Our guts are filled with lots and lots of microbes (aka gut microbiota or microbiome). Recently, more and more research is pointing towards these tiny inhabitants of our digestive system as a very relevant factor in various aspects of our lives. Including our brains and behavior. New data is revealing that the beasts in our gut influence the central nervous system. (So, it’s not just parasites that potentially control our brains… See previous posts here and here.)
What do we know so far about this? A review in Nature Reviews Neuroscience bundles current knowledge.
It’s been known for quite a while that the gut influences the brain, and vice versa, which has led to the development of the ‘gut-brain axis’ concept. What is new, however, is that our microbiome, more than 1000 species rich, plays an important part in this.
How, you ask, do we know this?
Good question. At the moment, there are four main approaches that are used to elucidate the complicated relationship between the many neurons and the many gut micro-organisms that are all encased by our bodies.
Germ-free animals. A mother’s womb is a sterile environment, so the gut is only colonized by its many inhabitants after birth. If you make sure that the animals (mostly rodents) are born into a sterile environment, voila, we have animals without gut microbiota. (Example: germ-free mice are more very sensitive to stress, and suffer from some cognitive deficits.)
- Bacterial infections. An infection changes the composition of the little society in your gut, and can impact brain and behavior. (Example: infected mice show more anxiety and cognitive dysfunction.)
- Probiotics. There are life organisms that, when ingested, con confer health benefits (just think of all those yogurt commercials). (Example: ingestion of a certain bacterial strain reduced anxiety and changed the presence and activity of receptors in certain brain regions which are associated with anxious and depressive-like behaviors in our animal cousins.)
- Antibiotics. If you take out some tiny gut-dwellers, this, of course, changes the community composition in your digestive tract. (Example: administration of certain antibiotics induces exploratory behavior in mice and changed the levels of a certain protein (BNDF) in the brain.)
What have we learned so far? In other words, if our gut guests can influence our central nervous system, then where do we already know them to be involved?
- Pain. Especially abdominal pain. Studies have shown that the ingestion of certain probiotics (for those interested: certain strains of Lactobacillus and Bifidobacteria) increases the pain threshold and can alleviate abdominal pain.
- Autism. Children with autism have been found to possess an altered composition of intestinal guests. (However, be careful here, as autistic children, on average, are administered more antibiotics than their non-autistic counterparts.) So, autism and different gut microbes, cause or correlation? Can’t say for sure, more (large-scale) studies required.
- Obesity. Here, causal links have been found in human beings (and their rodent fellow mammals). However, it’s more complex than you might think, since what you eat can also influence the composition of your internal society.
- Multiple sclerosis. Very early stages here. But studies on an animal (mice) model of the condition have shown that mice reared under sterile, and specific-pathogen-free, conditions, were more resistant to MS.
In general, our tiny gut inhabitants seem to have quite an impact on (the rest of?) us. However, research on the intricacies of the human (and others’) microbiome is fairly new, and a lot more is awaiting discovery.
The authors conclude:
Overall, it is becoming increasingly apparent that behaviour, neurophysiology and neurochemistry can be affected in many ways through modulation of the gut microbiota. Whether this translates to microbial-based CNS therapeutics remains a tempting possibility and one that is worthy of much further investigation.
Thus, be nice to your microbes. And hopefully, they’ll be nice to you too.
Cryan, J.F., & Dinan, T.G. (2012). Mind-altering microorganisms: the impact of gut microbiota on brain and behaviour. Nature Reviews Neuroscience, 13, 701-712 DOI: 10.1038/nrn3346