Intermittent Fasting After Sleeve Gastrectomy – A good idea or not?

I often get asked if it’s okay to do Intermittent Fasting after a sleeve. Let’s take a look! 

What is Intermittent Fasting? 

There are lots of different versions of Intermittent Fasting (IF) out there, doing the rounds at the moment. Essentially Intermittent Fasting suggests you fast (or not eat) during certain times of the day or week. 

For example, if you follow the 5:2 Diet you’ll eat normally (hopefully healthily) 5 days a week and eat only 500-600 calories on the other 2 days of the week. The 16:8 Diet is slightly different. You have 8-10 hours during the day where you will eat, then 14-16 hours overnight where you won’t eat. 

What are the proposed benefits of fasting? 

There is quite a bit of research into the benefits of Intermittent Fasting in the general population, but a lot of it is not great in terms of scientific rigour. In terms of weight loss in the general population, Intermittent Fasting seems to work quite well. This isn’t rocket science – the way IF works is by restricting the time you can eat during the day or week which means you eat less over the day/week resulting in a calorie deficit. 

There is also some interesting research suggesting IF could also speed up metabolism and burn fat whilst sparing muscle as well as a few links between IF and a reduction in chronic disease and reduced inflammation. However most of this research seems to be done in small, short term studies – there isn’t much that is big and robust.

Is IF appropriate for those with VSG?  

It sounds promising when we look at the research in the general population, even though more research is needed. The problem is there is not enough research in the bariatric population. When I look at any diet as a dietitian, I always look at safety first. This is where we need to be careful with IF especially depending on the stage you’re at post op. 

Fasting is limiting the time that you can eat over a day and we are already reducing your portion sizes and amount of food that you can eat over the day because of surgery which automatically puts you at risk of nutrient deficiencies. Because of this IF is not recommended for those after weight loss surgery, particularly if:

• You are within the first 12 months after surgery. It’s important that you are getting enough nutrients to heal post op and rather than fasting. The focus for the first 6-12 months post op should be on cementing those key healthy eating habits.  
• You have any existing nutrient deficiencies. Fasting limits the amount of food you can eat even more than you already have with the sleeve. Therefore fasting may compromise your ability to eat enough nutrients, further increasing your risk of deficiency.
• You have a history of bingeing or an eating disorder. Something like this can trigger some of that old dieting behaviour and trigger some of those habits especially if you have a history of anorexia, bulimia or binge eating disorder. You don’t want to get back into that  habit where you are associating what you eat with being “good”. 
• You need a reward after “being good”. You probably won’t be as successful with IF if you find you need a reward afterwards, especially if following the 5:2 diet. After those 2 fasting days, some people will have a rebound because they think they need a reward and then it turns out they eat more on those 5 days than they would have before starting the 5:2 which counteracts the whole point of the fasting. So if you are a reward person or you have tried diets before and they were not sustainable, you might find this diet won’t be sustainable for you.

If you do want to try Intermittent Fasting, be very careful. I would suggest that you try 16:8 rather than 5:2. With the 16:8 you still should be able to get in those 3 meals over the 8-10 hours, and it doesn’t restrict your ability to drink water at night. The 16:8 might suit you if you tend to graze at night and you might actually get more fluid in than you usually would! 

The bottom line… 

To summarise, there are certain circumstances where IF can be a useful tool in the general population and possibly the 16:8 for those after weight loss surgery. BUT I would be careful if you have a history of diet mentality, you need a reward after “being good”, if you’re pregnant, have any existing nutrient deficiencies, or have surgery less than 12 months ago.

Fasting encourages dieting behaviour and the aim after VSG is to get into long term sustainable behaviour rather than move from one fad to another. You need to make an educated decision when thinking about fasting. Talk to your team and make sure you are not at risk before you do anything major.