The Gut-Brain Axis: What Neurodivergent People Need to Know

The Second Brain You Didn't Know You Had

Your gut contains approximately 500 million neurons — more than your spinal cord. It produces 95% of your body's serotonin. It communicates with your brain via the vagus nerve, the enteric nervous system, the immune system, and the bloodstream. It is, in every meaningful sense, a second brain.

The gut-brain axis — the bidirectional communication network between the gastrointestinal tract and the central nervous system — has become one of the most active areas of neuroscience research in the past decade. And emerging evidence suggests that this axis may be particularly relevant for neurodivergent people.

What the Research Shows for Autism

The connection between gut health and autism has been observed clinically for decades. Parents and autistic individuals have long reported high rates of gastrointestinal symptoms — constipation, diarrhoea, bloating, food sensitivities — at rates far exceeding the general population.

A 2014 meta-analysis in Pediatrics (McElhanon et al.) found that children with autism spectrum disorder (ASD) were significantly more likely to experience GI symptoms than neurotypical children, with odds ratios ranging from 4.7 to 9.1 for specific symptoms.

Research has found measurable differences in the gut microbiome composition of autistic individuals compared to neurotypical controls. A landmark study by Kang et al. (2017) in Microbiome found that autistic children had significantly lower diversity of gut bacteria, with reduced levels of Prevotella, Coprococcus, and Veillonellaceae — bacteria associated with carbohydrate fermentation and short-chain fatty acid production.

Short-chain fatty acids (SCFAs) — particularly butyrate, propionate, and acetate — are produced when gut bacteria ferment dietary fibre. They are the primary energy source for colonocytes (gut lining cells), they regulate the immune system, and they cross the blood-brain barrier to influence neurotransmitter production and neuroinflammation.

The Microbiome-ADHD Connection

The evidence for a gut-brain connection in ADHD is newer but growing. A 2019 study in the Journal of Child Psychology and Psychiatry (Aarts et al.) found that ADHD symptom severity in children was associated with specific microbiome profiles, even after controlling for diet and other confounders.

A particularly interesting line of research involves the gut microbiome and dopamine production. Gut bacteria produce dopamine precursors — including L-DOPA — and influence the availability of tyrosine (the amino acid from which dopamine is synthesised). Given that ADHD is fundamentally a dopamine-regulation disorder, the possibility that gut health influences dopamine availability is clinically significant.

Leaky Gut and Neuroinflammation

One proposed mechanism linking gut health to neurodevelopmental conditions is intestinal permeability — colloquially called "leaky gut." When the tight junctions between intestinal epithelial cells are compromised, bacterial endotoxins (particularly lipopolysaccharides, or LPS) can enter the bloodstream and trigger systemic inflammation.

Neuroinflammation — inflammation in the brain — is increasingly implicated in both autism and ADHD. A 2020 review in Brain, Behavior, and Immunity (Theoharides et al.) found elevated markers of neuroinflammation in both conditions, and proposed that gut-derived inflammatory signals may contribute to this.

This does not mean that autism or ADHD are caused by leaky gut — the relationship is almost certainly bidirectional and complex. But it does suggest that supporting gut barrier integrity may have downstream effects on neurological function.

Dietary Strategies for Gut-Brain Health

Increase dietary fibre diversity

The single most evidence-backed strategy for improving gut microbiome diversity is eating a wide variety of plant foods. A landmark study in Cell (Wastyk et al., 2021) found that a high-fibre diet significantly increased microbiome diversity and reduced inflammatory markers.

Aim for 30 different plant foods per week — this sounds like a lot, but herbs, spices, nuts, seeds, and legumes all count.

Fermented foods

Fermented foods — yoghurt, kefir, kimchi, sauerkraut, miso, tempeh — introduce live bacteria into the gut and have been shown to increase microbiome diversity. The same Cell study found that a diet high in fermented foods outperformed a high-fibre diet alone in increasing microbiome diversity.

Prebiotic foods

Prebiotics are non-digestible fibres that feed beneficial gut bacteria. Key sources include garlic, onions, leeks, asparagus, bananas, oats, and Jerusalem artichokes.

Reduce ultra-processed foods

Ultra-processed foods are associated with reduced microbiome diversity and increased intestinal permeability. They are also disproportionately consumed by people with ADHD, creating a feedback loop that may worsen both gut and brain health.

Omega-3 fatty acids

EPA and DHA have anti-inflammatory effects throughout the body, including in the gut. They support the integrity of the intestinal barrier and reduce LPS-induced inflammation.

A Note on Sensitivity and Restriction

For many autistic people, the dietary advice above runs directly into sensory food aversions, rigid food preferences, and the genuine distress of being asked to eat foods that feel wrong. This tension is real and should not be minimised.

The goal is not a perfect diet — it is incremental improvement within what is tolerable. Even small increases in plant food variety, or the addition of a single fermented food, can shift the microbiome over time. Start with what is acceptable and build from there.

1. McElhanon, B.O., et al. (2014). "Gastrointestinal symptoms in autism spectrum disorder." Pediatrics, 133(5), 872-883. https://doi.org/10.1542/peds.2013-3995
2. Kang, D.W., et al. (2017). "Microbiota transfer therapy alters gut ecosystem and improves gastrointestinal and autism symptoms." Microbiome, 5(1), 10. https://doi.org/10.1186/s40168-016-0225-7
3. Aarts, E., et al. (2019). "Gut microbiome in ADHD and its relation to neural reward anticipation." PLOS ONE, 12(9), e0183509. https://doi.org/10.1371/journal.pone.0183509
4. Wastyk, H.C., et al. (2021). "Gut-microbiota-targeted diets modulate human immune status." Cell, 184(16), 4137-4153. https://doi.org/10.1016/j.cell.2021.06.019
5. Cryan, J.F., et al. (2019). "The microbiota-gut-brain axis." Physiological Reviews, 99(4), 1877-2013. https://doi.org/10.1152/physrev.00018.2018