Probiotics left the gut-linked blood marker TMAO unmoved

Trimethylamine N-oxide, usually shortened to TMAO, is a metabolite whose levels are thought to be shaped by the bacteria living in the gut, which is why probiotics have been proposed as a way to change it. Some clinical trials have reported that probiotic supplementation shifts TMAO through regulation of the intestinal microbiota, while others have found nothing, leaving the field contradictory. Probiotics are a huge consumer category, so whether a capsule can actually move a microbiota-linked blood metabolite is a fair test of what these products do inside the body. This review set out to settle the disagreement by pooling the randomised evidence.

The authors searched PubMed/Medline, Web of Science, Scopus, the Cochrane Library and Embase for randomised controlled trials published up to August 2021. Eligible studies compared adults over 18 taking probiotics against a control group, with TMAO measured as the final outcome. Eight trials with a combined 270 participants met the inclusion criteria. The results were pooled as a weighted average difference between the probiotic and control groups, and subgroup analyses then split the data by health status and by age to see whether particular groups responded differently to the supplements.

Probiotic supplementation had no significant effect on TMAO compared with control, with a pooled difference of 0.08 μmol/L lower, a figure indistinguishable from zero. Splitting the trials by health status did not change the picture: neither healthy nor unhealthy participants showed a significant shift in the metabolite after the intervention. The one flicker in the data was related to age, with probiotics reducing TMAO more in people under 50 than in those over 50. The authors present the overall result as a null and call for further studies to confirm the findings.

Individual trials in this area had pointed in different directions, with some reporting that probiotics lower TMAO and others finding no change, and this pooling comes down on the side of no overall effect. That complicates the popular idea that a probiotic capsule can meaningfully reshape what the gut microbiota produces, at least as measured by this particular metabolite. Real questions remain open, though: 270 participants across eight trials is a small evidence base, the hint of an age difference has not been tested directly, and the authors themselves say further studies are needed before the verdict is final.

If you take a probiotic and have come across TMAO as a marker worth watching, this pooled result is a reason not to expect the supplement to change it. The null held in both healthy and unhealthy adults, so this is not a case of probiotics failing in only one group. The suggestion of a stronger effect in people under 50 is interesting but unconfirmed, and nothing here says probiotics are useless for other purposes; it says only that on this one metabolite, the randomised evidence shows no significant movement.