5 Day Fast

My 5 day fast: an account of what happened when I stopped eating completely for five days, along with an in-depth analysis of the results.

5 Day Fast


I am not the first person to say this, but I am not a doctor and I don't play one on the internet. If you fall into any of the categories below:‍

  • You are under 18
  • You are underweight
  • You struggle with an eating disorder of any kind
  • You're pregnant or breastfeeding
  • You have a medical condition of any kind which might not play well with your not eating for a week

then please DO NOT try this yourself. It might be just fine - but then again, it might not, and it would make us both very unhappy if anything happened to you. Please exercise some caution.

There is a non-zero chance that you could actually die if you accidentally overlook something important (and I go into the reasons for this during the post), so if there's any doubt on your part whatsoever, HOLD OFF. Seriously.‍


From the evening of Sunday, 18 April 2021, until the evening of Friday, 23 April 2021, I did my second-ever 5-day fast. (The first one was in January). I didn't eat anything at all, and consumed only liquids such as water, tea and coffee, along with a few vitamin supplements for electrolyte balance.

I also want to demonstrate that this is well within the realms of possibility for most people - you don't need to do any crazy preparation for something like this.

‍Let me be clear: this wasn't some hyper-mystical retreat in the mountains, alone with only my thoughts, the sound of birdsong, and a wizened old guru egging me on. Rather, it was an ordinary work week (5 days of 8-hour work days), all done at home, with life continuing pretty much as normal except for the absence of food.

‍Three main reactions that fasting tends to elicit

Reaction 1: You're insane 😲

Reaction 2: This is exciting! Good luck, have fun, and let me know how it went! 😃

Reaction 3: Oh, the internet really needs another fasting post? 🙄

For whatever reason, fasting seems to be one of those things that's very common in certain social and cultural circles and yet simultaneously rare out there in the wider world of "normal people". (For simplicity, let's leave out the fasting that's done for religious reasons.)

This division became increasingly apparent when mentioning my upcoming fast to various people, generally manifesting in the form of two clear and separate groups.

Group One - the people for whom this kind of thing is apparently A Very Weird Thing Indeed™ - reacted to this experiment with a combination of bewilderment, concern, and derision. They generally hadn't had any personal experience with fasting, and didn't know anyone else who had, either. In terms of "goodness of intent", most of it was genuine concern (albeit somewhat misplaced), although it's generally impossible to do something weird without generating at least a little bit of pushback from a disgruntled minority as well. That's fine, and to be expected.

(Something that occurred to me during the fast: how might this same group have reacted if I had said that I would run a marathon - something which feels way more difficult to me on both a physical and mental level? Whilst I imagine I would get a similar number of "oh, I could never do that" responses compared to the fast, marathon-running somehow feels like it's much more culturally acceptable, and far less prone to "what the actual fuck?" types of responses.

For some reason, running a marathon is (rightfully!) something to be respected, yet not eating for a few days marks you out as wildly eccentric. Meh - what can you do?)

Group Two, on the other hand, were excited for me, and many of them had some general tips and advice based on either their own direct experiences (most had experimented with it already) or had some useful bits of guidance based on things that they had read. On a more significant note for me personally, it was something that they just "got" - it wasn't something that I had to defend or explain from scratch, which I was very grateful for.

(Never underestimate the power of social support. By complete coincidence, my friend Isabela had also decided to do a 5-day fast, and the dates coincided completely with the dates I had chosen for my own. Three cheers for serendipity! This was a totally new experience for me in that I was able to check in multiple times per day with someone who was going through the exact same process as me, and it helped a lot to have someone to message whenever I realised some new perspective or just simply needed to vent. The same was true for her, as it was her first 5-day fast and she was glad to have somebody to talk to about it!)

There is also a Group Three - a cynical bunch of people who, for whatever reason (some perfectly legitimate!) are sick to death of fasting and meditation and keto and other things they perceive as the worst clichés of all the Silicon Valley tropes. These people definitely exist, but thankfully, my path did not overlap with theirs during this particular fast. Perhaps this post will draw the ire of some? Who knows.

(Note: I'm not even on the same continent as Silicon Valley and don't have that kind of background, but I will admit that I can definitely find some sympathy for anyone who's been exposed to any echo chamber of any sort for an extended period of time.)

(There are, of course, many other categories of people aside from these three groups - such as a plausible, mostly-neutral Group Four who are quite familiar with the concept of fasting BUT either have no personal interest in trying it or are actively against the idea for other, specific reasons - but again, for simplicity's sake, I'm keeping it to three broad groups for now.)

Well-intentioned warnings

For the probable reason that extended fasting is still very much a niche activity, anybody engaging in such an experiment is likely to find themselves the recipient of much well-meaning but information-light advice. A few of the most common:

"But that can't be healthy..."

