The gut-brain axis is the two-way communication system between your digestive tract and your brain. It runs on nerves, neurotransmitters, immune signals, and microbial chemistry, and it’s a large part of the reason why so many neurodivergent people have persistent stomach problems, which are often not even connected to their neurodivergence.
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If you’re autistic or have ADHD, there’s a reasonable chance you know what those stomach problems feel like. Constipation, diarrhoea, bloating, abdominal pain, acid reflux, nausea — normalised, everyday experiences nobody connected to your neurodivergence.
But how are gut problems connected to neurodivergence in the first place? What does your neurodevelopmental condition has to do with having a “sensitive tummy” if it is connected to the brain?
When people say “gastrointestinal” (GI for short), they mean the digestive tract: your oesophagus, stomach, and intestines.
The gut-brain axis is what connects that system to the brain, and “axis” just means a two-way line between two things that influence each other.
Well, it turns out our gut is also connected to the brain.
Your gut has nerves
You might have heard the nickname “second brain” for our guts. It sounds like an exaggerated metaphor, but it is, for once, actually literal.
Your gastrointestinal tract has its own nervous system, the enteric nervous system, containing somewhere between 200 and 600 million neurons.1 Just so you have a benchmark, that’s more neurons than the spinal cord.2 These are the same type of cells your brain uses, communicating with many of the same neurotransmitters.
The enteric nervous system coordinates digestion, regulates blood flow, and manages immune responses, and it can do all of this independently of the brain in your skull. Both systems originate from the same embryonic cells, a population called the neural crest, and they never lost their connection.2 The main line between them is the vagus nerve.
The vagus nerve
The vagus nerve is the physical connection between the gut and the brain. It runs from the brainstem down through the neck, chest, and abdomen, and it carries information in both directions. Roughly 80% of its fibres are afferent, meaning they carry signals from the gut to the brain. The remaining 20% carry instructions from the brain back down to the gut.3 This means your gut is sending your brain far more information than your brain is sending your gut.
The vagus nerve also regulates gut inflammation and intestinal permeability (how tightly sealed the gut wall is) through a mechanism called the cholinergic anti-inflammatory pathway.3 When this pathway is underactive, a state called low vagal tone, inflammation tends to increase, and gut barrier function tends to decrease. Chronic stress suppresses vagal tone,34, which is one of the mechanisms through which prolonged stress affects digestion, immune function, and mood simultaneously. For a fuller picture of the vagus nerve and what it does across the body, see the [vagus nerve] glossary entry.
Serotonin and the gut
Serotonin is a neurotransmitter involved in mood, sleep, and emotional regulation. Approximately 90% of the body’s serotonin is produced in the gut by specialised cells called enterochromaffin cells in the intestinal lining.5 This gut-produced serotonin activates receptors on vagal afferent fibres, which transmit the signal up to the brainstem. It arrives at a relay point called the nucleus tractus solitarius, which projects to brain regions involved in mood, stress responses, and emotional regulation (the dorsal raphe nucleus and the locus coeruleus).5
The gut microbiome influences this process directly. Certain bacterial species — including Lactobacillus and Bifidobacterium — produce short-chain fatty acids that increase serotonin production by the enterochromaffin cells.5 Changes in your gut microbiota can change how much serotonin your gut produces, which changes what signals travel up the vagus nerve, which changes how the brain regions receiving those signals behave. It’s a chain, and what happens at every link has an effect on the next one.
The gut microbiome
Your gut is home to trillions of microorganisms (bacteria, fungi, viruses) collectively called the gut microbiome. These microorganisms are not the kind you want to get rid of, because they have very useful roles in our body. They produce neurotransmitters, metabolise nutrients, train the immune system, and influence how permeable the gut wall is. The serotonin pathway is just one example: gut bacteria produce the short-chain fatty acids that drive serotonin production in the intestinal lining.
