by Abhinn Gupta1, Charvi Trivedi1, Prakshal Parekh1, Vanshika Mistry1, Vishva Patel1 | February 29, 2024.

1Biological Research Laboratory, School of Arts and Sciences, Ahmedabad University, Gujarat, India.

Edited by ScienceRely.

Imagine a world where trillions of microorganisms orchestrate an unseen symphony, affecting everything from the food on your plate to the very air you breathe. Moving the spotlight to your gut, picture this—a bustling metropolis of bacteria, fungi, and even viruses, all harmoniously existing within your digestive system.

Science suggests that the ratio of gut microbe cells to human cells is almost equal, reaching trillions of cells. How fascinating is it to think that the human body can harbour such a huge number and diversity of microorganisms! The gut microbiota helps in maintaining gut integrity, wards off insidious pathogens and aids in a smooth digestion process. The gut microbiome of humans is known to be composed of bacteria from some of the common genera, namely, Lactobacillus, Staphylococcus, Clostridium and Prevotella, to name a few. But did you know that this microscopic universe is capable of influencing our emotional well-being as well?

Step into the classroom of human biology, where we’ve been introduced to a fascinating cast of characters, our organs. The heart guides the rhythm of blood, the lungs choreograph breath, and the stomach masterfully breaks down food. Then comes the director, the nervous system. It is divided into: the Central Nervous System (CNS) consisting of the brain and spinal cord, shaping thoughts and relaying messages and the Peripheral Nervous System (PNS) the messengers that bridge brain signals to coordinate actions across the body. In this article, we will explore the connection between our gut and the brain, a captivating concept known as the gut-brain axis

The Gut-Brain Link: A Covert Information Superhighway

Imagine this axis as a bridge, not of concrete and steel, but of neural pathways, hormonal highways, and nutrient-rich passages. On one end is the brain, directing our actions and reactions, thoughts and emotions. On the other is the intricate expanse of the digestive system, a bustling hub where nutrients are broken down and energy is harvested. It is a bidirectional highway, meaning, the traffic keeps on moving between the brain to the gut and back to the brain. This invisible highway of communication threads through our bodies, weaving together the narrative of our health and experiences. 

How did we come to know of its existence in the first place? The discovery of the gut-brain axis (GBA) traces its roots to the pioneering studies of the 19th and 20th centuries. Among these illuminating milestones stands the work of the renowned US Army surgeon William Beaumont, whose insights forever altered our understanding. He observed the association between mood and gut function by examining a fistula, an abnormal connection between two organs or vessels, in a patient’s stomach. His experiments opened the door to an entirely new realm of research.

The key mechanism by which our gut and the brain interact is via chemical signals termed neurotransmitters (Figure 1). The microorganisms in our gut release neurotransmitters in response to our dietary intake. These chemical messengers are taken up by a large nerve called the vagus nerve, which extends its path into the brain. They can enter the brain through two routes: the bloodstream or a network of local nerves. These chemical signals have a multitude of effects on brain functions ranging from inducing alterations in our mood to our overall personality. For instance, certain subspecies of the genus Lactobacillus release acetylcholine, a neurotransmitter responsible for regulating memory, attention, learning, and mood. Meanwhile, members of the Candida, Streptococcus, Escherichia, and Enterococcus genera are recognized for their secretion of serotonin, often referred to as the “happy hormone”.

Figure 1 illustrates the intricate communication between the gut and the brain through neurotransmitters (dopamine, glutamate etc) released by gut microorganisms in response to our diet. These chemical signals, carried by the vagus nerve, traverse into the brain via different pathways, impacting various brain functions, including mood and personality. (Image created using
Implications of diet on mood and mental health

It is well established that gut bacteria release a variety of chemicals responsible for our mood. But, how does this intricate mechanism operate? What sets it in motion? Our diet, of course! It is often emphasized that maintaining a balanced diet is important due to the nutritional value it offers. Now we know that these nutrients encompass not only vitamins and minerals but also beneficial gut bacteria.

