The Bidirectional Relationship Between Type 1 Diabetes Mellitus and Obesity in Pediatric Patients: A Systematic Review

Background/Objectives: The rising prevalence of childhood obesity has coincided with increasing incidence of type 1 diabetes mellitus (T1DM), raising questions regarding their potential bidirectional interaction. This systematic review evaluated the association between obesity and T1DM risk, as well as post-diagnostic weight trajectories and metabolic outcomes in pediatric populations. Methods: A systematic review was conducted in accordance with PRISMA 2020 guidelines. PubMed, Embase, and Scopus were searched for studies published between January 2010 and January 2026. Eligible studies included observational and interventional research in children and adolescents addressing T1DM and obesity; reviews, case reports, and non-English studies were excluded. Risk of bias was assessed using Joanna Briggs Institute tools. Due to heterogeneity, results were synthesized narratively. Results: Sixty-seven studies were included. Population-based data showed a positive association between higher BMI and incident T1DM, with obesity associated with a twofold increased risk (HR 2.05, 95% CI 1.58–2.66) and a 25% increase per 1-SD BMI increment. Insulin resistance (IR) indices correlated with BMI and predicted faster progression to clinical T1DM in autoantibody-positive individuals. At diagnosis, 20–30% of children were overweight or obese, increasing to 30–40% during follow-up. Excess adiposity was associated with higher insulin requirements and increased prevalence of hypertension and dyslipidemia. Longitudinal data indicate that BMI standard deviation scores rise with age, diabetes duration, and pubertal stage, with higher insulin doses and intensive insulin therapy contributing to weight gain. Conversely, some large cohort studies report no linear association between BMI and incident T1DM, indicating heterogeneity across populations. The limitations of this review include the predominance of observational studies, heterogeneous methodologies, and limited generalizability beyond predominantly European and North American pediatric populations. Conclusions: Overall, the evidence supports a bidirectional relationship: obesity may increase T1DM risk and accelerate disease progression, while T1DM-related factors promote weight gain after diagnosis. These findings highlight the importance of integrating weight management strategies into routine pediatric T1DM care.