While scientists have studied the interaction between our gut and our brain for many years, some researchers are beginning to think the connection is quite complex. It may be that our brains are significantly influenced by the bacteria in our gut.
Your gut has natural defenses against harmful pathogens. In addition, your gut encourages the growth and survival of healthy bacteria. The majority of these bacteria reside in the colon, which harbors around 1 trillion bacteria in each gram of intestinal content. It is difficult to accurately count the number of bacteria in your gut. However, a good estimate is that your gut is home to around 40 trillion of them. In contrast, your entire body is made up of around 30 trillion cells, not including the bacteria in your gut.
Most of the gut bacteria fall into one of 30 to 40 species. However, there can be up to 1,000 different species. Together, we call this the gut microbiome. Many of those bacteria give us positive benefits. Some species break down dietary fiber into short-chain fatty acids making them easier to absorb. They metabolize many compounds and play a role in the synthesis of vitamins B and K.
Other research shows that dysregulation of the gut microbiome can be a significant factor in inflammatory diseases and autoimmune disorders. As scientists delve more deeply into these interactions, they are finding that our gut bacteria can moderate our brain and behavior.
Why is the gut linked to the brain?
What happens in your gut can literally be a matter of life or death. If the gut is empty, it needs to relay that information to the brain. If the gut is fighting a pathogen, it needs to relay that information as well. The links between brain and gut are immunological, hormonal, and neural with the information relayed over the central nervous system and enteric nervous system, which controls gut functioning. They are termed the gut-brain axis.
The connections between brain and gut might surprise you. However, almost every human has experienced action when faced with extreme stress or anxiety. Your gut wants to empty itself quickly in those situations.
The relay of information between the gut and brain has been studied for years. Now, scientists are examining how the microbiome actually influences the gut-brain axis. In other words, researchers are asking: do the bacteria in our gut affect our psychology and behavior?
Stress, anxiety, and the gut
In humans, the hypothalamic-pituitary-adrenal (HPA) axis is the primary responder to stresses of any kind. It is one of the major players in the limbic system and is heavily involved in emotions and memory.
Stress activates the HPA axis and eventually results in the release of cortisol – the “stress hormone” – which has a variety of effects on many organs, including the brain and gut.
This is how the brain’s response to stress directly impacts the cells of the gut, including enteric neurons, interstitial cells of Cajal, immune cells, enterochromaffin cells, and epithelial cells.
By contrast, the microbiome also influences these cells. While the mechanisms by which they do this are less clear, the evidence is growing that the microbiota does indeed regulate the brain in some ways.
The first evidence that microbes may have some influence over our mental activity was discovered over 20 years ago. Patients with hepatic encephalopathy, which is a decline in brain function caused by severe liver disease, improved substantially after taking oral antibiotics.
Further studies suggested that the microbiome had a stronger influence on states of mind by impacting depressive-like behaviors and anxiety. Observations also pointed to dysbiosis with autism. Children with autism frequently have less diverse or abnormal communities of gut bacteria.
One researcher concluded:
“We suspect that gut microbes may alter levels of neurotransmitter-related metabolites, affecting gut-to-brain communication and/or altering brain function. […] Correlations between gut bacteria and neurotransmitter-related metabolites are stepping stones for a better understanding of the crosstalk between gut bacteria and autism.”
In 2004, scientists bred mice with no gut bacteria and demonstrated that the mice had an exaggerated HPA axis response to stress. Another study with similar mice showed that the lack of gut microbiome changed their memory function. These types of mice have been useful in studying the microbiome-gut-brain axis. They have helped prove that something is occurring, although it is impossible to apply the findings to humans because there is no such thing as a human without a gut microbiome.
Other studies have used different approaches. Some investigated the effects of the neuroactive compounds that gut flora produces. Others examined the differences in gut microbiome in people with neurological or psychiatric differences.
Still, the research is not conclusive. The question always persists: which came first? Does a psychiatric condition cause change to the gut microbiome, or does the gut microbiome cause a psychiatric condition and its resulting changes in behavior? Is it possible the interaction is two-way?
How can the gut microbiome moderate the brain?
We know that stress can increase the permeability of the intestinal lining, which makes it easier for bacteria, both good and bad, to access the neuronal cells of the enteric nervous system and the immune system. There may be other ways in which bacteria influence our systems. However, scientists have found a more direct route.
One study, using food-borne pathogens, provided evidence that bacteria in the intestines can activate stress circuits by directly activating the vagus nerve, which is connected to many organs including the upper digestive tract.
This more direct route may be the direct contact of the microbiome with the sensory neurons of the enteric nervous system. The studies with germ-free mice show that their sensory neurons are less active than normal mice. When they receive probiotics to reintroduce a microbiome to their system, the activity levels of the neurons return to normal.
Probiotics influencing psychology
If germ-free mice have altered behaviors, can adding gut bacteria to animals also alter behaviors? A meta-analysis published in the Journal of Neurogastroenterology and Motility collated the results of studies looking at the effects of probiotics on central nervous system function in both humans and animals.
The researchers evaluated 15 human studies and 25 animal studies. Most of the studies used Bifidobacterium and Lactobacillus over a 2-4-week period. The authors noted the difficulty of applying results of animal studies such as these into human terms.
They concluded:
“These probiotics showed efficacy in improving psychiatric disorder-related behaviors including anxiety, depression, autism spectrum disorder, obsessive-compulsive disorder, and memory abilities, including spatial and non-spatial memory.”
Another study, published in PLOS One, found that age-related decline in memory could be reversed in rats by altering the levels of Actinobacteria and Bacterioidetes in their gut with probiotics. The authors conclude: “The data support the notion that intestinal microbiota can be manipulated to positively impact neuronal function.”
The future of the microbiome-gut-brain axis connection
This is a complex issue. Scientists involved in studying the connections investigate the various ways that trillions of bacteria can alter our behavior have a long way to go. However, it is possible that further research may determine that medicines may specifically target the microbiome for psychiatric conditions. Or, they may be able to use the microbiome as a diagnostic tool.
In the far-flung future, perhaps medicines specifically targeting the microbiome will be created for psychiatric conditions; the microbiome may become an early warning system for certain diseases or even a diagnostic tool.
In the meantime, we can be proactive with probiotics and taking care of our guts by eating properly.