The Gut-Brain Axis: The Future of Neurological Health

Food is life. Ever since the beginning of time, almost all species need some sort of nutrients to survive. Humans have one of the most fascinating connections between our gut (stomach, intestines, and colon) and our brain called the gut-brain axis. This connection is a highway for certain neurotransmitters, which are particles used for communication throughout our nervous system and influence all aspects of our lives from learning to maintaining body temperature. However, many researchers have noticed that this connection has a direct link to many physical and mental disorders. 

The gut-brain axis is composed of three main parts: the enteric nervous system (ENS), the vagus nerve, and the central nervous system (CNS). The ENS, sometimes referred to as our “second brain,” is the largest neural network outside of our brain, with a whopping 500 million neurons. This network lines our gastrointestinal (GI) tract, which detects nutrients, immune response, secretes bioactive peptides (small proteins), and more. The vagus nerve carries connections between the CNS and the ENS; its main duty is to send the brain information about the gut, such as the presence of food. Finally, the CNS will pick up signals from the gut and act accordingly based on the type of signals sent to the brain. Though this is a simplified example of the complex communication between the brain and the gut, it shows the clear and powerful connection our gut has with our brain.

Through this connection, millions of signals are passed between the ENS and the CNS, regulating our daily lives. To fulfill this connection, neurotransmitters and bioactive peptides come into play. The gut produces more than 95 percent of our body’s serotonin, commonly known as the “happy hormone.” It affects body temperature, sleep, happiness, learning, and more. Additionally, the gut has a whole microbiome of gut microbiota consisting of bacteria, viruses, and fungi that have been developing since birth. However, negative shifts in the gut microbiome are linked to many mental health, neurological, and GI disorders. 

The primary mental health disorder that is linked to gut microbiota is depression. Studies have proven that people with depression tend to have a weaker gut biome. A research study conducted by Anna-Chiara Schaub, et al. noticed that an increase in a strain of bacteria called Lactobacillus is linked to a reduction in depressive symptoms amongst patients with mild depression. The study tested two groups: the placebo group, which had no gut bacteria, and the probiotic group. The Hamilton Depression Rating Scale (HAM-D) was used to determine how “depressed” a person is from 0, meaning normal, to 24, meaning severe depression. The results concluded that 80 percent of patients under prebiotic treatment showed a 57 percent decrease in HAM-D score, while only 48 percent of patients under the placebo group showed such a decrease. They also noticed that only the probiotic group had an increase in the Lactobacillus genus and that the increase in abundance of the Lactobacillus genus had a negative association with HAM-D ratings.

Additionally, generalized anxiety disorder (GAD) is another prevalent mental condition amongst four to six percent of the world’s population. In individuals with GAD, there is a decreased amount of short-chain fatty acid (SCFA) bacteria. This bacteria is important in regulating metabolism, improving the gut barrier, and more. Researchers Chen Yi-huan et al. noticed that people with active GAD have differences in the composition of the gut microbiome compared to people who don’t. Bacteria like Firmicutes and Tenericutes were more represented in healthy people, while Bacteroidaceae, Bacteroides Enterobacteriaceae, and Burkholderiaceae were more represented in people with active GAD. In total, 25 genera of bacteria differed between people with active GAD and those without GAD. Researchers Zu et al. further tested that probiotic treatment of Lactobacillus plantarum JYLP-326 could reduce the change in the gut microbiome to the regular state. For example, a bacteria called Roseburia decreased with probiotic treatment to close to normal levels in feces tests. They also noticed decreased anxiety symptoms in students who took the probiotic treatment.

Neurological disorders can also prevail from dysbiosis, an imbalance of bacteria in the gut. Alzheimer’s and Parkinson’s Disease (PD) are neurodegenerative diseases that are both mainly caused by the immune system targeting neurons, propagating the respective neurological disease. Many researchers have noticed that Parkinson’s begins in the gut. PD begins when a protein called alpha-synuclein, found in the gut and brain, gets misfolded and toxicates the cells. This quickly makes these neurons a prime target for the immune system. However, the immune system attacks the neurons directly and not the deposits of alpha-synuclein inside them. Researchers Dritan Agalliu and David Sulzer tested alpha-synuclein on mice and observed what occurred in both the gut and the brain. They noticed that the brain ceased to show signs of degenerating neurons, a sign of PD, but also, surprisingly, that the gut was being attacked. The gut showed a clear sign of an autoimmune attack, causing many GI effects resembling early signs of PD. Later, researchers noted that SCFAs can prohibit alpha-synuclein aggregation, decreasing the risk of PD and providing protection for the brain. Researchers also linked a healthier diet with improved conditions in people with PD and an increase in SCFA-producing bacteria that have anti-inflammatory properties. These anti-inflammatory properties protect the ENS from permanent damage. However, increased sugar content causes increases in bacteria like Romboutsia, which reduces cognitive capability and escalates depressive symptoms in people with PD.

It is vital to have a healthy diet to protect ourselves from disruptions on the gut-brain axis. Consuming products with fiber, probiotics, antioxidants, and anti-inflammatory foods all nurture our gut protecting our connection. It’s also important to avoid ultra-processed foods (UPF), which can alter the gut microbiota’s shape and metabolism. Eating UPF causes inflammatory responses, which are linked to various brain disorders. Unfortunately, there still needs to be more coercive research on our gut-brain to understand the impact of gut health on our overall health. Hopefully, future experiments will continue proving strong correlations between strains of bacteria like Lactobacillus and Rombusta on our gut health, thus improving our current knowledge. However, one thing is clear: maintaining a healthy diet is undeniably beneficial.