Effect of Protein Supplementation and Fluid Restriction on Plasma Sodium Levels in Patients Undergoing Pituitary Surgery

Inclusion Criteria: * Adult patients (aged 18 or older) undergoing trans-sphenoidal or trans-cranial surgery for sellar / suprasellar pathology * No pre-existing AVP-deficiency and SIAD or other causes of hyponatremia Exclusion Criteria: * AVP deficiency diagnosed before surgery based on established criteria * Other type of neurosurgery / intracranial pathology / pre-existing spontaneous CSF -rhinorrhea * Traumatic brain injury or intracranial hemorrhage * Systemic infection or other causes for systemic stress * Lactose intolerance, milk protein allergy, soy allergy, nuts allergy or known hypersensitivity or allergy to one of the components of the protein supplementation * Inborn metabolic disorders implying carbohydrate, lipid, or protein metabolism * Severe hepatic impairment (ALAT/ASAT \>3x upper limit) or advanced symptomatic liver disease defined as past or current hepatic encephalopathy, liver cirrhosis Child C, or decompensated (bleeding, jaundice, hepatorenal syndrome) * Reduction of eGFR \<45 mL/min/1.73m² (KDIGO G3b, G4 and G5) or end stage renal disease (dialysis) * Recently initiated (within the last 5 days) SGLT2 inhibitors, vaptans, or oral urea therapy * Pregnancy or breastfeeding * Lack of capacity or other reason preventing from giving informed consent or following study procedures. Post-Inclusion Exclusion criteria (day 3 post-surgery, i.e.,time of randomization): - AVP deficiency diagnosed on post-operative day 3