The quote heading this post - "A test that distinguishes ASD [autism spectrum disorder] fast response constipation from ASD persistent right-sided constipation" - comes from the findings reported by Stephen Walker and colleagues . The Walker results continue a theme from this research group (see here) whereby some important data is being generated on how to treat some fairly prevalent bowel issues that seem to accompany quite a few diagnoses of autism (see here).
Much like other research from this authorship group, the research material examined was biopsy tissue - "ascending colon biopsy tissues" - provided by 35 children diagnosed with an autism spectrum disorder "and chronic constipation on a background of enterocolitis." I know some people don't like the word 'enterocolitis' in the context of autism (see here) but prejudices aside, there is nothing in the current research literature to suggest that a diagnosis of autism is somehow protective against the development of inflammatory bowel disease and/or its symptoms. Nothing.
Anyhow, 20 of those 35 children were categorised as 'slow responders' on the basis of showing "recurrent right-sided fecal loading requiring regular colon cleanouts during treatment for enterocolitis" and 15 were defined as 'fast responders' as a function of experiencing "a sustained state of GI [gastrointestinal] symptomatic remission while on maintenance anti-inflammatory therapy." In effect the group was divided up into those whose bowel symptoms got better (n=15) and those whose bowel symptoms did not (even after multiple attempts) (n=20). Researchers analysed those biopsy samples with the expression of genes in mind as per other research occasions .
Results: "Significant differences were found between the two clusters with fast responder-predominant cluster showing an upregulation of transcripts involved in the activation of immune and inflammatory response and the slow responder-predominant cluster showing significant over-representation of pathways impacting colonic motility (e.g. genes involved in tryptophan and serotonin degradation and mitochondrial dysfunction)." Apologies for the long quote taken from the Walker paper, but they said it better than I ever could. The translation: gene expression data was different between the fast and slow responder groups.
Obviously more research is needed in this area with larger participant groups and perhaps using samples from other non-autism groups who present (or don't) with various types of bowel issues, whether sensitive to treatment or not. The cluster of genes that were used in the authors' modelling did all right when it came to talk of possible 'biomakers' - "The sensitivity (sensitivity = 0.88), specificity (specificity = 0.89), and kappa (kappa = 0.77) statistics all reflect a good strength of agreement between prediction and actual assignments" - but still need more work before any big claims are made.
There are a couple of other things to mention from the Walker results. So, results suggested that: "predominantly chronic constipation in fast responders is not only related to the inflammatory status of the right colon but is likely a direct consequence of this colonic inflammation." Inflammation perhaps equalling constipation? Interesting. And it not only offers lots more avenues for further study but also some important treatment options.
Next, the amino acid tryptophan was singled out as being potentially "especially significant." I've always been interested in the aromatic amino acids in relation to some autism (see here). Tryptophan is a particularly important aromatic amino acid because it's eventually metabolised into a whole slew of important compounds from serotonin (5-HT) to melatonin and beyond, with some interesting connections to autism (see here). Walker and colleagues mention how: "In the slow responder cluster of patients, there was a significant upregulation of transcripts in each of the metabolic degradation pathways for tryptophan, serotonin, and melatonin, suggesting that TRP [tryptophan] insufficiency (and therefore 5-HT insufficiency) may be an important factor in the sustained hypomotility seen in this patient cohort." There's some much more study that one could do in this area. Particularly when 'gut hypomotility' is a potential issue for quite a few people on the autism spectrum (see here).
There are other things to consider from the Walker paper - "A third relevant theme apparent from the slow response gene expression profile involves a number of pathways that converge in the mitochondria and impact mitochondrial function" - but I'll leave that for now (see here). Suffice to say that there is enough evidence emerging in the peer-reviewed domain to say that (a) pathological bowel problems are more than present alongside a diagnosis of autism, (b) said bowel issues also overlap with functional GI symptoms such as constipation in particular, (c) there are physiological reasons for such bowel issues outside of any psychobabble explanations, and (d) lots more research is required in this area without fear or favour pertinent to improving quality of life...
 Walker SJ. et al. A molecular biomarker for prediction of clinical outcome in children with ASD, constipation, and intestinal inflammation. Sci Rep. 2019 Apr 12;9(1):5987.
 Walker SJ. et al. A Putative Blood-Based Biomarker for Autism Spectrum Disorder-Associated Ileocolitis. Sci Rep. 2016 Oct 21;6:35820.