In recent years, biologists and nutritionists alike have been abuzz over the human microbiome: the thriving ecosystem of microorganisms colonizing the lining of our guts, the surface of our skins, and every other crevice and orifice our bodies have to offer. Study after study has proven how essential these microbes are to our wellbeing. Imbalances in gut bacteria have been implicated in everything from obesity to diabetes. Scientists have even successfully treated certain gut infections by “transplanting” gut bacteria from a healthy donor into the sick patient.

In the midst of all this excitement over our microbiomes came a surge in the popularity of probiotics. The idea behind probiotics is that they contain live microorganisms, which, when ingested, provide health benefits by modifying our gut flora, replacing harmful microbes with good microbes. Typical probiotic foods include yogurt, cheese, and pickled vegetables—and now there are also probiotics pills and supplements available. This idea sounds good in theory—even if it is a little simplistic—but do probiotics actually improve human health?

As it is with almost all nutritional questions, the answer seems to be: maybe. More specifically, researchers have found a few specific situations where probiotics offer a real benefit to human health—but in most cases, they don’t seem to do much.

Let’s begin with when probiotics do actually help. If probiotics function by acting as good microbes crowding out the bad, it makes sense that they would be most helpful for people suffering from existing imbalances in their gut flora. Such imbalances frequently occur in patients taking antibiotics. Antibiotics indiscriminately kill off both good and bad microbes, leaving a vacancy in the gut that can later be colonized by harmful bacteria like Clostridium difficile, leading to antibiotic-associated diarrhea (AAD). A 2012 systematic review found that probiotics reduced the risk of AAD by an absolute amount of 7%, which is pretty good. Probiotics seem to help with regular diarrhea too: a 2010 systematic review found that probiotics reduced the average duration of acute infectious diarrhea by around 24 hours. They also found no negative side effects that were attributable to the administered probiotics. So, given its relative cheapness, and the lack of side effects, probiotics may just be a decent option to alleviate diarrhea.

Unfortunately, this benefit doesn’t carry over to other ailments. There have been systematic reviews investigating the effect of probiotics on food allergies, urinary tract infections, Crohn’s disease, fatty liver disease, ulcerative colitis, bacterial vaginosis, hepatic encephalopathy, and so on and so on. None of these studies found evidence that probiotics had any beneficial effect. However, we shouldn’t immediately conclude that probiotics are useless for treating these diseases. Not all probiotics are the same—there are different species of bacteria and different methods of administration, each of which may yield different effects. Probiotic research may still yield valuable new therapies—but it won’t be as simple as eating some yogurt.

But perhaps some skepticism is warranted: supplements aren’t regulated by the Food and Drug Administration (FDA), and so there’s no requirement that these probiotics actually do anything.

What about probiotics for healthy people? Anyone who’s strolled through the health section of a retail store will have seen dozens of brands of probiotics. These aren’t meant for people suffering from diarrhea either—they are marketed towards healthy individuals as a supplement to boost health. But perhaps some skepticism is warranted: supplements aren’t regulated by the Food and Drug Administration (FDA), and so there’s no requirement that these probiotics actually do anything. The National Institutes of Health (NIH) emphasizes on its website that the research on the efficacy and safety of probiotics is still extremely lacking, and cautions that “the rapid growth in marketing and use of probiotics may have outpaced scientific research for many of their proposed uses and benefits.”

There’s a parallel here between probiotics and multivitamin supplements. Study after study has shown that regular vitamin supplementation, although not harmful, has no significant benefits for most people. However, multivitamins do help people who are chronically deficient in certain nutrients. Perhaps that is the takeaway here: supplements help to make up for something that’s missing. Just as multivitamins would help someone with vitamin D deficiency, probiotics would help someone with a damaged microbiome, be it from antibiotic usage or acute infection. But for a regular person with healthy gut flora, perhaps it would be wise to think more critically before medicating diseases we don’t have.

About The Author

Daniel Liu

Hailing from the quiet suburbs of Potomac, Maryland, Daniel is an editor and former writer for Innovation's health section. He studies molecular biology at Princeton, with a particular research interest in cancer stem cell biology and the molecular pathways governing metastasis. Outside of academics, Daniel also enjoys painting and drawing in his free time.