Why Do I Need a Pre-Operative Diet Before Bariatric Surgery?

9 July, 2026

Why do I need to follow a pre-operative diet?

If you’re preparing for bariatric surgery, your surgeon may ask you to follow a pre-operative diet in the weeks leading up to your procedure.

This is typically a very low calorie diet (VLCD), designed to reduce the size of your liver, making surgery technically easier for your surgeon while improving the safety of your procedure.

What is a very low calorie diet?

A very low calorie diet, commonly referred to as a VLCD, significantly reduces your daily calorie intake while also restricting carbohydrate intake.

Most VLCDs provide around 800 calories per day, with carbohydrate intake usually limited to less than 80 grams per day.

To achieve this, meal replacement products are commonly used alongside low-energy vegetables and either a small serve of lean protein or a complex carbohydrate, depending on your individual plan.

For some patients, a food-based VLCD may also be appropriate and can be discussed with your dietitian.

When carbohydrate intake is significantly reduced, your body switches from using carbohydrates as its main fuel source to using stored fat for energy. This process produces substances called ketones, which place your body into a natural metabolic state called ketosis.

Why is a VLCD important before surgery?

During bariatric surgery, your surgeon needs to gently move the liver to safely access the stomach. If excess fat is stored within the liver, it becomes enlarged and is more difficult to move during surgery. Fat stored around the stomach can also reduce visibility and make the procedure more technically challenging.

Although many people lose weight while following a VLCD, the primary goal is to reduce the size of the liver rather than achieve weight loss. Many patients lose around 5–10% of their body weight, depending on how long they follow the diet.

It’s important to remember that following the diet for longer than recommended doesn’t necessarily lead to greater liver shrinkage or additional weight loss. For the best outcomes, always follow the plan provided by your dietitian.

What side effects might I experience?

As your body adapts to ketosis, it’s common to experience temporary side effects during the first few days, including:

• Hunger
• Nausea
• Fatigue
• Headaches
• Difficulty concentrating

These symptoms usually improve as your body adjusts.

Some people also notice changes to their breath or experience constipation while following a VLCD. Maintaining good oral hygiene, drinking plenty of fluids, and including the recommended amount of low-energy vegetables can help minimise these symptoms.

How your dietitian can help

During your appointment, your dietitian will discuss:

• How long you’ll need to follow your VLCD
• The most appropriate meal replacement products for you
• Your personalised meal plan
• Strategies to help manage common side effects
• Any questions or concerns you have before surgery

Following your pre-operative diet as prescribed is an important part of preparing for surgery. By reducing the size of your liver and optimising your body before your procedure, you’ll help support a safer operation and the best possible surgical outcome.

Can a Dietitian Help After Surgery?

Yes. Dietitians can provide ongoing support to help patients meet their nutritional requirements during recovery and address any nutrition-related concerns that arise following surgery.

Dietitan consultation fees can be viewed under our appointment fee page.