Taking probiotics or changing the diet may help treat autism, suggest scientists who have found that altering the make-up of gut bacteria can reduce the symptoms of the disorder.
Until now, caregivers have relied on rehabilitation, educational interventions and drugs to reduce Autism Spectrum Disorder (ASD) symptoms.
A review of more than 150 papers on ASD and gut bacteria found that since the 1960s, scientists have been reporting links between the composition of bacteria in the gut and autistic behaviour.
The research highlights many studies showing that restoring a healthy balance in gut bacteria can treat ASD symptoms.
"To date there are no effective therapies to treat this range of brain developmental disorders," said Qinrui Li of Peking University in China.
"The number of people being diagnosed with ASD is on the rise. As well as being an expensive condition to manage, ASD has a huge emotional and social cost on families of sufferers," said Li.
The link between the gut and ASD is well-known among sufferers: problems like diarrhea, constipation and flatulence are commonly reported.
The root of gastro-intestinal problems like these is an imbalance of "good" and "bad" bacteria in the gut.
Many studies support the idea of a gut-brain axis - a way in which factors in the gut can affect processes in the brain.
These gastro-intestinal problems may have a more sinister side. The overgrowth of bad bacteria in the gut inevitably leads to an overproduction of by-products - including toxins.
These can make the gut lining more permeable.
Then toxins, by-products and even undigested food can get into the bloodstream and travel to the brain.
In a child under three years old, whose brain is at the height of development, the presence of these chemicals can impair neuro-development, leading to ASD.
"ASD is likely to be a result of both genetic and environmental factors" said Li.
"The environmental factors include the overuse of antibiotics in babies, maternal obesity and diabetes during pregnancy, how a baby is delivered and how long it is breastfed. All of these can affect the balance of bacteria in an infant's gut, so are risk factors for ASD," he said.
However, the researchers found a significant body of evidence that reverting the gut microbiota to a healthy state can reduce ASD symptoms.
"Efforts to restore the gut microbiota has been shown to be really effective," said Li.
"Our review looked at taking probiotics, prebiotics, changing the diet - for example, to gluten- and casein-free diets, and faecal matter transplants. All had a positive impact on symptoms," he said.