Research consistently finds that autistic and ADHD adults have a measurably different microbiome composition compared to non-neurodivergent controls.67 The differences show up at the microbiome community level: the overall balance of species is shifted, even when individual studies disagree about which specific bacteria are higher or lower.68
In autistic adults, lower microbial diversity has been correlated with more “severe” autistic traits.9 In adults with ADHD, a study of medication-naïve participants identified a four-genus bacterial signature that distinguished ADHD from controls with moderate accuracy.10 A separate finding linked changes in Bifidobacterium levels to altered dopamine precursor synthesis and reduced reward anticipation in the brain.11 That connection runs from gut bacteria through neurochemistry to one of the core experiences of ADHD.
Can the microbiome be directly connected to neurodivergence?
These are legit, quantifiable differences, but there is also no black-and-white answer. The specific bacteria that show up as different vary between studies. The results depend on geography, diet, medications, age, and how the samples were collected and analysed.678 So even if it would be very convenient, there is no single “autism microbiome” or “ADHD microbiome” that researchers can point to. The commonality in all the findings is a pattern of dysbiosis — a shift in the overall microbial community away from what’s typically seen in non-neurodivergent populations.67
What this means in practice is still being worked out. The research supports the idea that your gut microbiome is part of the picture, but it can’t yet tell you which specific bacteria to target or what to do about it.
Are probiotics worth taking?
The conclusion seems logical: if your gut microbiome is measurably different, and that difference is connected to how you feel, then changing the microbiome with probiotics should help. Right? Well, researchers have been testing this for over a decade, and the results are more complicated than the “this supplement will solve all your problems” marketing promises suggest.
The most consistent finding is that probiotics can improve gastrointestinal symptoms. Across multiple randomised controlled trials, constipation, diarrhoea, and overall GI discomfort improved in autistic participants taking various probiotic formulations compared with placebo.252627 If your gut feels bad and you try a probiotic, and your gut feels better, that’s great. But probiotics can also cause bloating, gas, and digestive discomfort, particularly in the first few weeks, so it’s not always obvious whether a new symptom is a side effect of the probiotic or the problem you were trying to solve not reacting to the supplement.
Probiotics are also marketed beyond gut problems, for example, for cognitive and behavioural struggles. The evidence is not as straightforward about these claims. Some trials report improvements in social behaviour, hyperactivity, or “oppositional behaviour” in autistic children taking specific strains.2528 Others, using different strains or different measures, find no change in core autism measures.26 The effects that do show up tend to be subgroup-specific rather than broad improvements across all participants. They appear in younger children, or in children who already had GI symptoms, or with particular bacterial strains.2629
For ADHD, the claim that probiotics can help is not backed by science. A 2024 meta-analysis of seven randomised trials in children and adolescents found no significant benefit of probiotics over placebo for total ADHD symptoms, inattention, or hyperactivity.30 Some individual trials report modest improvements when probiotics are added alongside stimulant medication, but these effects are small and not consistently replicated.3132
Once again, almost all of this research is on children; adult evidence is basically non-existent. In general, across trials in otherwise healthy people, probiotics are usually safe and well-tolerated, with side effects mostly limited to mild, transient digestive discomfort.2530 However, the case is different for people who are immunocompromised, on immunosuppressants, or living with serious intestinal conditions, where rare but real risks, including infections and immune disturbance, have been documented.33
What the bottom line is: probiotics may help your gut feel better, and that alone can make a meaningful difference to your quality of life. The evidence does not support them as a “treatment” for ADHD or autism. If someone is selling you a specific probiotic formulation for your neurodivergent brain, their claims are currently not backed by science. In any case, talk to your doctor before starting a probiotic.
When the gut barrier breaks down, aka “leaky gut”
The lining of your intestines is a barrier.
- On one side: the contents of your digestive tract, including food, bacteria, and bacterial byproducts.
- On the other side: your bloodstream and immune system.
The barrier is made of a single layer of cells held together by structures called tight junctions, which are protein complexes that control what passes through and what stays out.
When this barrier is working well, it lets nutrients through and keeps everything else contained. When it’s compromised, molecules that should stay inside the gut (bacterial fragments, food antigens, inflammatory compounds) can cross into the bloodstream. This is what researchers mean by increased intestinal permeability, and it’s what gets called “leaky gut” in popular language.