Given the rarity of extended fasts (or any fasts) within society in general, I can totally understand the level of anxiety around this, but the health benefits are fairly well-documented at this point (see below) and, given reasonable precautions, the downside appears to be minimal. Still, my longest fast before my first 5-day fast was around 23 hours, and I think it's entirely plausible that up to that point, I had gone my entire life without skipping a single day of food. The idea of doing this is quite alien to most people (and understandably so!), which means that it tends to elicit reactions which are more emotional than logical. At the root of it, it's simply fear of the unknown, for which I have a lot of sympathy. Be gentle with your refutations.

"5 days? I can understand 16-hour intermittent fasts, but everything in moderation, right?"

Sure - including moderation.

"Aren't you going to be hungry?"


What reasons did you have for doing a 5-day fast?

Here are my main reasons:


  • To see if I could do it. I wanted to know if I had the psychological force of will to begin a 5-day fast and see it through right until the end. Having now accomplished this twice, it's a serious confidence boost, and comes with increased self-trust in the form of having set a difficult goal and achieved it. (Note: The reverse is also true. My self-trust erodes a little bit every time I say I'm going to do something and then don't do it, so it's important to get this flywheel moving in the right direction.)

NOTE: When discussing this point with my friend Isabela who was fasting along with me, she pushed back on this point in a way that was super interesting. Her words:

"I am dying to really take a deep dive into this. I want to understand if you GENUINELY feel anxious about not being able to tolerate a few days of hunger or having a few cold showers. I have ZERO doubt I could deal with these stressors, without having to go through them. Like literally zero. I am a strong believer in people’s amazing capacity to rise up to insane challenges if forced. Most of us absolutely can deal with what we’re dealt."

I think she actually makes a good point. My only possible point of divergence is that although most people are made equal by brutal and forced circumstances, I'm not sure this applies if it's something that's optional (and let's be honest, for most people like myself who are fortunate enough to live in developed economies, a 5-day fast is totally optional).

Put another way: a group of people locked involuntarily in a room will all, without doubt, be 100% capable of getting through a 5-day fast. On the other hand, the number of people who will voluntarily opt to forego food for a week without being asked to do so is probably very small, especially when there are no penalties whatsoever for just stopping whenever you like.

The question isn't dealing with the problems dealt to us by other people. The real question is: what hand are we choosing to deal for ourselves?

  • To make intermittent fasting feel easier by expanding my circle of comfort and changing the relative feeling of the fasting timeframe. In other words, driving at 50mph feels fast, but after you speed up to 120mph and acclimate to that as a "new default", then even decelerating to 80mph is going to feel slow in comparison. In the same way, any intermittent fasting (16-20 hours) that I do in the future is going to feel ridiculously easy compared to the ordeal of a 120-hour fast, even if I had previously found it challenging.
  • To develop Stoic resistance. Sometimes, it's easy to fall into the cycle of not appreciating what you have, and there are many creature comforts that we take for granted which make it very easy to feel entitled. Now, after the fast, I have the certain knowledge that if I ever find myself in a difficult situation where I'm not able to eat for extended periods of time, such as travelling through multiple airports with no chance to stop for food, then it's not going to faze me. (Cold showers can do exactly the same thing, and actually, this really happened to me when visiting family abroad for New Year 2020. Their hot water system broke. Everyone else became dependent on setting up visits to other houses so that they could shower, whereas I just took cold showers and got on with my day. That, too, was optional, but it made me feel good.)
  • To reset my relationship with food. Perhaps it was just the general bleakness of the pandemic finally getting to me, but to my alarm, I found that food was becoming purely functional; a necessary checkbox on my to-do list rather than something to savour and take real pleasure in. Doing a longer fast has the wonderful benefit of resetting that relationship with food, and once you're able to eat again, you tend to be a lot more appreciative of even the smallest things.
  • To see if the stories about insane focus were true. Many people have reported experiencing sharply-honed levels of attention when fasting (probably related to being deep in nutritional ketosis), and I wanted to see if that was something that I could also experience.
  • To just generally reset my mind. The past year has been quite challenging, and I felt that having a completely new and sustained experience would give me a perspective shift of some kind and force me outside of my comfort zone. In a sense, I wanted to see to what extent I might be able to wipe the slate clean and start constructing some habits from scratch again, and this was just extreme enough that it might work.
  • To regenerate the feeling of possibility in an otherwise dark year. There's a funny psychological quirk that happens when you massively restrict something for an extended period of time and then allow yourself to lift that block. Even though the restriction (and thus, the feeling of liberation) is technically limited to that area - so, "no food" versus "food" - it's been my experience that your mind will perceive both the limitation and the unblocking across a much wider scope. In other words, you may feel like the removal of food feels like a removal of many of your freedoms, even though that's not the case. Conversely, the feeling of freedom and delight that you experience once you can eat again is multiplied across your whole life, and you start to feel like anything is possible.‍



