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.)