Indoxyl sulfate—a bacterially derived metabolite—has been identified as a toxin that is elevated in children with autism spectrum disorder (ASD). As a neurotoxin, uremic toxin, nephrotoxin, cardiotoxin, osteotoxin, and myotoxin, indoxyl sulfate has been associated with several other conditions, including chronic kidney disease, acute kidney injury, Parkinson’s disease, cognitive disorders, and mood disorders such as anxiety and depression. Indoxyl sulfate is derived from bacterial modification of host tryptophan, and elevated levels of indoxyl sulfate are associated with decreased levels of important neurotransmitters including serotonin, dopamine, and norepinephrine. This article will review what is currently known about indoxyl sulfate in relation to ASD and its comorbidities. A systematic review identified six studies of levels of indoxyl sulfate in children with ASD. All six studies found that indoxyl sulfate was significantly elevated in the urine of children with ASD compared to typically developing children. Through this review, indoxyl sulfate was identified as a toxic microbially derived metabolite that is significantly increased in a subset of children with ASD and may contribute to both core and co-morbid ASD symptoms.

Autism spectrum disorder (ASD) is a common condition with substantial personal and financial burdens of lifelong implication. Multiple twin studies have confirmed a genetic or inherited component at ~80%, higher than any other common condition. However, ASD’s rapidly‐accelerating prevalence, now at 1 in 31 in the USA, appears to defy a predominantly genetic basis and implements our rapidly‐changing environment. A potential explanation of this paradox is a recent increase in de novo variants (DNVs), which are “new” mutations present in the patient yet absent in both parents. The present authors recently reported using trio whole genome sequencing (WGS) that DNVs highly likely to be highly disease associated (“Principal Diagnostic Variants”, PDVs), mostly missense variants, are present in (25/50) 50% of the ASD patients clinically evaluated by our team. The current study was designed to support this observation with trio‐WGS in 100 additional unrelated ASD patients. De novo PDVs were identified in 47/100 (47%) of cases, in close approximation to our previous work. Using non‐transcribed (up and downstream) variants for all genes as a control group, these DNV‐PDVs were far more likely (P<0.0001, OR 5.8, 95% C.I. 2.9‐11) to be in SFARI‐listed genes associated with ASD. Consistent with the emerging polygenic model, using the same analyses, inherited missense variants are also associated with ASD (P<0.0001). Highly unexpectedly, silent variants, both inherited (P<0.0001) and de novo (P<0.007), were also statistically associated with ASD, and among inherited variants, silent variants are more associated with ASD than are missense variants (P<0.0001). Adding de novo silent DNVs as PDVs increases the proportion of our subjects with at least one DNV‐PDV to 55% of the subjects. Our proposed model for ASD, with prominent DNVs in most, that are genetic yet not inherited, predicts the known predominate genetic pathogenesis and the accelerating prevalence of ASD, presumably from chemical mutagenesis.