Participants in the intervention group will follow a low-calorie diet during 14 days before undergoing surgery. The diet will consist of both meal replacements and regular foods.

For women, the diet provides ± 900 kcal, 50 grams of carbohydrates, 85 grams of protein, 30 grams of fat and 25 grams of fibres.

For men, the diet provides ± 1000 kcal, 55 grams of carbohydrates, 100 grams of protein, 30 grams of fat and 30 grams of fibres.

Participants in the control group can eat according to the standard nutritional advices provided by their dietitian. These advices are intended to educate participants on the recommended eating pattern after surgery.

Participants will be asked to:

• Use 4 (women) or 5 (men) Modifast meal replacements per day.
• Daily consume a minimum of 250 grams of (raw) vegetables without dressing.
• Daily consume 100 grams of lean meat/fish/meat analogues.
• Daily consume two spoons of liquid oil
• Daily drink at least 2.0 litres of water, coffee or tea without added sugar or milk, bouillon or sugar-free drinks.

Duration of the surgical procedure, retrieved from the electronic medical record

Ten aspects of the RYGB procedure will be scored on a 5-point Likert scale. The total score can range from 10 to 50, with 10 being the easiest.

The ten aspects include:

• Exerting force on instruments (1=very low, 5= very high)
• Difficulty of moving the liver (1=very easy, 5=very difficult)
• Size of the liver (1=very small, 5= very large)
• Size of omentum (1=very small, 5= very large)
• Difficulty of approaching pouch (1=very easy, 5=very difficult)
• Accessibility of Treitz ligament (1=very easy, 5=very difficult)
• Difficulty of creating pouch (1=very easy, 5=very difficult)
• Difficulty of measuring small intestine (1=very easy, 5=very difficult)
• Difficulty of entero-entero connection (1=very easy, 5=very difficult)
• Difficulty of abdominal wall hernia repair (1=very easy, 5=very difficult)

Feasibility will be assessed via questionnaires

Compliance will be assessed via food records