Obesity in Type 1 Diabetes: Moving Beyond the "Lean" Disease Paradigm to Understand Risk, Complications, and Treatment.

PURPOSE OF REVIEW: Overweight and obesity are increasingly common in people with type 1 diabetes mellitus (T1DM). This narrative review synthesizes current evidence on the epidemiology and mechanisms linking excess adiposity with T1DM, the obesity-associated burden of complications, and approaches to obesity management in this population. RECENT FINDINGS: Excess adiposity is now frequent in people with T1DM, approaching general-population prevalence, and obesity may also increase the risk of incident T1DM. In T1DM, weight gain reflects intersecting drivers, including intensive insulin therapy, hypoglycemia-related compensatory intake and activity avoidance, obesity-related insulin resistance, and overlapping genetic/hormonal determinants. Across multiple cohorts, higher adiposity is associated with poorer glycemic control and increased risk of cardiovascular events and mortality, as well as higher risks of retinopathy, diabetic kidney disease, and neuropathy. Evidence to guide treatment in T1DM remains limited. Lifestyle approaches require individualization to minimize hypoglycemia and ketosis. RCTs of adjunct liraglutide demonstrate weight loss and insulin-sparing effects but signal dose-dependent risks of hypoglycemia and ketosis. Early RCTs with semaglutide and tirzepatide suggest substantial weight loss and improved glycemic metrics in selected settings, with vigilance for ketosis. Sodium-glucose cotransporter-2 inhibitors have shown modest HbA1c reductions with small but clinically meaningful weight loss in T1DM; however, they are associated with an increased risk of diabetic ketoacidosis. Bariatric surgery yields large weight loss but only modest glycemic benefit and carries risk of metabolic instability. SUMMARY: Obesity is a common, clinically important comorbidity in T1DM, linked to poorer glycemic outcomes and complications. Longer-term, T1DM-specific RCTs are needed to guide weight management and define the benefit-risk of emerging therapies.