Just smelling food can make you fat, UC Berkeley study says

These mice ate the same high-fat diet in a UC Berkeley study, but the sense of smell was removed for the one on the right, helping it stay slim. These mice ate the same high-fat diet in a UC Berkeley study, but the sense of smell was removed for the one on the right, helping it stay slim. Photo: Courtesy Of UC Berkeley / UC Berkeley Photo: Courtesy Of UC Berkeley / UC Berkeley Image 1 of / 39 Caption Close Just smelling food can make you fat, UC Berkeley study says 1 / 39 Back to Gallery

On the bustling streets of San Francisco, people can sense what’s grilling, baking and frying in restaurants from North Beach to Noe Valley without even looking at the menus, as myriad cuisines serve up a smorgasbord of aromas for the nostrils.

But what most foodies and waist-watchers strolling by don’t realize is that just smelling burgers, pizza, sushi, falafel or any of the other sweet and savory offerings could be causing them to gain weight.

A study by UC Berkeley researchers found that a sense of smell can influence the brain’s decision to burn fat or store it in the body — or a least the bodies of mice.

Researchers Andrew Dillin and Celine Riera studied three groups of mice — normal mice, “super-smellers” and ones without a sense of smell — and saw a direct correlation between their ability to smell and how much weight they gained from a high-fat, “Burger King diet,” Dillin said.

Each mouse ate the same amount of food, but those with a super sense of smell gained the most weight.

The normal mice ballooned, too — up to 100 percent from the weight they were when the research started.

But the mice who couldn’t smell anything gained only 10 percent of their weight. Obese mice who had their sense of smell wiped out slimmed down to the size of normal counterparts without a change in diet.

Riera said the study, which was published this month in the journal Cell Metabolism, reveals that outside influences such as smell can affect the brain’s functions related to appetite and metabolism.

Other studies have shown that olfactory systems, the organs and lobes related to smell, can influence things like sexual behavior or fertility, but this study isn’t related to internal functions or hormones, said Riera, a former UC Berkeley postdoctoral fellow now at Cedars-Sinai Medical Center in Los Angeles.

“In the context of food and appetite, this is really novel,” she said.

Using the study’s methods in humans could be possible.

After eating, a person’s sense of smell decreases. So, if a person was eating with a lessened sense of smell, the brain could be tricked into thinking it’s already been fed and choose to burn the calories instead of store them, Riera said.

People struggling with obesity could have their sense of smell wiped out or temporarily reduced to help them control cravings and burn calories and fat faster.

But there are risks.

“People that don’t have a sense of smell can get depressed, because the sense of smell is very important for behavior,” Riera said. “They lose all pleasure of eating.”

People who lose their sense of smell from aging, injury or diseases like Parkinson’s can also struggle with keeping weight on.

The mice in the study who lost their sense of smell also saw a significant increase in the hormone noradrenaline — a stress response from the nervous system that can lead to a heart attack if levels are too high.

Eliminating a human’s sense of smell would be a radical step, said Dillin, an expert in stem cell research. But it could also be an option for obese people who are considering such weight-reduction alternatives as stomach stapling or gastric-band surgery, even with the potential for high levels of noradrenaline.

In both humans and mice, decreasing or wiping out the sense of smell is only temporary. In the study, researchers injected a toxin that destroyed olfactory neurons in the nose, but they avoided the stem cells which allowed the neurons to grow back in three to eight weeks.

To use the method in a human population, scientists would need to know how many of the olfactory neurons to destroy and how often, Dillin said.

Because once the sense of smell comes back, the weight could also return.

“Maybe once a year you block your sense of smell for a while and then you lose the weight from the year and do it all over again,” Dillin said. “We don’t know yet. There’s a lot we still need to do.”

Alison Graham is a San Francisco Chronicle staff writer. Email: agraham@sfchronicle.com Twitter: @alisonkgraham