Autism may be helped by changing a patient's diet, new research reveals.
Taking probiotics and adopting a gluten-free lifestyle may improve sufferers' social behaviour and ability to express emotions, a study review found.
This is thought to be due to gluten causing a "leaky gut" - where toxins and even undigested food enter the bloodstream and travel to the brain, which may cause autism symptoms.
Probiotics are thought to reverse this by strengthening the gut's lining.
Study author Dr Qinrui Li from Peking University, said: "Efforts to restore the gut microbiota to that of a healthy person has been shown to be really effective.
"Our review looked at taking probiotics, prebiotics, changing the diet - for example, to gluten- and casein-free diets. All had a positive impact on symptoms."
HOW STUDY WAS CARRIED OUT
Researchers from Peking University analysed more than 150 papers dating back to the 1960s.
The studies investigated the impact of taking probiotics or prebiotics on autism symptoms.
Prebiotics are indigestible food ingredients, such as fibre, that act as fertilisers to stimulate the growth of bugs in the digestive tract. Probiotics specifically introduce new bacteria into the gut.
The studies also assessed the impact of gluten and casein-free diets. Casein is a type of protein found in milk.
KEY FINDINGS
Results revealed that improving the gut's bacterial health via dietary choices eases autism symptoms.
Supplementing autistic sufferers' diets with probiotics or prebiotics may help to strengthen activity in areas of the brain associated with emotion, the research revealed.
A gluten and casein-free diet was also found to improve symptoms and social behaviour.
The findings were published in the journal Frontiers in Cellular Neuroscience.
WHY IS A HEALTHY GUT IMPORTANT?
A healthy gut is thought to reduce autism symptoms via the so-called 'gut-brain axis' - where conditions in the gut influence the brain's processes.
The overgrowth of 'bad' bacteria in the gut leads to the production of toxins, which makes the gut lining more permeable.
This allows the toxins and even undigested food to enter the bloodstream and travel up to the brain, which may cause autism in young children.
Gluten and casein have previously been linked to 'leaky gut syndrome', where the permeability of the small intestine is increased.
Probiotics and prebiotics are thought to reverse this by strengthening the gut's lining.
Dr Li said: "Efforts to restore the gut microbiota to that of a healthy person has been shown to be really effective.
"Our review looked at taking probiotics, prebiotics, changing the diet - for example, to gluten- and casein-free diets.
"All had a positive impact on symptoms."
HOW MAY THE FINDINGS BENEFIT PATIENTS
Dr Li said: "To date there are no effective therapies to treat this range of brain developmental disorders.
"The number of people being diagnosed with ASD [autism spectrum disorder] 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."
Although simple dietary changes may benefit autism sufferers, the researchers add more research is required.
Dr Li said: "For now, behavioural therapies remain the best way to treat ASD.
"We would hope that our review leads to research on the link between the gut microbiota and ASD, and eventually a cheap and effective treatment."