There is a strong connection between gut health and depression. You know the gut is considered to be our second brain. Emotion like depression, anxiety or stress is also linked to our gut health as well as our mental health. This connection of our gut health and depression is in a bidirectional pathway. People with poor gut health easily experience mental issues like depression and anxiety. Depression also causes several gut health problem like the disruption of the composition of gut bacteria, leaky gut syndrome, and inflammation.
Link between gut health and depression:
The sympathetic and parasympathetic branch of the autonomic nervous system mediates the communication between the central nervous system (CNS) and the enteric nervous system (ENS).
A large amount of serotonin induces the receptors responsible for peristalsis and secretion in the gastrointestinal tract and thus improves the communication between the gut and brain.
The communication between the central nervous system (CNS) and enteric nervous system (ENS) is mediated by the vagus nerve, prevertebral ganglia, immune activation, intestinal barrier function, enteroendocrine signaling.
The nerve called vagus nerve transfer message from the gut to the brain and vice versa. This nerve also sends information from other organs (lung and heart) to the brain. The gut release hormone and neurotransmitter (dopamine, serotonin) to transfer information from gut to brain.
Several microbes release hormone and neurotransmitter to regulate mental health.
List of gut bacteria with their hormone and neurotransmitter 
BACILIUS Dopamine, norepinephrine
BIFIDO-BACTERIUM Gamma-aminobutyric acid (GABA)
ESCHERICHIA Norepinephrine, serotonin
LACTOBACILLUS Acetylcholine, GABA
The link between gut and brain in a bi-directional pathway:
The gut is connected with the brain in a bidirectional pathway. This pathway is called the gut-brain axis. The vagus nerve transfer message through this axis between the gut and brain. This nerve is considered to be the longest nerve in the human body which connect the gut directly with the brain.
The gut-brain axis comprises the CNS, the neuroendocrine and neuroimmune system, the autonomic nervous system, the enteric nervous system (ENS) and gut microbiota.
In one way, the central nervous system can indirectly influence gut microbiota by causing changes in gastrointestinal motility, secretion, and/or intestinal permeability, or may directly affect microbiota by stimulating the release of molecules from enterochromaffin cells, neurons, and immune cells.
In the other way, gut microbiota sends messages to the brain via multiple ways such as direct stimulation of receptor-mediated signaling and enterochromaffin-cell signaling.
Mental health problem is strongly connected to a gut health problem such as constipation, gas, bloating and chronic diarrhea.
On the other hand, dysbiosis (imbalance of gut bacteria) and gut inflammation can affect the nervous system and cause several mental health problems such as Parkinson’s disease, autism, anxiety, and depression.
Stress-induced alteration to gut microbiota can cause the possibility of intestinal permeability or leaky gut. A leaky gut can cause diarrhea, increased inflammation, abdominal pain/cramping, allergies, skin problem, gastrointestinal tract obstructions, and various diseases.
Stress can alter the composition of gut bacteria causing an imbalance of gut bacteria or dysbiosis.
Stress can cause a great change in the composition of gut microbiota.
Chronic stress also causes damage to the gut barrier and make it leaky. Therefore, it increases the number of immunomodulatory compounds of bacterial cell wall such as lipopolysaccharide.
Gut health and depression:
Scientists showed that mice with stress and autism have a lower level of Bacteroids fragilis in their gut.
They also showed that depressed people have a decreased amount of good bacteria such as Coprococcus and Dialister in their gut.
On the other hand, mentally healthy people have a sufficient amount of Faecalibacterium and Coprococcus.
When the scientists fed the mice, they recovered their mental health problems. They confirmed that treatment of mice with B. fragilis cause the mice to become social and mentally sound.
In other research, scientists collected bacteria from depressed people and inoculated it in mice. This cause depression in mice.
The composition of gut microbiota:
- There are 100 trillion microorganisms in the gut. They weigh approximately 2 kg. Among them, 1000 unique species of bacteria are found in the human gut. Gut microbiota comprises more than 1000 microbial species and more than 7000 strains.
- 95% of bacteria of our human body lives in our digestive tract.
- The composition of our gut microbiota is unique just like our fingerprints.
Comparison of Brain with gut (second brain):
- Central nervous system (CNS) contains 85 billion neurons. Enteric nervous system contains 500 million neurons.
- Researchers identified 100 neurotransmitters in CNS and 40 neurotransmitter in ENS.
- CNS produce 50% of all dopamine and ENS produce rest 50% of all dopamine.
- CNS produces 5% of all serotonin and ENS produce the rest 95% of all serotonin.
Probiotic bacteria help in reducing anxiety and depression:
- Microorganisms always play an important role in reducing depression and anxiety.
- Probiotic bacteria such as L. rhamnosus reduce stress. Moreover a combination of Lactobacillus helveticus R0052 and B. longum R0175 help in reducing anxiety and depression.
See also: 10 Best Probiotics for Gut Health
- Several studies say that Lactobacillus farciminis can prevent barrier permeability of the gut.
Reference of Hormone (or neurotransmitter)
1. Journal of Psychiatric Research, ‘Collective unconscious: How gut microbes shape human behavior’, April 2015