Gut Microbiome Composition Linked to Human Behavior

The gut microbiome refers to the trillions of beneficial bacteria, viruses, fungi, and other microorganisms that normally inhabit our gastrointestinal tract. These organisms, or microbes as they are called, are crucial to our digestive system and to the overall health of our body.

According to past studies, the gut microbiome also has a measurable impact on the brain, influencing stress, anxiety, depressive symptoms, and human behavior. This microbiome-gut-brain axis may be influenced by various mechanisms from the neural, immune, and endocrine systems. So far, the majority of research has been conducted in animal models while the limited number of human studies has focused on psychiatric conditions. (1)

Recent research from Katerina Johnson of Oxford University has revealed the possible relationship of the gut microbial community to personality.  Can a person’s behavior, which is governed by personality, be influenced by the state of their gut microbiome?  It was observed that people who have larger social networks are more likely to have greater gut microbiome diversity. Those exhibiting anxiety and stress showed an altered microbiome composition.  “This may shed new light onto known behavioral variations in the general population as well as to cases of psychiatric disorders,” states Johnson. (2)

Gut-Brain Connection

Called the gut-brain axis, other studies show that the brain affects gut health and that the gut may even affect brain health.

The vagus nerve is one of the biggest nerves connecting the gut and brain. It sends signals in both directions. The gut and brain are also connected through chemicals called neurotransmitters which are produced in the brain and control feelings and emotions.

The gut-brain axis is also connected through the immune system. Gut microbes play an important role in the immune system and inflammation by controlling what is passed into the body and what is excreted. Inflammation is already associated with several brain disorders like depression and Alzheimer’s Disease. (3)

Mental Health

Evidence from small studies of humans and decades of animal model research has begun to link the human microbiome to central nervous system disorders including autism, depression, schizophrenia, anxiety, multiple sclerosis, and possibly some personality disorders like obsessive-compulsive, paranoia, and antisocial. (3)

According to Cryan and Dinan, “…the emerging concept of a microbiota-gut-brain axis suggests that modulation of the gut microbiota may be a tractable strategy for developing novel therapeutics for complex CNS disorders.” (3)

Jane Foster Ph.D., a neuroscientist and researcher at McMaster University in Ontario, notes that the blood and tissue tests available to help diagnose cancer and other conditions are not yet available for mental health, “so these studies for individual differences [in the gut microbiota] are actually where this area of research is going to have the biggest impact.” (4)

As an example, in a mouse model of amyotrophic lateral sclerosis (ALS or Lou Gehrig’s Disease) animals that had ample levels of the bacterium Akkermansia muciniphila in their gut fared better than those carrying almost no members of the species. Dr. Elinav found lower levels of the bacterium in the stool of ALS patients and lower levels of nicotinamide in their blood and cerebrospinal fluid. Also, the levels of nicotinamide in the blood correlated with the severity of the patient’s condition. Patients with lower levels tended to have worse symptoms. (5)

Benefits of Exercise and Diet

People often feel better after exercising. It now appears a new benefit might be the maintaining of proper gut microbiome diversity through exercise, thus contributing to a healthy body.

One of the leading ideas in the field of exercise’s influence on the microbiome is that working out helps support the levels of gut microbes that produce butyrate, a short-chain fatty acid that has a whole host of health benefits in humans.  These benefits include producing satiety hormones that curb hunger to playing a role in supporting the survival of existing neurons and promoting the growth of new ones.

Jeffrey Woods, a researcher at the University of Illinois, proposed that exercise might alter the gene expression of immune cells in the tissues of the gut, leading to the production of fewer pro-inflammatory cell-signaling proteins and more anti-inflammatory ones, as well as antioxidant enzymes. The results make it clear that exercise, regardless of diet or body composition, changes the gut microbiota of humans. (6)

Foods like fruits, vegetables, nuts, seeds, whole grains, and legumes contain high levels of dietary fiber, which fuels the beneficial bacteria in the gut. By eating a diversity of plant-based food, different types of microbes can be nourished, leading to a more diverse microbial community. Studies in laboratory mice have shown that certain prebiotics and probiotics can increase the production of GABA, an amino acid that works as a neurotransmitter in the brain. This may help reduce anxiety and depression-like behavior. (7)

The reduced diversity and quality of our food today is a contributing factor to the state of our gut microbiome. Before the introduction of chemicals into the growing of food, our ancestors enjoyed mainly local farm-fresh products. With the advent of agribusiness, 75 percent of plant genetic diversity has been lost. Farmers worldwide have left their multiple local varieties for genetically uniform, high-yielding varieties and high production practices. According to the Food and Agricultural Organization of the United Nations, most of the world’s food today is generated from only 12 plants and five animal species. (8)

More research needs to be done to further prove that the variation in the gut microbial community is related to personality and that the behavioral differences between individuals relative to the gut microbiome are consistent over time and in different situations.  

Challenges remain. More human subjects need to be tested. More standardization of analytical tools needs to be developed. However, the work on the microbiome and behavior being done so far is very encouraging for the behavioral and mental health sciences and thus for the millions of people suffering from mental health disorders.