The Gut-Brain Axis: How Your Gut Bacteria Influence Your Mood

Key Takeaways

• The gut-brain axis is the two-way communication network linking your digestive system and your brain, running through the vagus nerve, your immune system, hormones and bacterial metabolites.

• Your gut contains its own nervous system of more than 500 million neurons, which is why it's often called the "second brain".

• Around 90% of your body's serotonin is made in the gut, though this serotonin doesn't cross into the brain, so its effect on mood is indirect.

• Gut bacteria influence mood through inflammation, the vagus nerve, and by shaping the supply of tryptophan your brain uses to make its own serotonin.

• Research on probiotics and mood is promising but early. Effects are strain-specific and clearest in people who already have symptoms, so a balanced gut is best treated as a foundation, not a quick fix.

Your gut and brain are in constant two-way conversation, and the bacteria in your digestive tract are active participants in it. This connection, called the gut-brain axis, helps explain why stress can upset your stomach and why your gut health can shape how you feel. This article breaks down how that communication works, what the science does and doesn't support about serotonin and mood, and the practical steps that keep the whole system running well. We'll separate the solid evidence from the hype so you know what's worth acting on.

What Is the Gut-Brain Axis?

The gut-brain axis is the constant, two-way communication system connecting your gastrointestinal tract and your central nervous system. Signals travel in both directions, which is why your brain can trigger a nervous stomach and your gut can send messages that shape how you feel. Your gut microbiome sits in the middle of this exchange, influencing the signals that pass along it.

This isn't a single channel. The gut and brain talk through several overlapping routes:

• The vagus nerve: A direct nerve line carrying signals both ways between gut and brain.

• The immune system: Gut bacteria influence inflammation, which affects the brain.

• Hormones: The gut releases signalling molecules that reach the brain through the bloodstream.

• Bacterial metabolites: Microbes produce compounds like short-chain fatty acids and neurotransmitters that carry information.

Why Is the Gut Called the Second Brain?

The gut is called the "second brain" because it contains its own dedicated nervous system that can operate largely on its own. This network, the enteric nervous system, holds more than 500 million neurons woven through the walls of your digestive tract. That's more neurons than your spinal cord.

This system runs digestion without waiting for instructions from your head. It also gives the gut the wiring to send a steady stream of information upward.

Here's what your enteric nervous system handles:

• Controls digestion independently, from muscle contractions to enzyme release.

• Senses conditions inside the gut, including what your microbes are producing.

• Sends signals to the brain, mostly along the vagus nerve.

• Responds to your microbiome, adjusting its activity based on bacterial activity.

The takeaway is simple. A large, active nervous system lives in your gut, and your bacteria have a direct line into it.

Is It True That 90% of Your Serotonin Is Made in the Gut?

Yes, around 90 to 95% of your body's serotonin is made in the gut, but the popular conclusion drawn from that fact is wrong. Gut-derived serotonin doesn't cross the blood-brain barrier, so it can't directly top up the serotonin your brain uses for mood. The two serotonin systems are separate.

Specialised gut cells called enterochromaffin cells produce this serotonin, and your gut bacteria help regulate that process. In the gut, serotonin mostly manages digestion, including how fast food moves through you.

So how does gut serotonin relate to mood at all? The link is real but indirect:

The honest version is that gut health influences mood through several indirect pathways rather than by pumping serotonin straight to your brain. That distinction matters, because it changes what you can realistically expect from looking after your gut.

How Do Gut Bacteria Influence Your Mood?

Gut bacteria influence mood mainly through inflammation, nerve signalling and the raw materials your brain uses to make its own mood chemicals. None of these is a switch that flips your mood on command. Together they help set the background conditions your brain works within.

The main pathways worth knowing:

1. Tryptophan supply: Gut bacteria help control how much tryptophan, the building block for brain serotonin, is available. Unlike serotonin, tryptophan can cross into the brain.

2. Inflammation: An imbalanced gut can drive low-grade inflammation, which is linked to low mood. A balanced microbiome helps keep inflammation in check.

