Content warning: difficult topics. This might get uncomfortable. Remember you always have the option to stop reading if it gets overwhelming.
Maybe this could go under a section for common co-occurring conditions? along with apraxia, EDS, etc
Autism and gastrointestinal problems have been linked to each other in multiple studies. In this article, we aim to give a short overview of their findings.
Nobody knows if gastrointestinal problems are a symptom of autism, a common co-morbidity of autism, a consequence of autism, or even a cause of autism. The ongoing research is at least in part aimed at identifying how exactly autism and gastrointestinal problems are related. Note that at least some of the research that is done into autism has questionable ethics. This article is not meant to discuss those ethics. Please read the page on ethics of autism research to learn more. Please also note that some readers might be triggered by reading about ethically questionable research, and should take caution when reading this article.
To start things off, here are some claims with references. We should turn them into a proper "story" at some point.
- A study from 2013 compared biopsies taken from the guts from children diagnosed with [ASD] and gastrointestinal problems, to biopsies taken from patients with Crohn's disease, ulcerative colitis, or no gastrointestinal problems ('histologically normal'). The aim of the study was to find out if there are genetic commonalities between the patients with Crohn's disease or ulcerative colitis, and the children with ASD. The study found that, while there was a significant overlap in the gene expressions of these groups, the ASD group also had distinctive features, which the authors of the study interpreted as evidence for the presence of an ASD-associated variant of inflammatory bowel disease (IBD).
- A study from 2014 found that children with ASD were at least three times more likely to experience frequent gastrointestinal symptoms than children with a "typical development". The study specifies these gastrointestinal problems as: abdominal pain, pain on stooling, constipation, gaseousness/bloating, diarrhoea, sensitivity to foods, as well as vomiting and difficulty swallowing. Each of these symptoms was very rare in the typical development control group. The most commonly reported gastrointestinal symptom in both groups was "sensitivity to food". In the group of children diagnosed with ASD, 31% reported having sensitivity to food "frequently" or "always" in the last three months, compared to 4.5% in the typical development control group. The second-most common symptom was "constipation", which was 15.5% of ASD children experienced "frequently" or "always" in the last three months, compared to 3.5% in the control group of typical development children. For some less common symptoms, children diagnosed with ASD were 6 (bloating) or even 8 (diarrhoea) times more likely to experience them than the children in the control group were. 
- A 2018 study investigated the relationship between autism, the immune system, gastrointestinal problems and gut bacteria. 
As a teenager, I was diagnosed with irritable bowel syndrome (IBS). I found that cutting diary from my diet helped a lot. I also found that cutting other foods that are known for causing irritation to the bowels, helped calm things down. I am now careful around cabbage, certain beans, all alliums, and chilli. However, stress, particularly stress caused by having to participate in too many social situations, increases my bowel problems. Stress will cause bloating, diarrhoea, nausea, vomiting and abdominal pain.
Please add more.
- ↑ Walker S.J., Fortunato J., Gonzalez L.G., Krigsman A. Identification of unique gene expression profile in children with regressive autism spectrum disorder (ASD) and ileocolitis. PLoS One. 8 Mar 2013. doi: 10.1371/journal.pone.0058058.
- ↑ Chaidez V., Hansen R.L., Hertz-Picciotto I. Gastrointestinal problems in children with autism, developmental delays or typical development. J. Autism Dev. Disord. May 2014. doi: 10.1007/s10803-013-1973-x.
- ↑ Rose D.R., Yang H., Serena G., Sturgeon C., Ma B., Careaga M., Hughes H.K., Angkustsiri K., Rose M., Hertz-Picciotto I., Van de Water J., Hansen R.L., Ravel J., Fasano A., Ashwood P. Differential immune responses and microbiota profiles in children with autism spectrum disorders and co-morbid gastrointestinal symptoms. Brain, Behavior, and Immunity, Volume 70, pages 354-368. May 2018. doi: doi.org/10.1016/j.bbi.2018.03.025.