  • Lots of recent research has pointed to fasting (and, more broadly, caloric restriction in general) as one of the single most important things that we can do when it comes to longevity, which has been replicated extensively across studies in both humans and animals. This has been comprehensively discussed in the work of David Sinclair, Peter Attia, Aubrey de Grey, etc.
  • Fasting is well-known to promote cellular autophagy, which is the body's process for clearing out damaged cells in order to clear the way for the regeneration of new and healthy cells. Think of it as a hard-drive defrag for the body.
  • Senescence - the process that we know as biological aging - involves senescence on the cellular level, which is the gradual deterioration of cell function and, specifically, the cessation of cellular division.
  • Autophagy is one biological function that the body is able to deploy in order to regulate and "recycle" these senescent cells. Therefore, fasting promotes autophagy, which clears out these clusters of old, dysfunctional cells, which involves a reduction (and, in the best-case scenario, a reversal) of biological aging.
“A significant part of aging consists of the body’s failure to eliminate damaged cells.”
- Aubrey de Grey
  • Telomere extension. This is hugely oversimplified, but think of a chromosome as being like a shoelace. Telomeres are like the little plastic caps that you find at the end of that shoelace. These protect the chromosome's genetic material from becoming damaged in the process of cell replication. Telomeres become shorter and shorter as the cell ages, and when they eventually become too damaged and fray at the ends, the cell dies. Fasting has been shown to slow the process of telomere aging.
  • Insulin regulation. Insulin is extremely important for the body, but a permanent excess of it can lead to serious health issues, with diabetes being the most prominent. Insulin resistance can cause your body to become indifferent to the hormone's effects, leading to a vicious cycle of ever-increasing insulin secretion. Fasting is very effective at regulating insulin, which in turn is the hormone which regulates blood sugar. Increased insulin levels lead to the body storing excess fat, which means that lower insulin levels can reverse this process and lead to greater fat loss.
  • Stimulation of hGH (human growth hormone) secretion, which - amongst many other biological functions - appears to improve cognitive function and plays quite a significant role in both physical and mental well-being. (Adults with growth hormone deficiencies appear to be significantly more prone to depression than those with normal levels.)
  • Ghrelin and leptin regulation. Briefly: ghrelin is the "hunger" hormone (i.e. increased ghrelin production makes you feel more hungry) and leptin is the "satiety" hormone (increased leptin production will make you feel full). Interestingly, these don't necessarily correlate with amounts of food consumed; high levels of leptin will make you feel full even if you haven't eaten anything, and high levels of ghrelin will stimulate feelings of hunger even if you've just finished a large meal.
    Fasting can stabilise the body's reaction to these two hormones, which is important because it's possible to develop ghrelin or leptin resistance. This resistance means that your body could be producing a lot of leptin but you won't be as sensitive to its effects, so you may not feel satiated even after eating. Fasting can reduce this problem.
  • Weight loss. This was not the primary goal, and I worry about some people diving in headfirst and unprepared because of the perception that it's some "get-thin-quick" trick. It is not.

After I finished the fast, I was slightly alarmed when someone asked how my "liquid diet" had gone, and I had to do some gentle explaining in order to correct the misapprehension before it went too far. It's important to note that most of the weight you lose during a fast is water weight and some portion of that will return when you start eating. However, there will be some real, bona-fide fat loss too, but be sure to calibrate your expectations accordingly. The scales will go up again.‍

Main takeaways

There were several things that I discovered during the course of these fasts. Here are some of the main ones:

Food cravings are not the same as hunger.

It was interesting to meditate on what I was experiencing during the fast, and I discovered that what I thought was "hunger" was actually comprised of three separate sensations.

  • Actual hunger. This is the physical discomfort caused by lack of food, and although people expect this to get linearly worse over the course of the fast, I didn't find this to be the case at all. In fact, Day 4 felt considerably easier than Day 1 for me.
  • Food cravings. This is the desire to eat food, either generally or with something particular in mind. In my case, I noticed that the two sensations were often not experienced simultaneously. Sometimes I felt I could've killed a man for a croissant although my actual hunger was relatively low, and at other times, I was desperately hungry but the idea of food was simply uninteresting.
  • Restlessness. This had nothing to do with either hunger or food cravings (directly, at least) and had much more to do with existing habits and daily routines. For instance, I noticed that I would start to feel edgy around lunchtime because I was so used to stopping to eat something. Without the food, I felt a little confused, as though my body didn't quite know what to do with itself.

Observation of your feelings can make them much easier to manage.

This ties closely in with some of the things that I'm gradually beginning to understand through meditation, but the rough gist of it is that you do not need to identify as strongly with your feelings if you're able to merely observe them in action without judgement. "Watching" the sensation of hunger very often allows it to just dissipate, and you recognise it as a transient state rather than as something that you need to strongly identify with.

Meditation has also helped to re-frame some of my perspectives. For instance, if you have a spare five minutes, here is a short lesson by Sam Harris on choosing the "right" problems to have. Basically: were you truly ever expecting to reach a point in your life where you had no more problems left?

