How Your Gut Influences Mental Health

Introduction: The gut and brain are in constant two-way communication – so much so that the digestive system is often called our body’s “second brain.” Tiny nerve cells and neurotransmitters in the gut send signals to the brain via the vagus nerve, while the brain sends back stress or hunger signals to the gut. This gut–brain axis affects everything from appetite and digestion to mood, stress resilience and overall mental health​hms.harvard.edu​nature.com. For women 35+, factors like hormonal changes, stress or diet shifts can disrupt gut flora and vagal signaling, impacting mood. Understanding this connection helps us make choices (diet, stress reduction, supplements) that support both gut health and a positive mood​​. Let’s take a look at how your gut influences mental health.

• Gut as Second Brain: The gut’s own nervous system (the enteric nervous system) contains ~100 million neurons – more than the spinal cord – and uses many of the same neurotransmitters as the brain​. In fact, about 90% of the body’s serotonin (a key mood-regulating chemical) is produced and used in the gut​. This means your gut literally “feels” hunger, pain or pleasure and can alert the brain. This extensive neural network lets the gut digest food autonomously, but it also “chatters” with the brain and influences how we feel.
• Bidirectional Gut–Brain Communication: The gut–brain axis is a two-way street​. When you encounter stress (fight-or-flight), your brain can signal the gut, causing butterflies, nausea or slowed digestion​. Conversely, gut signals – such as a message that you’re full, or signals from gut microbes – travel to the brain and affect emotions. Early research (Beaumont in the 1840s) already hinted that emotions change digestion​. Modern studies confirm that gut microbes, hormones, and immune messengers all feed back to brain centers for mood and cognition​​. This is the pathway that affects how your gut influences your mental health.

Gut as Your “Second Brain”

• Enteric Nervous System (ENS): The gut’s ENS is an independent nervous network lining the digestive tract. It has more neurons than the spinal cord​ and uses neurotransmitters like serotonin, dopamine and GABA – the same chemicals your brain uses​.
• What It Does: This “little brain” in your belly helps control digestion (enzymes, gut muscle contractions) and alerts your brain if something’s wrong (food poisoning, inflammation). It evolved so the brain doesn’t have to micro-manage digestion.
• Neurotransmitters & Mood: Because the gut makes ~90% of serotonin​, gut health directly influences brain chemistry. For example, low levels of gut-produced serotonin can contribute to anxiety or depression. In fact, gut illnesses like IBS often coincide with mood issues.

Serotonin in the Gut–Brain Axis

It’s often said that 90% of serotonin is made in the gut, and indeed, most of our body’s serotonin is produced by gut cells. However, this gut-derived serotonin is used mainly for digestive processes like motility and secretion, and cannot cross into the brain. In contrast, the serotonin that affects mood is synthesized inside the brain itself from the amino acid tryptophan. In other words, the gut and brain each have their own separate serotonin “supply lines.”

• Gut vs. brain pools: Over 90% of the body’s serotonin resides in the gut and is released by enterochromaffin cells. This serotonin helps regulate gut function, including intestinal contractions. Because of the blood–brain barrier, this serotonin stays outside the brain. The remaining ~10% of serotonin is in the central nervous system and is made by neurons using a specific enzyme called TPH2.
• Mood-related serotonin: The brain’s own serotonin affects mood, appetite, sleep, and cognition. It must be made locally in the brain from circulating tryptophan. Serotonin in the bloodstream – including gut serotonin – does not directly boost brain serotonin due to the protective blood–brain barrier.
• Blood–brain barrier: Under normal conditions, peripheral (gut) serotonin cannot cross this barrier, which keeps the brain’s serotonin pool separate from the rest of the body.

Indirect Gut–Brain Pathways

Even though gut serotonin doesn’t flow into the brain, the gut still influences brain chemistry indirectly through several fascinating routes:

• Tryptophan supply: The gut microbiome and diet determine how much of the amino acid tryptophan enters the bloodstream. Tryptophan crosses the blood–brain barrier and is the raw material for brain serotonin. Changes in gut bacteria or diet can impact how much serotonin your brain can make.
• Vagus nerve signaling: The vagus nerve is a major two-way communication line between the gut and the brain. Serotonin released in the gut can activate vagal afferent fibres, sending electrical signals to brain centers involved in mood and cognition – even though the actual serotonin molecules remain in the gut.
• Microbial metabolites: Gut bacteria produce molecules like short-chain fatty acids (SCFAs) that influence both gut and brain function. SCFAs can boost the expression of TPH1 (the enzyme that makes gut serotonin), and help regulate vagal tone and serotonin transporter levels. A diverse microbiome and its beneficial metabolites help protect the gut lining and reduce systemic inflammation – which in turn supports a healthy, balanced brain.

