Background To support the development of a national guideline on stem cell therapy, the Department of Health Research, India, commissioned this systematic review to evaluate the efficacy and safety of various stem cell types in patients with type 1 and type 2 diabetes mellitus (DM), focusing on patient-important outcomes. Methods Following PRISMA guidelines, a literature search was conducted in PubMed, Embase, Web of Science, and Cochrane databases from inception to August 30, 2024. Critical outcomes for type 1 DM included insulin-free periods, hypoglycemic episodes, quality of life, and serious adverse events. For type 2 DM, outcomes included HbA1c, insulin requirements at 6, 12, and 24 months, and serious adverse events. Meta-analyses used random- or fixed-effects models based on heterogeneity (Chi-square test and I2). Risk of bias was assessed using the Cochrane Risk of Bias Tool 2.0, and evidence certainty was evaluated with GRADE. Results The search identified 11,026 articles, of which 20 randomized controlled trials (RCTs) were included, encompassing 427 and 351 patients in the intervention and control groups, respectively, with follow-ups ranging from 3 to 96 months. Predominantly studied therapies included mesenchymal and bone marrow mononuclear stem cells. In type 1 DM, stem cell therapy showed no significant improvement in quality of life MD: 3.15% (95% CI: -0.80 to 7.10); 2 trials, n = 63 participants; I2 = 0%; GRADE: very low certainty or reduction in hypoglycemic episodes [RR: 0.90 (95% CI: 0.56 to 1.45); 3 trials, 68 participants; I2 – 0%; GRADE: very low. In type 2 DM, stem cell therapy significantly reduced insulin requirements at 6, 12, and 24 months, with MDs in IU/day of -14.42 (95% CI: -24.25 to -4.59); 6 trials, n = 167 participants; I2 = 91.64%; GRADE: low certainty; -17.79 (95% CI: -26.39 to -9.18); 6 trials, n = 212 participants; I2 = 70.96%; GRADE: low certainty; and -35.73 (95% CI: -40.82 to -30.64); 1 trial, n = 61 participants; I2 = NA; GRADE: very low certainty, respectively, with a low certainty of evidence. Conclusion Stem cell therapy did not achieve an insulin-free state or improved quality of life in type 1 DM patients. However, it reduced insulin requirements by 14–36 units over 6–24 months in type 2 DM patients, without significant glycemic control. Larger, high-quality RCTs with extended follow-ups are essential to determine the therapeutic potential of stem cell therapy in diabetes mellitus. Systematic review registration PROSPERO ID: CRD42023451602.
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