The imbalance makes the gut lining more permeable, and toxins, and even undigested foods, can get into the bloodstream and to the brain.
Many environmental factors can compromise the gut, from antibiotics to the health of the mother, and the length of time a baby was breastfed, said Dr Qinrui Li if Peking University in China, whose team reviewed more than 150 papers.
The conclusion is borne out by the experiences of many autistic children, who often complain of gut problems, such as diarrhea, constipation or flatulence.
But the cause also holds the key to a solution, he says. Restoring the gut microbiota can also go a long way to reversing the problem, and taking probiotics, prebiotics, or changing the diet to one that is gluten- and casein-free had a positive impact on symptoms.
Introducing these dietary changes improved sociability, reduced repetitive behaviour and enhanced social communication.
Dr Li described the findings as a "breakthrough" in autism treatment.