Sniffing out hunger: a nose-to-brain connection linked to appetite

(Press-News.org) No more hunger after cooking? A newly identified network of nerve cells is responsible, a research group at the Max Planck Institute for Metabolism Research has discovered in mice. They discovered a direct connection from the nose to a group of nerve cells in the brain that are activated by the smell of food and, when activated, trigger a feeling of fullness. This was not the case in obese mice. This discovery suggests that treating obesity might require different advice about smelling food before a meal based on a person's weight. 

The researchers used brain scans to investigate which regions of the mice's brains respond to food odours, and were able to identify a new group of nerve cells in the medial septum of the brain. These nerve cells respond to food in two steps: When the mouse smells food, the nerve cells fire and create a sensation of fullness. This happens within a few seconds because the nerve cells are directly connected to the olfactory bulb. The nerve cells react to different food smells, but not to other smells. When the mice started to eat, the nerve cells were inhibited. Overall, the mice ate less when these nerve cells are active before eating.

“We think this mechanism helps mice in the wild protect themselves from predators. By eating for shorter periods, they reduce their chances of being caught.," explains Janice Bulk, the first author of the study.

Excess weight disturbs perception

In obese mice, the same group of nerve cells was not activated when the mice could smell food. The mice did not feel fuller and did not eat less overall. The authors point out that it is already known that obesity disrupts the olfactory system, including neuronal activity in the olfactory bulb. The newly identified group of nerve cells could also be affected by obesity.

And in humans?

The human brain contains the same group of nerve cells as the mouse, but it is not yet known whether they also respond to food odours. Studies by other research groups have shown that smelling some specific odors before a meal can reduce people’s appetite. In contrast, other studies have shown that overweight persons eat significantly more in the same situation.

"Our findings highlight how crucial it is to consider the sense of smell in appetite regulation and in the development of obesity.  Our study shows how much our daily-lives’ eating habits are influenced by the smell of food. Since we discovered that the pathway only reduces appetite in lean mice, but not in obese mice, our study opens up a new way to help prevent overeating in obesity”, says Sophie Steculorum, the head of the study and research group leader at the Max Planck Institute for Metabolism Research.

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