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Study · BMC microbiology · 2024Conflicts declared

Walnut intake, gut bacteria and urolithin metabolites

Liu H, Birk JW, Provatas AA, et al · Free full text

What it found

In 39 healthy adults, eating 2 oz of walnuts daily for 21 days changed gut bacteria and raised urine levels of urolithin metabolites, especially urolithin A and iso-urolithin A.

What they found

Studies in people

Urolithin levels rise

After walnut consumption, total urolithins, urolithin A, and iso-urolithin A in urine increased compared to before.

Gut bacteria change

The overall gut bacterial community and its diversity differed after walnut intake. Thirteen types of bacteria became more abundant, including Gordonibacter, Roseburia, and Rothia.

Bacteria linked to urolithins

Twenty-six types of bacteria were associated with seven urolithin metabolites after walnut intake. For iso-urolithin A, 13 types of bacteria were linked to its production after walnuts, compared with only one before.

Microbial gene pathways

Predicted microbial gene functions differed before and after walnuts. After supplementation, seven gene functions were linked to iso-urolithin A, while none were linked before.

Studies in people and animals

Gordonibacter and urolithin A

The amount of Gordonibacter bacteria increased after walnuts and was linked to higher urolithin A levels. In lab tests, Gordonibacter can turn ellagic acid into urolithin C, which is later made into urolithin A or iso-urolithin A.

What the authors conclude

“In summary, our study showed that the intestinal microbiota and the levels of urolithin metabolites are modified by walnut supplementation in healthy subjects, with an increased relative abundance of Gordonibacter genera associated with urolithin A.”
Liu H, Birk JW, Provatas AA, et al, 2024

Also in their conclusions

  • They conclude that walnut supplementation modifies the gut microbiota and urolithin metabolite levels in healthy subjects, with an increased relative abundance of Gordonibacter associated with urolithin A.
  • They state that further studies are needed to investigate the functional impact of bacterial genes on walnut metabolism and urolithin levels.
  • They suggest that such studies may lead to the creation of specific bacterial designer consortium to augment the production of anti-inflammatory urolithins in humans.

How it was done

Walnuts are rich in ellagitannins, which gut bacteria convert into urolithins. The authors wanted to see how walnut supplementation changes gut bacteria and whether those changes relate to urolithin production.

They collected stool and urine samples from 39 healthy adults before and after they ate 2 oz of whole peeled walnuts daily for 21 days. They sequenced fecal DNA to identify bacteria and measured nine urolithin metabolites in urine.

What it can’t tell you

  • This study followed people over time without a control group, so it can show links between walnut intake, gut bacteria, and urolithins, but it cannot prove that walnuts cause these changes.
  • The findings are from 39 healthy adults, so they may not apply to other groups or to people with different gut bacteria.
  • The gene pathway results are predicted from bacterial DNA, not measured directly, so they cannot confirm which microbial functions are actually active.

The paper

Title
Correlation between intestinal microbiota and urolithin metabolism in a human walnut dietary intervention
Type
Study
Evidence
Studies in people · animals · lab
Summarised from
Full text
Cite
Liu H, Birk JW, Provatas AA, et al (2024). Correlation between intestinal microbiota and urolithin metabolism in a human walnut dietary intervention. BMC microbiology. doi:10.1186/s12866-024-03626-5Free full textPubMed 39548408DOI

Summary written 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error