In this study the investigators hypothesize that pathological alterations in adipose tissue biology already occur during the development and progression of obesity in children and adolescents. The investigators aim to identify and characterize mechanisms and molecular targets that affect the development of adipose tissue and ensuing obesity in childhood and adolescence.
Inclusion Criteria: * body weight: 3000 g - 180 kg * suitable surgical procedure * signed informed consent by the guardians and the study participant, if older 12 years Exclusion Criteria: * severe chronic and inflammatory diseases * acute or chronic infections * oncological diseases * clotting disorders * complications during surgical procedure * drug treatment * nil by mouth