Urolithin B
Urolithin B, gut microbial metabolite of ellagitannins
Written by Aaron CuhaReviewed Sep 2026
Also known as: UroB, Uro-B
Sold on the reputation of urolithin A, which has human trials. Urolithin B has none: the muscle result is C2C12 cells and mice given 10 micrograms a day through an implanted osmotic pump for 28 days.
Overview
Urolithin B is one of the compounds gut bacteria make from the ellagitannins in pomegranate, walnuts and berries. It is structurally simpler than urolithin A and has a different reported action: rather than mitophagy, the 2017 paper that put it on the map described it as a regulator of muscle protein balance acting through the androgen receptor.
That paper is the whole case. In C2C12 myotubes, 15 micromolar urolithin B for 24 hours increased growth and differentiation by raising protein synthesis and repressing the ubiquitin-proteasome pathway, with genetic and pharmacological evidence implicating the androgen receptor and crosstalk with mTORC1 possibly via AMPK. In mice, 10 micrograms a day delivered continuously for 28 days by implanted mini-osmotic pump induced muscle hypertrophy and reduced atrophy after sciatic nerve section. A 2025 study in vascular smooth muscle cells reports a separate anti-proliferative effect in vitro.
No human has been given urolithin B in a trial. Not for muscle, not for anything. The conflation with urolithin A in retail copy is routine, and it matters: urolithin A is the molecule with a first-in-human safety trial, a four-month randomised trial in older adults and a randomised trial in middle-aged adults, and even those trials missed primary endpoints. Urolithin B has none of that record and is frequently sold as though it shares it.
Mechanism of action
In muscle cells and mice, an increase in protein synthesis with repression of the ubiquitin-proteasome degradation pathway, with the androgen receptor implicated genetically and pharmacologically and crosstalk to mTORC1 signalling, possibly through AMPK. This is a different proposed mechanism from urolithin A, which is sold and studied as a mitophagy activator. Which urolithins a person produces from dietary ellagitannins depends on their gut microbiota, and the metabotype categories described in the polyphenol literature differ in whether urolithin A, urolithin B or the isourolithins dominate.
Reading the research record
There is no human evidence section on this page because there is no human evidence. No registered trial has given urolithin B to a person, and the searches behind this entry returned nothing beyond in vitro digestion chemistry and observational polyphenol metabotype work.
The route of administration in the only in vivo study deserves emphasis. The mice did not eat urolithin B. They had mini-osmotic pumps implanted that released 10 micrograms a day continuously for 28 days, which produces steady exposure that an oral capsule taken once a day does not replicate, in an animal whose muscle biology differs from ours anyway.
The commercial context is the reason this page exists. Urolithin A reached human trials, is sold as a branded ingredient, and even there the trials are honest about missing primary endpoints on peak power and six-minute walk. Urolithin B is cheaper, is chemically adjacent, and is sold in the same category with the same language. The two molecules have different proposed mechanisms, one through mitophagy and one through the androgen receptor, and only one of them has ever been in a person under study conditions.
A further practical point from the polyphenol literature: which urolithins you produce from pomegranate or walnuts is determined by your gut microbiota, and people fall into different metabotypes. That is a reason the field moved to giving specific urolithins directly, and it is also a reason not to assume eating the parent food delivers any particular one.
The evidence, charted
Fig. 1 · evidence composition
1of 4 citations (25%) are in people
Counted from the citation list on this page. The Human count is the same number shown in the badge at the top. Both understate any literature larger than the sources we cite.
Fig. 2 · evidence over time
Evidence spans 3 distinct years, 2017 to 2025, counted from the citation list on this page.
Fig. 3 · legal status at a glance
US
Not approved
UK
Not approved
AU
Not approved
CA
Not approved
Not approved in any of the four jurisdictions shown. A jurisdiction's classification is a regulatory fact, not a verdict on the science; see Legal status below for the full text and any notes.
Key studies & citations
- Animal2017
Urolithin B, a newly identified regulator of skeletal muscle mass
C2C12 myotubes treated with 15 micromolar urolithin B for 24 hours grew and differentiated more, through increased protein synthesis and repression of the ubiquitin-proteasome pathway, with the androgen receptor implicated. In mice, 10 micrograms a day delivered continuously by implanted mini-osmotic pump for 28 days induced muscle hypertrophy and reduced atrophy after sciatic nerve section. Cells and mice, by continuous infusion, not an oral human dose.
Journal of Cachexia, Sarcopenia and Muscle - In vitro2025
Urolithin B suppresses phenotypic switch in vascular smooth muscle cells induced by PDGF-BB via inhibiting the PI3K-AKT pathway
Vascular smooth muscle cells in culture. A separate proposed action, entirely in vitro, with no animal or human follow-up.
In Vitro Cellular and Developmental Biology. Animal - In vitro2017
Gastrointestinal stability of urolithins: an in vitro approach
In vitro digestion work on the stability of urolithins through the gastrointestinal tract. Relevant because the only in vivo urolithin B data came from a pump implanted under the skin, which bypasses the gut entirely.
European Journal of Nutrition - Human2022
Effect of Urolithin A Supplementation on Muscle Endurance and Mitochondrial Health in Older Adults: A Randomized Clinical Trial
Included as the contrast, and it is not urolithin B. This is a randomised trial of urolithin A in older adults, in which muscle endurance and plasma biomarkers improved while the six-minute walk and peak ATP did not differ from placebo. Urolithin B has no trial of this kind, or of any kind.
JAMA Network Open
Frequently asked questions
Is urolithin B the same as urolithin A?
No. They are different molecules with different proposed mechanisms, urolithin A as a mitophagy activator and urolithin B as an androgen receptor linked regulator of muscle protein balance. Urolithin A has completed randomised human trials. Urolithin B has never been given to a human in a trial.
Does urolithin B build muscle?
In cultured muscle cells and in mice receiving a continuous infusion from an implanted pump, yes. In humans, unknown, because it has never been tested. The mouse dose was 10 micrograms a day delivered continuously, which does not translate to an oral capsule.
Will eating pomegranate give me urolithin B?
Possibly, depending on your gut bacteria. People differ in which urolithins they produce from dietary ellagitannins, which is precisely why researchers moved to administering specific urolithins rather than the parent foods.
Is urolithin B safe?
Nobody knows. There is no human safety data, no human pharmacokinetics, and no dose in a person that anyone has characterised. Absence of reported harm in a compound nobody has studied in humans is not a safety record.
Why is it cheaper than urolithin A products?
Chemistry and evidence both. It is a simpler molecule, and it carries none of the trial programme that the branded urolithin A ingredient has behind it. You are paying less because there is less to pay for.