Given the magnitude of that perspective change, hunger ceased to be such a weighty "problem" for me and it became much easier to just observe the sensation for exactly what it was, instead of attaching unnecessary baggage to it.

Hunger does not increase linearly over a longer fast.

This is an understandable error, but an error nonetheless. The common anticipation that most people have is that they expect hunger to get gradually worse and worse over the course of the week, culminating in an agonising peak of hangry rage by the very end. Actually, it seems to vary quite a bit for different people, but not a single one of the 5-day fast accounts that I've read about has followed this pattern.‍

For example, Day 4 was the day that I felt the best, including the first day, but when my friend Bryan fasted for 5 days, he described 5pm on Day 4 with the sentence "I faced debilitating hunger, the worst of my life." (However, it did go away after twenty minutes.) Strong words, of course - but my point is that although the specifics are different for everyone, it's definitively not a linearly-worsening experience.

You'll probably be cold.

For me, this was actually one of the most annoying things about the fast, and it seems to be a common occurrence for others as well. On both of my 5-day fasts, I began to get cold towards the end of day 1 and, with brief respites, stayed cold for the duration of the fast. I found myself wearing thick jumpers, warming hot water bottles, and holding mugs of hot coffee just to keep my hands warm.

For this reason, exercise was extremely useful as a tool - not just for blunting hunger pangs, but also for keeping warm.

Exercise and workouts

I continued to work out as normal during the fast, focusing primarily on bodyweight exercises and weightlifting (using a kettlebell and dumbbell) as opposed to cardio. It seems that weight training / resistance training is actually quite effective in preventing the loss of excess muscle mass whilst fasting, as mentioned here by Peter Attia.

"It’s been observed that resistance training prevents most, if not all in some cases, the muscle loss that comes with CR. You may recall in one of my AMA’s when I discussed my quarterly 7-day water-only fasts, I’m amazed at the lack of muscle loss, provided I lift weights daily. In fact, with many cycles of such fasts, I see the same results over and over, suggesting that preservation of leucine alone is sufficient to maintain muscle mass."

Here's a sample of one of my workouts. It lasted roughly an hour and a half and was of moderate intensity.


  • Kettlebell Single Arm Farmer Walk (20kg, 4 sets)
  • Kettlebell Row (8x20kg, 4 sets)
  • Seated Dumbbell Curl (7x15kg, 3 sets)
  • Ab Rollout (10 reps, 3 sets)
  • Russian Twist (18-20 reps, 3 sets)
  • Dumbbell Bicep Curl (8x15kg, 3 sets)
  • Hammer Curls (8x15kg, 3 sets)
  • Single-Leg Romanian Deadlift (5x15kg, 4 sets)
  • Scissor Crossover Kick (11 reps, 3 sets)
Heart rate information showing which heart rate zones I was in.

Day-by-Day Rundown

Day 0 - Sunday

(I'm adding a "Day 0" because I technically started this fast on Sunday evening, not on Monday.)

It wasn't a great start to the fast. I had a busy evening, and had dinner a little later than I had originally intended, about 8:45pm. It also probably wasn't the most nutritionally intelligent meal I could have had...‍

That said, once finished, I started up the Zero app, adjusted it to 5 days, and pressed start. It's a very psychologically interesting feeling to watch a timer start counting down from 120 hours, knowing that that's the next time you're allowed to eat.

5 seconds in, and doing well!

I didn't get to bed until well past midnight, making this the only night of the fast where I got less than 6 hours of sleep.

Day 1 - Monday


The first morning was a fairly ordinary one, as the fast had barely started. I got up early, made coffee, and burned down quite a few tasks off my to-do list before working out and meditating. I was in a really good mood, and felt calm, but at the same time had a lot of energy reserves. I actually experienced a few small hunger pangs around 9-10am, but they passed quite quickly, which felt like confirmation that our existing mealtime habits contribute significantly to our expectations of hunger.


By the time the afternoon rolled around, I wasn't quite as perky as the morning. It was quite a chilly day, and fasting tends to drop my body temperature quite a bit, so I was starting to feel cold, especially in my hands. I also noted that I was feeling quite cranky at around 3pm - easily irritated and annoyed with my work, and although I took a midday break, I noticed that the absence of a proper lunch routine threw me off a bit. On the plus side, I wasn't hungry, and I didn't have any food cravings yet. It seemed to be purely the change in routine that was throwing me off.‍

After finishing work for the day, though, something surprising happened. Freed from the necessity of cooking an evening meal, I decided to make full use of the additional free time, and so I retreated to my home office to begin work on my site redesign. I'd been meaning to migrate from WordPress to Webflow for quite some time, and decided this was as good a time as any to stop putting it off and just get started.

I worked for a solid 6 hours straight with no break at all.

I don't know if fasting was a direct cause of flow or whether it was because of the additional free time and lack of distractions, but I got the majority of the site structure set up in one long, uninterrupted burst (whilst learning Webflow from scratch, as I'd never used it before).

