It is no secret that I am a huge proponent of fasting, which has been well documented across most cultures throughout history. My belief in its benefits come from a convergence of anthropology, biochemistry, complex systems theory, medical case studies, (low quality) scientific studies, and personal experience. This post is meant to be a practical introduction to fasting, not intended as a comprehensive literature review or justification. Read on if you would like some advice on how to get started and troubleshoot the initial adaptation period!
Note that this guide is under-specified, and I give lots of general guidance and a few different perspectives on fasting. If you want a hard and fast rule I could give you one, but I believe you will do best to approach it with an experimental mindset and do some tinkering. Work from these general guidelines, make tweaks, and soon you will be writing up your own protocol!
Obligatory note: I am not a doctor. This is all my personal experience and conjecture. Always consult a doctor before doing anything, ever. If you have medical conditions, especially consult a doctor, so they can convince you not to try fasting.
What is fasting?
By “fasting” I am referring specifically to water fasting, where you consume nothing but water, or at least nothing caloric in any form (during some fasts I drink coffee). Many people use the term fasting to refer to not eating any solid foods, or juice fasting, where you are still consuming sugar from fruits and vegetables – this is not what I mean, and I do not believe it has the same benefits.
Fasting has been making a comeback over the past several years along with other alternative hypotheses, especially under the term “intermittent fasting” (IF). Most of the mainstream advocates of fasting will advise you to set a daily window of eating and fasting, commonly 16/8 (16 hours of fasting, 8 hours of eating), or even 20/4, or a full 24 hours (usually dinner-to-dinner). Relatively few sources advocate fasting beyond a full day. Personally I believe there are additional benefits from fasting much longer than one day.
What about just eating a little food / caloric restriction?
You should fast completely if you want to get the maximal effect. Any calories on the margin will change your metabolism away from the fasted state in a variety of ways (metabolic and signaling pathways). If you’d rather have a very low calorie day instead, that’s still going to be better than a usual day of eating, but I really do recommend trying to do a complete water fast. Ultimately if you find that sporadic low-calorie days work for you, great, do what makes it work.
Chronic calorie restriction, while it has been shown to work well to extend life in short-lived organisms, has more ambiguous evidence in long-lived organisms, and the side effects do seem to be quite significant. I think there are good reasons to cycle between fasted and fed states, rather than being in a perpetual semi-fed state.
What is the optimal fasting routine?
My recommendation is to have a fairly regular intermittent fasting routine, along with occasional significantly longer fasts. I like to eat lunch and dinner, or only dinner. A number of practitioners have made good cases that one should only eat during the midday and not at night before bed – I think there is probably merit to this approach, even if I personally find it trickier.
There has not been enough research to pin down exactly optimal regimens, but it appears that there can be significant benefits even to moderate length fasts. Many studies done during Ramadan show that even a ~12 hour fast (along with many other lifestyle changes) significantly improves biomarkers of health.
Having a regular IF routine will keep your body accustomed to going without food, which will make subsequent fasts (including long ones) much easier, and you get to reap the metabolic benefits. The most common method of implementing 16/8 is to drop breakfast, and for 20/4 you drop breakfast and lunch (but drop dinner if that works for you!). You can do this on a daily basis, though a periodic refeeding where you eat as much as you want throughout the day could still be useful.
The optimal length of long fasts is even more understudied. I believe fasts of 2-3 days duration could be done frequently without much concern (I have personally done a 2 day fast weekly without issue). Fasting for 4 days or longer begins to downregulate a variety of bodily processes, which is very useful to do occasionally, but not regularly. One week-long fast per year has been advocated in the literature. There have been a large number of documented month-long fasts without issue.
Preparation for a fast?
If you’ve never tried fasting before it can seem quite daunting! The initial state of your metabolism will likely determine how hard or easy it will be to begin fasting, and you can do some initial preparation to make the transition easier. If you’re the kind of person who regularly forgets to eat meals, or doesn’t want to snack between widely spaced meal times, you’re likely able to start fasting immediately.
On the other hand, if you regularly want/need to eat every few hours, and routinely snack between meals, jumping right into fasting will feel quite unpleasant (though it’s certainly possible to do it regardless). My general advice would be to switch to a low-carb diet for a month preceding your first fast, to begin preparing your cellular machinery to be burning a primarily fat-based energy substrate. There are many other guides to going low-carb, so I won’t say much about that here, except to note that you will experience “low-carb flu” during the initial adaptation period. It sucks, but for most people it lasts a month, and you’ll appreciate the lack of wild energy swings down the road.
Fasting, especially over long durations, involves some degree of ketosis. You can begin priming your body to the presence of ketones by creating them from coconut or MCT oil, regardless of what diet you are eating. (MCT oil is notorious for causing GI upset, however, so do ramp up your consumption slowly.)
I would recommend getting comfortable with IF first before moving to multi-day long fasts. The best approach is usually to start off by delaying breakfast as long as you can, and then keeping it fairly small. Gradually fade out eating breakfast altogether. (You can also lengthen the fast from the other end by not eating too soon before you fall asleep.) Try to establish a habit of not eating breakfast, and then start to try skipping lunch once you feel steady.
There is an element of mental preparation as well. I personally find the precommitment to a given length of fast to be very helpful. Maybe you even stop thinking of food as edible, or eating is something that other people do. Also note that for many of us, myself included, eating can be a coping mechanism, or serve a wide variety of other psychological functions. Removing the ability to eat can cause things to come to the surface, the same as removing any other potential source of comfort/pleasure/habit. I recommend being cognizant of that, and giving yourself some space during your initial forays into fasting.
