Beyond the Scale: The Hidden Burden of Underweight and Cachexia in Adults with Congenital Heart Defects and Heart Failure-Results from the Pathfinder CHD-Registry.

Heart failure (HF) poses a major challenge in managing adults with congenital heart defects (ACHD). Emerging evidence suggests that HF in ACHD increases the risk of underweight due to heightened metabolic demands, gastrointestinal complications, and psychological factors such as anxiety and depression. Despite its critical implications, few studies have examined this association. This study evaluates the relationship between HF and underweight-defined as a body mass index (BMI) < 18.5-in ACHD. The Pathfinder-CHD Registry is a prospective, observational, web-based HF registry including ACHD with manifest HF, history of HF, or significant risk for HF. It documents congenital diagnoses, HF type, comorbidities, and treatments. Patients were categorized by BMI into mild (17.00-18.49), moderate (16.00-16.99), and severe (<16.00) underweight. As of September 2024, the registry enrolled 1420 adults (mean age 31.8±11.3 years; 49.2% female). Underweight was present in 59 patients (4.2%): 62.7% mild, 18.6% moderate, and 18.6% severe. Among the remaining 1361 patients, 52.8% had normal weight, 32.8% were overweight, and 14.2% were obese. Women had significantly lower metabolic body weight than men ( = 0.002). Underweight correlated with younger age ( < 0.001) and CHD type ( = 0.02). Notably, 42.9% of underweight patients had cyanotic CHD. Underweight is an underrecognized problem in ACHD with HF. Adults with complex CHD or connective tissue disorders are disproportionately affected. Underweight should be seen as an alarm sign requiring personalized, multidisciplinary management, including nutritional support, tailored therapy, and close monitoring to improve outcomes.