3. Vagus nerve signalling: Bacteria send signals along the vagus nerve that reach mood-related regions of the brain.

4. Neurotransmitter production: Some gut bacteria produce compounds like GABA and short-chain fatty acids that influence how nerve cells communicate.

This is why a poorly balanced gut can leave you feeling flat, and why steady gut habits tend to support steadier wellbeing. For more on the gut side of this equation, see our guide on how probiotics can support a healthier gut.

If low mood is persistent or affecting your daily life, the pathways above aren't a substitute for proper care. Speak with your GP or a mental health professional.

Looking after the foundation? A balanced microbiome is what these gut-brain pathways depend on. UltraBLIS® delivers two clinically studied gut strains plus an oral-health strain in one daily lozenge, supporting the everyday balance your gut-brain axis relies on.

Can Probiotics and Diet Support the Gut-Brain Axis?

Diet is the most reliable way to support your gut-brain axis, and probiotics are a promising but still-emerging addition. A varied, fibre-rich diet feeds the bacteria that produce mood-relevant metabolites, while fermented foods add live cultures. The evidence for probiotics and mood is genuinely mixed, so it's worth being clear-eyed about it.

A systematic review of probiotic trials found that most probiotics didn't shift mood in healthy people, with the clearest effects appearing in those who already had symptoms. Benefits, where they show up, tend to be strain-specific. That's a reason to treat gut care as a long-term foundation rather than a mood treatment.

Practical steps that support the whole system:

• Eat 30 different plant foods a week to build the microbial diversity mood-related pathways rely on.

• Include fermented foods like yoghurt, kefir, sauerkraut and kimchi for live cultures.

• Feed your bacteria fibre from oats, legumes, onions and bananas to fuel short-chain fatty acid production.

• Cut ultra-processed foods and excess sugar, which push your gut toward imbalance.

• Prioritise sleep and movement, both linked to a healthier microbiome.

If harmful bacteria have taken over, our guide on how to starve bad gut bacteria walks through resetting the balance through diet.

Your gut and brain are in a lifelong conversation, and the bacteria in between help shape its tone. Feed them well with fibre and variety, keep your gut balanced, and treat any probiotic as support for the foundation rather than a shortcut. 

Give your gut and oral microbiome daily backing with UltraBLIS®, BLIS's two-in-one gut and oral probiotic.

Frequently Asked Questions

What is the gut-brain axis in simple terms?

The gut-brain axis is the two-way communication line between your digestive system and your brain. Messages travel both ways through nerves, hormones, the immune system and compounds made by your gut bacteria. It's why stress can affect your stomach and gut health can affect how you feel.

Can improving your gut health improve your mood?

Supporting your gut may help set healthier background conditions for mood, mainly by reducing inflammation and supporting the supply of tryptophan your brain uses. The effect is indirect and varies between people. It works best as a long-term habit alongside other wellbeing basics, not as a standalone fix.

Does the serotonin made in the gut affect the brain?

Not directly. Around 90% of your body's serotonin is made in the gut, but it can't cross the blood-brain barrier to reach your brain. Gut serotonin mostly manages digestion, while your brain makes its own separate supply for mood.

Which foods support the gut-brain axis?

Fibre-rich plants, fermented foods and a wide variety of vegetables, fruits and wholegrains support the bacteria involved in gut-brain signalling. Aim for range rather than any single food. Cutting back on ultra-processed foods and added sugar helps too.

Do probiotics really help with mood?

The research is promising but early, and results are mixed. Most trials show little effect on mood in healthy people, with clearer benefits in those who already have symptoms, and effects that depend on the specific strain. A balanced gut is best seen as a foundation for wellbeing rather than a treatment.

Is the gut really a second brain?

The gut earns the nickname because it holds its own nervous system of more than 500 million neurons that can run digestion on its own. It can't think or feel emotions like your actual brain. It does constantly send signals upward that influence how you feel.