Supplements and VitaminsType 2 Diabetes (T2D)Behavioral Intervention
RESEARCH SUMMARY

Bifidobacterium longum BL21 probiotic may improve HbA1c when added to metformin in type 2 diabetes

Moderate confidence
some concerns bias
Last updated May 4, 2026

Key takeaway:

A 12-week double-blind RCT in adults with T2DM found that adding Bifidobacterium longum BL21 to metformin significantly lowered HbA1c by 0.3% and favorably shifted gut microbiota compared to placebo.

Study at a glance

What was studied

Whether adding BL21 probiotic to metformin improves glycemic control in T2DM

Study type

Randomized Controlled Trials (RCTs)

duration

Short-Term (≤3 mo)

Intervention

Probiotics

Outcomes

HbA1c, Fasting Plasma Glucose, Insulin resistance, Triglycerides, Gut microbiota diversity

Funding

Non-industry sponsored

Main effects

↓ HbA1c (0.3% reduction vs placebo, p=0.026, meeting ADA MCID threshold)

↑ Beneficial gut bacteria (Bifidobacterium, Faecalibacterium enriched)

→ Fasting glucose, insulin resistance, triglycerides showed non-significant improvement trends

Evidence Summary

InterventionOutcomeMeasured ChangeStudy Effect
Supplements
Probiotics
(Supplements)
Glycemic Control
Fasting Plasma Glucose
(Glycemic Control)
Decrease
Limited
Supplements
Probiotics
(Supplements)
Metabolic Health
Gut microbiota diversity
(Metabolic Health)
Increase
Strong
Supplements
Probiotics
(Supplements)
Glycemic Control
HbA1c
(Glycemic Control)
Decrease
Strong
Supplements
Probiotics
(Supplements)
Metabolic Health
Insulin resistance
(Metabolic Health)
Decrease
Limited
Supplements
Probiotics
(Supplements)
Metabolic Health
Triglycerides
(Metabolic Health)
Decrease
Limited

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evidence suggest

Evidence Suggest

  • BL21 significantly reduced HbA1c compared to placebo as an adjunct to metformin (p=0.026)
  • Favorable gut microbiota changes with increased SCFA-producing genera and reduced pathogenic taxa
  • Safety profile comparable to placebo with no serious adverse events reported
who this applies

Who this applies to

Adults with type 2 diabetes currently taking metformin who have room for improvement in their blood sugar control and are interested in evidence-based probiotic options as a supportive therapy.

keep in mind

Keep in Mind

The HbA1c reduction of 0.3% is modest compared to prescription diabetes medications. The non-significant findings for fasting glucose and insulin resistance suggest not all metabolic parameters benefited. The study was also relatively small and short, so larger and longer trials are needed to confirm whether this effect lasts and who benefits most.

between the lines

Between the Lines

  • Small sample size without formal a priori power calculation
  • 12-week duration may not capture long-term efficacy or sustainability
  • Gut microbiota analysis was taxonomic/descriptive without direct SCFA measurement

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Journal Reference

Zhu C, Liu Y, Chen Y, et al. Probiotic Supplementation With Bifidobacterium longum Subsp. Longum BL21 Improves Glycemic Control and Modulates Gut Microbiota in Type 2 Diabetes: A Randomized Controlled Trial. Food Sci Nutr. 2026;14(1):e71437.

Sources

Ranked by clinical relevance and evidence quality.

Key References

Most relevant evidence and guidance related to this research.

1
Guideline

American Diabetes Association Standards of Care in Diabetes—2025: Pharmacologic Approaches to Glycemic Treatment

The ADA's clinical practice guidelines establish the minimum clinically important difference (MCID) threshold for HbA1c reduction (0.3%–0.5%) and provide evidence-based recommendations for adjunctive therapies in type 2 diabetes management.
2
Guideline

Insulin Resistance & Type 1 | ADA

Insulin resistance is a problem found in people with prediabetes and type 2 diabetes, but people with type 1 diabetes can become insulin resistant, too.
3
Guideline

Standards of Care in Diabetes—2024

Annual ADA clinical practice guideline issue covering broad diabetes management.

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