I took my blood measurements around 10pm, and I was quite disappointed to see that I was barely in ketosis at all (0.2 mmol/L compared to Sunday's 0.0 mmol/L) and my blood sugar was identical to Sunday's measurement as well (5.4 mmol/L, or 97.2 mg/dL).

To shut down my mind a little, I went to bed with a cup of forest fruit tea and read some fiction for a while (Brandon Sanderson's excellent "Oathbringer"). I was slightly impatient about the lack of ketosis, so I added just a tiny bit (maybe half a teaspoon) of MCT oil to the tea in order to try to kickstart that process.

I took melatonin before going to sleep, which I had also done on my first 5-day fast. I had heard from several people that they had experienced disruption of sleep whilst fasting (both "onset" insomnia, or trouble getting to sleep, and "maintenance" insomnia, or disrupted sleep with periods of wakefulness during the night) and I decided I wasn't in a hurry to find out whether this would happen to me, too. I therefore supplemented with 5mg of melatonin before bed in order to ensure that my sleep cycle remained somewhat stable, and whilst you might call this cheating, I'm happy to say that I don't care at all. Fasting for a week is already hard enough without dealing with bad sleep on top of that.

Summary of Day 1: Hunger / Food Cravings / Thirst

Summary of Day 1: Energy / Focus / Happiness

Day 2 - Tuesday


Day 2 began, and I woke up early (with an alarm, but then again, I set alarms every single day, including days off, so this wasn't unusual).

I had focused so hard the previous evening on creating my site in Webflow that it actually carried over into my dreams, and I distinctly remember that even though I was fast asleep, I was thinking intensely about CSS padding, element adjustment, and so on. There's actually a name for this phenomenon: the Tetris effect.

"[The Tetris Effect] occurs when people devote so much time and attention to an activity that it begins to pattern their thoughts, mental images, and dreams."

(Side note: I'd forgotten that melatonin gives me extremely vivid dreams, so when I wasn't dreaming about Webflow, I spent the rest of the night experiencing some fairly intense nightmares, which wasn't the fault of the melatonin. It doesn't appear to make dreams "good" or "bad" - just very vivid and much more lifelike.)

I measured my weight, and I was down 1.7kg (3.7lb) (!!!), although this sounded more exciting than it actually was, as the majority would have been water weight and not true fat loss. Still, I can't deny it was quite motivating.

I made some coffee, enjoyed the morning sunshine pouring in through the window, and continued working in Webflow for a while, all of which which put me in a really good mood. There's something about creating things and using modern, streamlined tools which resonates deeply with me, and I was happy to get a few additional tasks checked off before the day had even begun.

After this, I worked out with a mixture of bodyweight exercises and dumbbell / kettlebell exercises. It was perfectly manageable and had the side-benefit of warming me up, as I was still feeling quite cold. Before the workout, I noticed a few very small (<30 seconds) spikes of hunger, but they receded almost immediately. After the workout, I got down to work and noticed that I was able to sink fairly easily into my work without getting distracted.


Around lunchtime, this is what I had written in my hour-by-hour notes‍

Thinking that the ritual of food is a good part of what makes it so enjoyable. You get to sit down, take a break from all else, and sample a different sensory perception. I'm still not really hungry, per se. I don't even have food cravings for anything specific, but I find myself pausing, almost slightly confused, around the times when I might otherwise go hunting around the kitchen for something to eat.

The afternoon continued well, with sharp focus and no hunger or food cravings, but by the time evening rolled around, I was feeling consistently cold and the food cravings were starting to creep in. My girlfriend and I watched the Apple event and she had dinner at the same time, which made it a little tougher. By the time the evening ended, the food cravings were the highest they had yet reached - a 9 out of 10, with actual hunger coming closely behind at 8 out of 10.

Thankfully, I was able to puncture some of the frustration by explaining my feelings of frustration to Isabela, who was also two days into her own fast. Simply being able to enunciate your feelings of frustration to somebody else is a solid way to put a little distance between you and your thoughts, which - in some cases - is just enough to allow you to avoid identifying with them completely‍

Ironically, the highest levels of hunger that I had yet experienced coincided with a significant improvement in my blood measurements. My blood sugar had dropped, just a little bit, to 4.6 mmol/L (82.8 mg/dL) and my ketone measurements had risen to 2.1 mmol/L, putting me firmly within the bounds of nutritional ketosis. Unfortunately, by this point, I was feeling weak and shaky and also experiencing a little bit of acid reflux (perhaps from drinking lemon water?) and so I decided to call it a night.

End of Day 2 - less than 70 hours to go!

Summary of Day 2: Hunger / Food Cravings / Thirst

Summary of Day 2: Energy / Focus / Happiness

Day 3 - Wednesday


Day 3 sucks.

