating disorders sometimes have a hard time controlling how much food they are eating and they have a lot of cravings,” Riera said. “We think olfactory neurons are very important for controlling pleasure of food and if we have a way to modulate this pathway, we might be able to block cravings in these people and help them with managing their food intake.”

Journal Reference: Celine E. Riera et al — The Sense of Smell Impacts Metabolic Health and Obesity. DOI: http://dx.doi.org/10.1016/j.cmet.2017.06.015.