Autism spectrum disorder (ASD) could be caused by a lack of "detoxifying enzymes" in the gut.
Toxins from the environment which would normally get cleaned up instead leak into the bloodstream, eventually reaching the brain and damaging mitochondrial DNA, new research suggests.
"Growing evidence suggests that autism spectrum disorder (ASD) is strongly associated with dysbiosis in the gut microbiome, with the exact mechanisms still unclear," a new paper published in journal Science Advances says.
Previous research had detected an overabundance of some microbes - such as the bacteria clostridium, and a lack of others in children affected by autism. But it's been hard to say this was behind the developmental disorder, since individuals' gut biomes vary so wildly.
The latest study involved dozens of children aged three to eight with and without autism, chosen because they would normally be expected to have similar gut biomes based on where they lived and other factors.
Looking at hundreds of different microbes in their stool, 18 of them standing out as being under- or over-represented in children with autism. Notably, the researchers found "a previously unidentified ASD-associated deficiency in microbial detoxification".
"When the intestinal microbial detoxification is severely impaired in ASD, more toxicants of external and internal origins might enter circulation and injure the mitochondria of various tissues."
Damaged mitochondrial DNA can cause a "low-grade inflammatory response" in the brain, a "well-documented" phenomenon amongst ASD patients.
Just how some kids end up with a lack of detoxifying microbes isn't clear, but the researchers say "various genetic and environmental factors such as altered diet and defects in the digestive system, which change the nutrients provided to the microbial inhabitants" could be to blame.
"Toxicants implicated in ASD include organochlorine pesticides," including glyphosate - commonly known as Roundup - "polycyclic aromatic hydrocarbons, automotive exhaust, and heavy metals."
These chemicals are hard to detect in the body though - even if the body lacks the microbes to clean them up, they "often rapidly react with surrounding biomolecules, making it difficult to detect these compounds in frozen samples".
"Second, the amount of toxicant exposure often fluctuates, and toxicants are possibly undetectable in feces once absorbed. However, the chronic impairment that they cause may last for a long time."
Though more research is needed, the scientists say their findings "suggest that it might be possible to create a therapy that assists in the detoxification process, thereby heading off the onset of ASD - or better yet, to overcome the elements that lead to detoxification problems in the first place".
The number of children being diagnosed on the autism spectrum has been increasing in recent years, though no cause has been found. Most of the rise is believed to be down to better understanding of the condition and widening diagnostic criteria, but it's not entirely clear.
No reputable evidence has ever been found for a link between vaccines and autism, as some have claimed.