The Second "Brain" in Our Gut

What do scientists mean when they say we have two brains? The more obvious brain is housed within the skull and is responsible for our thoughts, problem-solving skills, and planning. The brain connects to the spinal cord via the brain stem and continuously sends signals through peripheral nerves allowing us to be active, store memories, feel emotions, and even keep our hearts beating. Our gut is often referred to as the second "brain" and plays a crucial role in regulating many metabolic pathways. The gut does not only control our appetite and digestion, but it has been shown to contribute to psychiatric, neurodevelopmental, and neurodegenerative disorders. As we uncover the complexity of the gut, we begin to see how the gut affects our overall health.

The Enteric Nervous System

The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS). The CNS refers to our brain and spinal cord, and the PNS consists of all the peripheral nerves and pathways that connect our CNS to the rest of our body. The Enteric Nervous System (ENS) is a large part of our PNS composed of approximately 600 million neurons that play a primary role in regulating and supporting functions of the gastrointestinal tract. These functions include, but are not limited to breaking down food, absorbing nutrients, and secreting waste products. The main types of cells that compose the Enteric Nervous System include motor neurons, sensory neurons, and interneurons. Sensory neurons, also called afferent neurons, detect the initial physical changes and stimuli associated with digestion. The motor neurons can then coordinate peristalsis, the rhythmic contractions that move food through the digestive tract. The sensory and motor neurons are connected by interneurons coordinating the contractions in response to the incoming stimuli.

The Gut-Brain Axis (GBA)

The gut and the brain do not work independently of one another; they communicate extensively. The bi-directional communication between the brain and the gut is known as the Gut-Brain Axis (GBA) and contributes to our gastrointestinal homeostasis and higher cognitive functions. The complex communication between the CNS and ENS involves immune cells, neuroendocrine cells, and microbiota in the gut. Microbiota are microorganisms, such as bacteria, fungi, and viruses that contribute significantly to our brain and gut health. We have trillions of microbes in our gut, primarily bacteria, that are essential to supporting everything from digestive health to mental health.

Gastrointestinal Disorders

To better understand gastrointestinal disorders, we can look at gut microbiota composition and the subsequent connection between the gut and the brain through the GBA. Irritable Bowel Syndrome (IBS) is one of the most common gastrointestinal disorders with an unclear pathophysiology and involves many factors including genetics, age, diet, geographical origin, and use of antibiotics. Some causes of IBS are irregular muscle contractions in the intestine, impaired neural communication between the brain and the intestines, infection, life stressors, and dysregulation of gut microbiota. The dysregulation of gut microbiota is called dysbiosis and is a current focal point in research to understand the pathophysiology of IBS and other gastrointestinal disorders. Dysbiosis can be characterized by the lack of diversity or imbalanced composition of microbiota. The altered diversity and composition of microbiota can interfere with gut immune homeostasis, lead to inflammation, and ultimately exacerbate the symptoms of IBS.

The Gut and Mental Health

Gut microbiota has been shown to affect mental health conditions such as depression and anxiety through various pathways that include regulating neural signaling and neurotransmitters such as dopamine, serotonin, and glutamate. When analyzing gut microbiota and anxiety, patients with Generalized Anxiety Disorder (GAD) had altered gut microbiota characterized by less diversity and richness. In addition, these individuals had fewer species of microbiota that contribute to protective effects of mental disorders, and an increased count of microbiota that are positively correlated with anxiety. Similar trends in gut microbiota composition were observed in individuals with Major Depressive Disorder (MDD).

Conclusion

From gastrointestinal disorders to mental health, the significance of the gut in our overall health is very apparent. A healthy, balanced diet, is not just important for reducing the risk of developing cardiovascular diseases, hypertension, and obesity, but also for maintaining gut health. It is recommended to eat foods high in fiber and unsaturated fatty acids while reducing consumption of saturated fats and processed carbohydrates. Some examples of gut-healthy foods include fruits, vegetables, whole grains, legumes, nuts, and fermented foods as they contain probiotics and polyphenols that support the growth of good bacteria in the gut. In addition to our diet, we can work to lower our stress levels, exercise frequently, and prioritize sleep. Your second “brain” will thank you for living a healthy lifestyle.

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The Second "Brain" in Our Gut

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