Understanding the Gut–Brain Axis

• Bidirectional Pathways: The gut–brain axis involves nerves, hormones and immune factors. The vagus nerve is the main neural highway (see next section), but gut bacteria also influence hormones and inflammation that circulate to the brain​. This axis means the brain can adjust gut function (e.g. slow digestion when stressed) and the gut microbiome can regulate brain function via immune and endocrine signaling​.
• Stress and Digestion: Stressful events trigger the brain’s fight-or-flight response, which diverts blood from digestion and can cause stomach upset​. Conversely, when the gut is inflamed or upset (food sensitivities, IBS flare-ups), that signal can drive anxiety or low mood. Studies find that diets high in processed, inflammatory foods correlate with worse mood​. Researchers often say “fix the food first” before other therapies: a whole-food diet supports a balanced microbiome and stable mood.
• Microbiome Signals: Gut microbes produce vitamins, short-chain fatty acids (SCFAs), and neuroactive chemicals. These can cross the gut lining or activate the vagus nerve to alter brain chemistry. For example, certain bacteria ferment fiber into SCFAs that strengthen the gut barrier and reduce inflammation, indirectly benefiting the brain. Other microbe-produced molecules (like GABA or acetylcholine) bind receptors on gut nerves and may influence mood.

Meet Your Vagus Nerve: The Gut-to-Brain Superhighway

• What is the Vagus Nerve? The vagus nerve (cranial nerve X) is the longest nerve in the body, running from the brainstem down to the gut​. It’s the main parasympathetic (“rest and digest”) nerve and carries information both ways. In fact, about 80–90% of its fibres send signals from the gut to the brain, not the other way around​.
• Key Functions: The vagus monitors gut nutrient levels, gut stretch (fullness), inflammation and microbial activity. It also controls heart rate, breathing and digestion. When the vagus is stimulated (e.g. deep breathing, meditation), it can dampen stress responses and reduce inflammation, improving mood and anxiety symptoms​.
• Research Findings: Animal studies show the vagus is essential for gut-brain effects. For example, mice given probiotics (Lactobacillus) showed reduced anxiety-like behavior only if their vagus nerve was intact. Cutting the vagus (“vagotomy”) blocked the probiotics’ benefits on the brain​. This proves that many microbial and nutrient signals rely on an active vagus nerve to reach the brain.
• Vagal Tone and Mood: Doctors often measure “vagal tone” (strength of vagus signaling) as an indicator of stress resilience. Higher vagal tone is linked to better stress management and mood. Practices like slow breathing, yoga or cold exposure can boost vagal tone​, indirectly supporting gut-brain harmony.

How Gut Signals Influence Mood and Mental Health

• Neurotransmitter Production: Gut bacteria help produce or modulate mood chemicals. For instance, certain Lactobacillus and Bifidobacterium strains, like the ones found in Bioteem40, can increase GABA (a calming neurotransmitter) in the brain​. One study found that feeding mice Lactobacillus rhamnosus altered GABA receptor expression in the brain and reduced anxiety/depression behaviors, via the vagus nerve​.
• Inflammation and Immunity: A healthy gut lining limits inflammation. If “bad” bacteria leak through the gut wall (leaky gut), they can trigger systemic inflammation, which is linked to depression and brain fog. A strong vagus nerve also triggers anti-inflammatory reflexes​. Balanced gut flora and nutrients (like omega-3s, antioxidants) help keep inflammation down, which in turn protects mental health.
• Clinical Evidence: Human studies back up the gut–mood link. Reviews show that people with depression or anxiety often have distinct gut microbiota profiles, and treatments with probiotics or prebiotics can improve symptoms​. For example, one review found a “significant relationship” between gut microbiome composition and depression/anxiety symptoms, and noted that probiotics or prebiotics tended to reduce these symptoms​​. Another study of young adults found that those consuming probiotic foods (like yogurt) had lower depression, anxiety and stress scores on psychological tests​.
• Real-World Studies: In a Harvard Health report, women who ate a probiotic yogurt twice daily were calmer when shown stressful images, and their brain scans showed reduced activity in the insula (a region tied to gut sensations). Other trials have linked daily probiotics to slight improvements in mood and memory. While research is ongoing, it’s clear that “psychobiotics” – foods or supplements like Bioteem40, that alter gut flora – can be a useful tool alongside other therapies​. ​

