Indole-3-propionic acid (IPA, indole-3-propionate, or indole propionic acid), is a reductive product of tryptophan formed by bacteria in the gastrointestinal tract of mammals and birds (PMID:29168502). It is endogenously produced by human microbiota and has only been detected in vivo (PMID:19234110). While many microbial metabolites produced in the gut are toxic or act as uremic toxins (when they are reabsorbed through the gut epithelia), indole-3-propionic acid is a very beneficial microbial metabolite (PMID:30914514, 30862081, 29238104). In limited studies, urinary IPA correlates positively with disease and it remains unclear if this represents host bacteria responding to pathology via the production of IPA, or intestinal permeability changes leading to higher absorption and excretion of IPA, or inflammatory changes within kidneys leading to high excretion of IPA (PMID:32132996). Indole-3-propionic acid is a remarkably strong antioxidant (PMID:10721080). It is an even more potent scavenger of hydroxyl radicals than melatonin, the most potent scavenger of hydroxyl radicals synthesized by the human body. Similar to melatonin but unlike other antioxidants, indole-3-propionic acid scavenges radicals without subsequently generating reactive and pro-oxidant intermediate compounds (PMID:9928448, 10419516). Indole-3-propionic acid has been shown to prevent oxidative stress and the death of primary neurons and neuroblastoma cells exposed to the amyloid beta-protein in the form of amyloid fibrils, one of the most prominent neuropathologic features of Alzheimer's disease. 3-Indolepropionic acid also shows a strong level of neuroprotection in two other paradigms of oxidative stress. (PMID 10419516) More recently it has been found that higher indole-3-propionic acid levels in serum/plasma are associated with a reduced likelihood of type 2 diabetes and with higher levels of consumption of fibre-rich foods (PMID:28397877). Studies have shown that serum levels of indole-3-propionic acid are positively correlated with dietary fibre intake and negatively correlated with C-reactive protein levels (PMID:29795366). Indole-3-propionic acid is a marker for the presence of Clostridium sporogenes in the gut. Higher levels are associated with higher levels of Clostridium sporogenes (PMID:7378938). In addition to its useful physiological role in mammals, indole-3-propionic acid is a plant hormone with functions similar to indole-3-acetic acid (or IAA), the major plant auxin. Recent studies have shed some light on additional mechanisms of action of IPA. In the intestine, IPA could serve as a ligand to an adopted orphan nuclear receptor, Pregnane X receptor (PXR) and act as an anti-inflammatory agent (PMID:25065623). This property has allowed investigators to develop more potent analogs targeting PXR (PMID:32153125). Other tissues may also be targeted by IPA in a similar manner (PMID:31211619).