The gut microbiome refers to the numerous microbes, viruses, and bacteria that live in your intestines. Did you know there are over 100 trillion bacterial cells in your GI tract? Don’t be too creeped out. You and your gut bacteria have a harmonious relationship… most of the time.
The gut-brain axis refers to the connections between your GI tract, the gut microbiome, and your nervous system.
There are three main pathways that make up the gut-brain axis – nerves, the neuroendocrine system, and the immune system.
Nerves connect to the GI tract and allow communication signals to flow between the gut and the brain.
The vagus nerve acts as the major communication line to the gut and it is affected by movement of your GI tract, as well as the activity of your gut microbes, hormones, and neurotransmitters. Vagus nerve activity has been linked to mood regulation.
The enteric nervous system, which is a network of nerves found throughout the walls of your GI tract, also plays a key role in gut-brain communication.
The bacteria in your gut also make molecules that like short-chain fatty acids, neurotransmitters, and the materials to make them.
Neurotransmitters are chemicals that transfer impulses from nerve fibers. (Examples include serotonin and dopamine.) The activity of certain neurotransmitters affects areas of the brain, which can in turn, affect our mood and behavior.
Your gut microbes also affect the production of hormones involved in the stress response, metabolism, and mood.
Gut bacteria also make short-chain fatty acids (SCFAs) by breaking down fiber. SCFAs help your body by fighting inflammation. Many mental health disorders have been linked to high levels of inflammation in the body.
The gut microbiome is an important part of the protective mucus layer within the gut.
Having fewer or less healthy bacteria can lead to a weaker mucus layer in the GI tract, which makes it easier for inflammatory molecules to pass through. This can contribute to higher levels of inflammation throughout the body, including the brain.
Your gut microbiome is made up of countless species of bacteria. Each species thrives under certain conditions. The composition of your gut microbes can vary based on the foods you eat, your overall lifestyle, stress, and certain drugs.
Researchers are still working to determine what an “ideal” gut microbiome looks like, but we do know that more diverse and rich gut microbiomes are associated with better health outcomes, including a lower risk of depression.
In fact, the makeup of the gut microbiome can also vary in people with mental health conditions like ADHD, anxiety, and insomnia. Diet changes to manage gut health and/or inflammation (like taking probiotics, including omega-3’s, and eating more fiber) have been linked to symptom improvement in some mental health conditions.
Taken together, these connections give us clues that gut health is strongly connected to mental health.
Western diets tend to be low in fiber and high in fat, which affects the diversity, richness, and composition of gut microbes, including bacteria that produce helpful molecules like SCFAs and neurotransmitters.
Diets higher in fiber and lower in fat increase helpful bacteria like Lactobacillus and Bifidobacterium, which produce SCFAs that decrease inflammation. This increase in helpful bacteria strengthens the protective layer inside your gut, which decreases permeability and protects your body from inflammation.
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