The relationship between the gut and the brain is a growing area of research. Recently, the gut has been referred to as the second brain. In this article, we will explore the enteric nervous system and how it communicates with the brain. We will also take a look at the explanation behind common sensations like butterflies in the stomach, the role of gut microbiota, and whether the gut microbiome can influence intelligence.

Here are the key takeaways from this article about how the gut affects the brain:

• The relationship between the gut and the brain is an area that is of growing interest to researchers and practitioners.

• The enteric nervous system (ENS) is a complex network of neurons in the gastrointestinal tract that plays a role in digestion and communicating with the brain.

• The connection between the gut and the brain involves both physical and chemical pathways, including the vagus nerve, the ENS, neurotransmitters, and the gut microbiota.

• The sensation of "butterflies" in the stomach is linked to gut bacteria sending signals to the brain via the hypothalamic–pituitary–adrenal (HPA) axis and the vagus nerve.

• Research suggests that the gut microbiome can influence fluid intelligence, with a correlation found between the abundance of gut bacteria and cognitive performance.

The enteric nervous system (ENS) is a complex system of sensory neurons, motor neurons, and interneurons embedded in the wall of the gastrointestinal system, stretching from the lower third of the oesophagus right through to the rectum. It has been estimated that the ENS contains more neurons than the entire spinal cord. The ENS processes a range of sensations, such as the nature of gut contents and more. By doing this, the ENS can guide and optimise the muscular and secretory activity of the gastrointestinal tract. The ENS is thought to be able to learn changes in dietary habits, as well as disruptions in the gut, making it a highly complex and sophisticated system that could be linked with general health and brain activity [1].

The enteric nervous system that regulates our gut is often called the body’s "second brain." When people refer to the gut as the second brain, they are actually referring to the enteric nervous system. Although it can’t perform functions like our brain can, this ENS uses the same chemicals and cells as the brain to help us digest food and alert the brain when something is wrong or different. The gut and brain are in constant communication, sharing information to ensure the body is working properly. This close relationship highlights the gut's significant role in influencing our overall health and well-being [2].

The connection between the gut and the brain is intricate and sophisticated. The connection between the gut and the brain is made up of both physical and chemical connections, including [3]:

• The Vagus Nerve - The vagus nerve creates a physical link between the gut and the brain and is one of the longest nerves in the body. Messages and signals are sent in both directions along the vagus nerve, so both the gut and the brain are communicating

• The Enteric Nervous System - The Enteric Nervous System, also known as the ENS, is a network of over 100 million nerve cells in the gastrointestinal tract. The ENS primarily controls digestion, but it also communicates with the main brain, which is why poor gut health and digestive issues can trigger an emotional reaction.

• Neurotransmitters - Neurotransmitters are biochemicals that allow neurons in the body to communicate with each other. They transmit messages in the brain, many of which originate from the gut.

• The Gut Microbiota - The gut contains trillions of bacteria is microbes that are known as the gut microbiota. These microbes and bacteria create chemicals that interact with the brain, potentially changing emotions and mood.

The sensation of butterflies in your stomach is a result of gut bacteria and their metabolites sending signals to the brain and altering behaviour. One of the most significant routes for these messages being sent to the brain is the hypothalamic–pituitary–adrenal (HPA) axis. This axis links the gut microbes, gastrointestinal tract, brain, and behaviour, causing the fluttery feeling in the gut. As well as this, stress signals can be transmitted to the brain via the vagus nerve and affect neuron connections, contributing to the sensation of butterflies in the stomach [4].

Video: What is the gut-brain axis?

The PrecisionBiotics Group has created a helpful video to provide an overview of the gut-brain axis. Take a look below:

The gut microbiota can significantly affect the link between the gut and the brain [5]. It does this in a number of different ways. Specific gut microbes can modulate the immune system by producing molecules that signal neurons to regulate immune activity. Microbes also regulate the early development of neurons, as well as the production of certain biochemicals including serotonin, which is used to manage mood. Research also suggests that microbes communicate with the brain through a number of pathways, including the vagus nerve. Certain bacteria use these pathways to interact with the brain, for example, lactobacillus rhamnosus JB1 has been shown to improve the mood of anxious or depressed mice.

Yes, according to a 2022 study, your gut microbiome may influence your intelligence, more specifically your fluid intelligence. Fluid intelligence refers to the ability to think logically and solve problems in situ, away from learned knowledge. It involves reasoning, abstract thinking, and the ability to adapt to new information or circumstances. Among healthy young adults, research indicates a correlation between the abundance of a gut bacterial community and fluid intelligence scores. This suggests that cognitive performance could potentially benefit from interventions targeting the gut microbiome [6].

The gut-brain connection is a complex and increasingly significant area of research, revealing how closely our gastrointestinal system is linked to both our emotional and cognitive health. The enteric nervous system, often referred to as the "second brain," plays a central role in this communication, influencing everything from digestion to emotional responses, such as the sensation of "butterflies" in the stomach. The gut microbiota further impacts this connection by producing chemicals that can affect mood, immune function and even cognitive abilities. Research indicates that the gut microbiome may influence fluid intelligence, suggesting that maintaining a healthy gut could have broader effects on brain function.

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[1] Peripheral nerves - Science Direct

[2] The Gut and the Brain | Harvard Medical School

[3] What is the gut-brain connection? - Axa Health

[4] Butterflies in the gut: the interplay between intestinal microbiota and stress - PMC

[5] How gut bacteria are controlling your brain - BBC

[6] A prospective investigation into the association between the gut microbiome composition and cognitive performance among healthy young adults - Springer Nature