I woke feeling weak and nauseous, and this time even the further drop of 1.1kg (2.4lb) did nothing to brighten my mood. I made some coffee, which is generally acknowledged to be a fairly effective hunger suppressant, but it did nothing to make me feel better and in fact made the nausea stronger. I therefore found myself in a very odd state of mind; my actual hunger was at full-tilt, but because of the nausea, the idea of food was very off-putting, so my food cravings were extremely low.

(Interestingly, I referenced my notes from my first fast, and the low period was exactly the same, beginning on the evening of Day 2, continuing throughout most of Day 3, and abating from there. Whilst not exactly comforting, it was certainly interesting to note the consistent experience on both fasts, and it provided a little bit of determination to get through the day.)

Given that coffee failed to do anything to stem the hunger pangs, I decided to work out. This actually helped a little bit, but it didn't remove the hunger entirely. I was noticeably slower than normal during the workout, and although I could definitely still lift the weights, there was no explosive energy behind it, making the entire thing feel a lot more sluggish than normal.

Although the workout wasn't great, I meditated immediately after it, and to my surprise, the meditation session was fantastic. Rather than feeling grumpy about slogging through the fast, I felt a sudden sense of gratefulness. I was grateful for the heightened sense of awareness that I was experiencing, and I remembered that despite my temporary state of discomfort, my body was working hard for my benefit, engaged in autophagy, improvement of metabolic health, regulation of blood sugar, and so on.

Most importantly, I felt a sense of calm resolve which filled me with confidence: a strong feeling of "I am able to do this." To call this either arrogance or an affirmation would be to miss the mark completely; instead, it was more like a quiet, serene statement of fact.

The hunger and food cravings were still firmly present, though, so rather than dwelling on them, I decided to try the tactic of simply writing down whatever food I was craving - with the reassurance that if I still wanted it by the time the weekend arrived, I could have it then (which I duly did - see the end of this post). This was actually quite effective in taking my mind off the cravings - something which I completely failed to do on day 2 of my previous fast.

(If you're curious: I foolishly lost an entire afternoon and evening to increasingly-manic Google image searches for every type of food I could think of. Rookie mistake, and made my life five times harder than it needed to be.)


Work came and went and the evening arrived, and given that neither coffee nor exercise had blunted my hunger to the extent that I wanted, I decided to pull my third and final weapon out of the bag and went for a long walk (something recommended effusively by my friend Bryan during his own fasting experience).‍

This turned out to be a really good idea. It was a warm and sunny evening, and the fresh air and greenery did me a lot of good. Getting out of the house is also, it turns out, a good way of getting out of your own head, and I walked partly in silence and partly with an audiobook (Jan Swafford's excellent biography of Mozart).

Canals in Haarlem.
A peaceful lake.
Much-needed greenery.

By the time I got home, the hunger and restlessness had reduced a little, but I found myself feeling a little weak and tired, and wasn't able to be particularly productive.

I read somewhere that dizziness whilst fasting is often a sign of electrolyte imbalance, so I supplemented with a cup of bouillon in order to get some much-needed salt into my system.‍

The main feeling that summarised the evening of Day 3 was simply boredom. I was bored of the fast, bored of waiting around, and bored of the week feeling like it was dragging on. The only redeeming factor was that I finally felt a little warmer during the evening after suffering the cold solidly for the previous 3 days.‍

Blood sugar had fallen further to 4.1 mmol/L (73.8 mg/dL) and my blood ketone levels were only a little higher than the previous day at 2.6 mmol/L.

Summary of Day 3: Hunger / Food Cravings / Thirst

Summary of Day 3: Energy / Focus / Happiness

Day 4 - Thursday


Day 4 dawned, and I woke feeling glorious.

There was no grogginess - I was simply wide awake from the moment my eyes opened, and I felt this wonderful lightness that's hard to explain. No hunger. No nausea. I had been experiencing some pain in my leg from a workout injury a week prior, but that had also all but disappeared.

The scales showed that I was down another 1.1kg (2.4lb) and, again, whilst most of this would be water, there is also some real fat loss which happens over the course of these 5 days, small though it might be - and it was starting to show. On top of this, my skin looked very clear (another odd pattern that I've noticed amongst other people who have also tried a 5-day fast. Perhaps it's just to do with drinking a lot of water?)

I did another short meditation, and this was even better than yesterday's session. I felt totally calm, was able to observe my breath without any distracting thoughts, and even had a small physical sensation of euphoria at the end of it (something which has only happened to me a few times, usually after doing a meditation after some intense exercise. The euphoria normally manifests in the form of a happy, warm glow spreading throughout my body, and it feels really good).‍

Here are my post-coffee notes:

Still feeling great! Had a cup of coffee and I feel super awake and focused. My mood is great too - the bright sunlight outside helps a lot. No real hunger. Food cravings are kind of theoretical right now - what I mean by that is that this happy mood makes me excited for the world to open back up again, to go to nice restaurants and try new food and explore things in general, but the cravings aren't localised to any one particular food. Instead, I feel an optimism for exploring these things in the future in a way that doesn't have a negative effect on my current mood!

