Does the gut microbiome ultimately play a role in the manifestation of autism spectrum disorder? Despite the great research interest, experts emphasize that there is no scientific evidence to confirm that it causes the neurodevelopmental disorder.

It has been known for years that people on the autism spectrum have higher rates of gastrointestinal problems, such as irritable bowel syndrome and abdominal pain. Several studies have shown that the microbiome of these individuals differs from that of people without autism, leading some researchers to hypothesize that improving gut flora through dietary interventions may offer benefits.

However, according to developmental neurobiologists, this view is based on faulty assumptions, small samples, and inappropriate study designs.

“Despite what you may have heard, read or seen, there is no evidence that the microbiome contributes causally to autism,” says Dr. Kevin Mitchell of Trinity College Dublin. In the study published in Neuron, which he led, he also notes: “I don’t think it’s justified to invest more time and money into this issue. We know that autism has a strong genetic basis and there is still a lot to understand there.”

Autism rates have risen sharply in recent decades. But experts attribute the increase not to a real increase in cases, but to better information and broader diagnostic criteria. Experts say genetics, birth complications and air pollution may be responsible for the increase.

At the same time, interventions aimed at increasing “gut diversity,” from fecal transplants to prebiotics and probiotics, have been touted as potential treatments for autism, reigniting the debate over whether autism is curable.

The idea is based on the fact that autism spectrum disorder is one of the most heritable neurodevelopmental conditions. However, much of its heritability remains unexplained by the gene mutations that have been identified so far.

Thus, some researchers have hypothesized that there may be an environmental factor causing the "epidemic", such as a diet rich in ultra-processed foods with chemical additives that affect the microbiome.

In search of answers, the authors of the latest opinion study reviewed previous research comparing the gut bacteria of people with and without autism, studied mouse models, and conducted clinical trials.

They concluded that the results of all these studies were weak, disjointed, and often methodologically problematic.

Professor Dorothy Bishop, an expert in developmental neuropsychology, points out: “There is a great deal of variability across all three categories of studies and the results do not present any coherent picture.”

Even the most popular studies had very small samples – for example, from 7 to 43 participants – far below the statistical standards that require thousands of people.

In many studies, when factors such as diet or genetic differences were adjusted for, the apparent changes in microbial diversity disappeared, suggesting there was no causal relationship. Dr. Mitchell added: “If there is a relationship, it is probably the other way around: autism may affect diet, and that may affect the microbiome.”

Eating problems are common among individuals on the spectrum, while social communication difficulties, anxiety, and depression can also affect digestive function. Disorganized eating behavior, while likely to affect the microbiome, is unlikely to create consistent differences between individuals, given the wide variety of eating habits on the spectrum.

cONcluSiON

Based on all the data, the researchers conclude that the theory that the gut plays a crucial role in the etiology of autism spectrum disorders is now at an impasse.

“If you accept our message, there are two ways,” they say. “One is to stop research in this area — which would satisfy us.
The other, more realistic, is to finally start conducting studies with a much more rigorous and proper design."