Indole and I3A are agonists for the aryl hydrocarbon receptor (AhR), a transcription factor that regulates interleukin (IL)-22 expression, increases TH17-cell activity, and helps maintain intraepithelial lymphocytes. Indole upregulates the expression of tight junction proteins and modulates the expressions of pro- and anti-inflammatory genes in intestinal epithelial cells. These activities of AhR help ensure that commensal bacteria outcompete pathogenic bacteria in the gut microbiota. Recent studies show that indole also modulates GLP-1 release from L cells, so that indole formation may contribute to satiety and inhibition of obesity. IPA also potently scavenges hydroxyl radicals, thereby protecting against oxidative injury in various animal models. The oral charcoal adsorbent AST-120 binds indoles in the gut lumen and reduces plasma IndS levels, thereby reducing kidney damage and atherosclerosis associated with kidney injury.