My Experience with Fasting 36 Hours at a Time, 3 Days a Week

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by Nils Osmar. April 24, 2022

I’m getting back into doing regular 36 hour fasts after taking a break from them. (I did lots of 36 hour fasts in 2021, but shifted into doing shorter fasts this past January.) I’m wrapping up my latest 36 hour water fast this morning.

I’d actually forgotten how great it feels to take a complete break from food for 36 hours how sharp and clear the mind gets, and how completely hunger goes away during fasts of this length.

My Experience

My experience is that fasts of 36 hours or more feel qualitatively different from fasts of 24 hours or less. This is (I believe) because we’re moving into autophagy and ketosis at the same time, cleaning debris from our cells and fueling our life processes with ketones instead of glucose. (See article: How Autophagy Works – and Why It’s Beneficial to Longevity.)

36 hour water fasts also feel qualitatively different to me than fasting mimicking diets, also known as FMDs (in which people essentially go vegan/low protein/low carb for a short stretch of time). FMDs are easier for some people, and it’s possible that they may be safer than water fasts. But to me they don’t have the benefits in terms of greater mental clarity and energy that go along with zero calorie (water, coffee and tea only) fasts.

For folks interested in 36 hour fasts, in my experience they are much easier if you segue into them by eating a low carb diet (under 50 grams of carbs a day) for a few days, so you’ll be adapted to burning fat instead of glucose.

I find, interestingly, that the deeper I go into ketosis (as measured with urine-dip keto sticks), the greater my mental clarity. When I’m doing fasting-mimicking diets, even if they’re extremely low in protein and carbs, I see almost no ketones; when I do long water fasts following a day or two of low-carb eating, I go into very deep ketosis. I spent most of the day yesterday teaching, and notice a profound difference in my clarity and communication, something many people who do regular fasting have remarked on.

The Monk Fast

  • One easy way to dip your toes into 36 hour fasting (if you’d like to try it) would be to do the Monk Fast, which centers around just one 36 hour water fast per week.
  • The rules are simple: just drink water and other non-caloric drinks. (I just drink water, black coffee and tea when doing this kind of fast.) Skip the sweeteners, even non-caloric ones. No fat or oil either (for the 36 hours), particularly if you want to use up some of your own body fat.

Too Much Fasting?

  • The reason I took a break from doing regular 36 hour fasts for a while was a concern that I might be doing too much fasting. In much of 2021, I was doing 36 hour fasts a week. There can be a danger when we do this of building up micronutrient deficiencies over time.
  • I was doing fine as far as I could tell; I was losing a lot of body fat and simultaneously building a lot of muscle. I was taking in extra nutrients, including proteins, on my feasting days to make up for the nutrients I was missing when fasting.
  • What I found, though, was that a few months ago I suddenly started feeling an innate resistance to doing long fasts. I’d feel anxious when I tried fasting for more than 24 hours, and resistant to the idea of doing so.
  • I decided to “trust my gut” and assume that I might be feeling that way for a good reason. As Dr. Jason Fung, an advocate of fasting has said, there’s no need to force ourselves; we can always fast again later. (“When in doubt, go ahead and have something to eat.”)
  • All things considered, it seemed wise to take a break from that level of fasting till it felt right to start doing it again. So I cut back on the long fasts and started doing more short fasts and fasting mimicking diets for a few months. I also focused on increasing my nutrients during my non-fasting stretches, particularly my consumption of healthy fats and proteins. Then a few weeks ago, I felt more like doing long fasts again.
  • I’m not saying that how we feel when fasting is the only important criteria; but I think it’s one that’s worth paying attention to. How we look, our percentage of muscle and body fat, our mental clarity, and other factors, some of which can be monitored and measured, also matter.

The Monk Fast

  • This article (The Monk Fast) has an overview of the possible benefits and risks of 36 hour fasts. In my opinion is somewhat underplays the benefits and exaggerates the risks. As it correctly points out, though, some people, like those with certain types of eating disorders, should be extremely cautious about fasting. And of course, pregnant women shouldn’t do fasts of this length. If you have any medical reason not to fast, of course you shouldn’t do it. But for those who can do so safely, the evidence suggests fasting can be a powerful anti-aging, and possibly also life-extending, practice.

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