I worked out again, and it was very similar to Day 3 except without the hunger and in a much better mood. I could still lift the weights, but there wasn't much force behind it. Indeed, I was - quite literally - running on empty.

Interestingly, I actually felt worse after the workout - not due to the workout itself, but because there were some upcoming work-related tasks that I was dreading, and my mood dipped accordingly. Physically, I felt absolutely fine.


During the afternoon, my mood waxed and waned and eventually settled on "neutral", but the hunger receded almost completely until emerging again at a low level around 4pm. (I marked it a 6/10, and it soon returned to a 3).

Once again, I went on a walk for about an hour and a half, opting to get out of the city centre and spend some time on forested paths in order to reset my mind a little. I called a friend as I walked, and felt totally normal - great, even. In fact, it registered as quite strange - almost absurd - that it was Thursday and I hadn't eaten anything since Sunday.

My bloodwork, this time, was quite different from yesterday's. My blood sugar had dropped to 3.4 mmol/L (61 mg/dL) and my ketone levels had risen to 4.1 mmol/L, which put me firmly into "deep ketosis" territory. (For comparison, “optimal ketosis” is defined as having ketone levels between 1.0 mmol/L and 3.0 mmol/L, which is perfectly sufficient for something like a standard ketogenic diet. A blood ketone level higher than this is nothing to worry about until you start getting into and beyond the 8-10 mmol/L range, at which point it starts to become ketoacidosis, which can be very dangerous and even lethal.

(I'm serious about this. In fact, the very next day - Friday, 23rd April, or my Day 5 - my Twitter feed suddenly started to fill up with tweets mourning the passing of Daniel Kaminsky, a well-respected cybersecurity researcher who suddenly and tragically passed away at the age of 42. The cause of death? Diabetic ketoacidosis.)

By the end of the day, my energy and happiness had started to dip a little bit and I started to feel quite appreciative that the whole experiment would soon be coming to a close.

Well... "appreciative" is one word. "Impatient", however, is a more accurate one.

Aside from this, Day 4 was my favourite day of both 5-day fasts (with Day 1 coming a close second).

Summary of Day 4: Hunger / Food Cravings / Thirst

Summary of Day 4: Energy / Focus / Happiness

Day 5 - Friday


Final day of the fast!

Unfortunately, unlike the entirety of the previous day, this one started with quite a bit of hunger. It wasn't as bad as it could have been (I gave it between 7-8), and thankfully it faded again throughout the morning, but after the high of Thursday, it was a sharp reminder that I was well past the 100-hour mark. There was a small amount of weakness and nausea, but that passed quickly.

(Also, this was the only day of the fast that I didn't work out, although I probably should have done.)

The most interesting thing that happened during Day 5 (aside from being able to eat again, obviously) was a mindshift change that evolved slowly and gradually over the whole day‍

When you have a cold, you curse all of the times that you could breathe clearly and freely and yet took it completely for granted. When you injure your foot, the fact that you could previously walk for hours without even thinking about it seems like a marvel‍

Something similar happened for my relationship with food, and I realised that I had been in a kind of stagnation period. I'd call myself a mid-level foodie - it's far from the first thing I'd identify as, and there are many people who'd happily admit to being far more obsessed than I am - but I do enjoy cooking a lot. I own a sous-vide machine, I look up to people like Kenji López-Alt and Samin Nosrat and Jacques Pépin, and I use a recipe manager (Paprika 3) almost every day. Cooking is, generally, an activity that I enjoy.

Despite all of that, I'd been in a culinary funk, and hadn't even realised. I'd been falling into the same patterns and wasn't getting any enjoyment out of it. After 5 days of no food whatsoever, though, I found myself really excited to cook again, and I started to seek out new recipes to try and made lists of the items of kitchen equipment I'd been meaning to pick up for a while. Cooking was once again starting to feel like a playful idea - a fun process of experimentation where you got to create something real and physical with your bare hands, with the added bonus of being able to eat it afterwards.

Isabela experienced exactly the same thing. In her words‍

YESSSSSS. Totally agree, although I've been a foodie for a while. But this damn pandemic has made me very unappreciative of food. I used to LOVE cooking and now I am SO SO bored with it all. Just being able to go out once in a while to restaurants to have a break and enjoy something different made me super excited to cook more. It's become such a chore, getting off work calls at 5:30 then going down and being in my kitchen in 30 sec and starting to set up dinner. Now I am SUPER psyched to cook again. So yes, totally get the appreciation part and I'm VERY grateful for this learning.

Physically, although the hunger had receded again, I was feeling quite weak, and once or twice experienced feelings of lightheadedness.


I worked from my balcony in the bright sunshine for a little while, which was a great mood-booster. Unfortunately, the mood gains got erased pretty quickly after a less-than-inspiring Friday afternoon meeting, and the dip in my "happiness" measurements reflected this accordingly.

By the time I finished working for the day, the end of the fast was drawing very close indeed, and I hadn't yet worked out exactly how I was going to break it.

