No Impact of Probiotics on Gut Microbiota Composition

Everyone knows that probiotics “promote a healthy microbiota composition” right? This common but unsubstantiated generalization was challenged by a recent review.

Kristensen et al (2016) conducted a systematic review of seven randomized, controlled trials that assessed fecal samples using shotgun metagenomic sequencing, 16S rRNA sequencing or phylogenetic microarray methods. The probiotics used in the studies were different strains of Lactobacillus, Bifidobacterium longum or Bacillus subtilis. The subjects in the studies were healthy humans. The findings of the review – not surprising to those who follow the research – were that probiotics did not impact fecal microbiota composition when compared to a placebo, as reflected by different microbiota composition measures (alpha-diversity, richness or evenness).

The press release announcing this paper bore the regrettable, and inaccurate, title “Do probiotics have an effect on healthy adults? It’s too early to tell”. The text of the press release accurately reflected the findings of the paper. But the title led many to conclude that probiotics have no health benefit for healthy people. Articles such as Probiotics Are a Waste of Money for Healthy Adults hit the press. What these journalists failed to do was distinguish between probiotic impact on microbiota, which is one possible mechanism of probiotic-induced effects, and evidence for health benefits in healthy people.

Probiotics have been linked to benefits in healthy people. Some strains of probiotics are used for the following prevention/management situations:

  • Reduce incidence or duration of common respiratory tract infections
  • Improve blood lipid profiles
  • Improve digestion of lactose in lactose malabsorbers
  • Help improve immune function and inhibit pathogens
  • Reduce crying time in colicky babies
  • Manage mild digestive symptoms or intestinal regularity
  • Reduce antibiotic-associated side effects. Lest you object that antibiotics are not used in healthy people, remember that many of us carry on with normal lives while on antibiotics. For example, prophylactic use during dental procedures for those with joint replacements, or to treat ear or sinus infections.

The choice to use or not use probiotics when healthy depends on many factors. But I expect that consumers are more interested in WHAT probiotics can do for them rather than HOW they do it. The Kristensen meta-analysis looked at available evidence on one mechanism of action – changing microbiota composition. But many other mechanisms may mediate probiotic benefits. Probiotics may act through changing the function of the resident microbes, not their composition (Eloe-Fadrosh et al. 2015 and McNulty et al. 2011). They may interact with host immune cells. They may inhibit opportunistic pathogens that are not dominant members of the microbiota. They may promote microbiota stability (see Probiotics and Gut Microbiota Stability). They may change the composition of microbiota in difficult to sample areas of the intestine, such as the small intestine or the proximal large intestine. So the fact that these probiotics don’t change the composition of fecal microbiota does not mean that there is no means for them to impact host health.

Finally, this study looked at a narrow range of probiotics. Perhaps there are some strains/doses that will be found to impact gut microbiota composition.

Kristensen et al. have reminded us to not make assumptions about mechanisms of action for probiotics, and continue research into the what’s and how’s of probiotics. The aftermath of the Kristensen et al. paper should also remind science journalists to make it a priority to communicate research accurately.

Additional information:

 Sanders commentary on the article, published in BMC Medicine

 Probiotics for Healthy People blog post

Probiotics Impact Gut Microbiota: Not What’s There but What They are Doing blog post