Last week I wrote about the gut as a metaphor. The "gut feeling", the bodily read your nervous system issues before your analytical mind has finished its report. Today I want to talk about the gut as an actual organ. Or more precisely, the roughly 38 trillion bacteria living in it, and whether they are quietly running your mood, your focus and your decisions.

Because that is the claim being sold right now. Walk the wellness aisle and you will find "psychobiotics", "brain probiotics", "mood support" cultures, all promising to upgrade your cognition through your colon. The global probiotics market is worth tens of billions of dollars and climbing. The pitch is seductive, especially to a high performer looking for an edge: swallow this, feed the right bugs, think better.

Here is the reality. The gut-brain axis is very real. It is one of the most exciting areas in modern neuroscience, and it sits right at the heart of everything I teach, because it is physiology reaching up and shaping psychology. And most of what is being sold on the back of it is running years ahead of what the evidence shows.

Get a kombucha or kefir and settle in. This one separates the genuine, load-bearing science from the marketing, and gives you the handful of moves that actually move the needle.

Short on time? There is a TL;DR near the end.

The Basics: You Are Piloting An Ecosystem

Start with the thing that is not in dispute. Your gut houses a dense microbial community, and that community is in constant contact with your brain. Constantly, bidirectionally, and through channels we can now name and measure. This is the microbiota-gut-brain axis, and the definitive reference on it runs to 136 pages in Physiological Reviews for a reason (Cryan et al., 2019).

There are four main lines of communication, here are the cliff notes:

• The vagus nerve. The same highway from the interoception edition. About 80% of its fibres run body to brain, and it carries signals from the gut straight into the brainstem and limbic system (Bonaz et al., 2018). Your bugs have a direct line to your brain.
• Microbial metabolites. When your bacteria ferment fibre, they produce short-chain fatty acids like butyrate. These influence inflammation, the blood-brain barrier, and microglial function (Silva et al., 2020). This is the mechanism behind "feed your bugs and they feed you back."
• The HPA axis. The microbiota help calibrate your cortisol stress response. Germ-free animals, raised with no microbiome at all, show an exaggerated, dysregulated stress reaction (Cryan et al., 2019).
• The immune system. Your gut bugs shape systemic inflammation, and inflammation feeds directly back onto mood and cognition.

If you have followed this newsletter for a while, you'll know why I find this relevant. This is psychology following physiology at the most fundamental level there is. Literally a bug in the source code! So the mechanism is there and we understand it. The question is what happens when we go looking for it in actual humans, doing actual things, under actual load.

Where The Story Gets Honest: The Mouse Problem

Here is the pattern you'll see behind some of the more extravagant claims in the field.

Almost every jaw-dropping gut-brain headline you might have read traces back to a mouse. Transfer the microbiome of an anxious mouse into a calm one and the calm one gets anxious. Raise a mouse germ-free and its brain develops differently. These studies are real, elegant, and genuinely important for understanding the mechanism.

But they are also mice. Bred to be genetically identical, raised in sterile cages, sometimes with no microbiome whatsoever. I lived this live as a PhD student and post-doctoral researcher. A germ-free mouse is a laboratory instrument for answering highly specific questions in highly specific ways, they are not a model of your Tuesday or how you should structure your day. When these dramatic effects are tested in humans, living messy lives with varied diets and intact microbiomes, they shrink dramatically or fail to appear at all.

So let us look at what actually holds up in people.

What Actually Holds Up In Humans

I went through the human trial literature, and it sorts cleanly into three buckets. \

Bucket one: stress and mood. Modest effect, real, replicated.

This is the strongest human signal. Specific interventions reliably nudge the stress system:

• A prebiotic fibre called B-GOS lowered the salivary waking cortisol response and shifted attention away from negative stimuli in healthy volunteers, over just three weeks (Schmidt et al., 2014). Notably, a different prebiotic in the same study did nothing. Specificity matters.
• A specific bacterial strain, Bifidobacterium longum 1714, reduced cortisol output and subjective stress, and measurably altered neural activity during a social stress task (Allen et al., 2016; Wang et al., 2019).
• Meta-analyses of probiotics for stress, anxiety and depression find a genuine but small effect, strongest as an adjunct to existing treatment rather than a standalone fix, and limited by short, small, heterogeneous trials (Zhang et al., 2020; Sikorska et al., 2023).