Yoghurt is a rich source of probiotics

Chances are you have encountered an upset stomach at least once in your lifetime. During such times, the recommendation to opt for a light diet, including generous portions of yoghurt, might have been given. Yoghurt is a probiotic that helps promote the growth of ‘good bacteria’ in the gut which helps restore proper gastrointestinal functioning. Interestingly, it doesn’t stop there; yoghurt also positively affects brain function. A study by UCLA’s Digestive Diseases Department involved 36 women consuming probiotic-rich yoghurt for about four weeks. Brain MRI scans after this period showed increased activity in regions associated with cognition and mood. This could be due to chemicals like acetylcholine released by Lactobacilli in yoghurt, which help reduce stress and anxiety and even enhance memory.

Dark chocolate and good mood

Transitioning to a more captivating and healthful culinary delight, dark chocolate! It’s rich in cocoa, which contains essential compounds like polyphenols and flavanols. These substances are instrumental in fostering the growth of Lactobacillus and Bifidobacterium, key microorganisms involved in polyphenol metabolism. The resulting metabolites then journey to the brain, where they aid in the production of neurotransmitters such as serotonin and dopamine. These neurotransmitters, known for their mood-enhancing properties, help alleviate pain and anxiety while promoting positive emotions. So, the next time your partner is navigating her menstrual cycle, consider gifting her a generous bar of dark chocolate—a gesture that aligns with both taste and science! 

Exploring the role of the gut-brain axis in Autism

Diet not only influences your daily moods, but also has a deeper role to play in neurodegenerative disorders like Autism, Parkinson’s disease, and Alzheimer’s disease. Focusing on Autism Spectrum Disorder (ASD), characterized by language and learning difficulties, social interaction challenges, and repetitive behaviours, studies have highlighted the association between ASD symptoms and specific microbial species. This neurodegenerative disorder has been shown to correlate with many species living inside our bodies.

A study undertaken in 2015 by Kantarcioglu and colleagues, sheds light on how Candida albicans, a well-known yeast species, is involved in the aggravation of ASD symptoms. Similarly, Finegold and colleagues, in 2002, found an elevated number of Clostridium species in people with ASD and further showed how these bacteria produce toxin B which can cause cell death in neurons, leading to symptoms of ASD.

Clostridium and Autism

The looming question of how dietary intake influences the gut microbiome was also answered by a recent study by Tomova and colleagues, on children with and without ASD. They assessed different nutritional components in both groups of children, which included intake of fibre, carbohydrates and omega-3, to name a few. Some notable examples from their study indicated that high carbohydrate intake decreased Bacteroides while low fibre intake significantly increased the diversity of Hydrogenoanaerobacterium, Clostridium cluster IV, and Anaerotruncus bacteria. 

Iovene et al. reinforced the connection between Clostridium and ASD by observing higher occurrences of Clostridium in faecal samples from 33 autistic children. Let us now zoom into how these notorious gut bacteria are contributing to this disorder.  Certain species within the Clostridiaceae family produce toxins like phenols, p-cresol, and indoles, all of which affect the human metabolic system. Several studies have shown elevated P-cresol levels in the faecal and urinary samples of children with autism. It inhibits the effects of certain enzymes which are involved in the metabolic regulation of neurotransmitters like dopamine and norepinephrine. Reduced production of these neurotransmitters is known to increase the symptoms of ASD. This underscores the influence of dietary choices on gut health, which, in turn, affects mental well-being.

Take home message

The intricate relationship between gut microbiota and our understanding of its impact on the brain is likely to deepen in the coming years. This would lead to discoveries that influence the composition of intestinal contents to address chronic pain ailments or other brain-associated disorders. This could encompass conditions like Parkinson’s disease, Alzheimer’s disease, and autism, potentially paving the way for innovative therapeutic approaches. Therefore, let’s make mindful choices to adopt a nutritious and well-balanced diet. By doing so, we can foster the growth of good microorganisms within us, ensuring that they contribute to our present and future happiness. 

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