A decade ago, it might have been hard to find kefir outside health food stores or Eastern European groceries. Now the yogurt-like drink is in any dairy aisle, riding the surge of popularity of probiotics. By 2018, probiotic food sales in the U.S. may reach $10 billion; probiotic supplements may reach $2.5 billion.
Manufacturers tout the potential for the products, which are filled with live microorganisms, to help support a healthy gut and immune system. And for certain medical issues, such as irritable bowel syndrome or ulcerative colitis, there's evidence that probiotics can help. Whether they can help people who are otherwise healthy is another question.
A new study from researchers at the University of Copenhagen looked seven randomized controlled trials of fecal microbiota—i.e., the bacteria living in poop—and found that, so far, there's no evidence that consuming probiotics makes any difference for already healthy people.
Only one study showed more diversity of beneficial bugs in the poop of people who consumed probiotics; none of the others showed any difference in the number of different kinds of bacteria or how they were distributed.
That's not to say, as some headlines did, that probiotics are a waste of money. That is the "wrong interpretation" of the study, says senior author Oluf Pedersen. The trials that the meta-study examined had some issues, such as small sample sizes and no control of people's variations in diet, which can also affect gut microbiota. There's also no consensus on what a "normal" population of bacteria in the gut should be. The real conclusion: There's a need for better designed, carefully conducted clinical trials before it's possible to say what effect probiotics might have and whether buying supplements is money well spent.