We rely on our ability to orient ourselves in space, but what do our guts have to do with it?
In a famous scene from the French novel In Search of Lost Time by Marcel Proust, the narrating character takes a bite out of a madeleine (a small, traditional French sponge cake) that he had previously dunked in a little tea.
Having done so, he begins to remember snatches of his childhood spent in the countryside.
"No sooner had the warm liquid mixed with the crumbs touched my palate than a shudder ran through me and I stopped, intent upon the extraordinary thing that was happening to me," he says.
He goes on, saying, "And suddenly the memory revealed itself. The taste was that of the little piece of madeleine which on Sunday mornings at Combray [...] my aunt Léonie used to give me."
The link between food or drink once tasted and the memory of places or things is something that all of us will be familiar with, and much has been made of it in literature and the arts.
But there is more to the way in which food jogs our memory. In fact, it seems that the signals that our guts send to our brains have been serving us well in terms of how we orient ourselves in the world that we inhabit, and they have been doing so for thousands of years.
This is what researchers from the University of Southern California in Los Angeles have found, at least, in a study recently published in Nature Communications.
How the gut orients the brain
Lead investigator Andrea Suarez and team suggest that the signals that our guts send to our brains play a key role in how well we remember landmarks that allow us to navigate the world, spatially.
The way the gut communicates with the brain, say the researchers, is through the largest nerve of the autonomic nervous system, the system that helps to automatically regulate basic bodily functions: the vagus nerve.
This nerve also connects the gut to the brain or, more specifically, the brainstem, which is the part of the brain found at the lower back of the head. The brainstem is also thought to be the "oldest" part of our brains — that is, the brain that our ancestors' ancestors first developed.
Suarez and colleagues believe that, through the vagus nerve and then the brainstem, the gut sends signals to another part of the brain called the hippocampus, the seat of memory formation and recall.
In doing so, the gut "prompts" the brain to take special notice of where we ate particular foods.
An age-old mechanism
But what is the relevance of this mechanism? According to the authors, its importance stems from the role it played in humans' far history, when we had to forage or hunt for food daily.
"When animals find and eat a meal, for instance," says corresponding study author Scott Kanoski, "the vagus nerve is activated and this global positioning system is engaged."
"It would be advantageous for an animal to remember their external environment so that they could have food again," he adds. This would also be the case with humans.
In short, this gut-brain signaling allowed us to learn where we would find a ready source of food, thus saving us the trouble of potentially starting our search from scratch, at great expense of energy and time.
Implications for obesity treatments?
The research team tested some of these ideas by conducting a series of experiments working with a rat model.
The scientists found that animals in whom they disconnected gut-brain communication through the vagus nerve had trouble remembering key information about the space in which they moved, and they therefore could not orient themselves.
"We saw impairments in hippocampal-dependent memory when we cut off the communication between the gut and the brain. These memory deficits were coupled with harmful neurobiological outcomes in the hippocampus."
At a closer look, Suarez and team found that, in the brains of rats in whom gut-brain communication had been disrupted, the markers of brain cell development, and the development of new neural connections, had been affected. However, the disruption did not impact the animals' level of anxiety or their weight.
"These findings may have clinical relevance in relation to current treatments for obesity that involve disruptive manipulation of the vagus nerve, such as bariatric surgeries [...] and chronic electrical disruption of vagal nerve signaling," the researchers note in their paper.
That is why they advise that future research should focus on gaining a better understanding of how gut-brain signaling via the vagus nerve functions, and what biological mechanisms it might affect.