Leaky gut and autism
In autistic people, this barrier is measurably different. Studies using sugar absorption tests, which measure how easily molecules cross the gut wall, find abnormal permeability in roughly 37–43% of autistic participants, compared with fewer than 5% of non-autistic controls.1213 Gut tissue samples from autistic individuals often show reduced levels of barrier-forming tight junction proteins and increased levels of pore-forming ones.14 The seals between cells are weaker, and the gaps are wider, which means the movement through the barrier is working differently.
What happens next is a domino effect.
- When bacterial products like lipopolysaccharides cross into the bloodstream, the immune system responds with inflammation.
- Pro-inflammatory cytokines (signalling molecules including IL-1, IL-6, and IL-17) rise.1513
- This systemic inflammation can reach the brain, where it activates microglia, the brain’s resident immune cells, and drives neuroinflammation.1617
And now we have come full circle; the gut-brain axis closes into a loop. The blood-brain barrier controls what enters the brain from the bloodstream, and it uses the same type of tight junctions as the gut lining. Fiorentino et al. (2016) examined both intestinal tissue and brain tissue from autistic individuals and found the same pattern of damage in both: reduced barrier-forming tight junction components and increased pore-forming ones.14
Mast cells and the gut barrier
Mast cells are immune cells concentrated just beneath the gut lining. They regulate intestinal permeability directly: their enzymes and signalling molecules can remodel tight junctions, and when they’re overactive, they weaken the barrier.18
Mast cell activation syndrome (MCAS) is a condition in which mast cells release excessive inflammatory mediators (including histamine, serotonin, and cytokines) without a clear allergic trigger. MCAS frequently co-occurs with hypermobile Ehlers-Danlos syndrome (hEDS) and dysautonomia, particularly postural orthostatic tachycardia syndrome or PoTS.19 All three conditions overlap with autistic populations.20 ASD appears roughly 6–10 times more often in people with mastocytosis, a related mast cell condition, than in the general population, though these estimates come from small, selected samples rather than population-level studies.21
The research connecting MCAS specifically to autism is still early. But the pattern is consistent enough to name: mast cells driving gut barrier dysfunction, clustering with conditions common in autistic people.
Why the gut-brain axis is important for neurodivergent people
Gastrointestinal symptoms are reported in 25–70% of autistic individuals, depending on the study.22 That range is wide because measurement methods and populations vary, but even the lower end means a quarter of autistic people are dealing with gut problems. Constipation, diarrhoea, abdominal pain, and reflux are all more common in autistic people than in the general population.
Sensory issues and gut problems
One of the most obvious mechanisms connecting autistic traits to gut problems is food selectivity. Many autistic people choose foods based on sensory properties like texture, temperature, colour, and smell rather than on nutritional content.23 This is a sensory processing difference, not a preference or “picky eating”. But it can lead to restricted diets that alter the gut microbiota, which worsens GI symptoms, which makes eating even harder, which further restricts food choices.
It’s a feedback loop: autistic traits shape eating, eating shapes the microbiome, the microbiome shapes gut function, and gut function shapes what food feels tolerable.23
Unfortunately, at the time of writing, little research exists on autistic adults and nutrition. A recent review found only 43 studies across all databases examining food and nutrition in autistic adults.23 Most of what we know about the gut-brain axis in autism comes from research on children. The adult experience is largely unstudied. So science still has no answer to the question: What happens to the microbiome after decades of sensory-driven food choices and restricted diets?
The gut-brain connection is less of a focus for ADHD research, but it is still relevant. The gut produces precursors to dopamine, and gut inflammation affects how well nutrients are absorbed. Bifidobacterium levels in the gut have been linked to dopamine precursor synthesis and reward anticipation in the brain.11 The microbiome-immune-brain loop is increasingly recognised as a factor in attention and emotional regulation.24
And it is not just ADHD and Autism: a 2023 study found that children with Tourette’s also had gut microbial compositions different from matched healthy controls.34
What you eat matters, but so does whether your gut can absorb it and what your microbiome does with it.
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