Probiotics did not make healthy guts more diverse

Probiotics are widely used dietary supplements promoted as good for gut health, and they are especially popular among healthy people with nothing specific to treat. One of their purported benefits is enhancing gut microbiota diversity, a feature associated with improved resilience and overall health, and a claim that appears routinely in marketing. Yet the evidence behind that specific promise has remained inconclusive. This review set out to answer a clean question: do probiotic supplements actually change the diversity of the gut microbiota in healthy people?

The authors ran a systematic search of the MEDLINE, Embase and Cochrane databases in April 2024, following a strategy registered in advance on PROSPERO. Out of 9217 identified articles, 47 met the inclusion criteria for the review, and 22 randomised trials with data from 1068 individual healthy subjects were eligible for the pooled analysis. Diversity was assessed through standard indices used in microbiome research, and the team used a random-effects model to estimate differences between probiotic and unsupplemented control groups, anticipating variation between studies.

There was no statistically significant effect of probiotics on any of the four indices analysed: Shannon diversity, observed operational taxonomic units, Chao1 or Simpson's index of diversity, compared with unsupplemented controls. Further analyses showed that the taxonomic family of the probiotic, the risk of bias in the trials and the duration of supplementation did not change the result. Other diversity indices could not be pooled for lack of data, but most of the included studies reported no difference between probiotic and control groups on those measures either.

This directly challenges one of the most common claims made for probiotics in healthy people, and it does so with a reasonably large pooled dataset and a result that survived every sensitivity check the authors ran. It does not close the book on probiotics: the authors themselves call for research into whether specific strains or formulations might influence diversity in targeted subgroups or under specific conditions. It also says nothing about people with health conditions, where separate evidence suggests probiotics can shift the gut microbiota, nor about probiotic effects that have nothing to do with diversity.

If you are healthy and take a probiotic partly because you have heard it will diversify your microbiome, this is a reason to let go of that specific expectation: across 22 trials, the diversity needle did not move. That is not the same as saying probiotics do nothing at all, and it does not apply to people taking them for a diagnosed condition. It is, though, a useful corrective to a claim that has been repeated far more confidently than the evidence supports.