It is ancient wisdom that human brain and gut are in constant chitchat. An upset stomach disturbs one's mood and when in foul mood everything tastes bitter. Add to this the gut feeling about someone or something which at times proves right. The concept of a gut-brain axis (GBA) took shape during the 19th century which now stands scientifically proven. Follow up studies have established that the gut has its own nervous system and it can function autonomously. In other words we do have a second brain in the gut.
The gastrointestinal tract ( or GI tract for short), irreverently referred to as the gut, is a muscular tube that stretches from the esophagus to the rectum. The ingestion, digestion, absorption, egestion processes happen in stages aided by the enzymes secreted by the stomach, pancreas, and liver etc. The inner wall of this tract is lined with a fine mesh of millions of nerve cells. This meshwork called the Enteric Nervous System (ENS) independently ensures and coordinates the efficient functioning of all the biochemical and muscular activities in the gastrointestinal tract. Hence ENS is referred to as the gut brain or the second brain. The ENS communicates with the main brain via a two-way hotline called the vagus nerve. The vagus nerve, one of the longest nerves in the body, begins at the brainstem and stretches down to the abdomen, exchanging information with various organs all along its path.
The term microbiota refers to the collection of trillions of bacteria, fungi, archaea and viruses etc. residing in the gut. Microbial colonization in the human GI tract begins right after birth. By age of six a child will have a thriving colony of heterogenous good, and bad microbes. These minute inhabitants are not passive observers of the happenings in the GI tract. They aid in the breakdown down of complex carbohydrates and tough fibrous matter. They have other responsibilities too: facilitating the development and modulation of immune system, enabling the production of certain essential amino acids and vitamins, helping to maintain the integrity of the gut barrier, protecting the body from pathogenic organisms. etc.
The power of the microbiota lies in its diversity and number. More than 2000 known species totaling trillion plus are living in the human intestinal tract. Collectively they contain more than 100 times the human genomic DNA . In healthy individuals there is a fine balance between the good and bad microbial communities. When this balance is upset ( this is referred to as dysbiosis) a variety of maladies including mood disorders, depression, anxiety, etc. manifest. Because many of the microbial metabolites and secretions are potent neurotransmitters such as serotonin, dopamine, oxytocin, short chain fatty acids (SCFA), tyramine, acetylcholine, norepinephrine and many more. These molecules are the main propellants of various brain functions like emotions, learning, and memory. However these neuroactive molecules generated in the gut don't reach the brain instead they modulate/manipulate the communication along Gut-Brain Axis and thus indirectly exert influence. Imbalances in the microbial diversity and in the level of neurotransmitters have been detected in the gut of patients afflicted with neurological and psychological disorders such as Alzheimer's and Parkinson's diseases, autism , anxiety and depressive disorders.
As a correction mechanism attempts are on to enrich the microbial diversity by including food rich in probiotics (beneficial live microorganisms), prebiotics (substances that promote beneficial bacteria growth), and postbiotics (microbial metabolites) in the diet. Indications are that a microbial approach to the management of physical and mental health might soon evolve.
The communication along the Microbiota-Gut-Brain Axis, (MGBA for short) is bidirectional hence the brain can as well drive changes in the gut and alter the microbial composition. “The complex interplay between our microbiome and brain is a testament to the body’s remarkable interconnectedness. It’s not just about gut health or mental health; it’s about how each influences the other in profound ways,” says Sean Spencer, MD, PhD, Gastroenterologist and Physician Scientist at Stanford University.
TAILPIECE
Dr Jake M. Robinson poses a relevant question: If neurotransmitters and hormones secreted by microbiota can influence the emotional status of the host then why not explore the microbiology of Love? Perhaps Ophelia's depression and Hamlet's despair had a microbiological cure.
REFERENCES:
1. The Gut-Brain Paradox : Steven R Gundry, Harper (2025); ISBN-13 : 978-0063471252
2. The Mind-Gut Connection: How the Hidden Conversation Within Our Bodies Impacts Our Mood, Our Choices and Our Overall Health : Emeran Mayer : HarperCollins (2016) ISBN-13 : 978-0062376589
3. How gut bacteria are controlling your brain
4. Does microbial-endocrine interplay shape love -associated emotions in humans? A hypothesis
5. The gut microbiome connects nutrition and human health
6 .Microbiota–gut–brain axis and its therapeutic applications in neurodegenerative diseases