Nutrients & Lifestyle: Feeding Your Gut–Brain Axis

To keep the gut–brain axis healthy and your mood balanced, focus on lifestyle habits that nourish the microbiome and support vagal signaling:

• Fiber-Rich, Whole Foods: Eat plenty of fruits, vegetables, legumes and whole grains. These foods contain prebiotic fibers that feed beneficial gut bacteria (Bifidobacteria, Lactobacilli, etc.). For example, onions, garlic, bananas, oats and apples are great prebiotics. A fiber-rich diet also produces SCFAs (like butyrate) that strengthen the gut lining and can improve mood​.
• Fermented Foods and Probiotics: Include fermented foods (yogurt, kefir, sauerkraut, kimchi, miso) which supply live probiotics. These add helpful bacteria strains and enzymes. Even cultured beverages like kombucha or kefir can boost microbial diversity. (Notably, Bioteem40 uses coconut milk kefir as its base to deliver probiotics – see below.) Consuming fermented foods has been associated with reduced stress and better mood in some studies​.
• Healthy Fats and Omega-3s: Fatty fish (salmon, mackerel) and flaxseeds provide omega-3 fatty acids, which support brain cell membranes and reduce inflammation. Research suggests that omega-3s can benefit depression and cognitive health. Also, gut microbes help break down some fats and produce anti-inflammatory molecules.
• Minimize Processed Sugars and Ultra-Processed Foods: Diets high in sugar, refined carbs or preservatives can disrupt the microbiome and promote inflammation​. Harvard researchers emphasize “fixing the food first”: prioritize whole foods and avoid ultra-processed, inflammatory items​. Even replacing sugary snacks with nuts or berries can feed good gut bugs and stabilize blood sugar, which supports a steady mood.
• Stay Hydrated: Water helps digestion and keeps the mucous layer of the gut healthy. Even mild dehydration can stress the system.
• Stress Management: Chronic stress weakens the vagus nerve and shifts gut bacteria toward “bad” species. Practices like deep belly breathing, meditation, yoga or gentle exercise all enhance vagal tone​. These activities can directly soothe the gut (less cramping or IBS symptoms) and improve mental well-being.
• Regular Sleep and Exercise: Good sleep helps regulate gut hormones (like ghrelin/leptin) and encourages a healthy microbiome. Similarly, regular moderate exercise has been shown to increase microbiome diversity and boost endorphins, improving both gut function and mood.

Tips to strengthen your gut–brain axis:  In short, feed the gut food your brain (“second brain”) likes – whole foods, fibre and fermented stuff – and give your vagus nerve plenty of relaxation and breathing. These simple steps can ripple into a better mood, clearer thinking and resilience against anxiety or low feelings.

Conclusion: Nourish Your Gut–Brain Connection with Bioteem40

Keeping your gut healthy is a powerful way to support your mood and mental health. In addition to diet and lifestyle changes, targeted supplements can help. Bioteem40 is an all-in-one probiotic shot designed to fuel the gut–brain axis. It starts with coconut milk kefir – a rich source of diverse live cultures (bacteria) that colonize the gut. On top of that, Bioteem40 is fortified with key vitamins, minerals, omegas and antioxidants that help feed your gut microbes and the vagus nerve. 

By providing 30+ beneficial bacterial strains plus nutrient cofactors in each shot, Bioteem40 helps restore microbial balance and improve nutrient absorption. A well-nourished microbiome can signal through the vagus nerve more effectively, releasing brain-healthy compounds (like serotonin precursors and short-chain fatty acids)​​. In practical terms, that means better digestion, less inflammation, and a calmer stress response – all of which support a brighter mood and mental well-being. For people who want to feel their best mentally and physically, a supplement like Bioteem40 can be a convenient way to bolster the gut–brain axis. 

Take Action: Try incorporating Bioteem40’s kefir-based probiotic shake into your morning routine. Its mix of probiotics and nourishing micronutrients is formulated to “heal” the gut first so it can absorb and use brain-supporting nutrients effectively​. With consistent use, you may notice improvements in digestion and mood stability as your gut and brain communication strengthens. Embrace your gut as your second brain – feed it well, and it will help lift your mood, sharpen focus, and support overall mental health. 

Sources: Peer-reviewed research on the gut–brain axis and vagus nerve​pmc.ncbi.nlm.nih.gov​nature.com​pmc.ncbi.nlm.nih.gov​pmc.ncbi.nlm.nih.gov​health.harvard.edu, as well as expert health summaries (Harvard Health)​health.harvard.edu​health.harvard.edu, were consulted to compile this blog.