A large-scale scientific review has confirmed what the Internet has been shouting about and parents of kids with autism spectrum disorder (ASD) have been trying to manage for years: a link between the make-up of gut bacteria and autism. Until now, autism treatment has focused on rehabilitation, educational interventions and drugs to manage autism, but the review suggests that managing autism could be as simple as changing a kid’s diet.

In a review of more than 150 papers on ASD and gut bacteria, the author found that scientists have been reporting links between the composition of gut bacteria and autism since 1960. The review highlights many studies showing that restoring a healthy balance in gut flora can treat autistic behaviour.

“To date there are no effective therapies to treat this range of brain developmental disorders,” said Dr Qinrui Li of Peking University, China, the review’s lead author. “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.”

The link between the gut and ASD is well-known among sufferers: problems like diarrhoea, 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 of the papers reviewed support the idea of a gut-brain axis — a way in which factors in the gut can affect processes in the brain. The overgrowth of bad gut bacteria can lead to an overproduction of by-products — including toxins, which 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.

How the gut gets this way is where the review links the gut-brain axis back up with prior understanding of what causes autism.

“ASD is likely to be a result of both genetic and environmental factors,” Dr Li said. “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.”

The researchers found a significant body of evidence that reverting balancing gut bacteria can reduce autistic behaviour.

“Efforts to restore the gut microbiota to that of a healthy person has been shown to be really effective,” Dr Li said. “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.”

The positive impact being increased sociability, a reduction in repetitive behaviour, and improved social communication: all hugely beneficial to the life of a person with ASD.

The researchers are calling for new clinical trials to develop methods of balancing gut bacteria for autism treatment.

“We are encouraged by our findings, but there is no doubt that further work needs to be carried out in this field,” Dr Li said. “We need more well-designed and larger-scale studies to support our theory. 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.”