New insights on the brain’s impact on excessive hunger and appetite regulation

New studies on rats and mice suggest that the brain creates meal memories to help control hunger and eating behavior and activates a feeling of fullness after smelling food. Though more research is needed, the findings could inform new approaches to treating obesity and weight management.

Scientists have identified meal memory neurons that encode what food was eaten and when. This could explain why people with memory problems often overeat and why forgetting a recent meal can trigger excessive hunger and lead to disordered eating. 

These neurons in the ventral hippocampus become active during eating and form meal engrams, specialized memory traces that store information about the experience of food consumption. 

“An engram is the physical trace that a memory leaves behind in the brain,” says co-author Scott Kanoski, professor of biological sciences at Dornsife College of Letters, Arts, and Sciences at the US University of Southern California. “Meal engrams function like sophisticated biological databases that store multiple types of information, such as where you were eating and the time you ate.”

Meal memory mechanism

The study published in Nature Communications used advanced neuroscience techniques to observe laboratory rats’ brain activity as they ate. The study suggests that meal engrams are formed during the pauses between bites when the brain surveys an eating environment. 

When the researchers selectively destroyed these meal memory neurons, the rats showed impaired memory for food locations but remembered locations of non-food-related tasks. 

The team explains that meal memory neurons communicate with the lateral hypothalamus, a brain region known to control hunger and eating behavior. When this connection was blocked, the rats overate and could not remember where they consumed meals. 

Similarly, earlier research identified hippocampal neurons that drive overeating by encoding sugar and fat memories, shaping feeding behavior and metabolic health. The scientists said that silencing these neurons likely reduces sugar consumption as people cannot recall sugar-related memories. 

When the researchers disrupted the connection between meal memory neurons and the hypothalamus, mice overate.Another study on mice highlights overactive food-seeking neurons as a potential driver of post-meal snack cravings instead of an overactive appetite. The mice’s brain circuit makes the animals crave and seek food, even if they are not hungry. 

New weight management strategy

Kanoski says that “it can be assumed” that human brains also make meal engrams. 

The authors highlight how distracted eating can impair meal memories by compromising these encoding moments, thus contributing to overconsumption. 

They suggest that enhancing meal memory formation could be an essential weight management strategy in addition to food restriction or exercise. 

“We’re finally beginning to understand that remembering what and when you ate is just as crucial for healthy eating as the food choices themselves,” adds Kanoski.

Moreover, the team notes their findings could help understand human eating disorders. For example, patients with memory impairments like dementia may often consume multiple meals quickly because they cannot remember eating. 

Food smells trigger “feeling of fullness”

Meanwhile, researchers at the German Max Planck Institute for Metabolism Research found a direct connection from the nose to a group of nerve cells in the brains of mice that trigger a feeling of fullness when activated by the smell of food. However, this was not the case in obese mice. The results are published in Nature Metabolism. 

“Our study shows how much the smell of food influences our daily eating habits,” says Sophie Steculorum, the head of the study and research group leader at the institute. 

Smelling food activates brain nerve cells that trigger a feeling of fullness within seconds, which could open a new strategy to prevent overeating.Using brain scans, the team identified a group of nerve cells in the medial septum of mice’s brains. When the animals smell food, these cells activate and create a sensation of fullness within seconds. The cells are directly connected to the olfactory bulb. 

Although the cells react to different food smells, they don’t respond to other smells. When the mice started to eat, the cells were inhibited. If the cells were active before eating, the mice ate less. 

“We think this mechanism helps mice in the wild protect themselves from predators. Eating for shorter periods reduces their chances of being caught,” explains Janice Bulk, the study’s first author.

Obesity impact

However, in obese mice, these nerve cells were not activated when the mice smelled food. These mice did not feel fuller and did not eat less overall. 

“Our findings highlight how crucial it is to consider the sense of smell in appetite regulation and the development of obesity,” adds Steculorum. “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.”

As in the research on meal memory, the authors note that more research is needed to determine if humans respond similarly. They posit that human brains have the same group of nerve cells as mice, but it is unclear whether they also respond to food odors. 

Other research indicates that smelling specific odors before a meal can reduce appetite, but different studies note that overweight persons eat significantly more in the same situation. 

In related news, earlier this year, researchers revealed that appetite-suppressing GLP-1 medications may weaken taste function. The study highlighted that 85% of participants using these drugs had significantly diminished taste perception.