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RESULTS: 2-Day Water Fast Wearing a CGM

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  • August 22, 2020April 22, 2021

This past weekend, I experimented with a 2 day (mostly) water fast, while wearing my CGM (continuous glucose monitor). My primary goal of the fast was for the autophagy, longevity, and mental clarity benefits. But, unlike other fasts that I’ve done, this one I was wearing a CGM and could monitor my blood glucose levels during the fast, which gave me a great insight into my body.


I had originally listed a 3-day fast as one of my goals to track while wearing my CGM, but the timing of this fast came a week earlier than anticipated. With the air quality poor due to the fires, I had to cancel my distance run for this weekend and opted to do the fast this weekend instead. I made the call to do a 36-72 hour fast, with the option to stop at any time but was leaning towards 48 hours. This sounds really cheap but I had a LOT of food in the fridge that I didn’t want to go back, and had initially wanted to taper off caffeine and coffee prior to the fast because I didn’t want to have any caffeine withdrawal during the fast.


I ended up doing a 48-hour liquid fast, with the first 44 hours being water and tea only, and drinking a keto shake at hour 44 and 46 to prep me for my first meal to break the fast. Luckily (as you’ll read more below), I think I was getting the benefits of autophagy quite early, which made it easier to stop at 2 days. Overall, the fast wasn’t too bad as I was able to work out and do yoga during the fast and I think that was due to having a prior background in fasting and keto, and entering the fast in a light-state of ketosis (0.5mmol).


Background in Fasting/Keto:

Fasting: I regularly practice intermittent fasting. I’m not too strict on tracking my hours; it’s more of a lifestyle practice that’s built into my life where I don’t really have to think/track/stress out over it. So on average, I would say I fast between 14-18 hours 5-6x a week (the wide range being due to the fact that it’s kind of freeform without any planning).


I think the term “intermittent fasting” really shouldn’t even be a thing. It really is how we are evolutionarily built to eat, so we shouldn’t need to stress while “counting down” our hours to the next meal. I recommend not to eat 3 hours before bed, or 3 hours after waking. Assuming you sleep 8 hours a night, that would get you to a 14 hour fast. The term breakfast literally means the meal that you break your fast. So pretty engrained into my routine is a window of time where I’m not eating.


Keto: I’ve written about my keto(-ish) journey in 2018 where I was measuring ketones and blood sugar every morning. Dating back further, I’ve been using some form of a low-carb lifestyle since high school, intuitively figuring out that it gives me the highest levels of energy and mental clarity. I would say that I’ve never truly been keto, by definition of the diet and macronutrient percentages, but am regularly in and out of ketosis with a combination of my nutrition, meal timing, and activity levels. As you can imagine, having a fat-adapted body would make the transition to a fast much easier than someone who is used to snacking every 2 hours and has blood sugar spikes throughout the day.



Prior to the fast, I downloaded the app Zero and watched this interview (amongst others) with Peter Attia.

Dr. Peter Attia Zero Q+A | Part I




There were quite a few helpful tips relating to supplementation, building muscle, macros, and more, but the two main takeaways for me in my particular case were the following:

  • Pre-fast and post-fast meals: They discuss how important it is to enter and exit the fast, to make the transition easy and avoid refeeding syndrome. The biggest takeaway there is to enter while already in a mild ketosis and exit with a high fat meal. After extended day fasting, you will be in an insulin resistant state, so it is best to consume some fatty foods at that time.
  • Autophagy Quantified: The average person will not be able to decisively test for when their body is in a state of autophagy (“self-eating”), or the process in which your body cleans out and recycles damaged cells. But Peter Attia made an interesting point that if you have a CGM, you can track when your blood glucose flatlines at around 60 (he said at about 3 days into a water fast), your body is probably at a statistically significant level of autophagy.


Luckily, I had been eating keto-ish and intermittent fasting in the weeks prior to the fast, and was able to enter the fast in a light state of ketosis (0.5mmol). My pre-meal dinner was Boston Butt, carrots, onions, and brussel sprouts.


