AHR and fatty liver disease: Although the bacterial species are potentially harmful, the tryptophan metabolite IAA has been shown to bind AhR, which, through binding, sets up an anti-inflammatory signal cascade, for example, to alleviate hepatic steatosis (Xu et al. 2021) and to reduce fatty acid and LPS production of pro-inflammatory cytokines in macrophages (Krishnan et al. 2018).