Researchers looked at 47 studies involving over 1,000 healthy people to see if probiotic supplements (the “good bacteria” pills many people take) actually make their gut bacteria more diverse. Gut diversity is thought to be important for health. After analyzing the data, scientists found that probiotics didn’t significantly change how diverse people’s gut bacteria were compared to those who didn’t take them. While probiotics are popular and heavily marketed, this large review suggests they may not work as advertised for healthy people. However, researchers say more studies are needed to see if certain types of probiotics might help specific groups of people.

The Quick Take

  • What they studied: Whether taking probiotic supplements changes the variety of bacteria in your gut, which is believed to be good for your health
  • Who participated: Over 1,000 healthy people across 22 different research studies. These were people without serious health problems who took probiotics or a placebo (fake pill) for various lengths of time
  • Key finding: Probiotics did not make a meaningful difference in gut bacteria diversity compared to people who didn’t take them. The differences found were so small they could have happened by chance
  • What it means for you: If you’re a healthy person considering probiotics mainly to improve gut bacteria diversity, this research suggests they may not deliver on that promise. However, this doesn’t mean probiotics have no benefits—just that increasing bacteria variety may not be one of them for healthy people

The Research Details

This was a meta-analysis, which means researchers searched through thousands of published studies and combined the results from the best ones to get a clearer picture. They looked at studies published through April 2024 in major medical databases. They found 47 studies that met their quality standards, and 22 of these had enough detailed information to include in their final analysis.

The researchers used strict rules to decide which studies to include. All studies had to test probiotics in healthy people (not sick people), measure how diverse the gut bacteria were using scientific methods, and compare probiotic takers to people taking a placebo. This careful selection process helps ensure the results are reliable.

To combine the results fairly, researchers used statistical methods that account for differences between studies. They measured diversity using four different scientific scales (Shannon diversity, observed operational taxonomic units, Chao1, and Simpson’s index) to make sure their findings were consistent across different measurement methods.

A meta-analysis is powerful because it combines many smaller studies into one big picture. Individual studies can sometimes give misleading results by chance, but when you combine dozens of studies with thousands of people, you get a much clearer answer. This approach is especially important for probiotics because there’s a lot of marketing hype around them, and people need reliable information to make good choices about their health.

This study is strong because it was published in BMC Medicine, a respected medical journal. The researchers registered their plan ahead of time (in PROSPERO), which prevents them from changing their methods based on results. They searched multiple major databases and included 47 studies total, with 1,068 people in the final analysis. The fact that results were consistent across different measurement methods and different types of probiotics makes the findings more trustworthy. However, the studies included varied in quality, and some were quite small, which is why the researchers noted the need for more research.

What the Results Show

The main finding was clear: probiotics did not significantly change gut bacteria diversity in healthy people. When researchers looked at Shannon diversity (a common measure of bacteria variety), the difference between probiotic takers and non-takers was essentially zero (-0.08 on the scale). The confidence interval around this number was -0.16 to 0.01, which means even the upper range of possible benefit is tiny and could easily be due to chance.

The same pattern held true for three other diversity measures. For observed operational taxonomic units (a count of different bacteria types), the difference was 2.19, but the range was -2.20 to 6.57, meaning it could go either direction. For Chao1 index, the difference was -3.19 with a range of -27.28 to 20.89. For Simpson’s index, the difference was -0.01 with a range of -0.02 to 0.00. In all cases, the ranges were so wide that the true effect could be zero or even slightly negative.

When researchers looked at subgroups (different types of probiotics, different study lengths, studies with different quality levels), the results didn’t change. Probiotics still didn’t significantly improve diversity. This consistency across different analyses strengthens the conclusion that probiotics simply don’t increase gut bacteria diversity in healthy people.

The researchers also looked at other measures of bacteria diversity that weren’t included in the main analysis because there wasn’t enough data. Most of these studies also reported no meaningful differences between probiotic and control groups. This supports the main finding that probiotics don’t significantly change the makeup of gut bacteria in healthy people. The consistency of “no effect” across many different studies and measurement methods is important—it suggests this isn’t just one fluke result.

This finding is important because probiotics have been heavily marketed as a way to improve gut health and increase bacteria diversity. Many companies claim that more diverse gut bacteria lead to better health, and they use this as a reason to buy their products. This meta-analysis suggests that at least for healthy people, probiotics don’t deliver on the diversity promise. However, it’s worth noting that some previous smaller studies claimed benefits, which is why this larger combined analysis is valuable—it gives us a clearer, more reliable answer.

The researchers couldn’t analyze all the studies they found because some didn’t report their results in a way that could be combined with others. The studies included varied quite a bit in how long people took probiotics (from weeks to months), what types of probiotics were used, and how they measured diversity. Some studies were small, which means their results are less reliable. The researchers only looked at healthy people, so these results may not apply to people with digestive problems or other health conditions. Finally, probiotics are a huge category with many different strains and combinations, so it’s possible that some specific types might work even if most don’t.

The Bottom Line

For healthy people: Probiotics are unlikely to increase gut bacteria diversity based on current evidence (low confidence in this specific benefit). If you’re considering probiotics mainly for diversity, this research suggests you probably won’t see that benefit. However, some people report other benefits from probiotics, and they’re generally safe for healthy people. If you want to try them, do so knowing that diversity improvement isn’t a proven outcome. For people with digestive issues or other health conditions: This study doesn’t apply to you, and you should talk to your doctor about whether probiotics might help your specific situation.

This research matters most for healthy people considering probiotics as a preventive health measure. It’s especially relevant if you’re thinking about spending money on probiotics specifically to improve gut bacteria diversity. People with digestive diseases, weakened immune systems, or serious health conditions should not use this study to make decisions—they need to talk to their doctors. The findings don’t tell us anything about whether probiotics might help with specific health problems.

If probiotics don’t increase diversity, you wouldn’t expect to see this benefit at any point. The studies looked at ranged from short-term (weeks) to longer-term (months), and none showed significant diversity changes. So whether you took probiotics for 2 weeks or 3 months, this research suggests diversity wouldn’t improve.

Want to Apply This Research?

  • If users want to track probiotic use anyway, suggest logging: (1) which probiotic product they’re taking and the strain name, (2) daily adherence (did they take it?), and (3) any digestive symptoms they notice (bloating, regularity, comfort). While diversity won’t change, they might notice other effects.
  • Instead of focusing on probiotics for diversity, users could track other evidence-based gut health habits: fiber intake (aim for 25-30g daily), water consumption, sleep quality, and stress levels. These factors actually influence gut health and are measurable in an app.
  • Create a gut health dashboard that tracks multiple factors (fiber, water, sleep, stress, probiotic use, digestive comfort) rather than focusing only on probiotics. This gives users a more complete picture of their gut health and helps them see what actually makes a difference for them personally.

This research applies to healthy adults and does not apply to people with digestive diseases, compromised immune systems, or other medical conditions. Probiotics are generally safe for healthy people but are not proven to increase gut bacteria diversity based on this evidence. If you have health concerns or are taking medications, consult your healthcare provider before starting probiotics. This article summarizes research findings and should not replace professional medical advice. Individual results may vary, and some people may experience benefits from probiotics even if diversity improvement isn’t one of them.