Neurodivergence is defined as a difference in mental or neurological function from what is considered typical (Oxford English Dictionary, 2013). It encompasses a range of conditions that affect individual brain function and behavioural traits, including dyslexia, autism spectrum disorder (ASD), obsessive compulsive disorder (OCD) and attention-deficit hyperactivity disorder (ADHD).

Around 1% to 2% of the global population is estimated to have ASD (Maenner, 2025; Roman-Urrestarazu et al., 2021), 2% of the global population lives with OCD (Brock et al., 2024) and around 5% of all children and 3% to 4% of all adults live with ADHD (National Institute of Mental Health, 2024). However, the epidemiological estimates for both ASD and ADHD diagnosis have risen dramatically within the last 40 years (Yoo, 2013), sparking increased efforts into researching the underlying causes of these conditions. Thus far, evidence has suggested that both genetic factors, such as chromosomal abnormalities (Gillberg, 1998), and environmental factors, such as stress and diet (Matsuzaki et al., 2012), play a role. However, a definitive cause behind the development of ASD, ADHD and other neurodivergent conditions is still not yet fully understood.

Emerging research suggests a potential role for the gut-brain axis in the development of neurodevelopmental conditions (Hsiao et al., 2013). Patients with ASD, OCD and ADHD respectively have demonstrated an alteration in gut microbiome population compared to neurotypical controls (Checa-Ros et al., 2021; Grassi & Pampaloni, 2024; Yap et al., 2021). Recent evidence from a harmonised meta-analysis of adult ADHD cohorts also supports the presence of microbiome alterations in ADHD (Jakobi et al., 2024), which provides further evidence towards a potential link between gut microbial environments and neurodevelopmental differences. This theory has been researched further, particularly in respect to ASD, with a recent study using a valproic acid-induced autism mouse model finding that modulation of gut dysbiosis by faecal microbiota transplantation (FMT) from a healthy mouse seemed to improve ASD-related behaviours (Wang et al., 2023). In clinical settings, FMT has been investigated in autistic children primarily for the management of gastrointestinal (GI) symptoms, rather than to modify the symptoms of autism itself (Zhu et al., 2023).

In addition to the presence of a link between ASD diagnosis and altered gut microbiome population, there is also an association between ASD and the prevalence of GI symptoms (Leader et al., 2022). Specifically, those diagnosed with ASD are far more likely to experience symptoms such as constipation, diarrhoea and abdominal pain compared to neurotypical controls (Leader et al., 2022). This is also the case for patients diagnosed with OCD, with research consistently pointing towards the presence of gut microbiome population alterations and high rates of bowel syndromes (Grassi & Pampaloni, 2024). This association between gut dysbiosis and GI symptoms has been the target of extensive research.

The current dominant hypothesis proposes that the imbalance in gut bacterial communities leads to activation of the gut immune system, causing low-grade inflammation and subsequent GI symptoms, such as irritable bowel syndrome (IBS; Ohman & Simrén, 2010).

A recent systematic review and meta-analysis found that cohorts with ADHD are 1.25 times more likely to be diagnosed with intestinal disorders, such as irritable bowel syndrome, inflammatory bowel disease, celiac disease and peptic ulcers (Ng et al., 2025). This study represented a large study sample and encompassed a multi-national population, thus providing a highly generalisable body of evidence that further supports the hypothesis between altered gut microbiome and gastrointestinal issues.

See the full report/study from Journal of attention disorders here. https://journals.sagepub.com/doi/10.1177/10870547261469626