The sense of smell affects the causes of obesity

The sense of smell is especially important
for many animals, such as mice, when looking for food.
But a study published Monday (Nov.
14) in the journal Nature Metabolism shows that food smells don't just give these animals environmental information: it also affects their body's use
of fat.

Minhong Ma, a neuroscientist at the University of Pennsylvania, said the study, led by Toshiyasu Sasaoka of the University of Toyama in Japan, provides new insights
into neural pathways between olfactory perception and regulating the body's efficiency in converting food into energy, known as metabolic adaptations.

Previous studies have shown a close relationship
between olfactory neurons (responsible for detecting odors and transmitting this sensory information to the brain) and metabolism.
For example, a 2017 study showed that rats that temporarily lost their ability to smell food lost weight, while rats that could smell food gained
weight on the same diet.

But odor perception is a complex process, and understanding the underlying mechanisms linking odor perception to the hypothalamus, the brain region that controls appetite, cravings, and metabolic signals, has been challenging
.
The study is one of the first to systematically study this link, linking it to metabolic diseases such as obesity and diabetes
.

The researchers first divided the mice that had been fasted for 24 hours into two groups
.
One group was placed in
a cage with four tubes.
One is the smell of normal food, one is the smell of high-fat food, and the other is the smell
of high-sucrose food.
The fourth tube had no odor and was used as a control
.
The other group of mice were all placed in a similar cage, but exposed to visual stimuli of food through transparent bottles placed in cages, each containing one of three diets, and an empty control bottle
.

The researchers found that seeing food had no effect on the exploratory behavior of hungry animals, and that mice stimulated by smell spent the most
time exploring pipes that smelled of fatty food.
The study's authors concluded that fasted mice looked for known foods
based on smell rather than visual information.

From there, the scientists used the same two groups of mice to see how the smell of food or the visual stimuli of food affected the distribution
of body fat during fasting.
When the body lacks energy, fatty acids are released from adipose tissue and quickly mobilized for use by tissues throughout the body
.
The researchers measured levels of these fatty acids in mice after 6 hours of fasting and 16 to 24 hours of fasting
, respectively.

They found that in mice that fasted for 24 hours, exposure to familiar food odors for 60 minutes increased their fatty acid levels, but had little effect
on mice that fasted for 6 hours.
In addition, food odors increase the ratio
of oleic acid (thought to be beneficial for metabolic health and the prevention of type 2 diabetes) to other lipids (stearic acid fatty acids).

On the other hand, during fasting, the visual stimulation of food does not affect the distribution
of fat in the body.

The researchers wrote in their paper that because obesity and type 2 diabetes are global health risks, their findings could lead to the development of odor-based strategies to help combat metabolic disorders
.

But rodents like mice rely heavily on their sense of smell to find food, and it remains to be seen
whether the relationship between metabolism and smell also exists in humans who rely on more vision.
Haijiang Cai, a physiologist at the University of Arizona who was not involved in the study, agrees that this is a limitation
.
According to him, all odors are encoded in an overlapping way in the cerebral cortex, rather than in discrete areas of the brain, and determining which foods or possible non-food odors affect the metabolic process is challenging
.