Every now and then, I get a "gut feeling" about an issue, topic, or information. A sense of its veracity from somewhere other than my conscious mind.
This gut/brain balance has been well studied and described by the likes of Daniel Kahneman and Malcom Gladwell, among others. We are constantly processing the billions of bits of information we receive every second at a level below our consciousness. It is only when these computations lead to a revelation we become aware of their existence and we say our gut is trying to tell us something.
But over the past decade, researchers have become increasingly interested in the connection between our actual guts and our brain. Our bodies house an immense microbiome - approximately 10 trillion microbes with 95 per cent of them living in our gastrointestinal tract. Some scientists have gone so far as to think about re-categorizing humans as "composite animals" as the number of microbes rivals the number of cells in our bodies.
What has become increasingly clear is the connection between the microbes in our guts and our overall behaviour. This is perhaps not too surprising. The insides of our intestines have a surface area of about 32 square metres, on average. It is through this surface we absorb all of the chemicals essential for life - amino acids to make proteins, sugars to provide energy, fats to build cells, etcetera. We absorb all of the nutrients for life through our intestinal lining.
It is perhaps not surprising, then, to find our intestines also interact with the microbiome living within us. These organisms take up nutrients and pump out byproducts which we can then absorb. We wouldn't be able to live without our microbiome doing a lot of the heavy lifting with respect to digestion. And in return, we provide these microbes with a safe and stable place to flourish.
That these creatures may also influence our overall mental health is a question open for examination. There are a number of possible pathways by which they may influence our overall feelings.
They may interact with the immune cells lining the gut, prompting the production of cytokines which are released into the blood. These may then circulate through the body to the brain and impact brain chemistry.
Microbes might also interact with another type of cell lining the gut called enteroendocrine cells which produce neuroactive molecules and short chain proteins called peptides. These molecules can then interact with the vagus nerve which is one of the major neural pathways to the brain. The vagus never is bit of a superhighway linking various body parts to the medulla oblongata (a grape-sized bit of tissue at the base of the brain). It provides us with a sense of how our bodies are doing. Or, as one researcher puts it, "Sometimes you feel good; sometimes you feel crappy. That is your vagus nerve telling you what's going on."
The microbiome might also be producing neurotransmitters directly which are then able to cross the gut-body barrier. Some molecules, such as butyrate, circulate to the brain and cross the blood-brain barrier while other metabolites might induce changes in the cellular activity of the barrier itself. In either case, such molecules may have a direct impact on the brain's chemistry.
It was Louis Pasteur who first speculated in 1885 on the positive role microbes might play in our lives. But it wasn't until as recently as 2004 that research appeared showing so-called germ-free mice responded to stress much more strongly than mice raised in a normal environment. Without the micro-organisms inside them, the animals appeared to be more susceptible to behavioral abnormalities.
In 2009, John Cryan, a neurobiologist at University College Cork, published research showing that when young rat pups were separated from their mothers, the stress could lead to long term changes in their microbiomes.
In 2011, Cryan's group reported that when Lactobacillus rhamnosus JB-1 was given to mice, their stress behaviours decreased with accompanying changes in their brain chemistry. Further, mice with a severed vagus nerve did not respond in the same way.
Other researchers have also investigated the connection between the vagus nerve, the brain and the microbiome. For example, in 2015, Mauro Costa-Mattioli and colleges at Baylor College of Medicine fed female mice high-fat diets to make them obese before they got pregnant. They found their off-spring initially exhibited problems with social interactions. However, once these mice were weaned, they were reunited with the control animals and four weeks later, all the mice were interacting normally. These results can be explained by the animals ingesting each other's poop and thereby developing a normal microbiome.
There are still many researchers skeptical of the extent of the gut-brain connection and the field is not without its critics. But it is just possible our guts may be telling us more than we think.
