The researchers conducted a systematic review and meta-analysis of 30 randomized controlled trials, of which 25 were double-blinded, 1 was single-blinded, and 5 were unblinded. The study locations also varied: 24 studies were conducted in Asia, 3 in Europe, 2 in South America, and 1 each in Africa and Australia. No studies were conducted in the U.S.

The daily doses ranged from 10 million (1x10⁷) to 600 billion (6x10¹¹) colony-forming units. The participants ingested the probiotics as foods (bread, yogurt, and fermented milk) or as supplements (capsules, powder, and tablets). Most studies used capsules (9 studies), followed by yogurt, powder, and fermented milk (4 studies each), and also tablets, bread, and milk (2 studies each). The study duration ranged from 4 to 36 weeks (11 weeks on average).

Biomarkers of glycemic control included insulin, fasting glucose, hemoglobin A1c (HbA1c or glycated hemoglobin, a measure of average blood sugar levels over the past 2–3 months), and insulin resistance assessed by the homeostasis model assessment index for insulin resistance (HOMA-IR). The effect of probiotics on these glycemic biomarkers was evaluated compared to placebo in all studies. The researchers also determined the degree of heterogeneity to assess the variability in the effect sizes between the meta-analyzed studies.

Subgroup analyses were performed to evaluate whether the outcomes were influenced by the following factors:

Dose of probiotics: higher doses (>10 billion CFU) vs. lower doses (≤10 billion CFU)
Study duration: longer (>8 weeks) vs. shorter follow-ups (≤8 weeks)
BMI: participants with obesity (BMI >30) vs. those without obesity (BMI ≤30)
Ethnicity: participants of European origin vs. those of Asian origin
Type of probiotics: food-type probiotics (e.g., yogurt and fermented milk) vs. processed probiotics (e.g., powder, capsules, and tablets)
Genus of probiotics: Bifidobacterium (one of the major genera of beneficial bacteria found in the human gut)^[1] vs. Lactobacillus (another major genera of beneficial bacteria found in the gut, mouth, and genital tract)^[1] vs. a combination of both

The researchers also evaluated the quality of the studies to identify potential selection bias, detection bias, attrition bias, reporting bias, and performance bias.

Supplemental probiotics may improve glycemic control Original paper