Effects of Cross-sex Hormone Therapy on Eating Behavior, Metabolism, Energy Balance and Cardiovascular System

Inclusion Criteria: * transsexual patients undergoing cross-sex hormone therapy versus controls * able to give informed consent, if \<18 years from all legal guardians * only for part B (patients will participate in all other study parts): weight stable (± 5%) for last 3 months, BMI ≤ 30 kg/m2 Exclusion Criteria: * severe medical impairments (e.g., uncontrolled cardiovascular disease, severe heart failure, uncontrolled hypertension, cerebral insult, active malign disease, etc.) * self-initiated cross-sex hormone therapy before study start only for part B (patients will participate in all other study parts): Insufficiently controlled endocrine disorders (Cushing's disease, other uncontrolled pituitary disorders, uncontrolled hypothyroidism, hyperthyroidism, etc.) * Chronic pulmonary disorders, including chronic obstructive pulmonary disease that would limit ability to follow the protocol (investigator judgment) and obstructive sleep apnea syndrome; only subjects with mild or exercise-induced asthma on no medications or on beta-adrenergic agonists only will be allowed to enter the study (provided use of these agents is not required for 1 week before * Diagnosed gastrointestinal diseases, including inflammatory bowel diseases (e.g. Crohn's disease and ulcerative colitis), malabsorption syndromes (e.g. celiac disease), gastric ulcer (active); only subjects with gastro-esophageal reflux will be allowed to enter the study entry). * History of HIV infection or ongoing chronic infection (such as tuberculosis) * only for part D (patients will participate in all other study parts): Contraindication against performance of an MRI scan (i.e., presence of metal in body, tattoos in head/neck region, claustrophobia etc.)