The human body is home to trillions of microorganisms. They carpet the skin, coat the nose and mouth, and saturate the gastrointestinal tract. Researchers have long recognized that these microbes — particularly the ones in the gut — play a role in human health. But it wasn’t until June 2012 that the veil was truly lifted from the eyes of the general public and the medical science community.
That month, the National Institutes of Health announced preliminary findings from its $170 million Human Microbiome Project (HMP). The project’s stated goal was “to further our understanding of how the microbiome impacts human health and disease,” and it did that in part by mapping the genetic makeup of the microorganisms living in and on 242 healthy volunteers. Those maps revealed that the microbiome contributes more genes that are “responsible for human survival” than do the body’s own cells. The HMP also provided scientists with a healthy reference model against which to compare the microbiomes of sick individuals.
Right away, researchers started contrasting the microbiomes of the well and unwell. (This line of research wasn’t new, but it became both more reliable and more commonplace.) Many of these studies found that the gut microbes of sick people differed from those of healthy people in predictable ways. These discoveries helped popularize the idea that by improving the makeup of the microbiome, a person could sidestep or treat a wide range of medical conditions — everything from obesity and viral infections to depression.
Enter probiotic supplements. By definition, a probiotic is something that contains live microorganisms that confer a health benefit on their “host.” And the presumption among consumers and, at least initially, among many doctors was that ingesting certain bacteria could reshape or repopulate the gut’s ecosystem of bacteria in ways that improve a person’s health. Unfortunately, this presumption has turned out to be wrong.
“The old story that you can just pump a bunch of good bacteria in and they’ll replace the bad is not accurate,” says Dr. Emeran Mayer, a professor of medicine and co-director of the Digestive Diseases Research Center at UCLA. “People think that taking a consortium of 10 or 15 probiotic strains at maximal doses is helpful, but the benefits of this over and above their placebo effect have really not been demonstrated.”
Mayer’s position may seem to contradict the studies that have linked probiotic supplementation with health improvements. And those studies are numerous. But he says the vast majority of those experiments have involved animals, not people.
“When you look at humans, it becomes a much more difficult story,” he says. “We’re finding that if you add in an organism, it may have a huge effect or it may have no effect whatsoever, and that effect could be good or bad.”
The idea that taking a probiotic could do more harm than good is relatively new. But it has some strong evidence to back it up.
For example, some of the most robust research on the benefits of probiotics has involved people who are recovering from a course of antibiotics. While antibiotics are aces at slaying infectious bacteria, they also kill plenty of good, health-supporting microorganisms. This can lead to “dysbiosis,” which is an unhealthy reduction or reshuffling of a person’s community of gut microorganisms. These changes can lead to diarrhea and other unwanted gastrointestinal issues, and some studies have found that taking a probiotic alongside an antibiotic can mitigate these problems by aiding the gut’s reconstitution of healthy bacteria.
But last year, a study in the journal Cell found that, compared to people on antibiotics who did not take a probiotic, those who did experienced a “markedly delayed and persistently incomplete” microbiome recovery. Far from helping the people’s guts bounce back from antibiotics, probiotics seemed to interfere with this process. That Cell study and others like it have prompted expert warnings about the potential risks of probiotics. A 2019 editorial in The Lancet argues that the clinical evidence in support of probiotic supplementation is “mixed and often of low quality,” and that “both commercial and clinical use of probiotics is outpacing the science.”
“What we’re learning is that probiotics come with catches,” says Dr. Mark Moyad, director of preventive and alternative medicine at the University of Michigan Medical Center. While over-the-counter probiotic supplements are widely available, Moyad says people — and also doctors — would be better off treating these supplements as though they were prescription drugs. “They may work for specific conditions in specific circumstances, but not everyone should be taking them,” he says.
One of the problems with using today’s probiotics is that each person’s microbiome is unlike any other individual’s. “The current approach of using the same probiotic to treat people with different microbial and metabolic profiles is unlikely to work the same for everyone, and that’s what we’re seeing in the clinic,” says Mayer, who is the author of “The Mind-Gut Connection.” While it’s likely that probiotics can help some people with some gut-related medical conditions, such as inflammatory bowel disease, there are no guarantees, he says.
His warnings are echoed by the U.S. National Center for Complementary and Integrative Health: “We still don’t know which probiotics are helpful and which are not. We also don’t know how much of the probiotic people would have to take or who would most likely benefit from taking probiotics. Even for the conditions that have been studied the most, researchers are still working toward finding the answers to these questions.”
Meanwhile, the idea that a healthy person can derive a benefit from a probiotic supplement is even more suspect. “If you’re healthy, we have a number of studies now that show probiotics do nothing,” says Dr. Andrew Szilagyi, a gastroenterologist at Canada’s McGill University and its affiliated Jewish General Hospital.
“I tell patients to save their money,” says Dr. Raymond Cross, a professor of medicine and director of the Inflammatory Bowel Disease Program at the University of Maryland Medical System. Cross says probiotics hold immense promise, and that they one day may be used effectively to treat or prevent a number of health conditions. “I just hope we don’t ruin the whole field because we’re taking and prescribing these things willy-nilly,” he says.
“What people don’t understand is that they’re their own walking probiotic,” Moyad says. “When people eat right and exercise and lose weight — when they get healthier — this changes their gut flora in favorable ways.” If someone really wants to improve the health of their microbiome, the best way to go about it is to follow “all the old boring advice,” he says. “Diet, exercise, a good night’s sleep — all the stuff we’ve been talking about forever.”
That said, there may be one way to capture the potential benefits of probiotics while avoiding any downsides: Eat fermented foods that naturally contain probiotic bacteria — stuff like sauerkraut, kefir, kombucha, and kimchi.
“These foods have been around for hundreds and in some cases thousands of years, and they’re very safe,” Mayer says. “Each has a different combination of microbes, and so if you rotate those — say you eat them each once or twice a week — that’s probably your best chance to get a benefit.”
Eventually, he says that doctors may be able to quickly and cheaply map a patient’s microbiome and order a probiotic tailored to that person’s unique community of gut organisms. But we’re not there yet. “At this point,” he says, “we’re dealing with a very complex ecosystem that we don’t fully understand.”
Markham Heid, a reporter and writer, is a long-time contributor at TIME, Men’s Health, Food & Wine, and other outlets. He has received reporting awards from the Society of Professional Journalists and the Maryland, Delaware, and D.C. Press Association.