Fasting Day 1:

My blood sugar levels looked like an average day during Day 1 of my fast. I woke up with a morning blood sugar of 68 (ketones at 0.5mmol) and mostly stayed in the 70’s and 80’s and had a spike up to 100-102 during a kettlebell workout. My blood glucose before bed was 87 and my ketones were at 2.4mmol. I found it very interesting that I could get my ketones that high in the first day.


Fasting Day 2:

Day 2 is where it got interesting. I dreamed a lot and had an about average night of sleep (8 hours, 1:37 of REM and 1:59 of Deep Sleep) according to my Oura, but had my blood sugar in the 50s the entire night. My waking blood sugar was at 59 and ketones at 1.8mmol. My blood sugar remained below 70 until I decided to break the water-only portion of my fast, and consumed a keto shake (200 calories) that had some protein which raised my blood sugar levels to the 70s. While my ketones were in the low 60’s, my ketones got as high as 2.8mmol.


This period of time where my blood sugar was pretty much flat in the 60s is the period that Peter Attia talks about when autophagy is occurring at a statistically significant level. I was happy to experience this early into the fast, which I presume to be due to entering the fast already in ketosis and being able to kick up ketones pretty fast. Typically, Peter Attia mentioned that this occurs 3 days into a fast (and would have been the driving factor for me extending the fast to a 3rd day).


I also liked my strategy of breaking the phase with keto liquids, because when I did consume foods later, my blood sugar only went up to 92. My first real food after 48 hours was grass-fed beef, pork-shoulder, riced cauliflower, yellow squash, tofu, onions, and zucchini followed by a keto cheesecake. I probably went above the limits of the recommended quantity of food, but I really enjoyed it!


Day After:

  • The night after my first meal, I slept well and got a lot of REM, but was surprised to see my Oura results. My resting heart rate shot up to 50 (it’s normally between 38-42), body temperature was high by about 0.4C, and my HRV was 44 (when it’s normally 70). I quickly scrambled over to Peter Attia’s IG and was relieved to see that this is completely normal. Interesting to see how the body works!


My Observations from the Fast:

  • Blood Sugar<60 as a Threshold: I definitely don’t like how I feel when my blood sugar gets to the 50’s (mine only got to 58-59). Even with ketones at levels above 2, I would get a bit of a headache. At the higher 60’s I felt great (or in general, in the 70s having ketones makes me feel great as well).
  • Caffeine withdrawal: Thinking back, my caffeine consumption has been on the higher end lately, so I don’t like that I wasn’t able to do the fast knowing caffeine withdrawal could be a factor in how I felt. I wanted to be able to definitely know if I was getting a headache due to lack of caffeine or dehydration – and unfortunately I could not tell for sure. Prior to the fast, I had been getting a few headaches (2 bad migraines in the past 3 weeks), so I couldn’t isolate what exactly caused that. If I had tapered off caffeine correctly, and had not been having recent migraines prior to my fast, I would easily be able to correlate X with Y, but cannot say for sure. This will definitely be factored into the timing of my next fast.
  • Chronic Cough: I’ve had a chronic cough since I was very young. I noticed that I stopped needing to clear my throat during my fast, which may suggest my cough may be some sort of a food sensitivity. Definitely an idea to explore!
  • Digestion: It’s interesting to how your body frees up energy when you stop eating. I could do a separate post on this, you should also monitor your poop as a marker for your digestion and overall health (we do it for our dogs, don’t we?) One thing I noticed after my first meal is that I go within 2 hours of eating, which by all measures is very fast – but nothing would seem abnormal otherwise. Just another thing to look into.


What Questions I have:

  • If autophagy occurs at this level were glucose flat lines in the 60s, what does that mean for those on a 14-18 intermittent fasting protocol? In the interview noted above, Peter Attia noted that autophagy may not be a binary outcome, and may happen in scales. So my vague understanding is that some level of autophagy is occurring at that 14-18 hour window, with the most pronounced benefits occurring at 2-3 days when glucose flatlines. Definitely would like to read up more on this.


What I Would Change for next Fast:

  • As noted above, prep my meals (and week prior grocery shopping) and do a slow caffeine taper prior to the fast! My next one will be between 3-7 days.