Urolithins, the rescue of "old" metabolites to understand a "new" concept: Metabotypes as a nexus among phenolic metabolism, microbiota dysbiosis, and host health status

Urolithins are dibenzo[b,d]pyran-6-one derivatives that are produced by the human gut microbiota from ellagitannins and ellagic acid (EA). These metabolites are much better absorbed than their precursors and have been suggested to be responsible for the health effects attributed to ellagitannins and EA that occur in food products as berries and nuts. In the present review, the role and potential of urolithins in human health are critically reviewed, and a perspective of the research approach needed to demonstrate these health effects is presented, based on the existing knowledge. The analytical methods available for urolithin analysis, their occurrence in different tissues and biological fluids, and their metabolism by human gut microbiota are considered. In addition, the interindividual variability observed for the production of urolithins (metabotypes) and its relationship with health status and dysbiosis are also reviewed. The potential mechanisms of action of urolithins are also critically discussed, paying attention to the concentration and the type of metabolites used in the in vitro and in vivo assays and the physiological significance of the results obtained. The gut microbiota metabolism of EA to urolithins and that of daidzein to equol, their individual variations, and the effects on health are also compared.