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  • Writer's pictureEsther

CGM and fasting

Now that I am adapted to some fasting, it's time for some science and self-experiments!

With a CGM, I want to track what´s happening inside me.

You might know, that I was already wearing a CGM a couple of times and making tests on how I react to different foods and activities. This time I want to focus more on the other side of the coin and shed some light on how my body reacts and adjusts to fasting and breaking a fast. For this, I am playing around with my fasts a bit and present you with some results as well as some screenshots from my CGM readings. I am looking at the following fasts:

  • 29-hours fast

  • 40-hours fast

  • 18-hours fast

  • 45-hours fast

  • 28-hours fast

Sensor and Disclaimer

I am using the Abbott FreeStyle Libre 2 again. The sensor is designed to provide 14 days of data what allows me to get a pretty good overview.

Please refer to "Wearing a CGM" if you want to read more about the Abbott Sensor and how I used and installed it. As a side note - I am not sponsored by Abbott, and I paid for the sensor privately.

I am non-diabetic. I do not take any medications, and I do not have any medical conditions that would hinder me from fasting.

This blog does not contain any medical advice and is meant for entertainment and education only.

The timeframe

The start of this self-experiment was Tuesday, May 11, 2021 - at noon.

The self-experiment ended 14 days later on Tuesday, May 25, 2021, at 9 am.

First findings in a 29-hours fast

My first measurement on May 11th took place after approximately 22 hours of fasting and this was the result: 87 mg/dL (=4.8 mmol/L).

Let´s have a look at the development of my blood glucose levels on this first day.

As we are most curious to see, how fasting is having an impact on glucose, I continued to fast until 5:30 PM (=17:30) that day. The fast lasted for close to 29 hours.

As you can see in the image below, my blood glucose levels were pretty stable since the first measurement and rose a bit after my meal that was breaking my fast at 5:30 PM. As this meal was ketogenic, the reaction on my blood glucose level is minor and only scratching the 100:

This is the graph from the CGM showing my blood glucose on May 11, 2021
Blood glucose measurements - May 11, 2021

A 40-hours fast

After the above-mentioned meal, I immediately started another fast of 40 hours. The following graph shows how my blood glucose was developing under this slightly extended fast.

I did a short 2,5 km morning walk between 7 and 8 am and I am wondering why my levels are falling there. Usually, the levels are going up a bit during an exercise 🧐

However - while I stayed within the "green range" between 70 and 110 mg/dl (3.9 - 6.1 mmol/l) close to all day, the level fell under 70 mg/dl starting around fasting-hour 27 and stayed there all night:

This graph shows how my blood glucose levels were during my water fast on May 12, 2021
Blood glucose measurements - May 12, 2021

(There is a small empty spot in the above picture caused by me not taking a measure within the 8 hour-range the sensor is able to save data - I was sleeping too long 😉)

Let´s check the image below still showing me in my 40-hour fast: The glucose level stayed under 70 all night and only went up a little during a one-hour morning hike approx. between 8 and 9 am (blue line). I think here we can see nicely my liver producing some glucose (Gluconeogenisis) to feed my muscles for the activity.

This graph shows my blood glucose on May 13, 2021 incl a hike and  two meals
Blood glucose measurements - May 13, 2021

However, the level fell again right after stopping the activity, and at that point, I wasn´t feeling too great and so I decided to break my fast after 40 hours and 15 minutes (the red vertical lines in the image above indicate the meal timing). The first meal had approx. 30 gr of carbs and my glucose went up slightly above 100.

I don´t know whether it is related to the carbs I had or because I didn´t eat enough during that meal, but I wanted to eat more throughout the day. That´s why I had a second meal on that day (mainly meat and cheese with some onions). Interestingly even though I stayed in deep ketosis all day, I really wanted to eat.

An 18-hours fast

Let´s have a look at the next day, Friday, May 14, 2021: I ate early after 18 hours of fasting at 9:55 am (Pulled pork with a green salad). As I was pretty busy all day, I forgot to make regular measurements and therefore there is another blind spot in the morning and a second one in the evening as I was sleeping quite long again.

As the missing time frames are pretty small and the other data indicating rather regular levels, I am assuming we are not missing some relevant ups or downs here.

If I didn´t know when exactly I had my meal I would find it hard to say when I ate. Can you tell whether or when I had a second meal based on the graph below?

This graph shows my blood glucose on May 14, 2021.
Blood glucose measurements - May 14, 2021

Well - I had. Around 6:30 PM.

A 45-hours fast

For this 45-hours fast, I need to upload 3 images as the fast started on Sunday afternoon shortly after 3:00 pm and went on to Tuesday noon - I guess you can see my fast-breaking meal clearly in the last image:

So - what happened during the fast? Not too much, however: I´d say that I spend some time dealing with my meal on Sunday with some ups and downs before I found a solid level during the night. However - before I hit the 24-hour mark, the level went down again and stayed pretty solid until the end of the fast.

A 28-hours fast

I wanted to check an OMAD day as well in which I am only having one meal a day. The fast started on Wednesday at 1 pm and ended on Thursday at 5 pm.

It feels like we found a pattern: After having found a balanced level after some adjustment time after the meal, the glucose level stays at a low-normal level until the next meal.

Fasting Insulin

It´s not all about glucose and to get the full picture, I went to a doctor to get some blood tests done. One of the tests I asked for was fasting insulin. Here´s the result: 7.3 mU/l

I think I am on the edge here but still at a rather normal level and won´t be seen as insulin resistant at this point. I am definitely aiming towards a level between 2 and 7 as it seems to better from all that I´ve learned so far.

But I am anyways happy with the result as I feel that´s proof that I am doing at least something right.


After 14 days of wearing a CGM while fasting, I think that fasting is great for stable blood glucose levels. It is even a bit boring - but I´d say: Boring is a good thing in this case.

For me, it is clear that fasting - including longer fasts - does not have a negative impact on my glucose levels. Well, that´s no surprise as we know about gluconeogenesis. Happy we found a proof it´s actually working 🤓

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