The reproducible thread across all of it: a calmer cortisol response and a shift in emotional processing toward the positive. This is where the 'centre of mass' of the research is pointing.

Bucket two: cognition and performance. Weak, and mostly not you.

Here is where I have to be blunt. As of writing, there is no credible evidence (in my mind) that a probiotic makes a healthy, high-functioning adult think faster or sharper. Yet I see claims that do exactly this everywhere.

The positive cognition trials almost all sit in older adults or people with mild cognitive impairment, where there is genuine room to move (Kim et al., 2021; Aljumaah et al., 2022). The tell is a 2024 study that gave healthy adults GABA-producing bacteria: it improved their emotional reactivity to a bad mood but produced no improvement in actual cognitive performance (Casertano et al., 2024). Again, showing evidence in mood/stress but not cognition.

Bucket three: whole-food diet. The actual leverage.

Here is the (unsexy) reality the supplement industry would rather you missed. The largest gut-brain effects in humans come from food, not pills.

The landmark SMILES trial put adults with major depression on a modified Mediterranean diet for twelve weeks. The effect on depression was large, a Cohen's d of about 1.16, with roughly a third reaching remission versus 8% of controls (Jacka et al., 2017). No strain, no capsule. Just food.

And a beautiful Stanford trial pitted high-fibre against high-fermented-foods for seventeen weeks. Fermented foods, ramped up to around six servings a day, increased microbiome diversity and lowered inflammatory markers (Wastyk et al., 2021). Diversity of the ecosystem went up, inflammation came down, from sauerkraut and kimchi rather than a bottle.

Food beats supplements. Every time you follow the evidence in this space, it will lead you back to your plate.

The Hype Filter: Keep This, Drop That

Before the protocol, the same myth-correction pass I run on every topic, because your credibility depends on knowing where the line sits.

Keep: the axis is real. Fibre and fermented foods measurably shift your microbiome, your inflammation and your stress physiology. Specific strains can dampen a cortisol response. Diet has a real, sizeable effect on mood.

Drop: "brain probiotics" that make healthy people smarter. There is no human evidence for it. Drop the idea that a mood result for one strain applies to another, it does not, efficacy is strain-specific and does not transfer.

One flag worth naming for those of you who, like me, eat plant-based. A lot of the classic gut-brain studies, including the famous brain-imaging one, used dairy fermented-milk products, and some commercial strains are cultured on dairy. You lose nothing by going plant-first here. Sauerkraut, kimchi, tempeh, miso, and water kefir seem to do the same microbial job.

The Protocol: Four Moves, Bottom Up

True to form, this is sequenced from foundation to fine-tuning. Do them in order. The last one only matters once the first three are in place.

1. Diversify The Substrate

The Science

Your bugs are only as good as what you feed them. Microbiome diversity, which tracks with resilience and lower inflammation, is driven more by the variety of plants you eat than by any single "superfood" or supplement. Different fibres feed different bacteria, and it is those bacteria that ferment fibre into the short-chain fatty acids doing the downstream signalling (Silva et al., 2020). Diversity in, diversity out.

The Protocol

Count plants, not calories. Aim for 30 or more different plant foods across a week: vegetables, fruit, legumes, nuts, seeds, wholegrains, herbs and spices all count. Push total fibre toward 30 to 40 grams a day. The practical move is variety at the point of shopping: rotate what goes in the basket rather than buying the same five vegetables on repeat.

Expected Outcome

This is the upstream lever. It is unglamorous and it is the highest-confidence intervention in the whole edition. Like a lot of things in life, the most effective interventions are the basics and are not sexy.

2. Add Fermented Foods, And Escalate

The Science

This is the one intervention with a clean, causal human trial behind it. Ramping fermented foods up to around six servings a day increased microbiome diversity and reduced inflammatory markers over seventeen weeks (Wastyk et al., 2021). Note the word escalate: the effect scaled with the dose, and most people eat almost none.

The Protocol

Start with one serving a day and build over weeks, because going hard from zero is a reliable way to upset your gut. Plant-first options: sauerkraut, kimchi, tempeh, miso, water kefir, and unsweetened plant yoghurts with live cultures. A serving is small, a couple of forkfuls of kraut alongside a meal counts. Stack it onto food you already eat rather than treating it as a separate ritual. If you want to get right into it, making your own sauerkraut is fun, delicious and easy.

