Gut-Brain Axis

Your gut and your brain are in constant conversation, and the gut does most of the talking. Here’s what that actually means — and what it doesn’t.

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The connection between the gut and the brain is a two-way street, and the street has a name: the gut-brain axis. It’s one of the more genuinely interesting things in human biology, and also one of the most enthusiastically oversold — so this is the version with the receipts attached.

The wiring, briefly

Your gut has its own nervous system. Not a metaphorical one — an actual mesh of neurons woven through the wall of your digestive tract, called the enteric nervous system. For decades nobody had properly counted it, and wildly different figures got quoted. Somebody finally did the work in 2022: about 168 million neurons in the human gut, which is roughly as many as there are in your spinal cord.¹

That’s why people call it the "second brain," a phrase that took hold after neurobiologist Michael Gershon published a book by that name in 1998.² It’s a good phrase as long as you don’t take it too literally. Your gut is not weighing up career decisions down there.

And here’s the part people find surprising: the traffic is lopsided. Roughly 80% of the fibers in the vagus nerve are afferent — carrying signals up from the body to the brain rather than down.⁴ Your brain spends more time listening to your gut than instructing it.

Your brain spends more time listening to your gut than instructing it.

What is your gut telling you?

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Ever had a "gut feeling"? Felt butterflies before something that mattered? That is the gut-brain axis doing its thing, and the mechanism is well described.

Under acute stress your brain releases corticotropin-releasing factor, the opening move of the stress response. Among its effects: your stomach empties more slowly and your colon speeds up.⁵ Which is a tidy physiological explanation for why nerves feel like a knot in your stomach and an urgent need to find a bathroom, sometimes within the same ten minutes.

Worth saying plainly, because it cuts both ways: gut trouble can be the cause of stress and anxiety, or the product of it.⁶ Persistent digestive symptoms deserve a proper look from a doctor rather than a theory from a blog post.

The serotonin fact, and what it actually means

You have probably seen this one: 90 to 95% of the body’s serotonin is made in the gut, not the brain.⁷ It’s true, it’s remarkable, and it is almost always used to say something it can’t support.

Here’s the missing piece. Gut serotonin and brain serotonin are two separate supplies. The gut’s version is made by enterochromaffin cells lining your intestine, and it stays there, running digestion — motility, secretion, gut reflexes.⁸ It can’t cross the blood-brain barrier.⁷ The brain manufactures its own, on site, using a different enzyme.

So the gut isn’t posting serotonin up to your head. What it can do is influence the raw material: tryptophan, the amino acid the brain builds serotonin from, does cross into the brain, and gut bacteria have a hand in how much of it is available.⁹ Add the nerve and immune signals traveling up the vagus, and you have a real connection — just a more roundabout one than the headline implies.

The same applies to GABA, the neurotransmitter that damps down fear and anxiety. Gut bacteria genuinely make it — Lactobacillus brevis, Bifidobacterium dentium and various Bacteroides among them, and the genes for it are switched on in healthy human stool.^10,11 Whether that GABA reaches the brain is a different question, and an unsettled one.¹² The better-supported route, as you’ll see in a moment, is a nerve signal rather than a delivery.

Can probiotics actually help your mood?

This is the question everyone actually wants answered, so here is the honest state of play.

The famous study is from 2011. Mice fed Lactobacillus rhamnosus showed less anxiety-related behavior, lower stress hormone after a stressor, and changed GABA receptor expression across several brain regions. Then the researchers cut the vagus nerve — and the whole effect vanished.¹³ That’s the elegant bit: it demonstrated the gut was signaling the brain along a nerve, not shipping chemicals to it.

Then the same team gave the same strain to 29 healthy men for eight weeks. Nothing. No change in mood, anxiety, stress response, sleep or cognition. They called the paper "Lost in translation?", which tells you how they felt about it.¹⁴

That gap between mice and healthy humans is the thing to hold onto. The word for probiotics aimed at mental health is psychobiotics, and the researchers who coined it defined it narrowly: organisms that produce a benefit in patients with psychiatric illness.¹⁵ It was never a claim about generally well people. (Science’s 2020 feature on the "psychobiome" is a good tour of the field if you want one.¹⁹)

The pooled human evidence tracks that distinction closely. Across 34 trials, probiotics produced a small effect on depression (d = −0.24) and a negligible one on anxiety (d = −0.10) overall — but the effect grew to d = −0.45 in clinical samples and to −0.73 in psychiatric ones.¹⁶ An umbrella review of 30 meta-analyses found a moderate benefit for depressive symptoms and a small, inconsistent one for anxiety, while noting that 76.6% of the reviews it pooled were of moderate, low or critically low methodological quality.¹⁷

Diet, inflammation and the gut

If the probiotic route to mood is thinner than advertised, what about food? This is where the picture gets more interesting, and also more specific than the usual advice.

