New research funded by the European Research Council and published in the journal Nature Neuroscience claims to have shown how the endocannabinoid system controls food intake using the sense of smell.
Endocannabinoids are cannabis-like chemicals that are made in the body and are used to send "messages" between cells.
The endocannabinoid system is a network of neuron receptors, enzymes and endocannabinoids that exists both in animal and human brains. The receptors in the endocannabinoid system are associated with sensations such as euphoria, anxiety and pain.
Scientists know that when we are hungry, our bodies boost the performance of our sense of smell in order to improve our chances of finding food.
"Increasing smell allows better retrieval of food sources," study author Giovanni Marsicano, PhD, told Medical News Today, "but it also increases the attractiveness of certain odors. I would imagine that there is a sort of positive feedback: you are hungry, you smell more and, therefore, you look better for food and you are more attracted by it."
But the mechanisms involved in how the brain co-ordinates the sensation of hunger, sense of smell and food intake has not been well understood.
During hunger, brain mechanism boosts sense of smell
Dr. Marsicano and his team found - through experiments in mice - that this feeding mechanism is initiated in the endocannabinoid system.
They found that the "CB1" cannabinoid receptors in the brain connect the nervous system, which processes smell ("the olfactory bulb"), with higher brain structures associated with smell ("the olfactory cortex").
The sensation of hunger activates the CB1 receptors, which then activate the olfactory bulb and cortex. So it is this brain mechanism that increases our sensitivity to smell when we are hungry, which in turn increases our craving for food.
Animals and humans are thought to process smell differently - particularly when it comes to finding food.
But Dr. Marsicano argues that rather than human behavior being less driven by a sense of smell than animals, our sense of smell is processed at a lower level of consciousness than other sensory information.
In their study, the authors use the example of partner selection in humans being guided unconsciously by sense of smell as evidence of this.
"For sure smell is one of most important senses for rodents and we tend to think that it is less important for humans," acknowledges Dr. Marsicano. He adds:
"However, it is clear from experimental psychology studies in humans that we take many important decisions based on smell, but we just do not realize it. In the specific case, the impact of cannabinoids on food intake is very similar between humans and rodents. Therefore, we can assume that similar mechanisms apply also to humans. Indeed, a potentiation of smell is a well-described effect of cannabis in humans."
Dr. Marsicano notes that although there is evidence that THC (the ingredient of cannabis that provides a "high" when used recreationally) can boost both sense of smell and the sensation of hunger in humans, it does so independently - there is no scientific evidence as yet that these two functions are linked.
In people with eating disorders, such as obese or anorexic patients, the researchers speculate that this olfactory circuit is altered. Sensitivity to smell may be stronger or weaker in these patients, compared with other people.
The researchers think this could provide the basis for future research into how the endocannabinoid system works in humans.
In 2013, Medical News Today reported on a study claiming to have found the area of the brain that triggers overeating.