Effect of Protein Supplementation on Plasma Sodium Levels and Urinary Urea Excretion in Patients With SIAD

Inclusion Criteria: * previous documented diagnosis of chronic SIAD * confirmed diagnosis of SIAD at screening visit defined as: * plasma sodium concentration \<135 mmol/L, measured in lithium heparin plasma * Plasma osmolality \<300 mOsm/kg * Urine osmolality \>100 mOsm/kg * Urine sodium concentration \>30mmol/l * Clinical euvolemia, defined as an absence of signs of hypovolemia (orthostasis, tachycardia, decreased skin turgor, dry mucous membranes) or hypervolemia (edema, ascites) Exclusion Criteria: * lactose intolerance, celiac disease, milk protein allergy, soja allergy, nuts allergy or known hypersensitivity or allergy to one of the components of the protein supplementation (Whey Protein®, foodspring GmbH, Germany or Clear Whey Isolate®, MyProtein THG Company, United Kingdom) * inborn metabolic disorders implying carbohydrate, lipid or protein metabolism - severe symptomatic hyponatremia in need of treatment with 3% NaCl-solution or in need of intensive/intermediate care treatment at time of inclusion * Risk factors for osmotic demyelination syndrome: hypokalaemia (K \<3,4 mmol/L), malnutrition, advanced liver disease, alcoholism. * contraindication for lowering blood pressure * type 1 diabetes mellitus * uncontrolled type 2 diabetes mellitus (defined as HbA1c \>8.0%) * uncontrolled hypothyroidism * uncontrolled adrenal insufficiency * reduction of eGFR \<60 mL/min/1,73 m2 (KDIGO G3, G4 and G5) or end stage renal disease (dialysis) * severe hepatic impairm…