In a 17-week randomized trial at Stanford, one group ate more fermented foods and another ate more fiber. The fermented-food group’s microbiome diversity rose steadily and their inflammatory markers fell. The fiber group’s diversity didn’t rise at all — though their microbes’ capacity to break down plant fiber did change.²⁰ Both are worth eating. They are not doing the same job, and the fermented-food result is the one usually attributed to both.

On inflammation, the association is solid. Pooling more than 5,000 patients and 5,000 controls, people with depression have measurably higher CRP, IL-6 and TNF-α than those without.²¹ Markers suggesting a more permeable gut lining also tend to run higher in depression.²²

What isn’t settled is the direction. Increased gut permeability could contribute to low mood, or be a consequence of it, or both could be downstream of something else entirely. Anyone telling you the arrow points one way is ahead of the evidence.

One more finding worth knowing, because it undercuts a popular shortcut: across 59 case-control studies, gut microbiota differences in psychiatric disorders turned out to be transdiagnostic rather than disorder-specific, and two of the four standard diversity measures showed no significant difference at all.²³ "More diverse microbiome equals better mental health" is a marketing sentence, not a finding.

So what does actually help?

A fair question after all that hedging. There is a body of research on what raises wellbeing, and some of it is very good indeed. It is also wildly uneven — a few interventions are backed by hundreds of thousands of people, others by effects that shrink to nothing under a fair comparison.

So here they are in order of how much the evidence actually supports them, which is not the order these lists usually run in.

A note on the whole field: when a 2019 analysis corrected the positive-psychology literature for small-sample bias, the average effect on wellbeing came out around r = 0.10 — real, but a third of the figure that had been widely quoted, and the effect on depression became variable and generally non-significant.²⁴ Read everything below with that in mind, including the parts we like.

1. Invest in your relationships

Photo: Zeynep M./ https://www.pexels.com

If you change one thing on this page, change this one. It is not close.

Pooling 148 studies and 308,849 people followed for an average of seven and a half years, those with stronger social relationships had a 50% greater likelihood of survival (OR 1.50). The effect held across age, sex, starting health status and cause of death.²⁵ A later analysis of 70 studies covering 3.4 million people found social isolation, loneliness and living alone each independently raised mortality risk even after full adjustment.²⁶

The Harvard Study of Adult Development has followed the same men for more than eighty years and keeps landing in the same place: the quality of close relationships in midlife predicts later health and life satisfaction better than cholesterol does.²⁷ Worth noting the cohort was all male and overwhelmingly white, recruited in the 1930s and 40s, and the study is observational — it predicts rather than proves.

The practical version is unglamorous. Check in with someone you have not seen. Cheer for your friends out loud. A ten-minute call counts. If someone crosses your mind while you are making dinner, tell them so.

2. Be kind — it works on you

Photo: Julia M Cameron/ https://www.pexels.com

Performing acts of kindness measurably lifts the mood of the person doing it. Across 27 studies and about 4,045 people, the pooled effect was small-to-medium (δ = 0.28), unmoderated by age, sex or the type of kindness, and with no indication of publication bias.²⁸

Whether kindness genuinely spreads is harder to prove than the phrase "pay it forward" suggests. The best experiment on it found that contributions in a public-goods game rippled outward to three degrees of separation, with each extra contribution effectively tripled by the behavior it prompted in others.²⁹ That is a clever result and a real one — but it studied money in a lab game with reassigned strangers, not holding a door for a neighbor.

One practical detail from the giving research: the effect is most reliable when you actually chose to give, and when you get to see what it did.³⁰ Anonymous obligation works less well than deliberate generosity you can watch land.

3. Savor the good bits

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Savoring — deliberately stretching out a good moment rather than letting it slide past — holds up better than most wellbeing advice. Across 20 randomized trials and 4,805 participants, the pooled effect was moderate (g = 0.51), and the authors ran four separate tests for publication bias, all of which came back clean.³¹

4. Gratitude — pleasant, and smaller than advertised

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The founding study is genuinely charming: people asked to count blessings rather than burdens showed improved wellbeing across several, though not all, measures, with the effect on positive mood the most robust.³⁴

Then the meta-analyses arrived, and the honest picture is more modest. Against a measurement-only control, gratitude interventions produced d = 0.31. Against an activity-matched control, d = 0.14. Against a psychologically active control — another exercise that does something — d = 0.03, which is to say nothing at all. The authors’ own conclusion was that the evidence is weak and the interventions "may operate primarily through placebo effects."³⁵ A later meta-analysis of 27 studies found the same shape: effects were larger against waitlists (g = −0.51) than against active controls (g = −0.18), and recommended that anyone actually trying to reduce depression or anxiety use something with stronger evidence.³⁶

So: keep the gratitude jar if you enjoy it. Noticing what went well is a nice way to live. Just do it because you like it, not because it is a treatment.

