This systematic review and meta-analysis observed a small, statistically significant reduction in HbA1c (≈0.25%) with probiotic supplementation in children and adolescents with type 1 diabetes; however, the certainty of this evidence is low, the clinical importance is uncertain (below commonly cited ≈0.5% thresholds), and no consistent effects were detected for fasting blood glucose, C-peptide, or insulin requirements. This evidence concerns the gene INS and type 1 diabetes mellitus.