Now I’m fasting and I feel really bad, help!
First of all, you will be okay. Most of your ancestors going back to single cell organisms have survived short-term energy deficits. We are not designed to drop dead from missing a few meals, or a few days of food.
Did you jump right into fasting too quickly? There really is an adaptation period, and you won’t feel good at first. You can push through this without harm, or you can ramp up to things slowly (see above).
The number one reason I feel bad during a fast is dehydration. Actually, almost every single time it’s dehydration. We are used to getting a large amount of our water from food, so you will need to be drinking more fluids to reach the same level of hydration as before. Try drinking a few glasses of water, and wait 30 minutes, then reevaluate.
If something feels really wrong and you want to eat or think it would help, just eat. Don’t beat yourself up, don’t get too discouraged, give it another try later.
Now I’m fasting and I feel lethargic, meh.
This one seems to be very different across people. Some people feel awake and energized (for the first few days), other people are tired, lethargic, foggy from the beginning and it doesn’t seem to go away. From my anecdata I think women are slightly more likely to feel sluggish than men, but I’ve found few other correlations yet. (For what it’s worth, you still should be getting the physiological benefits, but if you don’t also enjoy doing it I could see it being hard to stay motivated.)
One thing to potentially try would be supplements. Some people report positive effects from taking electrolytes, either a mixture or one of them individually. This seems unlikely to be harmful, but from the long medically supervised fasting literature it does not appear to have much effect. If you’re suffering from brain fog specifically, people report good effects with glutamine – which is a protein and will partially shift you out of the fasted state.
You can also try stimulants. A lot of people report that fasting is significantly easier with caffeine, for example, and that certainly has wakefulness promoting effects. Nicotine is also very potent at mobilizing fat stores. A wide variety of other natural and pharmacological substances interact with energy-hunger-alertness pathways, experiment as you feel comfortable. I don’t worry much about this affecting the metabolic benefits of the fasted state.
Now I’m fasting and I feel hungry!
Great! Now is the perfect time to start making a new relationship to food, eating, and hunger. :)
I think we often conflate hunger with multiple different things: habitual meal timing cues, gastric emptying (“stomach grumbling”), metabolic distress (pre-adaptation), caloric deficit, et al. My experience of fasting taught me how to piece those apart, and respond to each in turn. Gastric emptying is an interesting sensation, but says nothing about whether you should or need to eat, for instance. Habitual meal times can become an opportunity to observe the feeding behavior of the humans around you from a detached, almost anthropological perspective. Etc.
What about exercise while fasting?
During intermittent fasting, you can exercise as you normally would (except for terminally liver-glycogen-depleting world-record-speed marathons). You can even do strength training or bodybuilding during a fasting window – I highly recommend the site Leangains for a lot of helpful protocols and perspectives on maximizing muscle gain and fat loss using strength training during an IF protocol.
If you do a long fast, I generally would not recommend heavy exercise after day two, or only do so after paying attention to how your body feels. Anything below the aerobic threshold is okay, or even better, since you’re already burning fat almost completely for energy. Personally I enjoy exercise more during the first day of a fast, and I find high-intensity interval training will often reduce my hunger for the rest of the day. I also think HIIT and fasting go well together metabolically, and you will get synergistic effects from doing so, particularly when it comes to weight loss.
But my muscles!!
Don’t worry about it.
Really, don’t worry about it. I know you’ll worry about it anyway, but don’t.
Our bodies are not designed to be permanently crippled after a period of scarcity. If anything the opposite, your body requires a compensatory response upon re-entering the fed state, so as to prepare for the next time this happens. Our muscle mass is closely linked to the movements and loads they are required to execute on a regular basis, not our short term dietary intake. After my week-long fast ended, I appeared to have spontaneous body recomposition with no exercise in the adaption period afterwards – my fat was still gone (for a while anyway), and my muscles were significantly larger after refeeding than before. This overcompensation was maybe just glycogen, but it wouldn’t surprise me to get effects in muscle tissue as well.
On a scientific level, amino acid synthesis seems closely related to the change in amino acid blood levels, which means upon refeeding you will be going from an extremely low to and extremely high blood level, and replenish muscle rapidly. While I know less about the specific pathways, I suspect with high confidence that myocytes will be in a state of highly upregulated transporters of various types, and be eagerly awaiting the refeeding period.
If you are extremely worried / are a bodybuilder professionally / don’t mind blunting some of the effects of a fast, then you can listen to the Leangains guy and supplement with BCAAs. Go read his stuff, really, he’s a great fasting guru if you like his methodology.
I’m done fasting now, how do I break my fast?
For fasts of short duration I think you can eat roughly as normal, or as you feel like. Personally I developed a bit of a gorging habit (maybe out of necessity) when I only ate one meal/day for a long period of time, but it didn’t carry the usual discomfort I associate with being too full. Eating became something fun to look forward to, rather than as a time of restraint or necessity.
For longer fasts, where you’ve been purely fueling on internal muscle and fat tissue, then I recommend breaking the fast gradually. Your body will be less accustomed to glucose, so you should introduce it back over time, beginning your refeeding with almost entirely proteins and fats at first. Sometimes your hunger may not return immediately or as intensely, listen to your body, eat only as much as you want in the following days – that is also an opportunity to maintain that caloric deficit even into the refeeding period! Note that if you feel an urge to gorge after a very long fast, I recommend restraint, as that can be a very unpleasant experience.
This sounds awesome!
It is awesome. I highly encourage you to give it a try.
Please reach out with any questions/concerns, and I will add them to this post as I go!