5. Practice optimism, with realistic expectations

Short writing exercises — imagining your best possible future self — do nudge optimism scores upward, with a pooled effect of d = 0.33 across 29 studies and 2,909 people, and a larger one on positive affect.³⁷ The caveats are substantial: mostly students, mostly short, and only three studies followed anyone past three months.

Separately, people who score higher on optimism have fewer cardiovascular events and lower all-cause mortality across 229,391 people followed for nearly 14 years.³⁸ Every one of those studies is observational, though, and nobody has randomized anyone into becoming an optimist and then measured their heart. The two literatures have never been joined up.

And the daily basics

Colorful food. Movement. Enough sleep. Turning off the news occasionally and pointing your attention at something better. None of this is a revelation, and all of it is better evidenced than most of what gets sold as a shortcut.

A daily probiotic can sit alongside that as digestive and microbiome support. What we are not going to tell you is that it will make you happier — see the boxed section above, which is the whole reason this article exists.

Is Flourish living probiotic for you?

Flourish is a living liquid probiotic, delivered in the liquid it was fermented in rather than freeze-dried into a capsule. Each tablespoon carries 11 strains from 5 families, grown together as one culture on non-GMO sugarcane molasses or sweet sorghum, with every batch third-party tested and cleared for delivery of billions of live CFUs per dose.

Because nothing is dried out and reconstituted, the bottle also carries the metabolites the culture produced while it grew — short-chain fatty acids, bioactive peptides, amino acids and enzymes among them. Those short-chain fatty acids are, as it happens, one of the five routes on the list above. More on them in Flourish Probiotics and Postbiotics.

One point of precision, since the word gets used loosely: under the international consensus definition, a postbiotic is a preparation of inanimate microorganisms or their components with a demonstrated health benefit — metabolites on their own do not qualify.³⁹ So we will tell you what is in the bottle, and leave the label off it.

What we’d claim for it is digestive and microbiome support, taken consistently — which is why consistency is key. What we wouldn’t claim is a mood effect. See above.

For context on where probiotics sit clinically: the American Gastroenterological Association makes no recommendation for probiotics in adults and children with IBS, and its conditional support is limited to specific situations such as preventing C. difficile infection during antibiotic treatment.⁴¹ That is the frame we think is honest for a general-audience product.

Flourish is a dietary supplement made to support digestive and microbiome health. It isn’t a treatment for any condition, mental health conditions included. Follow the dosage on the label. Check with your doctor before starting a new supplement, and with your pediatrician before giving one to a child. If you are being treated for anxiety or depression, do not change or stop that treatment without speaking to the person managing it.

Ready to try a living probiotic?

Eleven living strains. One daily shot. For the whole family — grown-ups and older kids alike.

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And since taking it regularly is the part that actually matters, that’s what Subscribe & Save is for: 5% off all Subscribe & Save orders, plus free shipping on subscriptions of 2+ Flourish.

A few questions we hear a lot

Is most of my serotonin really in my gut?

Yes — 90 to 95% of it.⁷ But it stays in the gut, where it runs digestion, and it can’t cross into the brain. Your brain makes its own separately. The gut’s influence on mood runs through tryptophan supply and nerve and immune signaling, not through shipping serotonin upstairs.^7,9

Will a probiotic make me less anxious?

In healthy people, the pooled trial evidence says probably not to any noticeable degree.¹⁶ The effects that do show up cluster in people with a clinical diagnosis, and even there the underlying research quality is mixed.¹⁷ If anxiety is affecting your life, the person to talk to is a clinician.

What are "psychobiotics"?

A term coined in 2013 for live organisms that produce a mental health benefit in patients with psychiatric illness.¹⁵ It’s a real research field, not a marketing word — but note how specific the definition is about who it applies to.

Why do nerves upset my stomach?

Acute stress triggers corticotropin-releasing factor, which slows gastric emptying and speeds up the colon.⁵ Butterflies and urgency are the same signal arriving in two places.

Does the gut really have a "second brain"?

It has about 168 million neurons — comparable to the spinal cord, not to the brain.¹ It runs digestion largely on its own, which is impressive enough without overstating it.

Do gut bacteria make neurotransmitters?

A number of them, including GABA, dopamine, serotonin and acetylcholine — though most of that has been shown in culture rather than measured in a living gut.¹⁸ Making a molecule and delivering it to the brain are two very different achievements.

If probiotics won’t lift my mood, what will?

On the current evidence, the things with the strongest support are unfashionable: close relationships, physical activity, sleep, and treatment from a clinician if you need it. Relationships in particular carry an effect size that nothing in the supplement aisle comes close to.^25,26

Important information

This article is for general education and isn’t a substitute for professional medical advice, diagnosis, or treatment. Probiotic effects are specific to the strain and to the outcome studied; evidence for one product does not transfer to another. Nothing here is a treatment for anxiety, depression or any other mental health condition. If you are struggling with your mental health, please speak to a qualified professional. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

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