The gut-brain axis is real, it is genuinely interesting, and it has been used to sell an enormous amount of product that the evidence does not support. Both of those things are true, and the useful work is separating them rather than picking a side.
This piece is about the mechanism and the marketing — what's solid, where the leap gets made, and why we don't sell a probiotic. If what you specifically want is the trial evidence on probiotic strains and mood, that's a narrower question with its own answer: what probiotics can and can't do for mood and focus goes through the randomised trials strain by strain.
There is a physical communication system, and it's substantial. The gut has its own nervous system — the enteric nervous system, with something on the order of 500 million neurons embedded in the wall of the digestive tract. It coordinates digestion largely without instruction from the brain, which is why it's sometimes called the second brain. That nickname is doing more marketing work than scientific work, but the anatomy behind it is real.
The vagus nerve connects the two directly, and the traffic is mostly upward. The large majority of vagal fibres are afferent — carrying information from gut to brain rather than the other way. Your brain receives a continuous stream of information about the state of your digestive tract, almost none of which reaches consciousness. (What the vagus nerve actually is and does, if that's new.)
The gut produces neurotransmitters. A large share of the body's serotonin — commonly cited at 90% or more — is made in the gut by enterochromaffin cells. This is true, and it is the single most misused fact in the field. More on that below.
Bacteria produce compounds that affect the brain. This is the most credible mechanistic route. When gut bacteria ferment dietary fibre they produce short-chain fatty acids — butyrate, propionate, acetate. These are absorbed, they have effects on the integrity of the gut lining and on immune signalling, and there is reasonable evidence they influence the brain's own immune cells. If any part of this story turns out to matter clinically, this is the likeliest candidate, and note what drives it: fibre, not a capsule.
The relationship is bidirectional. Stress changes gut function through the HPA axis; gut inflammation affects mood and cognition. Anyone with IBS recognises the first half without needing a study. The inflammation half of that sentence has its own literature and its own marketing problem, which we've gone through separately in inflammation and the brain: a real mechanism and a marketing frame.
Animal work is striking. Transferring gut bacteria between mice can transfer behavioural traits. Genuinely remarkable, and the reason the field took off.
"90% of your serotonin is made in your gut, so probiotics improve mood." The first half is roughly right. The conclusion doesn't follow, for a specific reason: gut serotonin largely does not cross the blood-brain barrier. It is doing a different job down there — mostly regulating gut motility and secretion, and it's also involved in platelet function. The brain manufactures its own serotonin from tryptophan, behind that barrier. Two separate pools, one molecule, and the sentence quietly assumes they're the same pool.
Mouse to human is a long way. Mice are not small people. Germ-free mice — the workhorse of this research — are animals raised in conditions no human has experienced, and behavioural findings in them have translated poorly and inconsistently.
Strain specificity gets ignored. Where human trials on mood exist, they test specific strains at specific doses — sometimes called psychobiotics. A general probiotic from a shelf is not the thing that was tested, any more than "a drug" is the thing tested in a drug trial. Different strains of the same species can behave differently, and a benefit demonstrated for one does not transfer to another. We've gone through the specific trials and what they found separately — including one where some effects faded within days of stopping and others were still measurable weeks later.
"Leaky gut" needs unpacking. Increased intestinal permeability is a real, measurable phenomenon that occurs in certain conditions and is an active research area. "Leaky gut syndrome" as a consumer diagnosis — the cause of your fatigue, your mood, your brain fog, treatable with a specific supplement bundle — is not a recognised condition, and the products sold for it are ahead of the science by a distance.
Microbiome tests sold direct to consumers generally cannot tell you anything actionable. There is no agreed reference for a healthy microbiome; variation between healthy people is enormous; results shift substantially with what you ate that week and with the sequencing method used. A report telling you which foods to eat based on your bacteria is generating confident output from an input the field cannot yet interpret.
And one genuine clinical success is routinely misused as support. Faecal microbiota transplant works, and is an established treatment for recurrent Clostridioides difficile infection. That is a real demonstration that the microbiome can be therapeutically manipulated — for that infection. It is not evidence for anything about mood or cognition, and it gets cited as though it were.
The interventions with the least hype and the most support are unglamorous:
None of that is sold in a bottle, which is part of why it gets less airtime.
We get asked. The honest answer is that we haven't found one we could sell without overclaiming.
To do it properly we would need a specific strain, at the dose it was studied at, for the outcome it was studied for, with viability at the point you swallow it rather than the point it was manufactured. That last item alone disqualifies a lot of the category — colony-forming-unit counts on labels frequently refer to time of manufacture, and living organisms die in storage.
That is a meaningfully higher bar than most of this market clears, and until we can clear it, a Proco probiotic would be a product that exists because customers expect one rather than because the evidence asked for it. If that changes, we'll say so.
Probiotics are, for the record, generally safe in healthy people. They warrant more caution in anyone immunocompromised or seriously unwell, where they are not a casual purchase.
The reason people arrive at gut-brain content is usually not curiosity about bacteria. It's low mood, anxiety or brain fog, and the microbiome is being offered as the explanation.
Worth checking the more mundane explanations first, because they're testable and treatable. Four cheap blood tests cover the most common causes of persistent fog — and note that coeliac disease and B12 deficiency are both genuine gut-related causes of cognitive symptoms, with actual diagnostic tests, which is a more useful kind of gut-brain connection than the one being marketed. If anxiety is the issue, here are the interventions ranked by evidence, and probiotics are not on the list.
This article is not medical advice. Persistent gut symptoms — particularly any change in bowel habit, bleeding, unexplained weight loss or pain — deserve medical assessment rather than a supplement.