What Your Gut Bacteria Are Doing to Your Mood Right Now

There are roughly 38 trillion bacteria living in your digestive tract. That number is not a curiosity. These organisms produce chemicals that affect your mood, regulate your immune system, shape how you respond to stress, and send signals directly to your brain through a dedicated nerve pathway. What they are doing right now depends almost entirely on what you have been feeding them.

The gut microbiome produces neurotransmitters, the same chemicals the brain uses to regulate mood, anxiety, and motivation. About 90 percent of the body's serotonin is produced in the gut, as covered in the Mind and Nervous System series. GABA, which calms the nervous system, is also produced in significant quantities by gut bacteria. The gut is not just a digestive organ that occasionally sends a signal to the brain. It is an active neurological participant in how you feel.

The composition of the microbiome shifts in response to diet, sometimes dramatically and within days. A diet rich in fiber and diverse whole foods supports a diverse stable community of bacteria capable of producing a broad range of beneficial compounds. A diet high in refined carbohydrates, seed oils, and ultra-processed foods selectively feeds the bacteria that produce inflammatory signals, reduces overall diversity, and shifts the balance away from the organisms that support mood and neurological function. The emotional and cognitive consequences of this shift are measurable and they arrive faster than most people would expect.

You feed the bacteria. The bacteria shape how you feel. Fed well Fiber and diverse plants Bacteria produce mood chemicals and calm signals Brain receives supportive input Vagus nerve Direct line to the brain Fed poorly Sugar and processed foods Inflammatory bacteria thrive Inflammatory signals rise Brain receives distress input

This is why treating mood and mental health without ever considering the gut produces limited results for so many people. The gut is not a separate system sending occasional updates to the brain. It is a continuous input shaping neurological output moment to moment. Treat the gut and the brain often changes, not because the mind was directly targeted, but because what was feeding the brain's environment changed.

Improving the microbiome does not require a complex protocol. It requires reducing the inputs that feed inflammatory bacteria, increasing dietary fiber and diversity, and being conservative with anything that disrupts the ecosystem. Antibiotics, chronic stress, alcohol, and inadequate sleep all reshape the microbiome in ways that take significant time to recover from.

"The gut is the second brain only in the sense that it was discovered second. In terms of its influence on mood and neurological function, it has always been a primary player. We just were not looking."
© 2026 Dr. Tommy Siu / Siu Chiropractic · siuchiropractic.com