You mail in a stool sample. A few weeks later, you get a personalized report about your gut health — maybe a list of bacteria, a “health score,” and recommendations to buy certain supplements or change your diet. It sounds like science. But a major new study suggests those results may tell you very little about what’s actually living in your gut.
The gut microbiome boom
Interest in the gut microbiome — the community of trillions of bacteria and other microbes living in your digestive tract — has exploded over the past decade. A landmark moment came in 2012, when the National Institutes of Health released the Human Microbiome Project’s reference database. Researchers mapped the microbial makeup of healthy people for the first time. Since then, studies have linked the gut microbiome to digestion, immune function, mental health, and more.
The media picked up on this research enthusiastically. So did the wellness industry. Today, dozens of companies worldwide sell direct-to-consumer (DTC) gut microbiome tests. Verified Market Reports valued the home testing market at around $1.8 billion in 2025, and projected it to more than triple by 2034.
But popularity and reliability are not the same thing.
Evaluating home gut microbiome tests
A team led by Stephanie L. Servetas, PhD, at the National Institute of Standards and Technology (NIST) set out to evaluate whether these tests actually work. They published their findings in February 2026 in the peer-reviewed journal Communications Biology.
The study tested seven DTC gut microbiome testing companies. Researchers ordered three kits from each company — 21 kits total. Then they did something clever: they sent each company the exact same stool sample. Not samples from different people. One homogenized, carefully prepared sample, split into identical portions. If the tests were reliable, the results should look similar. They did not.
Researchers report shocking results
The differences were striking. Across all nine analyses (seven companies plus NIST’s own lab using two methods), researchers identified 1,208 unique microbial groups. Only three appeared in every company’s results. That’s a remarkable lack of agreement.
The variability between companies was as large as the biological difference between completely different people. In other words, sending the same sample to two different companies gave you results as different as if two separate people had taken the test.
A single company labeled the same identical sample as “healthy” in two reports — and “unhealthy” in a third.
One result was especially troubling. A single company labeled the same identical sample as “healthy” in two reports — and “unhealthy” in a third. That’s the kind of inconsistency that could push someone toward unnecessary supplements or interventions.
The researchers traced the problem to differences in how each company collects, ships, and processes samples. Companies use different DNA sequencing methods, different software, different databases, and different thresholds for reporting results. Any one of these steps can introduce errors. Together, they produce wildly different outputs from the same source material.
This stacked bar graph shows how different the results were when identical stool samples were analyzed by different companies that provide home test kits for the gut microbiome. The x-axis (across the bottom of the graphic) shows identifiers for the samples; each company is represented by a letter, and each replicate sample is represented by a number after a dash. The y-axis (the left side of the graphic) shows the relative abundance of the 35 most prevalent genera of bacteria found in all samples. At right, a color code appears for these genera. The grey area above each bar represents unclassified or uncommon genera. Graphic courtesy of Stephanie L. Servetas, PhD, NIST.
The consequence of false gut microbiome results
These tests are not just curiosities. They’re marketed to people looking for answers about their health. In an article by Joedy McCreary in MedPage Today, study co-author Diane Hoffmann of the University of Maryland put it plainly: the hype around gut microbiome testing “doesn’t really match the evidence.” Many people who seek these tests, she noted, are those with chronic gut conditions who have struggled to find effective treatments. For them, unreliable results could mean unnecessary costs, wrong interventions, and delays in getting real medical care.
Many companies also sell supplements alongside their tests — creating a conflict of interest. Some recommend costly probiotics based on results that, as this study shows, may not be reproducible.
A 2024 editorial in The Lancet Gastroenterology & Hepatology raised similar concerns. It called for tighter regulation of these products and noted that, in the US, microbiome tests are not regulated as medical devices. The FDA only steps in when a company makes explicit diagnostic claims. In the UK, the situation is similar — these tests are classified as “lifestyle” or “wellness” products, sidestepping medical requirements. That gap in oversight, the editorial argued, allows companies to market products that lack scientific support.
What the researchers are not saying
This study evaluated one important question: Are these tests reproducible? It found they often are not. What it could not determine is which company, if any, produced results closest to the actual truth. NIST is developing standardized reference materials that could eventually help answer that question.
The researchers also emphasized that this study used laboratory-prepared samples, not real-world stool samples collected at home. Real samples carry additional sources of variability. The results here likely represent the best-case scenario for reproducibility, yet the companies utterly failed.
In the MedPage Today article, co-author Scott Jackson, PhD, was careful to frame the study’s intent: “We want the industry and the scientific community to do better.” He called for standardized protocols, quality benchmarks, and independent oversight — not to discredit microbiome science, but to push it toward the rigor it needs before people can rely on it for health decisions.
What actually works for gut health
While gut microbiome tests face serious reliability questions, the science on improving gut health points more clearly in certain directions. The best-supported approaches don’t require a mail-in kit.
Research consistently shows that dietary diversity is one of the most powerful tools. Eating a wide variety of plant-based foods — vegetables, legumes, whole grains, fruits, nuts, seeds — supports a more diverse and beneficial microbiome. Some researchers suggest aiming for 30 or more different plant foods per week. Regular exercise also appears to improve microbial diversity. Fermented foods like yogurt, kefir, and kimchi may introduce beneficial bacteria. Managing chronic stress and getting adequate sleep round out the picture.
None of these strategies requires spending money on a home test. And unlike a test result that may vary depending on which company processes it, these lifestyle changes have a consistent body of evidence behind them.
How to evaluate health claims
The gap between what a product promises and what science supports is wide in this space. Before spending money on any health test or supplement, it’s worth checking the quality of the evidence. The Parsemus Foundation offers a practical guide to finding reliable, evidence-based health information at parsemus.org/how-to-find-evidence-based-health-information.
DTC gut microbiome tests may prove useful one day. Researchers are working to develop the standards that would make that possible. But no such tests have been approved or recommended yet by the U.S. Department of Health and Human Services. For now, the science doesn’t support using them to make health decisions. Save your money, eat your vegetables, and talk to a doctor if you have gut health concerns.
Source: Servetas, S.L., Gierz, K.S., Hoffmann, D. et al. Evaluating the analytical performance of direct-to-consumer gut microbiome testing services. Commun Biol 9, 269 (2026). https://doi.org/10.1038/s42003-025-09301-3
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