Probiotics Are Useless* | Vol. 3 / No. 32.2

*for healthy adults. | Photo: Ryan Snyder, CC BY 2.0


Motherboard announced rather triumphantly that “Probiotics are useless, GMOs are fine, and gluten is necessary” last week, but as usual, claims like that require a few minor caveats.

I, too, enjoy celebrating when studies demonstrate things that I already believe, but it’s at these times that we need to remain skeptical, because confirmation bias is a very real thing.

So Motherboard was referring to three studies that came out last week, and one of them, a meta-analysis of a number of other studies, found that (as Motherboard reported): “there is no convincing evidence for consistent effects of probiotics on fecal microbiota composition in healthy adults.” But that’s a very specific claim, and quite a way off from “useless.”

What they did was start by searching for studies of probiotic use, and they found 1368 they could use. But that shrank to 1287 after the removal of duplicates. That shrank by (not to, by) 1256 after review for the following reasons: it was an animal study, it was a meta-analysis (so not itself a study involving subjects), it wasn’t about healthy adults, it didn’t involve intervention using probiotics, or it didn’t look in the feces for microbiota composition. That left just 31 studies. Then they removed a further 24 studies (leaving just 7) because they had “no assessment of fecal microbiota composition, assessment of single-strain survival only, inclusion of non-healthy participants, non-randomized controlled design, provision of only within-group results, and combined intervention of probiotic with prebiotics or other foods.”

Looking at those remaining seven studies, they were able to say there wasn’t enough evidence to support the claim that probiotics have a consistent effect on the composition of bacteria in the feces of healthy adults.

Now interestingly enough they do point out that “where dysbiosis is present or where the microbiota is perturbed, there is some evidence for a restorative or protective effect of certain strains of probiotics, both on the fecal microbial community itself, but more importantly, also on host physiology, e.g. alleviation of gastrointestinal symptoms.” This is because there does appear to be some promising research that, in people who have problems in their guts (i.e. not healthy people), probiotics might be useful. One good example is this meta-analysis that showed some reduction in the risk of antibiotic-associated diarrhea in children, from 28.5% to 11.9% (relative risk).

Now I don’t know if 7 studies — which were pretty heterogeneous in their methodologies — are enough to prove for sure that probiotics don’t have a positive health effect on already-healthy people, but it doesn’t bode terribly well for the claim. It certainly does prove, as they say, the evidence is insufficient at present to demonstrate consistent benefits for healthy adults.

So long story short, there isn’t any evidence that, if you’re healthy already, probiotics will change much of anything in your guts, for good or ill. On the other hand, they might not technically be “useless,” especially if you’re a kid on antibiotics.


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Richard Ford Burley is a human, writer, and doctoral candidate at Boston College, as well as an editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.