Being able to smell food before eating can increase weight gain independently of how much fat is in it, according to a new study carried out on mice, which scientists think points to a link between a sense of smell and metabolism.
Mice fed the same high-fat diet showed big differences in weight gain depending on whether or not their sense of smell had been temporarily switched off.
The researchers from the University of California, Berkeley, say their study hints at a connection between what animals can smell and how they burn calories: so if they can't smell their food, they burn off fat rather than storing it.
"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," says one of the team, Céline Riera.
Using gene therapy, olfactory neurons in one group of test mice were temporarily zapped before eventually growing back after several weeks.
These smell-deficient mice started turning their beige fat cells, used to store fat around the body, into brown fat cells, which burn up fatty acids. What's more, their white fat cells – which store fat around the internal organs and can cause health problems if too many build up – got smaller.
In summary, these smell-deficient mice turned into lean, fat-burning machines. They did this by turning up the dial on their sympathetic nervous system, which increases fat burning.
Over in the group of untreated mice, who were given food with the same levels of fat in it as those with their sense of smell knocked out, weight gain spiralled up. These mice gained about 100 percent of their original weight, compared with a maximum of 10 percent in the other group.
The researchers also found that mice who were already obese and had their sense of smell removed showed signs of losing weight, while a group of "super smeller" mice tested in Germany, bred with acutely sensitive olfactory nerves, gained even more weight.
"Sensory systems play a role in metabolism," says one of the researchers, Andrew Dillin. "Weight gain isn't purely a measure of the calories taken in; it's also related to how those calories are perceived."
"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."
Such a drug might help those who struggle to lose weight by turning up their fat-burning systems. The research could also be used to improve treatments for those who've lost their sense of smell through ageing or diseases like Parkinson's, a loss which often corresponds with a dip in appetite.
One reason why we should be cautious about using the same approach in humans is that when the sympathetic nervous system gets more active, we also produce more of the hormone noradrenaline – and that can trigger a heart attack.
Still, given enough time and research, this is a technique that could work in humans too: not necessarily by blocking off the nerves in the nose but by tricking our bodies into thinking we're not smelling anything.
The scientists think there's a key connection between our sense of smell and the regions of the brain that regulate metabolism, particularly the hypothalamus – we're more sensitive to smells when we're hungry, for example. How exactly these systems link to each other still isn't clear though.
"People with eating disorders sometimes have a hard time controlling how much food they are eating and they have a lot of cravings," says Riera.
"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."
The findings have been published in Cell Metabolism.