Ketogenic Diet Prior to Bariatric Surgery

Inclusion Criteria: * Eligible for bariatric surgery (≥35 kg/m2 with obesity-related comorbidities, or a BMI of * 40 kg/m2 with or without comorbidities) * Roux-en-Y Gastric bypass (RYGB) Exclusion Criteria: * Weighing over 150kg because this amount is a limitation by the DXA device * Diabetes mellitus type 1 * Allergic to milk proteins * A recent history of a heart attack (\< 12 months), heart failure or cardiac arrhythmias * Kidney and/or liver failure (creatinine levels \>1.3 mg/dl or liver enzyme levels (AST, ALT, GGT)) less than three times over the upper normal threshold * Current infectious, sepsis or malignant disease * Rare condition like galactosemia, phenylketonuria or porphyria * Persistent diarrhoea * Hypokalaemia, chronic therapies with diuretics as furosemide and hydrochlorothiazide * Pregnancy or plans to get pregnant in the coming months * Patients who did not meet criteria to be eligible for bariatric surgery (BMI \<35, psychological or unstable psychiatric disorders, inadequate dietary regimen or inadequate exercise pattern which can't be resolved in the upcoming 6 months)