Standard dietary regiments for bariatric are advised prior to surgery. The most used diet is a very low calorie diet (VLCD). These diets potentially have multiple pittfalls.

Very low-calorie ketogenic diets (VLCKD) have been proposed as a new regimen for achieving weight- and liver volume loss in patients undergoing bariatric surgery. The beneficial effect of VLCKDs compared to (V)LCDs is the aimed preservation of FFM and RMR, while still reducing FM. Only a few small studies addressed the role of VLCKDs prior to bariatric surgery, and the data including FFM and FM is actually scarce. Therefore, a well-designed randomised controlled trial is necessary to establish the efficacy of a VLCKD.

According to the World Health Organization, obesity rates have nearly tripled since 1975, resulting in the mortality of four million individuals each year. Obesity contributes to a reduced quality of life and is a significant risk factor for multiple chronic diseases such as type 2 diabetes mellitus, cardiovascular disease, osteoarthritis and certain types of cancer . Bariatric surgery is regarded the most effective treatment for extreme obesity because it achieves significant long-term weight loss and improves or even eliminates obesity-related complications. Based on the American Society for Metabolic and Bariatric surgery, approximately 230.000 bariatric procedures are performed annually, and those procedures predominantly included sleeve gastrectomy and Roux-en-Y gastric bypass (RYGB) procedures. Bariatric surgery can be challenging because of an increased thickness of the abdominal wall, the presence of intra-abdominal obesity and hepatomegaly. These anatomical changes are associated with more anaesthetic and surgical manoeuvres during the procedure. Losing weight prior to the procedure is frequently advised because it may help to overcome technical challenges. It may furthermore improve short-term outcomes like surgical time, blood loss, hospital stay and postoperative complications, as well as long-term outcomes like weight loss.

In the preoperative phase, weight loss can be realised by multiple dietary regimens, including low-calorie diets (LCD) (800-1500 kcal/day) and very low-calorie diets (VLCD) (<800 kcal/day). In Máxima Medical Center and a few other hospitals performing bariatric surgery in the Netherlands, the VLCD is the standard diet of choice, which begins two weeks prior to surgery. Systematic reviews reported, as result of these diets, a reduction in liver size (5-20% with a VLCD; 12-27% with a LCD), intrahepatic fat (43% with a VLCD; 40-51% with a LCD) and body weight (2.8-14.8kg with a VLCD; 5.4-23.6kg with a LCD). These diets varied in duration, from two weeks to four months. One of the primary drawbacks of these diets is the loss of metabolically active fat free mass (FFM) and lean body mass (LBM), which are crucial for whole-body protein metabolism, in addition to fat mass (FM). In contrast to FFM, LBM contains bone mass, connective tissue, internal organs and essential fat stored in these tissues. It has been hypothesised that LBM will contain 3 to 5% more fat than FFM. A significant loss of FFM may negatively affect the resting metabolic rate (RMR), slow the rate of weight loss and predispose weight regain in the long-term. A significant loss of FFM, in continuing presence of an excessive FM, may furthermore contribute to sarcopenic obesity. A second important downfall is that some patients may not tolerate a (V)LCD regime due to side-effects leading to poor compliance and subsequently poor weight loss outcomes. For clinical trials, the reported attrition rate for a (V)LCD regime is around 20%, raising concerns about the study validity.

Very low-calorie ketogenic diets (VLCKD) have been proposed as a new regimen for achieving weight- and liver volume loss in patients undergoing bariatric surgery. A VLCKD is characterized by a very low carbohydrate content (<50 g/daily), a low-fat content (15-30 g fat/daily) and a high amount of proteins (1-1.5 g protein/kg ideal body weight). The beneficial effect of VLCKDs compared to (V)LCDs is the aimed preservation of FFM and RMR, while still reducing FM. In addition, the compliance of patients might be improved by VLCKDs, possibly due to the anorexigenic effect and hunger reduction of ketone bodies. Only a few small studies addressed the role of VLCKDs prior to bariatric surgery, and the data including FFM and FM is actually scarce. A retrospective cohort study found that a three-week lasting VLCKD resulted in more weight loss (5.8kg vs. 4.8kg) compared to a VLCD. In addition, two prospective cohort studies showed as result of a VLCKD, FFM was reduced by 7.6 kg (10.0%) and 0.7kg (1.1%) , whereas FM was reduced by 10.5kg (17.9%) and 5kg (8.8%). What is important to note is that these studies lack a control group, randomisation and are subjected to limitations in FFM and FM measurement due to the use of single-frequency bioelectrical impedance analysis (BIA). Therefore, a well-designed randomised controlled trial is necessary to establish the efficacy of a VLCKD.

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)