The idea the food we eat influences how we perceive the world and act upon it has been put forward on this blog several times (e.g. ‘are we what we eat?’, ‘food for switching’, ‘more charity through food’, ‘the good of chocolate cognition', ‘getting creative through food’, and ‘refueling your mental engine’). In most of these blogs, the focus was on how specific food components (e.g. the amino acid tyrosine) has a direct effect on the chemical messengers in the brain (e.g. dopamine), as such affecting your thoughts and behavior.

Besides certain components having direct effects on the brain, the idea that food influences your well-being, cognition, and behavior is further supported by the existence of the ‘gut-brain’ axis: a direct, physical connection between the intestinal tract and the brain.

Gut-brain communication
The gut and the brain communicate with each other via neural, endocrine, and immune signaling pathways. For example, the vagal nerve (often thought to be the physical link between the gut and brain) communicates information about the body’s organs to the central nervous system. The other way around, it also interferes with the parasympathetic control of the heart, lungs, and digestive tract - for example: the brain needs the vagal nerve to communicate to the diaphragma that it should 'do something', in order for you to breathe. Another way for the gut to influence the brain is via certain bacteria in the digestive tract that can actually produce neuroactive substances such as precursors of neurotransmitters (for example tryptophan, as found by Desbonnet and colleagues in 2008), which may influence neurotransmitter levels in the brain and, a such, subsequent behavior.
Indeed, in an earlier blog I already described how consuming certain good bacteria can decrease vulnerability to depression. However, research into how exactly the gut-brain communication works, especially in humans, is still scarce, despite the many interesting theories that have emerged.

The leaky gut
One way the gut and the brain communicate is because of what is called the ‘leaky gut’. Our gastrointestinal tract is separated from the rest of the body by what we call a ‘gut barrier’ or ‘intestinal epithelium’. Even though this barrier is a little permeable (i.e. porous, enterable, passable), allowing nutrients to get out, it mainly serves to keep harmful or toxic substances inside of the intestine. Stress, drugs, unhealthy lifestyle, together with bad eating habits, contribute to an increased permeability of this gut barrier, which then allows more substances to pass through it and start spreading throughout the body via the bloodstream. When this happens, toxic substances (‘endotoxins’), like for example lipopolysaccharide, may end up in the blood circulation. This evokes the release of pro-inflammatory cytokines (which act to make a disease worse by promoting inflammation) by the immune system, which may lead to neuronal distortions in for example neurotransmitter functioning. This may contribute to decreases in cognitive functioning and the subjective experience of stress, anxiety, and depression, which in turn may increase the intake of medication (e.g. painkillers), and may even affect dietary decisions in a negative way (don’t we all just want fast-food and chocolate when we feel down?).
These behavioral decisions in turn may contribute to even more ‘leakiness’ of the gut, as such creating a vicious circle.

Disorders and diseases: epiphenomena?
This mechanism may explain why intestinal problems often co-occur with symptoms of anxiety and stress, and why some psychological disorders and gastrointestinal diseases are even referred to as ‘epiphenomena’. Although the idea of the brain and gut being intimately connected and communicating with each other is relatively new, for most of us this comes as no surprise. Let’s just say we already had a ‘gut feeling’.

However, we need a lot more research before we can draw conclusions on the exact pathways, causal factors, and factors that may influence the connection between the gut and the brain. This means bridging the gap between animal studies and application for treatment and enhancement in humans will be the next big challenge. There are still many mysteries when it comes to understanding how the gut might affect the brain.

If research is able to solve even just few of the above mysteries, the gut-brain axis could be a powerful target for enhancing mood, cognition and behavior, for example in treating disorders, as well as preventing decline and enhancing functioning of the brain.

Interested? Visit www.mindmoodmicrobes.org