Input and output are subjects of intense interest in the scientific study of probiotics. Since probiotics are, by definition, “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”, knowing their effects is crucial. So when someone ingests a particular probiotic, what is the potential health benefit, or output?
What scientists know is that probiotics in healthy individuals are associated with a number of benefits. Meta-analyses of randomized, controlled trials show that probiotics help prevent upper respiratory tract infections, urinary tract infections, allergy, and cardiovascular disease risk in adults. But between the input and the output, what happens?
A common assumption is that probiotics work by influencing the gut microbe community, leading to an increase in the diversity of bacterial species in the gut ecosystem and measurable excretion in the stool.
But this theory doesn’t seem to be true, according to a recently published systematic review by Kristensen and colleagues in Genome Medicine. Authors of the review analyzed seven studies and found no evidence that probiotics have the ability to change fecal microbiota composition. So even though individuals in the different studies were ingesting live bacterial species, the bacteria didn’t stick around to increase the diversity of the gut fecal microbiota.
“Do probiotics alter the fecal composition of healthy adults? The answer seems to be no,” says Dr. Mary Ellen Sanders, Executive Science Officer for the International Scientific Association for Probiotics and Prebiotics (ISAPP), in an email to GMFH editors. But, she adds, “I don’t think the results of the study are a surprise.”
Dr. Dan Merenstein, Research Division Director and Associate Professor of Family Medicine at Georgetown University Medical Center in Washington, DC (USA), agrees. “Initially when probiotics were studied, some people expected to see permanent colonization. We now realize that is unlikely to occur,” he says. “This study shows that the probiotics tested to date do not result in overarching bacterial community structure changes in healthy subjects. But clinical effects are clearly demonstrated for probiotics, and likely some are mediated by microbiome changes.”
At issue, then, is not what probiotics do for healthy individuals, but exactly how they work: the so-called ‘mechanism’. Sanders, who described some alternative mechanisms in her BMC Medicine commentary about the Kristensen review, points out a logical error in news stories worldwide that covered the article: the assumption that if probiotics fail to change the microbiota composition, they fail to have any health effects. Sanders emphasizes that probiotics might work in many possible ways.
“Probiotics may act through changing the function of the resident microbes, not their composition. They may interact with host immune cells,” she says. “They may inhibit opportunistic pathogens that are not dominant members of the microbiota. They may promote microbiota stability… So the fact that they don’t change the composition of fecal microbiota does not mean that there is no means for them to impact host health.”
Merenstein stresses that the task ahead is to find out how the outputs occur after taking probiotics. “More refined assessments of the microbiome, including metabolomics measures and tracking specific target microbes, will shed more light on this issue,” he says.
Kristensen NB, Bryrup T, Allin KH, Nielsen T, Hansen TH, Pedersen O. Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials. Genome Medicine 2016; 8:52. doi: 10.1186/s13073-016-0300-5