Probiotic supplements are often sold with highly specific promises: support your gut, balance vaginal health, boost immunity or help your microbiome. But a new product analysis suggests the science behind those claims may be less precise than the labels make it seem.
In a study published in Nature Microbiology, researchers at the University of Virginia School of Medicine analyzed 352 over-the-counter probiotic supplements sold at CVS, Walgreens and Walmart. The study was not a clinical trial testing whether the supplements improved people’s health. Instead, it examined what bacteria the products contained, how those bacteria were grouped across different products and whether those combinations appeared to line up with the health uses being marketed.
Across all 352 products, the researchers found only 36 unique bacterial species. More than half of the products contained just one probiotic species, while the product with the most species contained 17.
The researchers concluded there was no clear consistency in how bacterial species were combined for different health categories, including gut health and vaginal health. That does not mean probiotic supplements never help. It does mean the formulas behind many products may not be as specifically matched to their marketed uses as shoppers might assume.
Probiotics are live microorganisms intended to provide a health benefit when consumed or applied to the body. They are found in some foods, such as yogurt and other fermented foods, and they are also sold as dietary supplements. Many of the bacteria used in probiotic products are similar to microbes that naturally live in or on the body.
But dietary supplements are not regulated like drugs in the United States. That means probiotic products can be sold for general wellness support without the same level of evidence required for a medication to treat or prevent a disease. According to the UVA researchers, the Food and Drug Administration has approved only two microbial products for therapeutic purposes, both for recurrent C. difficile infections in the colon.
That leaves a large gap between what many probiotic products suggest and what scientists fully understand.
“It is truly fascinating to discover that these probiotic bacteria hold a unique, specialized niche among the trillions of microbes in and on the human body,” said Glynis Kolling, PhD, a research faculty member in UVA’s Department of Biomedical Engineering who works closely with Jason Papin, PhD, who led the research.
The team also developed HaPaPro, a collection of more than 1,000 computer models of bacterial metabolism. The goal was to better understand what different microbes may actually do, rather than grouping them mainly by broad labels such as “gut health” or “women’s health.”
As a test case, the researchers used the models to look at vaginal health. The vaginal microbiome is a natural ecosystem of bacteria, fungi and other microbes. When that ecosystem is disrupted, bacterial vaginosis can occur, which may cause discomfort and is associated with pregnancy complications, pelvic inflammatory disease and higher risk of sexually transmitted infections.
The researchers identified microbes that may have the potential to help prevent bacterial vaginosis by competing with or limiting the growth of less desirable microbes. But those findings came from computational modeling and laboratory analysis, not from a clinical trial showing that a supplement prevents bacterial vaginosis in people.
That distinction is important. The study points to a possible way to design more targeted probiotics in the future. It does not prove that today’s probiotic products can deliver the specific results they often imply.
“It is remarkable how much microbes play a role in human health and well-being,” Papin said. “I love seeing how computational models of these complex biological systems are leading to new ideas for therapies and helping us understand such fundamental biological processes.”
The new study adds to a larger point about microbiome science: The field is promising, but still complex. The microbiome is not a single switch that can be flipped with one capsule. It is a living ecosystem shaped by diet, medications, illness, environment, hormones and many other factors.
That complexity matters in a supplement aisle where products often use broad language such as “gut health,” “immune support” or “women’s health.” The evidence for probiotics is often strain-specific, meaning one microbe studied for one purpose cannot be assumed to work the same way for another condition. A supplement marketed for general gut health, for example, may not be supported by the same kind of evidence as a specific probiotic strain studied for a specific digestive issue.
The same caution applies when comparing probiotic supplements with fermented foods. Yogurt, kefir, kimchi, sauerkraut and other fermented foods can be part of a healthy eating pattern, but not all fermented foods contain live microorganisms by the time they are eaten. Eating fermented foods is also not the same as taking a supplement designed to deliver a specific dose of specific bacteria.
The study does not close the case on probiotics. If anything, it shows why the next phase of microbiome science may need to become more precise than many current supplement labels suggest.
The work was supported by the National Science Foundation and the National Institutes of Health. The authors reported no financial interest in the probiotic industry. Jason Papin, PhD, disclosed a stake in Cerillo, which made instrumentation used in some of the analyses.