## 1 Introduction  
The management of adults with congenital heart disease poses an increasing challenge in 21 st-century cardiovascular medicine, particularly when complicated by heart failure [1, 2, 3, 4]. Advances in pediatric cardiology, congenital cardiac surgery, and improved long-term care have significantly improved the life expectancy of individuals born with CHD. As a result, the population of ACHD is increasing, currently exceeding 350,000 in Germany and 50 million worldwide [5]. These patients present with a unique clinical profile, characterized by a high burden of sequelae from the underlying CHD as well as associated complications, such as heart failure, pulmonary hypertension, arrhythmias, and non-cardiac comorbidities, including metabolic syndrome and obesity.  
The PATHFINDER-CHD Registry (\"Patients with Heart Failure Due to Congenital Heart Disease\"), a prospective, non-interventional, multicenter cohort study aimed at collecting structured real-world data on ACHD with current or previous HF, or those at risk due to structural or functional abnormalities, was established in 2022. In particular, the registry focuses on the epidemiology, risk factors, and management patterns [6]. PATHFINDER-CHD seeks to address key knowledge gaps by providing robust real-world evidence from specialized tertiary care centers in Germany.  
Recent studies highlight the high prevalence of cardiometabolic risk factors in ACHD populations. In a meta-analysis including 110,469 ACHD, 33% had hypertension, 18% were obese, and 7% had diabetes mellitus [7]. In a large cohort of 3905 ACHD, the estimated prevalence of heart failure was 6.4%, with a mean BMI of 25.2 \(\pm\) 4.9 kg/m\({}^{2}\)[8].  
The role of overweight and obesity in this vulnerable patient group has only rarely been investigated to date [7]. While the relationship between obesity and acquired cardiovascular disease is well documented, little is known about its prevalence and clinical implications in adults with CHD and HF. In adults without congenital heart disease, the co-prevalence of heart failure and obesity is well documented, with obesity contributing significantly to the pathogenesis and progression of heart failure [9]. However, data regarding this association in adults with congenital heart disease remain scarce, despite the growing number of patients reaching adulthood. The unique anatomy and physiology of CHD patients, combined with their lifelong disease burden, require tailored approaches for prevention and management. Obesity may exacerbate symptom burden, reduce functional capacity, and increase the risk of HF and procedural complications. Furthermore, the emergence of new pharmacological treatments in obese patients is often more challenging due to altered drug metabolism and dosing considerations, necessitating individualized therapeutic approaches. Stratification by body mass index (BMI) or body composition is therefore essential to better understand the heterogeneity of comorbidities and treatment patterns within the ACHD cohort [10].  
This current cross-sectional analysis specifically examines the prevalence of overweight and obesity in the cohort, by type of surgical status, by ventricular morphology, by functional status, and by cardiac and non-cardiac comorbidities, as well as medication usage across different BMI categories. It aims to identify potential disparities in care and to provide a basis for more personalized, preventive, and therapeutic strategies in ACHD.