The impact of probiotics or synbiotics on individuals with type 1 diabetes mellitus (T1DM): a systematic review and meta-analysis

Type 1 Diabetes Mellitus (T1DM) is a chronic autoimmune condition characterized by insulin deficiency. Probiotics and synbiotics are believed to impact glycemic control. This study aims to evaluate their effects on individuals with T1DM. A systematic search was conducted from inception to January 2026 in Cochrane Central, Web of Science, Scopus, and PubMed, with additional screening of gray literature and reference lists. Randomized controlled trials evaluating probiotics or synbiotics in individuals with type 1 diabetes mellitus (T1DM) were included. Risk of bias was assessed using the Cochrane ROB-2 tool. Data were synthesized using weighted mean difference (WMD) with 95% confidence intervals under a random-effects model in STATA-17. Subgroup, sensitivity, and publication bias analyses were also performed. From 2,722 records, 10 randomized controlled trials were included. Meta-analysis showed that probiotics or synbiotics significantly reduced HbA1c (WMD = -0.58%, 95% CI [-1.00, -0.17]) and fasting blood sugar (WMD = -43.34 mg/dL, 95% CI [-81.71, -4.96]) compared with controls. No significant effects were observed for C-peptide (WMD = -0.03 ng/mL, 95% CI [-0.11, 0.05]) or insulin requirement (WMD = -0.09 U/kg/day, 95% CI [-0.19, 0.01]). A significant reduction was found in total cholesterol (WMD = -7.92, 95 mg/dL CI [-15.73, -0.12]), whereas no significant changes were found for CRP, TG, HDL, LDL, or TNF-α. Probiotics or synbiotics may improve glycemic control in T1DM, particularly by reducing HbA1c and fasting blood sugar. The observed HbA1c improvement may be clinically meaningful if sustained. However, evidence for effects on beta-cell function, insulin requirement, inflammatory markers, and most lipid outcomes remains limited. Larger, high-quality trials with standardized interventions are needed to confirm these findings. Not applicable.