The link between the gut microbiome and autism not backed by science, experts say

The role of the gut in autism continues to attract widespread attention, but experts now say there is no scientific evidence that the gut microbiome causes the neurodevelopmental condition. 

It’s long been known that people with autism have higher rates of digestive issues—including irritable bowel syndrome and abdominal pain. 

Some studies have even shown they have a different make-up of gut bacteria to people without the neurodivergence, with researchers claiming that improving their gut flora through dietary interventions could offer benefits. 

But today, developmental neurobiologists said this is a flawed assumption, based on small sample sizes and inappropriate study designs. 

‘Despite what you’ve heard, read or watched on Netflix, there is no evidence that the microbiome casually contributes to autism,’ said Dr Kevin Mitchell from Trinity College Dublin. 

Writing in the journal Neuron, he added: ‘I don’t think it’s warranted to spend further time and funding on this topic. 

‘We know that autism is a strongly genetic condition, and there’s still loads to be worked out there.’ 

Rates of autism have soared in recent decades, which researchers put down to greater awareness and a broadening of the criteria used to diagnose the condition. 

In the UK, around one in 100 people are thought to be on the spectrum with experts suggesting that genetics, birth complications and air pollution could be behind the rise.

Fecal transplants and other treatments to boost gut diversity, including pre- and probiotics, have recently garnered attention as a potential treatment for autism among other conditions, reigniting the debate over whether it is even a treatable condition. 

The hypothesis that autism is caused, at least in part, by the gut is also being driven by the idea that it is one of the most heritable neurodevelopmental conditions yet much of its heritability remains unaccounted for by gene mutations that have been highlighted thus far.

Therefore, some scientists have concluded, there must be an important environmental trigger driving the ‘epidemic’—such as diets rich in ultra-processed foods packed with industrial chemicals that disrupt the gut microbiome. 

Looking for answers, the authors of this most recent opinion paper studied previous research which compared the gut bacteria of people with and without autism, studied mouse models, and conducted clinical trials.

The researchers found  the results from all of these studies to be largely flawed and unconvincing. 

Professor Dorothy Bishop, an expert in developmental neuropsychology and senior author, explained: ‘There’s variability in all three of those areas, and the studies don’t form a coherent story at all.’   

Even in the most highly cited studies, which compared the gut microbiomes of people with and without autism, researchers used sample sizes ranging from just seven to 43 participants, way below statistical guidelines which call for sample sizes in the thousands. 

Commenting on this inadequacy, Dr Darren Dahly, an expert in nutritional epidemiology and study co-author, said: ‘Autism is not rare, so there’s no reason to be having studies with only 20, 30 or 40 participants.’ 

Often, when studies accounted for potentially confounding variables such as diet or genetics, the apparent difference in microbial diversity also disappeared, suggesting there is no causal relationship between gut microbiome and autism. 

Dr Mitchell added: ‘If anything, there is stronger evidence for a reverse causal effect, in that having autism can affect someone’s diet, which can affect their microbiome.’ 

Eating problems are common in people with autism. Children with autism often experience difficulties with social communication, which researchers say could contribute to higher rates of depression and anxiety which can have an impact on the gut. 

Disordered eating might also result from dysfunctional methods of coping, which could affect their microbiome—though, according to the researchers, this is unlikely to cause a consistent difference in the gut microbiome, given that dietary preferences in people with the spectrum disorder are so diverse. 

Based on the lack of convincing evidence, and the lack of progress in the field, the researchers conclude that the theory that the gut causes autism has reached a dead end. 

They added: ‘If you accept our message, there’s two ways you can go. One is just stop working on this areas, which is something we would be quite happy to see. 

‘But given that realistically, people are not going to stop, they need to at least start doing these studies in a much more rigorous way.’ 

It comes as the Children’s Commissioner last year warned that kids forced to wait years for autism diagnoses on the NHS are being ‘robbed’ of their childhoods.

Data, accessed by the independent watchdog, shows one in six children seeking an autism diagnosis waited more than four years via the NHS’s community services.

Autism is not a disease and people have it from the moment they are born, although it may not be spotted until childhood and sometimes much later.

Autism exists on a spectrum. Some people will be able to lead fully functioning lives with no additional help. Others may need full-time assistance.