urolithins

Urolithins are a group of gut microbial metabolites derived from ellagitannins and the related compound ellagic acid, found in a variety of plant foods. They are most studied for their potential health effects and for reflecting how the gut microbiome transforms dietary polyphenols.

Formation and sources: Ellagitannins are hydrolyzed in the digestive tract to ellagic acid, which is then transformed by certain intestinal bacteria into a range of urolithins, most notably urolithin A and urolithin B. Some individuals’ microbiota can also produce other forms such as isourolithin A. Foods rich in ellagitannins include pomegranates, raspberries, strawberries, walnuts, and other berries; ellagitannins in these foods are the dietary precursors to urolithins.

Metabolism and circulating forms: After formation in the gut, urolithins are absorbed and predominantly present in the bloodstream as phase II conjugates (glucuronides and sulfates). They are excreted in urine and feces. The extent and pattern of urolithin production vary among individuals, influenced by the composition and activity of the gut microbiota.

Biological activity and evidence: In vitro and animal studies have suggested anti-inflammatory, anti-oxidant, and anti-cancer properties for urolithins, and urolithin A has been linked to improvements in mitochondrial function through promotion of mitophagy in aging models. Human data are limited and mixed; some small clinical studies report potential benefits in muscle function or metabolic markers, but results are not consistent. The health relevance of urolithins therefore depends on individual gut microbiota–mediated metabotypes that determine whether and how much urolithins are produced.

Research context: Urolithins continue to be studied for their role in mediating the effects of ellagitannin-rich diets, with ongoing research needed to clarify their health impacts, optimal intake, and the influence of microbiome diversity.