Answers to
Top 5 Bariatric Preop Diet Questions

bariatric diet questions mark

Does the thought of the bariatric preop diet make you want to run for the hills?

Allow me shed some light on the much talked about and often feared diet before bariatric surgery. Here are answers to the most common questions I hear as a bariatric dietitian:

1. How Many Calories? Carbs? Protein?

The bariatric preop diet isn't standardized, so how many calories, carbs, and protein you’re allowed will vary slightly from surgeon to surgeon.

These are typical guidelines:

  • Less than 1000 calories per day
  • Less than 50 grams of carbohydrate per day*
  • Protein 60-100 grams per day**

*Note: If you’re at risk of hypoglycemia, you may need to adjust medications and/or slightly increase carbohydrates. Be sure to check with your doctor before starting the diet.

**Maintaining high protein while you restrict calories and carbs is important for preserving lean muscle mass.

But what am I going to be able to eat?

2. WHY Do I Need a Pre Bariatric Surgery Diet?

The short term pre weight loss surgery diet has nothing to do with weight loss and everything to do with your liver. 

Your liver is located near your stomach so it kinda blocks the surgeons view if it’s big and fatty! He/she must lift the liver during surgery to see your stomach. Obviously, the surgery is more difficult, and the risk of complications is greater if your liver is large.

The most effective way to shrink the size of your liver quickly is by slashing carbs and calories. Enter the “liver shrinking diet before bariatric surgery,” also known as the “bariatric preop diet.”

Good news...the pre bariatric surgery diet can be done with meal replacements OR food!


  • Some insurance companies require a “medically supervised/physician monitored diet” for 3-6 months prior to surgery. This is different from the short-term liver reducing preop diet.

3. How Long is the Bariatric Preop Diet?

Most surgery practices require you to follow the liver reduction diet for 1-2 weeks prior to surgery. Studies show a low carb, low calorie diet can significantly reduce the size of your liver this fast!

4. Is it True That It's a Liquid Diet Before Gastric Bypass, Sleeve, Or Band?

Limiting yourself to only liquids before surgery is not the point. Drinking any ol’ liquids doesn’t shrink your liver. 

Achieving the macronutrient composition: <1000 calories, <50 grams carbohydrate, and 60-100 grams protein in order to reduce the size of your liver is what’s important. 

This can be achieved in different ways and a liquid diet involving high meal replacements is just one of the ways.

Good News

The bariatric preop liver shrinking diet can be done with liquid meal replacements OR real food!

Examples of what to eat on the 2 week pre op diet

5. What Happens If I Cheat on My Bariatric PreOp Diet?

Don’t forget that the purpose of the diet before weight loss surgery is to make your liver smaller so your surgeon can see better and get to your stomach easier. If your surgeon has an easier surgery, then you have an easier surgery.

The danger of “cheating” and gaining weight preop (and thus enlarging your liver), is the increase likelihood of having more complications post op and thus a longer hospital stay.

Giving up what you want most for what you want in the moment is hardly worth it!

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