The role of the gut microbiome in autism has attracted significant attention, yet developmental neurobiologists now firmly state there is no scientific evidence to suggest it causes the neurodevelopmental condition. It has long been recognized that individuals with autism frequently experience higher rates of digestive issues, including irritable bowel syndrome and abdominal pain. While some studies have indicated a differing composition of gut bacteria in people with autism compared to those without the neurodivergence, leading to claims that dietary interventions improving gut flora could offer benefits, experts now deem this a flawed assumption based on inadequate research methodologies.

Dr. Kevin Mitchell from Trinity College Dublin explicitly stated, "Despite what you've heard, read or watched on Netflix, there is no evidence that the microbiome casually contributes to autism." Writing in the journal Neuron, he further advocated against allocating more time and funding to this area, emphasizing that autism is a strongly genetic condition with much remaining to be understood about its genetic underpinnings. The increasing rates of autism in recent decades are primarily attributed to greater awareness and a broader set of diagnostic criteria.

The consensus among experts against a causal link stems from a critical review of previous research. Authors of a recent opinion paper meticulously examined studies comparing gut bacteria in individuals with and without autism, along with mouse models and clinical trials, finding their results largely flawed and unconvincing. Professor Dorothy Bishop, an expert in developmental neuropsychology and senior author, highlighted that there is significant variability across these studies, preventing them from forming a coherent narrative. A major criticism, highlighted by Dr. Darren Dahly, an expert in nutritional epidemiology and study co-author, is the use of statistically inadequate sample sizes in even the most cited studies, which ranged from a mere seven to 43 participants, far below the thousands typically required for robust statistical analysis.

Moreover, when studies appropriately accounted for potential confounding variables like diet or genetics, any apparent differences in microbial diversity often vanished, strongly suggesting an absence of a direct causal relationship between the gut microbiome and autism. Dr. Mitchell even proposed a stronger case for a reverse causal effect, where having autism can influence an individual's diet, which subsequently impacts their microbiome. Eating problems are prevalent in people with autism, and challenges in social communication can lead to increased rates of depression and anxiety, both of which can affect gut health. Disordered eating could also result from dysfunctional coping mechanisms, which might influence the microbiome; however, researchers believe this is unlikely to cause a consistent difference given the diverse dietary preferences among individuals on the autism spectrum.

The hypothesis that autism is caused, at least partially, by the gut was also fueled by the idea that despite being a highly heritable neurodevelopmental condition, much of its heritability remains unexplained by identified gene mutations. This led some scientists to postulate an important environmental trigger, such as diets rich in ultra-processed foods disruptive to the gut microbiome, driving an "epidemic." However, based on the lack of convincing evidence and the stagnation of progress in this specific field, researchers conclude that the theory linking the gut as a cause for autism has reached a dead end. They encourage a cessation of work in this area but acknowledge that if research continues, it must adopt significantly more rigorous methodologies.

Autism is defined as a neurodevelopmental condition, not an illness or disease, meaning an individual's brain functions differently from others. It is something one is born with or that appears in early childhood, and it lasts a lifetime. Autism is not a medical condition with treatments or a "cure," though many people benefit from support to navigate specific challenges. Autistic individuals may find it difficult to communicate and interact with others, experience sensory sensitivities to things like bright lights or loud noises, struggle to understand others' thoughts or feelings, become anxious or upset in unfamiliar social situations, and engage in repetitive behaviors or thoughts.

The exact causes of autism remain unclear, but it is known to be a strongly genetic condition that can affect people in the same family. It is definitively not caused by bad parenting, vaccines (such as the MMR vaccine), diet, or infectious diseases. In the UK, approximately one in 100 people are thought to be on the spectrum, with experts suggesting genetics, birth complications, and air pollution as potential factors behind its rise. Additionally, the Children's Commissioner warned last year about significant delays in autism diagnoses within the NHS, where some children wait over four years, effectively "robbing" them of their childhoods.