Why the gut keeps coming up in ADHD

If you or your child has ADHD, chances are most of the focus so far has been on the brain. Medications, strategies, routines, sensory tools. And all of that absolutely has its place. But there’s another system worth paying serious attention to, and it lives a lot further south.

The gut.

Over the last decade, research into ADHD gut health has grown enormously. What’s emerging is genuinely fascinating: the gut communicates constantly with the brain, produces a significant portion of the body’s neurotransmitters, and houses an immune system that can either support or undermine how we think, focus and feel. When the gut is struggling, the brain often is too.

In clinic, I see this pattern regularly. Kids and adults with ADHD who also deal with constipation, bloating, food sensitivities, or a history of frequent antibiotic use. It’s rarely a coincidence.

The gut-brain axis: a two-way conversation

The gut-brain axis is a sophisticated communication network linking the digestive system and the central nervous system. It works through the vagus nerve, the enteric nervous system (the gut’s own independent nerve network), immune signalling, and hormonal pathways.

The vagus nerve is the superhighway of this system. Here’s what surprises most people: roughly 80 to 90 percent of the signals travelling along it go from the gut up to the brain, not the other way around. The gut is quite literally briefing the brain all day long.

For people with ADHD, vagal tone — how efficiently the vagus nerve functions — is an area of growing research interest. Poor vagal tone has been associated with difficulty regulating the nervous system, increased impulsivity, emotional dysregulation, and reduced attentional control.

The enteric nervous system adds another layer. This network of over 500 million neurons lining the gut wall operates largely independently of the brain. When its environment is disrupted, the signals it sends upstream change accordingly.

The microbiome and ADHD: what your gut bacteria are doing

The gut microbiome — the trillions of bacteria and other microorganisms living in the digestive tract — has an outsized influence on brain function. Here’s how:

Leaky gut, neuroinflammation, and ADHD

The gut lining is designed to be selectively permeable — allowing nutrients through while keeping bacteria, toxins, and larger molecules out. When that lining becomes compromised (leaky gut), things get through that shouldn’t.

This triggers an immune response. Inflammatory signalling compounds called cytokines increase in circulation, and inflammation begins to affect the brain as well as the gut. This state of neuroinflammation is increasingly understood as a meaningful contributing factor in ADHD symptoms.

The current model gaining traction in research: gut dysbiosis leads to increased intestinal permeability → systemic inflammation → impaired blood-brain barrier function → neurological environment associated with worsening of symptoms associated with ADHD. It’s not the whole picture, but it’s a significant part of it.

Microbiome testing: actually seeing what’s going on

One of the most valuable things we can do when ADHD and gut symptoms overlap is to look at what’s actually happening in the microbiome — rather than applying a generic protocol and hoping for the best.

This is where I find gut microbiome testing genuinely useful. I currently use Microba, a gut testing service that uses deep shotgun metagenomic sequencing — a significantly more detailed method than standard testing. Rather than just identifying which bacterial families are present, it can identify bacteria at the species and strain level, measure functional capacity, and assess markers like SCFA-producing potential and inflammation-related species and metabolites.

Testing is done at home with a stool sample and returned to the lab via post — straightforward for both adults and kids. The results are detailed but translated into accessible language, which I find really useful for explaining findings to patients.

This information takes the guesswork out of gut support. Rather than applying a one-size-fits-all approach, we can see exactly where the gaps are and tailor support accordingly — specific probiotic strains, prebiotic foods, dietary shifts, and targeted herbal or nutritional support where indicated.

It currently costs ~$400-$500 depending on which level of test you choose which is not feasible for some people, but certainly gut health work can still be done without this, just not with the same level of fine tuning.

Common questions about ADHD and gut health