Beyond Neurons and Hormones: The Gut–Brain Axis

The nervous and endocrine systems keep your body balanced, but researchers have uncovered another network that connects the brain and body—the gut–brain axis.

This system links the digestive tract, immune system, and central nervous system through a constant exchange of chemical and neural messages. The gut doesn’t just digest food; it also produces hormones, neurotransmitters, and immune molecules that can affect mood, stress, and thinking. What happens in the gut can influence the mind—and vice versa.

What Is the Gut–Brain Axis?

The gut–brain axis is a two-way communication system between the brain and the digestive tract. The brain can influence digestion and stress responses, while the gut microbiota—the trillions of microorganisms living in your intestines—can affect brain chemistry and behavior through immune, hormonal, and neural pathways (Doenyas, Clarke, & Cserjési, 2025).^[1]

Your gut hosts more than 1,000 bacterial species, dominated by Firmicutes and Bacteroidetes, with smaller groups like Actinobacteria and Proteobacteria (Vlăduțu et al., 2025).^[2] Because it contains the body’s second-largest cluster of neurons after the brain, scientists call it the “second brain.”

But How Do the Gut and Brain Communicate?

• Neural Pathways: The vagus nerve acts as a communication highway, carrying signals in both directions—allowing the brain to regulate digestion and the gut to report on what’s happening inside the intestines.
• Immune System Interactions: Gut bacteria produce metabolites that influence immune responses, which can affect brain activity and mood (Wang, Buffington, & Salas, 2024). ^[3] When this balance is disrupted—a condition called dysbiosis—inflammation may rise and contribute to anxiety or depression.
• Neuroendocrine Signaling: The hypothalamic–pituitary–adrenal (HPA) axis coordinates the body’s stress response. Stress hormones such as cortisol can change the gut’s microbial composition, while some microbes can influence HPA activity, creating a feedback loop between stress and digestion (Kim & Shin, 2018.^[4]
• Neurotransmitter Production: Many gut bacteria help make neurotransmitters such as serotonin, dopamine, and GABA. Remarkably, about 90 percent of the body’s serotonin is produced in the gut, not the brain (Yano et al., 2015).^[5]
• Stress Response: Chronic stress activates the HPA axis and disrupts gut bacteria, increasing gut permeability and inflammation. This disturbance may intensify anxiety and reinforce a brain–gut feedback loop (Doenyas et al., 2025).
• Social Behavior and Personality: People with larger social networks tend to have more diverse gut microbes, suggesting that social interaction itself may shape the gut microbiome (Johnson, 2020).^[6]

Therapeutic Implications

• Psychobiotics: These probiotic supplements aim to support mental health. Meta-analyses show that certain strains—such as Lactobacillus plantarum PS128 and Bifidobacterium breve—can modestly reduce depression and anxiety, though effects are small and vary by strain (Pan et al., 2025).
• Dietary Interventions: High-fiber, plant-rich diets—like the Mediterranean diet—promote microbial diversity, lower inflammation, and correlate with improved mood (Vlăduțu et al., 2025).
• Fecal Microbiota Transplantation (FMT): FMT transfers gut microbes from a healthy donor to a patient’s intestines. Early trials show possible mood benefits, but this approach remains experimental for mental-health treatment (Vlăduțu et al., 2025).

Current Research and Future Directions

Scientists continue to study how the microbiome interacts with the brain in neurodegenerative, psychiatric, and metabolic disorders—from Parkinson’s disease to mood disorders (Doenyas et al., 2025). These insights may lead to microbiome-based therapies that complement existing psychological and medical approaches.

The gut–brain axis demonstrates a core biopsychological principle: the mind and body are interconnected. Mental health depends not only on brain activity but also on the ecosystems within our bodies—ecosystems shaped by diet, stress, sleep, and lifestyle.