I'm no doctor or biochemist but propionic acid seems way too natural and common a chemical to be a likely cause. It's not the first preservative I'd investigate either. For example, why not investigate benzoic acid?
I can't speak about benzoic acid, but the article notes that propionic acid is the most common compound produced by bacteria unique to the gut microbiomes of ASD children:
>Previous studies have demonstrated increased levels of PPA, a short chain fatty acid (SCFA), in the feces of children with ASD; we also know that the gut microbiome in these children is also quite distinct in terms of the type of bacteria that inhabit their intestines.
>Clostridia, Bacteriodetes, and Desulfovibrio bacteria are unique to patients with ASD. What’s interesting is that these bacteria are also known to be fermenters of carbohydrates that produce PPA, and other SCFAs as well. Ironically, while PPA is the most common compound produced by bacteria in ASD patients, it is also widely used in the food industry as a preservative due to its ability to inhibit grown of fungi (mold).
So following up I found an article on the etiological history of the PPA link [0]. From that article, there was a 2008 study of PPA injections in rats (spoiler, observed to cause ASD-like symptoms)[1], and a case study in 2012 of an ASD child with propionic acidemia, a rare metabolic condition that affects PPA metabolism [2], which led to more recent studies establishing a connection between behavioral disruptions and propionic acidemia, even in children who did not meet the diagnostic criteria for ASD [3].
ASD can also exhibit GI tract issues like constipation and IBS as symptoms, which would have supported some kind of gut microbiome hypothesis. Regarding what you're saying about how common PPA is, one research group speculates a "two-hit" mechanism behind ASD:
>Al-Owain’s group suggests that PA may thus act as one of the drivers of developing ASD under the two-hit model of ASD pathology, which posits that two separate factors must be present before ASD is developed. Under this model, one problem—such as PA in isolation—is insufficient to cause ASD, but when paired with another problem—such as another metabolic disorder affecting the gut-brain axis, like biotinidase deficiency—the combined detrimental effects of the two pathologies can cause ASD.
>Previous studies have demonstrated increased levels of PPA, a short chain fatty acid (SCFA), in the feces of children with ASD; we also know that the gut microbiome in these children is also quite distinct in terms of the type of bacteria that inhabit their intestines.
>Clostridia, Bacteriodetes, and Desulfovibrio bacteria are unique to patients with ASD. What’s interesting is that these bacteria are also known to be fermenters of carbohydrates that produce PPA, and other SCFAs as well. Ironically, while PPA is the most common compound produced by bacteria in ASD patients, it is also widely used in the food industry as a preservative due to its ability to inhibit grown of fungi (mold).
So following up I found an article on the etiological history of the PPA link [0]. From that article, there was a 2008 study of PPA injections in rats (spoiler, observed to cause ASD-like symptoms)[1], and a case study in 2012 of an ASD child with propionic acidemia, a rare metabolic condition that affects PPA metabolism [2], which led to more recent studies establishing a connection between behavioral disruptions and propionic acidemia, even in children who did not meet the diagnostic criteria for ASD [3].
ASD can also exhibit GI tract issues like constipation and IBS as symptoms, which would have supported some kind of gut microbiome hypothesis. Regarding what you're saying about how common PPA is, one research group speculates a "two-hit" mechanism behind ASD:
>Al-Owain’s group suggests that PA may thus act as one of the drivers of developing ASD under the two-hit model of ASD pathology, which posits that two separate factors must be present before ASD is developed. Under this model, one problem—such as PA in isolation—is insufficient to cause ASD, but when paired with another problem—such as another metabolic disorder affecting the gut-brain axis, like biotinidase deficiency—the combined detrimental effects of the two pathologies can cause ASD.
[0]: https://www.foundationalmedicinereview.com/blog/the-link-bet...
[1]: https://www.ncbi.nlm.nih.gov/pubmed/18395759
[2]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3573175/pdf/978...
[3]: https://www.sciencedirect.com/science/article/pii/S109671921...