It's easy to be confused by the frenzy of information surrounding probiotics — the "good" bacteria (sometimes called "live" bacteria) that are most often taken in pill form or through foods such as yogurt. Some people swear by them to treat a range of ailments from gut health to allergies, and those loyal fans have transformed probiotics into a $35 billion industry. But while anecdotal evidence may support those theories, the scientific evidence is still catching up.
Sure, previous studies have shown promise. For example, a 2013 study from the California Institute of Technology found that probiotic therapy reduced autism-like behaviors, but the study was performed on mice, so the results may not translate to humans. And a 2008 study published in the American Journal of Clinical Dermatology showed that probiotics could successfully treat atopic dermatitis. However, that study was based on a review of three small studies of pregnant women and babies only, and the authors wrote in their conclusion that "given the limitations of the review, methods should be treated with caution."
However, other studies have shown more concrete health benefits, especially when it comes to using probiotics to treat gastrointestinal issues, such as irritable bowel syndrome (IBS) or diarrhea. But as Scientific American recently reported, most studies have not shown health benefits for people who are already healthy:
"There is no evidence to suggest that people with normal gastrointestinal tracts can benefit from taking probiotics," says Matthew Ciorba, a gastroenterologist at Washington University in St. Louis. "If you're not in any distress, I would not recommend them." Emma Allen-Vercoe, a microbiologist at the University of Guelph in Ontario, agrees. For the most part, she says, "the claims that are made are enormously inflated."
So who benefits from probiotics?
Experts generally agree that probiotics have proven power in treating these three groups of people:
Pre-term infants. A 2017 study based on a large clinical trial of 4,500 newborns in India found that babies who took a probiotic that contained a strain of Lactobacillus plantarum were significantly less likely to develop sepsis — a life-threatening condition where infections cause inflammation and organ failure. It kills around 600,000 infants every year.
In the trial, only 5.4 percent of the infants who took the probiotic developed sepsis, which was a reduction of 40 percent. "Given the clear evidence of benefits, independent experts who were monitoring the study decided to stop the trial early: It would have been unethical to continue depriving half the newborns of the treatment," The Atlantic reports.
Necrotizing enterocolitis (NEC) is another sad and often fatal disease that infects the bowels of premature babies during their first few weeks of life. Doctors don't know exactly why it happens, but they know it has to do with bacteria in their still-developing bellies. A 2014 review found that giving those infants probiotics prevented the condition and reduced the number of preemies who developed it.
However, a 2016 study questioned previous findings because they "were summaries of different probiotic products and treatment protocols and did not allow drawing conclusions on the most beneficial strains and strategies." While that may be true, the authors seem to agree that some strains of probiotics have been proven to help some babies prevent a life-threatening disease, which is still a good thing.
People with IBS. The International Foundation for Functional Gastrointestinal Disorders (IFFGD) says that using probiotics to improve your gut bacteria can relieve the symptoms of IBS, and that advice is backed up by multiple studies. In particular, Bifidobacterium infantis has been shown to reduce pain, gas and bloating in IBS patients.
However, as Scientific American points out:
Microbiologists often caution that a promising study on a single strain of a particular species of bacteria should not be taken as proof that all probiotics work equally well. "Bacterial strains are so genetically different from one another, and everybody has a different gut microbiota," Allen-Vercoe says. "There will probably never be a one-size-fits-all probiotic."
It's worth noting that probiotics have not been proven to benefit people with other gut-related ailments, including Crohn's disease and inflammatory bowel disease (IBD).
Anyone taking antibiotics. Antibiotics kill gut bacteria, both good and bad, changing the bacterial makeup of your gut. Plus, antibiotics are known to cause diarrhea in one third of all patients. Multiple studies have found that taking a probiotic along with your antibiotic can prevent antibiotic-associated diarrhea (AAD).
The right microbes in the right gut
Why is it so tricky for the medical community to tell us whether taking probiotics will help with a certain condition or not? Because between 300 to 500 bacterial species exist in our gut, and the number of individual bacteria reaches into the trillions. Plus, each person's gut is different. So figuring out the magical combination of which bacteria strain needs to be targeted to fix which condition in which person is a complicated equation.
"Right now, we can't predict who's going to feel better and who's not," Purna Kashyap, an assistant professor of medicine at the Mayo Clinic and a member of the scientific advisory board for the American Gastroenterological Association's Center for Gut Microbiome Research and Education, told The Washington Post. "There's no way to confirm what is listed on the box is what is in the box, that it's safe and useful, and that it will help you," Kashyap said. The Post points to a 2015 examination of 16 probiotic products that found only one contained the bacterial strain listed on the label of every tested sample.
That example is one of many reasons why doctors and medical organizations say that probiotics are not a substitute for medication, and that you should consult your doctor before taking them.
Editor's note: This story has been updated since it was originally published in July 2017.