Effects of Probiotic and Synbiotic Supplementation on Glycemic Indices in Adult Individuals with Prediabetes and Type 2 Diabetes Mellitus: A Grade-Assessed Systematic Review and Dose–Response Meta-analysis of RCTs

Probiotic and synbiotic supplementation has garnered attention for its potential to modulate metabolic parameters, including glycemic control, in individuals with prediabetes and type 2 diabetes mellitus (T2DM). This systematic review and dose-response meta-analysis aims to evaluate the effectiveness of these supplements on glycemic indices, synthesizing evidence from randomized controlled trials (RCTs) with grade-assessed methodologies. A comprehensive search of scientific databases was conducted from inception through January 2025 to identify RCTs assessing the effects of probiotic and synbiotic supplementation on glycemic indices, including fasting blood glucose (FBG), fasting insulin (FI), hemoglobin A1c (HbA1c), and homeostatic model assessment for insulin resistance (HOMA-IR). Eligible studies were screened; relevant data were extracted, and the quality of evidence was evaluated using the GRADE approach. Statistical analyses were performed using STATA to generate pooled results. The pooled analysis of 48 RCTs demonstrated that probiotics and synbiotics supplementation significantly decreased FBG (WMD: - 7.00 mg/dL), FI (WMD: - 1.72 µIU/mL), HbA1c levels (WMD: - 0.34%), and HOMA-IR (WMD: - 0.91), with a substantial heterogeneity across all outcomes (p < 0.001). Subgroup analyses indicated reductions in FBG, FI, HbA1c, and HOMA-IR across varying baseline characteristics, including glycemic levels, trial durations, and participant health status. Both probiotics and synbiotics were effective, with notable benefits for patients with T2DM, longer intervention durations, and higher baseline BMI. Meta-regression and dose-response analyses revealed no significant association between the duration of probiotic and synbiotic supplementation and changes in glycemic indices. Probiotic and synbiotic supplements significantly reduce FBG, FI, HbA1c, and HOMA-IR, leading to improvements in glycemic markers in individuals with prediabetes and T2DM. These supplements can be used as effective adjunctive strategies to enhance glycemic control, highlighting the promising role of gut microbiota modulation in diabetes management.