Researchers reviewed 48 studies about special supplements containing helpful bacteria (probiotics and synbiotics) and how they affect blood sugar control in people with prediabetes and type 2 diabetes. They found that people taking these supplements had lower blood sugar levels, better insulin control, and improved markers that measure how well their body handles glucose. The improvements were seen across different groups of people and were especially noticeable in those with type 2 diabetes who took the supplements for longer periods. While these results are encouraging, the supplements appear to work best as an addition to regular diabetes treatment, not as a replacement.

The Quick Take

  • What they studied: Whether taking supplements with helpful gut bacteria (probiotics) or combinations of these bacteria with special food for them (synbiotics) could help people with prediabetes and type 2 diabetes control their blood sugar better.
  • Who participated: The analysis combined results from 48 different research studies involving adults with prediabetes or type 2 diabetes. The studies varied in size and how long people took the supplements.
  • Key finding: People taking probiotic or synbiotic supplements had meaningful improvements in blood sugar control. Their fasting blood sugar dropped by about 7 points, their insulin levels improved, and their HbA1c (a measure of average blood sugar over 3 months) decreased by 0.34%, which is a clinically meaningful change.
  • What it means for you: If you have prediabetes or type 2 diabetes, these supplements may help improve your blood sugar control when used alongside your regular treatment plan. However, they should not replace your current diabetes medications or lifestyle changes—talk to your doctor before starting any new supplement.

The Research Details

This was a systematic review and meta-analysis, which means researchers searched through all available scientific studies on this topic and combined the results from 48 high-quality randomized controlled trials (the gold standard of research). They looked for studies published from the beginning of scientific literature through January 2025. Each study they included had people randomly assigned to either take probiotic/synbiotic supplements or a placebo (fake pill) to make the comparison fair.

The researchers carefully measured four different blood sugar markers: fasting blood glucose (your blood sugar when you haven’t eaten), fasting insulin (the hormone that helps control blood sugar), HbA1c (which shows your average blood sugar over three months), and HOMA-IR (a calculation that shows how well your body is using insulin). They used a special method called GRADE to evaluate how reliable and trustworthy each study was.

By combining results from many studies instead of looking at just one, researchers can see the bigger picture and be more confident about whether something actually works. This approach helps separate real effects from lucky coincidences. The researchers also looked at how different factors—like how long people took the supplements, how overweight they were, and what type of diabetes they had—affected the results.

The researchers used the GRADE method to assess study quality, which is considered the best way to evaluate research. They found substantial variation between studies (called heterogeneity), which means different studies got somewhat different results. This variation could be due to differences in which bacteria strains were used, how long people took supplements, or differences in the study populations. The fact that improvements were seen consistently across many different studies and different groups of people makes the findings more reliable.

What the Results Show

The combined analysis of all 48 studies showed that probiotic and synbiotic supplements produced measurable improvements in blood sugar control. Fasting blood glucose (the amount of sugar in your blood when you wake up before eating) decreased by about 7 mg/dL on average. This might sound small, but it’s clinically meaningful—it’s the kind of improvement doctors look for.

Insulin levels also improved, dropping by about 1.72 units, which means people’s bodies didn’t need to produce as much insulin to control their blood sugar. The HbA1c level, which shows your average blood sugar over three months, decreased by 0.34%. While this might seem modest, it represents real improvement in long-term blood sugar control. Additionally, HOMA-IR (a measure of insulin resistance—how well your body uses the insulin it makes) improved by 0.91 points.

When researchers looked at different groups of people, they found that the benefits appeared across the board: people with different starting blood sugar levels, people who took supplements for different lengths of time, and people with different body weights all saw improvements. The benefits were especially strong in people with type 2 diabetes (as opposed to prediabetes) and in people who took the supplements for longer periods.

The analysis revealed that both types of supplements—probiotics alone and synbiotics (probiotics plus their food source)—were effective, though the results were similar between the two types. People with higher starting body weight seemed to benefit more from the supplements. The duration of supplementation mattered: longer studies tended to show better results, suggesting that these supplements need time to work. Interestingly, when researchers looked at dose-response relationships (whether taking more bacteria for longer periods produced better results), they didn’t find a clear pattern, suggesting that moderate doses taken consistently may be just as effective as higher doses.

This research builds on growing evidence that gut bacteria play an important role in blood sugar control and diabetes. Previous smaller studies had suggested benefits, but this comprehensive review combining 48 studies provides stronger evidence. The findings align with what scientists know about how gut bacteria influence metabolism and insulin sensitivity. This research adds credibility to the idea that modifying gut bacteria through supplements could be a useful tool in diabetes management, though it’s still considered a complementary approach rather than a primary treatment.

The studies included had significant variation in their methods, which makes it harder to draw definitive conclusions. Different studies used different bacterial strains, different doses, and different treatment durations, which could explain why some studies showed bigger benefits than others. The researchers couldn’t find a clear dose-response relationship, meaning we don’t know the optimal amount or duration of supplementation. Most studies were relatively short-term, so we don’t know if benefits continue long-term or if people need to keep taking supplements indefinitely. Additionally, the quality of evidence was sometimes moderate rather than high, meaning some studies had design issues that could affect reliability. Finally, the analysis couldn’t determine which specific bacterial strains work best or which types of people benefit most.

The Bottom Line

For people with prediabetes or type 2 diabetes: Probiotic and synbiotic supplements appear to be a safe, evidence-based addition to your diabetes management plan. They may help improve blood sugar control when combined with medication, diet, and exercise. Start with a moderate dose and take consistently for at least 8-12 weeks to see benefits. However, these supplements should never replace your prescribed diabetes medications or lifestyle changes. Confidence level: Moderate to High (based on 48 studies, though with some variation in quality).

People with type 2 diabetes or prediabetes who want additional tools to help manage their blood sugar should consider discussing these supplements with their doctor. People with higher body weight may see more benefit. However, people with severe diabetes complications, those taking certain medications that interact with probiotics, or those with compromised immune systems should consult their healthcare provider before starting. Pregnant women and young children should also check with their doctor first.

Most studies showed measurable improvements within 8-12 weeks, though some benefits appeared earlier. For the best results, plan to take these supplements consistently for at least 2-3 months before evaluating whether they’re helping. Some people may see benefits sooner, while others might need longer. Remember that these supplements work gradually by changing your gut bacteria, so patience is important.

Want to Apply This Research?

  • Log daily probiotic/synbiotic supplement intake and track fasting blood glucose readings weekly (if you monitor at home). Create a simple yes/no daily checklist for supplement adherence and record blood sugar numbers each morning before eating to see trends over 8-12 weeks.
  • Set a daily reminder to take your probiotic supplement at the same time each day (consistency matters for effectiveness). Pair it with an existing habit like breakfast or brushing teeth. Also increase fiber intake from foods like vegetables, fruits, and whole grains, since probiotics work better when they have adequate food sources.
  • Track supplement adherence weekly and blood glucose readings bi-weekly. After 8-12 weeks, review your average fasting blood sugar and any HbA1c test results with your doctor to assess whether the supplements are helping. Continue tracking monthly to ensure sustained benefits and adjust other diabetes management strategies as needed based on results.

This research summary is for educational purposes only and should not replace professional medical advice. Probiotic and synbiotic supplements may interact with certain medications or be inappropriate for people with specific health conditions. Before starting any new supplement, especially if you have diabetes, take medications, or have a compromised immune system, consult with your healthcare provider. These supplements are intended to complement, not replace, your prescribed diabetes medications, diet, and exercise plan. Results may vary between individuals, and what works for one person may not work for another. Always inform your doctor about any supplements you’re taking.