Urolithin A vs Urolithin B: What's the Difference?

Urolithin A and urolithin B are close chemical cousins, both made in your gut from the same dietary precursors. But they are not interchangeable. Urolithin A is the one carried into human clinical trials, and there is a clear reason nearly every serious product standardizes on it.

Same family, different molecules

Urolithins are a family of compounds your gut bacteria produce when they metabolize ellagitannins and ellagic acid, the polyphenols found in pomegranate, walnuts and berries. As bacteria break these precursors down, they generate a sequence of related molecules. Urolithin A and urolithin B are two members of that sequence. Structurally they are very similar, differing by a single hydroxyl (–OH) group: urolithin A carries two hydroxyl groups, while urolithin B carries one. That small difference in structure appears to matter for how the molecules behave and, importantly, for how much research each has attracted.

Both are produced along the same microbial pathway from the same foods. In simplified terms, ellagic acid is converted step by step, and different people's microbiomes settle on different dominant end-products, one of the reasons researchers describe distinct urolithin metabotypes. You can read more about that upstream chemistry in our guide to foods high in ellagic acid.

Why urolithin A became the studied one

The reason products standardize on urolithin A is not marketing, it is where the evidence sits. Urolithin A is the urolithin that has been the focus of mechanistic and, crucially, human clinical research, particularly around mitophagy, the cellular recycling of worn-out mitochondria. When scientists identified a urolithin that reliably promoted this housekeeping process in laboratory models, it was urolithin A, and that is what moved forward into people.

Preclinical origin (animal and cell, not human proof)

The foundational study by Ryu and colleagues (Nature Medicine, 2016) showed that urolithin A induced mitophagy, extended lifespan in the worm C. elegans, and improved muscle function in rodents. This preclinical work established urolithin A, specifically, as the candidate worth testing in humans. It is animal and cell evidence, presented here as the reason the field concentrated on urolithin A rather than its cousins.

Human trials (the strongest evidence, all on urolithin A)

Every major human trial in this area has used urolithin A. Andreux and colleagues (Nature Metabolism, 2019) reported it safe and well tolerated up to 1,000 mg per day, with a mitochondrial gene-expression signature. Liu, D'Amico and colleagues (JAMA Network Open, 2022) reported improved muscle endurance in older adults over about four months. Singh and colleagues (Cell Reports Medicine, 2022) tested 500 mg and 1,000 mg over four months in middle-aged adults and reported gains in muscle strength and exercise performance. A 2024 trial in resistance-trained men (Journal of the International Society of Sports Nutrition) looked at endurance and recovery markers over eight weeks, and a 2025 Nature Aging trial reported support for immune-cell mitochondrial health in midlife adults over roughly 28 days. Notice the common thread: the human record is built on urolithin A, not urolithin B.

Where urolithin B stands

Urolithin B is a genuine gut metabolite and has been examined in laboratory research, including some cell and animal work exploring muscle and other pathways. But it is far less studied than urolithin A, and it lacks the body of human clinical trials that urolithin A has accumulated. In honest terms, urolithin B is at an earlier, more preliminary stage of investigation. It would be inaccurate to claim it delivers the same benefits shown for urolithin A, because the human studies that would support such a claim have simply not been done to the same degree.

This is why you rarely see urolithin B sold as a standalone standardized supplement in the way urolithin A is. When a company has the option to formulate around the molecule with human safety and efficacy data, that is the responsible choice, and for the urolithin family that molecule is urolithin A.

Does your body convert one into the other?

A common point of confusion is whether urolithin A and urolithin B interconvert. They are both products of the same microbial breakdown pathway, formed as bacteria progressively modify ellagic acid, but which urolithins you end up producing depends on your particular microbiome. Some people's gut communities favor urolithin A; others produce different profiles. This individual variability, and the fact that many people convert precursors poorly overall, is exactly why relying on food to reach a specific urolithin A level is unreliable, a point we cover in our page on berries and urolithin A. A standardized urolithin A supplement removes that variability by delivering the finished, studied compound directly.

Honest limitations

Even for urolithin A, it is important to stay measured. A 2024 systematic review in Ageing Research Reviews that examined urolithin A in humans concluded the evidence is still emerging: the trials so far have been relatively short and modest in size. Urolithin A supports mitochondrial and muscle-related outcomes in these studies; it is not a treatment for any disease. If your interest touches areas like muscle loss, joint discomfort, immune health or metabolic health, treat urolithin A as a supportive supplement rather than a therapy, and speak with your doctor about any medical concern, this is not a substitute for professional care.

What this means for choosing a supplement

The practical takeaway is straightforward. If you are shopping the urolithin category, look for standardized urolithin A in the clinically studied range of 500 to 1,000 mg per day, backed by third-party testing. Urolithin B, whatever its future promise, does not yet have the human evidence to justify choosing it over urolithin A today. Our best urolithin A gummies comparison and our full urolithin A guide break down dose, form and quality so you can compare options with clear eyes. For dosing specifics, our urolithin A dosage page goes deeper.

SOMA HEALTH Urolithin A Gummies deliver 1,000 mg of urolithin A per four-gummy daily serving, the top of the studied range, in a raspberry-flavored, sugar-free, third-party tested format, 120 gummies per bottle for a 30-day supply. The formulation centers on urolithin A for exactly the reason this whole page describes: it is the urolithin with the human research behind it.

Urolithin A and urolithin B are close relatives, but they are not equals in the evidence that matters. When a page, product or claim leans on urolithin A's research, make sure the product actually contains standardized urolithin A, which is what the human trials used.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This guide is educational and is not a substitute for advice from a qualified healthcare provider.