What is already known on this topic
The gut microbiota has been linked to a series of brain conditions, including autism. Several studies have found that certain gut bacteria are more common in people with autism, suggesting that the gut microbiota may at least in part contribute to the condition. However, most studies have not been designed to address potential confounding factors.

What this research adds
Researchers studied stool samples from nearly 250 children aged 2 to 17, 99 of whom had been diagnosed with autism. After accounting for diet, age and sex, the researchers found limited evidence for a direct association of autism with the gut microbiota. However, they did find an association of autism with diet. Because children with autism tend to prefer a less-diverse, poorer-quality diet than non-autistic children, the findings suggest that the differences in gut microbial composition between autistic people and controls are due to the restricted dietary preferences associated with autism.

Conclusions
The findings highlight the relevance of often underappreciated confounders such as diet in studies involving autistic people. They also indicate that nutrition for children with autism is an important contributor to overall health.

Gut microbes have been linked to a series of brain conditions, including autism. But findings from a new study indicate that the differences in gut microbial composition seen in autistic people may be due to the restricted dietary preferences associated with autism, rather than being a potential contributor to autistic traits.

The findings, published in Cell, highlight often underappreciated confounders such as diet in studies involving autistic people. They also indicate that nutrition for children with autism is important for overall health, the researchers say.

“There’s a lot of interest surrounding the role of the gut microbiome in autism, but not a lot of hard evidence,” says study senior author Jacob Gratten at The University of Queensland in Brisbane, Australia. “Our study, which is the largest to date, was designed to overcome some of the limitations of prior work.”

Past studies have found that certain gut bacteria are more common in people with autism, suggesting that the gut microbiota may at least in part contribute to the condition. However, most studies have not been designed to address potential confounding factors.

Gratten and his colleagues analyzed stool samples from 247 children aged 2 to 17, 99 of whom had been diagnosed with autism. The children included in the analysis are participants of the Australian Autism Biobank and Queensland Twin Adolescent Brain Project.

Dietary preferences

Because the microbiota is affected by the environment, the researchers accounted for diet, as well as age and sex, in all their analyses. The team found limited evidence for a direct association of autism with the gut microbiota. However, they did find an association of autism with diet.

Children with autism tend to prefer a less-diverse, poorer-quality diet than non-autistic children, the researchers found. Because of this, the results suggest that the differences in gut microbial composition between autistic people and controls may be due to the restricted dietary preferences associated with autism.

“Taken together, the data support a strikingly simple and intuitive model, whereby autism-related traits promote restricted dietary preferences,” says study first author Chloe Yap

Replicating results

The findings are contrary to claims of the microbiota having a causal role in autism, the researchers say. “Instead, we find evidence that restricted dietary diversity and poorer quality—which is associated with specific [autism] features such as restrictive-repetitive behaviors—is a significant mediator of taxonomic diversity, and in turn, stool consistency,” they say. 

However, the team notes, the study has some limitations. One is that the researchers couldn’t rule out contributions of the microbiota prior to the autism diagnosis, nor the possibility that diet-related alterations of the gut microbiota could impact children’s behavior. 

Another limitation is that no comparable datasets are currently available to confirm the results. “We hope that our findings encourage others in the autism research community to routinely collect metadata in ‘omics’ studies to account for important, but often underappreciated, potential confounders such as diet,” Gratten says. The researchers plan to analyze a larger number of children to replicate their findings.