Urolithin A vs. C and D — What's the Difference?

If you've searched online for information about urolithins, you've probably run into the names urolithin A, urolithin B, urolithin C, and urolithin D — sometimes used interchangeably, as if they were just version numbers of the same ingredient. That's a misleading simplification. Urolithins are a family of distinct chemical compounds, formed in the colon from the same plant precursors (ellagitannins and ellagic acid, found in pomegranates, walnuts, raspberries, and strawberries), but differing in chemical structure and — most importantly for consumers — in a vastly different level of actual research behind them.

Urolithin A is today the only member of this family with published, randomized clinical trials in humans, including trials run on a specific, patented supplement form (Mitopure, by the company Amazentis). Urolithins C and D remain at a completely different stage — their descriptions in the scientific literature are currently limited to studies on cell lines, isolated tissue, and animal models. This isn't a subtle difference in strength of evidence — it's the difference between 'studied in humans under controlled conditions' and 'observed in a test tube.'

This article isn't an encyclopedia entry about one specific urolithin — that's covered in our separate knowledge-base entries on urolithin C and urolithin D. Instead, it's a guide to why this distinction matters practically, whenever you're standing in front of a supplement shelf or an ad promising 'mitochondrial support through urolithins' without clearly specifying which molecule it's actually talking about.

None of the urolithins are present directly in food. Pomegranates, walnuts, and berries contain ellagitannins and ellagic acid — large, poorly absorbed compounds that reach the colon largely unchanged. There, gut bacteria, mainly from the genera Gordonibacter and Ellagibacter, carry out a multi-step breakdown of these compounds, whose intermediate and final products are precisely the individual urolithins — A, B, C, D, and a few less commonly discussed ones.

What proportion of each urolithin a given person actually produces after eating the same portion of pomegranate depends heavily on their individual gut microbiota composition — a phenomenon described in the literature as a urolithin metabotype. A portion of the population (estimated at tens of percent in studied cohorts) produces very little urolithin A regardless of precursor intake, simply because they lack the gut bacterial strains capable of carrying out the full conversion. That's one practical reason why patented supplements containing pure, already-converted urolithin A (rather than raw ellagitannins from pomegranate extract) became the subject of separate clinical trials — they sidestep the uncertainty of whether someone's microbiota will produce the desired compound at all.

Urolithin A owes its position as the best-characterized member of the urolithin family mainly to research on mitophagy — the process by which a cell recognizes and clears damaged, inefficient mitochondria, replacing them with new ones. Animal models showed that urolithin A activates this process, which in theory translates to a better quality mitochondrial pool in high-energy-turnover tissues such as skeletal muscle. What sets urolithin A apart from C and D is that this hypothesis was actually tested in controlled human trials — not just suggested based on animal models.

Urolithin C and urolithin D form via the same metabolic pathway as urolithin A, from the same plant precursors — but they're studied in entirely different contexts and at a completely different stage of the science. As we describe in more detail in our entry on urolithin C, the available data on it concerns mainly glucose-dependent stimulation of insulin secretion from pancreatic beta cells and antioxidant activity — exclusively in studies on isolated cells and animal models, without a single clinical trial in humans.

Urolithin D, as described in our entry on urolithin D, is even less characterized — fewer publications exist about it than about urolithin C, and its standout feature in available studies is strong antioxidant activity in test-tube assays (a lower IC50 than ellagic acid itself) and selective inhibition of the EphA2 receptor in prostate cancer cell lines — observations purely at the cellular level, with no confirmation in living organisms.

The practical problem looks like this: a manufacturer of pomegranate extract or an ellagitannin blend can truthfully and honestly write 'contains urolithins' on the label, because plant raw material, after passing through the microbiota, really will produce some mix of different urolithins (A, B, C, D) in proportions that depend on the consumer, not the manufacturer. Then invoking clinical trials on urolithin A — trials that studied a specific, pure, patented form given at a set dose, not a mixture randomly formed from plant extract in someone's gut — is an unjustified logical leap, even if every individual statement on the label sounds true on its own.

Mitopure is the trade name for pure, synthesized (not fruit-extracted) urolithin A, developed by Amazentis and used in all three clinical trials described above. The key difference from pomegranate-extract supplements is that the dose given to trial participants was precisely known and identical for everyone in a given group — it didn't depend on how efficiently someone's microbiota would convert the plant raw material. That dosing precision is exactly what enabled reliable, placebo-controlled trials analogous to new-drug studies, rather than just observational dietary research.

This doesn't mean every supplement labeled 'urolithin A' actually contains Mitopure or a comparably pure form at the studied dose — the supplement market is bigger than one manufacturer, and verifying a specific product's actual content and quality goes beyond what the ingredient name alone on a label can guarantee.

The urolithin family is a good example of how one real, interesting scientific story (mitophagy and urolithin A) can be marketing-stretched to neighboring, far less studied compounds just because they share the first part of a name and a common chemical origin. Urolithin A deserves to be treated as a compound with real, though still limited, clinical support in humans. Urolithins C and D deserve an honest label: promising at the basic cell-study level, but not yet confirmed in any human trial.