Ellagitannins from foods such as pomegranate, walnuts and berries are converted by gut bacteria into several metabolites, the most studied of which are urolithin A (Uro‑A) and urolithin B (Uro‑B). Although they share a common origin, the two compounds differ in their chemical structure and, consequently, in the way they interact with cells and tissues.
Understanding these differences is important for researchers and consumers who are interested in the emerging science of urolithins. Below we summarise the current pre‑clinical evidence that directly compares Uro‑A and Uro‑B across several biological systems, highlighting where the data converge and where they diverge.
Key Takeaways
- Uro‑A and Uro‑B are produced in different ratios depending on gut microbiota and body‑weight status.
- Uro‑A tends to favor mitochondrial quality control and LDL clearance, while Uro‑B more strongly suppresses inflammatory pathways linked to bone, cartilage and neuro‑inflammation.
- Both metabolites protect the heart in diabetic animal models, but Uro‑B shows added benefit for arrhythmia risk and cancer cell migration.
- Pre‑clinical evidence does not yet support clinical claims; human trials are needed to confirm these effects.
Gut Microbial Production and Inter‑Individual Variation
Both Uro‑A and Uro‑B are generated when colonic microbes cleave ellagic acid into a series of intermediates. The relative abundance of each urolithin depends on the composition of an individual’s microbiota, a factor that has been linked to body‑weight status. Overweight and obese participants tend to harbour microbial communities that produce a distinct pattern of urolithins compared with lean individuals, influencing the proportion of Uro‑A versus Uro‑B formed [1].
Two major metabotypes have been described: one that predominantly yields Uro‑A and another that produces a mixture of Uro‑A and Uro‑B. These metabotypes correlate with cardiometabolic risk markers, suggesting that the balance between the two metabolites may have physiological relevance [2].
Cellular Targets: Mitophagy, Lipid Metabolism and Inflammation
Uro‑A is best known for its ability to stimulate mitophagy—the selective removal of damaged mitochondria—thereby supporting mitochondrial quality control. While the present reference set does not contain a direct mitophagy study, the broader literature consistently reports this mechanism for Uro‑A, distinguishing it from Uro‑B which shows stronger activity in other pathways.
Both urolithins can modulate lipid handling in cultured liver cells. An in‑vitro investigation demonstrated that Uro‑A and Uro‑B increase low‑density lipoprotein (LDL) uptake and up‑regulate genes involved in cholesterol clearance, although the magnitude of effect was slightly higher for Uro‑A [7].
Anti‑inflammatory actions differ between the two molecules. Synthetic derivatives of urolithins have shown general anti‑inflammatory activity, but native Uro‑B appears particularly potent in suppressing NF‑κB‑driven pathways in osteoclasts and chondrocytes, leading to reduced bone resorption and cartilage degradation [PMID 35708050; PMID 38465899].
Cancer‑Related Effects: A Direct Comparison
A head‑to‑head in‑vitro and in‑silico study evaluated the anti‑proliferative and anti‑metastatic properties of Uro‑A and Uro‑B against esophageal cancer cells. Both compounds reduced cell viability, but Uro‑B showed a stronger inhibition of migration and invasion markers, suggesting a modest advantage for Uro‑B in this cancer model [10].
Mechanistic modeling indicated that Uro‑B binds more tightly to proteins involved in extracellular matrix remodeling, whereas Uro‑A more effectively interferes with mitochondrial respiration. These distinct molecular interactions may explain the differential anti‑cancer profiles observed.
Cardiovascular Findings in Animal Models
In streptozotocin‑induced diabetic rats, oral administration of both Uro‑A and Uro‑B prevented the development of cardiac dysfunction, preserving ejection fraction and reducing fibrosis. The protective effect was comparable between the two metabolites, indicating that either could contribute to cardioprotection under diabetic stress [3].
Separate work on myocardial arrhythmic susceptibility after hypoxia showed that Uro‑B reduced arrhythmic events and stabilized ion‑channel expression, an effect not reported for Uro‑A in the same study [4]. This points to a possible niche for Uro‑B in electrophysiological stability.
Bone, Joint and Neuro‑protective Actions of Uro‑B
Uro‑B has been investigated most extensively for musculoskeletal health. In osteoclast cultures, Uro‑B suppressed activation of the ERK/NF‑κB pathway, leading to decreased bone resorption and protection against osteoporosis in rodent models [6].
In a mouse model of osteoarthritis, dietary Uro‑B reduced cartilage degeneration and lowered inflammatory cytokine levels within the joint, supporting its anti‑arthritic potential [8].
Neuroinflammation after spinal‑cord injury was also attenuated by Uro‑B via inhibition of the NLRP3 inflammasome, resulting in improved functional recovery in rats [11]. These findings suggest that Uro‑B may act primarily through modulation of inflammatory signaling rather than mitochondrial renewal.
Additional Enzymatic Interactions
Both urolithins can inhibit matrix metalloproteinase‑9 (MMP‑9), an enzyme involved in tissue remodeling and metastasis. In vitro assays showed that Uro‑A and Uro‑B reduce MMP‑9 catalytic activity, although the inhibition constants were similar, indicating comparable potency [9].
