MSc . Neuroscience
The gut is one of the main digestive organs in the human body. For the longest time, we were well aware of its digestive functions. However, the gut is responsible for many more processes apart from just digesting food. The condition of one’s gut is known to have an effect on the brain and the individual on the whole. The expanse of our guts is colonized by approximately 100 trillion microorganisms which are called ‘gut microbiota.’ With such a huge number, microbial genes are known to outnumber human genes in the body. Thus, that gives us an idea that the gut and the gut microbiota might have a profound effect on us.
When a baby is born, its gut is completely sterile. A vaginally born child is known to acquire its first microbial colonies from the mother’s birth canal. On the other hand, a baby born via C-section is known to acquire its microbial population from the mother’s/ nurse’s skin and the hospital environment.
By the age of one, the infant’s gut population completely flourishes like that of an adult. The human gut is colonized majorly by Bacteroidetes and Firmicutes, and consists of relatively smaller amounts of Proteobacteria, Actinomyces, Fusobacterium, and Verrucomicrobia. Every person’s microbiota is more or less the same yet unique to the individual. This is because everyone’s guts are colonized by the above-mentioned bacteria but dynamicity in one’s gut bacterial profile is brought about by the following factors: genetics, age, diet, personal ailments like intolerance, autoimmune disorders, use of antibiotics, and stress.
Gut microbiota helps in digesting food substances by taking them up in their metabolic cycles and breaking them down. It is seen to help in metabolism and drug detoxification. However, our gut microbiota has multiple other functions as mentioned below:
- maintains intestinal permeability and stimulates intestinal epithelial cell regeneration.
- produces mucus and nourishes the mucosa by producing short‑chain fatty acids (SCF’s).
- helps in the maturation of the immune system by vitalizing innate immunity leading to the maturity of the intestinal‑related lymphoid tissue.
- contributes towards innate immunity by colonizing and filling voids in the gut which would otherwise be colonized by opportunistic microbial populations.
The gut and the brain are always in communication. The gut sends signals to the brain and vice versa. This is called bidirectional signalling. Dysbiosis of the gut microbiota is seen to give rise to various conditions like irritable bowel syndrome (IBS), obesity, depression, and autism, of which depression and mood disorders are seen very often. The actual mechanism on how one’s gut microbiota is affecting one’s state of mind is not known yet. It is suggested that certain gut microbial populations might have an effect on neurotransmitters like serotonin. The level of serotonin in the body is responsible for determining one’s mood. However, scientists believe that this could be one of the many ways in which microbial populations affect the host. Gut microbiota is also believed to have an effect on Brain-derived neurotrophic factor (BDNF) which helps in neuronal survival, growth, and aids in neural plasticity.
The gut-brain interaction is brought about by many afferent nerves carrying information to the spinal cord and the brain. Most of the interaction is brought about by the vagus nerve which acts as an information highway between the gut and the brain. This was proven by an experiment wherein mice who underwent subdiaphragmatic vagotomies exhibited far less anxiety like behaviour’s than the other mice with intact vagus nerves. A disturbed gut is seen to affect the Hypothalamus-Pituitary-Adrenal Axis (HPA), leading to an increase in the level of the stress hormone cortisol. This results in increased stress levels in the individual. IBS and depression are thought to be outcomes of the breakdown in intestinal permeability that leads to a ‘leaky’ gut. Microbial imbalance in the gut population affecting intestinal permeability could explain why depression is comorbid with IBS.
If poor gut health could lead to deterioration of one’s mental health, then promoting gut health could help in easing these effects. This school of thought is what led researchers to dig deep into the beneficial effects of probiotics and prebiotics which would help in promoting gut health and improving mental health at large.