Expected Outcome

Rising diversity and falling inflammation over one to three months. This is where you get the most measurable microbiome change for the least effort.

3. Match The Strain To The Goal, Or Skip It

The Science

If you are going to spend on a supplement, spend with precision, because efficacy is strain-specific and the goal is narrow. The human evidence supports specific strains for stress and cortisol regulation, not for cognitive enhancement (Allen et al., 2016; Schmidt et al., 2014).

The Protocol

Only reach for this if your goal is stress reactivity, and match the tool to it. For the cortisol response, the evidence points to the prebiotic B-GOS. For stress and mood, specific strains like Bifidobacterium longum 1714. Read the label for the exact strain designation, the letters and numbers, not just the species. If it does not name the strain, it likely has not earned your money. And be honest with your expectations: you are smoothing a stress response, not buying IQ points.

Expected Outcome

A modest, real reduction in stress reactivity over three to four weeks, if the strain matches the evidence.

4. Protect The Whole System

The Science

The microbiome does not operate in isolation. A 2023 trial found the psychological benefit of a gut-targeted intervention was conditional on lifestyle: sleep, exercise and diet moderated whether it worked at all (Morales-Torres et al., 2023). A gut intervention layered onto a dysregulated system underperforms or worse does nothing.

The Protocol

Treat the microbiome as one system inside the entire network, not a standalone hack. One piece of a large puzzle. And like most things we talk about in this newsletter, sleep governs it. Exercise diversifies it. Chronic stress degrades it, and the vagal tone you build with slow breathing is the same nerve carrying gut signals north.

Expected Outcome

Compounding. The gut work pays off in proportion to the foundation underneath it. This is why I always start at the biology and build up: get the substrate right and the ecosystem largely tends itself.

The Bigger Picture

The gut-brain axis is exactly the kind of story I love, and exactly the kind that gets ruined in the telling. The science is genuine and profound: an ecosystem inside you, wired directly into your brain, shaping your inflammation, your stress response and your mood. That is physiology reaching up to author psychology, which is the whole thesis of this newsletter.

But the honest read is narrower than the marketing. The mechanism is strong. The human effects are real but modest, they live in stress and mood rather than raw cognition, and they come overwhelmingly from what you put on your plate rather than what you buy in a capsule. No bacterium is going to make a well-rested, well-fed, high-functioning leader think faster. What a well-tended gut can do is lower the noise floor: a calmer stress response, less inflammation, a steadier baseline to perform from.

That reframe is the useful one. Stop shopping for a brain upgrade in the supplement aisle. Start feeding the ecosystem that regulates the system you already have. Thirty plants a week, fermented foods you actually enjoy, sleep and movement to hold the whole thing together, and a strain-specific supplement only if smoothing your stress physiology is the goal.

Ferocious when it matters, fully present when it does not. Both of those run on a nervous system, and that nervous system is being fed, quite literally, three times a day.

TL;DR

• The gut-brain axis is real and mechanistically solid: your microbiome talks to your brain via the vagus nerve, microbial metabolites, the HPA axis and the immune system (Cryan et al., 2019).
• Most dramatic gut-brain headlines are mouse studies. In humans, living real lives, the effects are much smaller. Keep the mechanism, discount the hype.
• Stress and mood is the strongest human signal: specific strains and prebiotics can lower cortisol reactivity and shift emotional processing, though effects are small and strain-specific (Schmidt et al., 2014; Allen et al., 2016).
• There is no credible evidence probiotics enhance cognition in healthy high performers. The positive trials are in ageing and cognitive impairment (Casertano et al., 2024).
• Food beats supplements. The Mediterranean-style SMILES trial produced a large effect on depression (Jacka et al., 2017), and fermented foods raised diversity and lowered inflammation (Wastyk et al., 2021).
• Four moves, bottom up: (1) diversify the substrate, 30+ plants a week; (2) add fermented foods and escalate; (3) match a strain to the goal or skip it; (4) protect the whole system with sleep, exercise and stress work.
• The goal is not a brain upgrade in a bottle. It is a lower noise floor: a calmer, better-regulated system to perform from.