(NEWS) Probiotics & Gut Microbiota: Meta-analysis shows no effects on diversity in healthy individuals – What the research says

Probiotics are considered "good gut bacteria" and are often taken to increase the diversity of the gut microbiota—a characteristic associated with better health. However, a new meta-analysis of 22 randomized controlled trials (1,068 healthy participants) shows that probiotic supplementation does not significantly alter microbiome diversity in healthy individuals. Neither the Shannon index (p>0.05), nor the Chao1, Simpson, or other alpha diversity indices showed any effects. Study duration, probiotic family, or risk of bias did not change the results. But what does this mean for you? Are probiotics useless? And for whom might they still be beneficial? This article explains the latest findings—practical and evidence-based.

What the meta-analysis shows

The meta-analysis examined 22 RCTs with a total of 1,068 healthy adults. The key findings:

• Shannon Diversity Index: Median difference (MedD) = -0.08 (95% CI: -0.16 to 0.01; p>0.05) – no significant effect

• Observed OTUs (Operational Taxonomic Units): MedD = 2.19 (95% CI: -2.20 to 6.57; p>0.05) – no significant effect

• Chao1-Index: MedD = -3.19 (95% CI: -27.28 to 20.89; p>0.05) – no significant effect

• Simpson Diversity Index: MedD = -0.01 (95% CI: -0.02 to 0.00; p>0.05) – no significant effect

• Subgroup analyses: Neither the taxonomic family of the probiotics, the bias risk of the studies, nor the intervention duration changed these results.

• Beta diversity: Most of the included studies also reported no differences between probiotic and control groups.

• Overall conclusion: Of 9,217 identified articles, 47 met the inclusion criteria; 22 studies were included in the quantitative analysis.

Mechanism: What does microbiome diversity actually mean?

The diversity of the gut microbiota describes the variety of different bacterial species in the gut. Higher diversity is often associated with better health.

• Shannon index: Measures both the number of species (richness) and their evenness – a higher value means greater diversity.

• Chao1 index: Estimates the total number of different species (richness) – also takes rare species into account.

• Simpson Index: Highlighting common species – value from 0 to 1, where 1 represents maximum diversity

• Alpha diversity: Describes the diversity within a sample (such as Shannon, Chao1, Simpson)

• Beta diversity: Describes differences between different samples or groups.

• Why is diversity important? Greater diversity is associated with resilience to disorders, better metabolic health, and a lower risk of disease.

• Probiotic expectation: It was assumed that probiotics would increase diversity by introducing new types of bacteria.

• Reality: In healthy individuals with an already stable microbiota, probiotics show no measurable diversity effect.

What these results mean for you

The study raises important questions:

• Possibly unnecessary for healthy individuals: If your gut health is already good, standard probiotics will not additionally increase microbiome diversity.

• Mechanism unclear: Probiotics could have other effects (e.g., immunomodulation, production of metabolites) that are not measurable via diversity.

• Microbiome stability: A healthy microbiome is resistant to changes – this is actually positive, but makes modulation by probiotics more difficult.

• Transient colonization: Most probiotic strains do not remain permanently in the intestine, but only pass through it.

• No increase = no deterioration: The study also shows that probiotics do not reduce diversity – so they are safe.

• Possible functional effects: Probiotics could improve symptoms (e.g., bloating, diarrhea) even without changing the diversity of bacteria.

• Individual differences: Some people respond to probiotics (“responders”), others do not (“non-responders”) – the initial state of the microbiome plays a role.

For whom might probiotics still be beneficial?

Although diversity does not increase in healthy individuals, there are situations in which probiotics can be helpful:

• After antibiotic use: Restoration of disrupted microbiota (evidence-based for C. difficile-associated diarrhea)

• Irritable bowel syndrome (IBS): Symptom relief for certain subtypes (e.g., IBS-D with diarrhea)

• Chronic inflammatory bowel diseases: As an adjunct to therapy for ulcerative colitis (certain strains such as VSL#3)

• Acute infectious diarrhea: Reduction of illness duration in children and adults

• Disrupted microbiota: Probiotics could have a stabilizing effect in cases of proven dysbiosis.

• Not for healthy individuals for prevention: Current evidence does not support routine use in healthy people to "improve" diversity.

• Personalized approaches: Future research could show which strains are suitable for which microbiome profiles.

Comparison: Probiotics vs. other interventions for gut health

If probiotics don't increase diversity, what does?

• A diet rich in fiber: Prebiotics (e.g., inulin, resistant starch) promote the growth of existing beneficial bacteria – a stronger diversity effect than probiotics.

• Fermented foods: yogurt, kefir, kimchi, sauerkraut – provide probiotics plus metabolites that are already produced during fermentation

• Mediterranean diet: Increases microbiome diversity through a variety of plant-based foods, omega-3 fatty acids, and polyphenols.

• Exercise: Regular physical activity increases microbiome diversity regardless of diet.

• Avoiding unnecessary antibiotics: Protects existing diversity from disruption

• Synbiotics: Combination of probiotics + prebiotics – theoretically more effective, but evidence is still limited.

• FMT (Fecal Microbiota Transplantation): Highly effective in cases of severe dysbiosis (e.g., recurrent C. difficile infection) – not for healthy individuals.

Limitations of the meta-analysis

The authors identify five key limitations:

• Heterogeneity of the studies: Different probiotic strains, dosages and formulations make comparability difficult.

• Subjective endpoints: Microbiome analyses are based on stool samples – may not represent mucosa-associated bacteria.

• Short-term studies: Intervention duration often only a few weeks – long-term effects unclear

• Healthy populations: Results not applicable to people with dysbiosis or disease.

• Lack of mechanistic data: Study only measures diversity – other functional effects (metabolites, immune function) were not investigated.

⚠ Important note:

This information is for general informational purposes only and does not constitute medical advice. Probiotics can have different effects depending on individual health conditions. If you have a pre-existing intestinal disorder, immunosuppression, or are considering taking probiotics, you should consult your doctor. This study refers exclusively to healthy adults.

SOURCES (3 with DOI):

1. Elias AJ, et al. Effect of probiotic supplementation on the gut microbiota diversity in healthy populations: a systematic review and meta-analysis of randomized controlled trials. BMC Med. 2026;24(1):71. DOI: 10.1186/s12916-025-04602-0

2. Marco ML, Cunningham M, Bischoff SC, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of gut health. Nat Rev Gastroenterol Hepatol. 2026. DOI: 10.1038/s41575-026-01176-x

3. Van Hul M, et al. What defines a healthy gut microbiome? Good. 2024;73:1893–1908. DOI: 10.1136/gutjnl-2024-333378