Written by: Anika Jagasia
Edited by: Zahra Khan, Maria Levit, Apurva Oak
Illustrated by: Xinyi Niu
The human microbiome refers to the collection of microorganisms, including bacteria, archaea, viruses, and eukaryotic microbes that reside inside and outside our bodies [1]. These microorganisms play a critical role in supporting human health by assisting with digestion, regulating the immune system, and protecting us from harmful bacteria [1]. The microbiome is individually unique and can be influenced by factors such as diet, environmental chemicals, and genetics [2].
The gut microbiome refers to the microorganisms and microbes that reside in the human digestive tract (which we will refer to as the “gut”). Our gut microbiome helps our bodies break down and digest food, produce essential nutrients such as vitamins and amino acids (which form proteins), and regulate our immune system [3]. Understanding the gut microbiome and how to support its health is an area of ongoing interest and research in the scientific and medical communities. Research has found that disturbances to the gut microbiome from causes such as antibiotics, a poor diet, or other environmental factors can contribute to a variety of health problems such as autoimmune disorders due to the gut microbiome’s connection to the immune system [4].
Recent research on the gut microbiome has identified a connection between the bacteria in our digestive tract and our brain. This connection is defined as the gut-brain axis, which refers to the bidirectional communication between the gut and the Central Nervous System (CNS), which consists of the brain and spinal cord [5]. This axis is gaining an increasing amount of attention as studies show that the gut microbiome can influence brain behavior and function through a myriad of pathways such as the immune system and influence the production of neurotransmitters among other signaling molecules [6]. These neurotransmitters are chemical molecules that allow neurons to communicate and can cause certain pathways involved in the immune response to activate [6, 7]. Further, ongoing research demonstrates a strong interest in investigating the intricacies of the gut-brain axis as a means of expanding our understanding of our health. This review explores recent findings on the gut-brain axis with a particular focus on the mechanisms by which the gut and the brain communicate via hormones and the immune system, which can impact energy and mood regulation, among other components of mental and physical health.
Communication Between the Digestive System and the Brain
A crucial aspect of the gut-brain axis is the dynamic communication between the digestive system and the brain through pathways including nutrient-sensing paths, the enteric nervous system, the vagus nerve, and the release of hormones.
The communication between the digestive system and the brain begins even before food enters the digestive tract; the smell, sight, or anticipation of food can prompt the body to release digestive enzymes and hormones that will prepare for digesting food intake [8]. Therefore, nutrient-sensing pathways enable communication and signals to be sent between the brain and the gastrointestinal tract, which is composed of the mouth, throat, stomach, intestines, and rectum [9].
The Enteric nervous system (ENS) is composed of a network of neurons that reside in the digestive tract [10]. It functions as a catalyst, coordinating digestion and the absorption of nutrients and is composed of an extensive network of neurons and cells in the gastrointestinal tract that contributes to functions such as the chemical digestion process and rate through the integration of multiple signals from the ENS and the CNS [10]. One of the crucial gut-brain axis pathways where communication occurs is via the vagus nerve, which connects the gut and brain and plays a role in communicating signals of hunger, mood, stress, and more through hormones [11].
The gut-brain axis enables the communication and signals for the production and release of hormones such as serotonin, which can influence mood and emotions [11]. Serotonin is a neurotransmitter that has been shown to affect a multitude of functions including digestion, immune function, and emotional processing, and up to 90% of the body’s serotonin is produced in the gut; there are also harmful bacteria in the gut that may cause a reduction in serotonin [12]. Imbalances in the production and release of serotonin have been linked to a multitude of mental health disorders such as depression and anxiety [13]. This link between the production of serotonin in the digestive tract with the link to the emotional and cognitive functioning of the brain exemplifies the importance of understanding the communication network between the gut-brain axis for supporting human health and wellness, as research has begun to look at using the gut-brain axis in a therapeutic manner for mental health issues [12] .
The Role of the Endocrine System in the Gut-Brain Axis
The endocrine system is composed of a network of eight major glands and organs that produce and release hormones throughout the bloodstream, and in the gut-brain axis, the endocrine system plays a vital role as a pathway for communication between the gut and the brain [14]. Hormones that are produced and released in the gut such as ghrelin and leptin influence feelings of hunger and satiety respectively, and may participate in reward, motivation, and similar mood signaling pathways [14 ,15]. Particularly, ghrelin and leptin interact with receptors in the brain that send signals to communicate the state of digestion, which influences the release of hormones produced by the endocrine system that impact the function of the gut [16]. Ghrelin and leptin release is caused by the same areas of the brain that have important roles in mood disorders, and the subsequent relationship between food digestion and mood seems to depend on each individual [15]. The endocrine system and the gut-brain axis work together to maintain physiological balance in the body, impacting functions and processes of digestion, mood, and the gut.
The Role of the Immune System in the Gut-Brain Axis
The immune system is composed of an intricate network of cells, tissues, and organs that work together to protect the body from harmful invaders. It helps to identify and fight foreign substances that can threaten infections or diseases [17]. The immune system plays a key role in the gut-brain axis through the immune cells that reside in the gut, and maintaining the health of our gut in turn promotes a healthy immune system [18]. In our gut, there are immune cells that can produce and release signaling molecules such as cytokines that interact with the receptors in the brain and can influence the function of the gut-brain axis by activating the immune system [18]. In turn, inflammation occurring in the gut can prompt production and release of pro-inflammatory cytokines that triggers systemic immune responses thereby interacting with brain receptors [18]. Pro-inflammatory cytokines are molecules associated with dysregulation of the immune system, which can ultimately lead to health issues such as gastrointestinal disorders and mental health disorders [19]. This communication between the immune system and the gut-brain axis highlights the role of the gut microbiome in maintaining the health of our gut and immune system.
In conclusion, as ongoing and future research continues to investigate the gut-brain axis, we will gain a better understanding of the complex, interconnected interactions between the gut and the brain as a means of expanding our knowledge on how to better promote overall mental and physical health. We can apply the learnings of recent research on the gut-brain axis to our everyday lives by taking better care of our gut microbiome by supporting a balanced diet that may ultimately translate to improvements in our mental health and well-being.
References
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