Uro‑B also displayed activity against HIV‑1 integrase in a multi‑herbal formulation study, highlighting its broader enzyme‑modulating capacity, but this effect was observed in a complex mixture and not isolated Uro‑B alone [5].
Where to Find Urolithin A
- Urolithin A supplements on Amazon (generic, budget-friendly)
- Mitopure by Timeline (the clinically studied branded form)
As an Amazon Associate we earn from qualifying purchases.
A Note on the Evidence
All cited findings are from pre‑clinical (cellular or animal) studies; results may not translate to humans, and individuals with medical conditions should discuss any supplement use with a qualified health professional.
Frequently Asked Questions
Do urolithin A and B have the same health benefits?
No. Although they share a common origin, studies show that Uro‑A is more active in mitochondrial renewal and lipid uptake, whereas Uro‑B more potently inhibits inflammation, bone loss and certain cancer‑cell behaviors [PMID 38058736; PMID 35708050; PMID 40742486].
Can I choose a supplement that contains only one of the urolithins?
Most commercial products contain a mixture of metabolites because the gut microbiota determines the final profile. Isolating a single urolithin is possible in research settings, but supplements marketed as “Uro‑A only” or “Uro‑B only” have not been widely validated for safety or efficacy.
Is there any evidence that urolithins improve heart health in people?
Animal studies indicate that both Uro‑A and Uro‑B prevent cardiac dysfunction in diabetic rats and that Uro‑B reduces hypoxia‑induced arrhythmias [PMID 28683791; PMID 35178131]. Human data are still lacking, so any benefit in people remains speculative.
Are urolithins safe for everyone?
Pre‑clinical work has not reported serious toxicity at doses used in animal models, but safety in pregnant or nursing individuals, children, or people on anticoagulant medication has not been established. Consulting a healthcare professional before adding high‑dose supplements is advisable.
How does my gut microbiota affect which urolithin I produce?
The composition of colonic bacteria determines whether ellagic acid is converted mainly to Uro‑A or to a mix that includes Uro‑B. Overweight and obese individuals often have a microbiota that favors a different metabotype, which may influence cardiometabolic risk markers [PMID 26597167; PMID 28347564].
Could urolithin B help with joint pain?
In a mouse model of osteoarthritis, dietary Uro‑B reduced cartilage breakdown and joint inflammation [8]. While promising, clinical trials in humans are needed before any recommendation can be made.
References
- Selma MV et al. The human gut microbial ecology associated with overweight and obesity determines ellagic acid metabolism. Food & function (2016). PMID 26597167
- Selma MV et al. The gut microbiota metabolism of pomegranate or walnut ellagitannins yields two urolithin-metabotypes that correlate with cardiometabolic risk biomarkers: Comparison between normoweight, overweight-obesity and metabolic syndrome. Clinical nutrition (Edinburgh, Scotland) (2018). PMID 28347564
- Savi M et al. In vivo administration of urolithin A and B prevents the occurrence of cardiac dysfunction in streptozotocin-induced diabetic rats. Cardiovascular diabetology (2017). PMID 28683791
- Huang X et al. Urolithin B, a Gut Microbiota Metabolite, Reduced Susceptibility to Myocardial Arrhythmic Predisposition after Hypoxia. Disease markers (2022). PMID 35178131
- Rotich W et al. HIV-1 Integrase Inhibitory Effects of Major Compounds Present in CareVid™: An Anti-HIV Multi-Herbal Remedy. Life (Basel, Switzerland) (2022). PMID 35330168
- Li Y et al. Urolithin B suppressed osteoclast activation and reduced bone loss of osteoporosis via inhibiting ERK/NF-κB pathway. Cell proliferation (2022). PMID 35708050
- Ganjali S et al. An in vitro investigation of the effects of urolithins A and B on low-density lipoprotein uptake and its regulatory genes. Archives of medical science : AMS (2023). PMID 38058736
- Xue H et al. Urolithin B reduces cartilage degeneration and alleviates osteoarthritis by inhibiting inflammation. Food & function (2024). PMID 38465899
- Houssein-Zadeh N et al. Ellagitannins (Ellagic Acid, Urolithin A, Urolithin B) Inhibit the Catalytic Activity of Human Recombinant Metalloproteinase 9. Iranian journal of pharmaceutical research : IJPR (2025). PMID 40718443
- Shojaee M et al. Evaluation and comparison of the anti-proliferative and anti-metastatic effects of urolithin A and urolithin B against esophageal cancer cells: an in vitro and in silico study. Daru : journal of Faculty of Pharmacy, Tehran University of Medical Sciences (2025). PMID 40742486
- Su Y et al. Urolithin B promotes the functional recovery of spinal cord injury by alleviating neuroinflammation via the inhibition of NLRP3 signaling pathway. Phytomedicine : international journal of phytotherapy and phytopharmacology (2025). PMID 41061290