The "correct" way of breaking a fast is something that tends to spur some fairly heated arguments online, with some factions devising complex formulae for drawing the entire process out gradually over several days. I'm a little skeptical of this approach. Here's why.

Imagine that our hunter-gatherer ancestors (who were almost biologically identical to modern-day man) finally managed to track and kill a gazelle after not eating for a week. It seems to me highly unlikely that they would first begin by taking hours to boil up some bone broth, sipping it slowly over the course of two days, before seeking out some plants to supplement with and finally introducing tiny scraps of meat spread out throughout the day. By that time, they'd be so weak that the chances of catching any further food would diminish rapidly. It makes a lot more sense that they'd just eat the meat, and it would make no sense if evolution had furnished us with digestive systems incapable of handling this.

To be clear, I'm not saying you should bolt down a vindaloo curry after having nothing in your system for a week.  It probably makes sense to err on the side of eating some fairly neutral, unprocessed foods at first. But provided that you're relatively sensible about what you're introducing to your body, you will likely be fine.

(The exception to this is something called refeeding syndrome, which can indeed be severe enough to kill you. However, this is very unlikely to happen after a 5-day fast; it's generally limited to people who are seriously starved or critically malnourished, such as concentration camp prisoners. Still, it was enough to worry me when I discovered the Wikipedia page for it during my first 5-day fast.)

I do think that it's a good idea to break your fast with a small amount of easily-digestible food and let that settle for an hour or two before moving on to the main meal.

Before doing any of this, though, I wanted to take my final blood measurements before ending my fast. My ketone levels came in at 5.7 mmol/L, which meant that I was in very deep ketosis (unsurprising, after a full week of not eating). I then moved on to my blood sugar levels, which came out as 2.4 mmol/L (or 43.2 mg/dL).

I realised at this point that I had a relatively good intuition for what the blood ketone levels meant (0 is "not in ketosis", 0.5-1.5 is "light ketosis", 1.5-3 is "optimal ketosis", beyond 3 is "deep ketosis", beyond 8 is starting to approach ketoacidosis, and beyond 10 is dangerous to the point of being life-threatening).

However, I didn't have any kind of mental comparison for equivalent ranges for blood sugar. I jumped online to find out what 2.4 mmol/L of blood sugar meant, and this is what I found after a quick Google search:

"Are you sure your information is correct? If a person's blood sugar level were 2.4 mmol/L they would need immediate medical attention."


"That blood sugar level is considered critically low. A person at that level would in many cases be in hypoglycemic shock."


My favourite answer, however, was the following:

"It would be the patient's cause of death. Just kidding. But seriously the person would be dead."

Uh. What.

The odd thing was that I felt absolutely fine. Not even just "fine" by fasting standards, but objectively fine - a tiny bit weaker than normal, but I certainly wasn't feeling bad. And I definitely didn't feel like I was in hypoglycemic shock.

I still don't have an explanation for why I felt so normal with such low blood glucose, but my suspicion is that elevated ketone levels likely had something to do with it (although this is just a hunch). If you're reading this and you can give me some deeper insight into why this might have happened, please let me know!‍

Breaking the fast

I tracked my mood in more detail in a spreadsheet, but here's the rough version directly from the Zero app.

To break the fast, I started with an electrolyte sachet dissolved in a glass of water to make sure that I was hydrating properly, and then measured out a little bit of kefir yoghurt, which is both gentle on the digestive system and a good source of probiotics in order to repopulate weakened gut flora. I had this along with a handful of macadamia nuts for a relatively high-fat / low-carb base of stability, in order to prevent any huge spikes in blood sugar when beginning the refeed proper.

Electrolyte sachet with orange flavouring. It was quite strange - salty and sweet at the same time.
A small serving of kefir yoghurt for gut bacteria repopulation.
Salted macadamia nuts (only 100g, not the entire pack!)

Although the portions were quite small, I noticed that I already felt quite surprisingly full after finishing these. My notes:‍

"I genuinely can't tell if I'm hungry at this point. My body feels a bit confused after starting to eat again..."

After around an hour, the rest of my dinner was ready: a sous-vide chicken breast with some Mexican taco seasoning, an oven-roasted sweet potato with a little butter, and some broccoli. And so my 120-hour experiment came to an end. I felt fairly - but not uncomfortably - full after finishing dinner, and even satisfied some of my week-long cravings by having a little chocolate afterwards.

Chicken, sweet potato, and broccoli.
I was also hugely in the mood for these cocoa truffles, so I (cautiously) went for it.
Cocoa truffles.

At the end of my first 5-day fast in January, I noticed that my stomach was a little upset as I began to eat again, but thankfully this didn't seem to happen on this fast and I felt almost totally normal. Still, if you try this yourself, I would advise that you stay in a comfortable environment - preferably your own home - when you're breaking your fast, with quick access to a bathroom (just in case 😬).‍

Finally, just for comparison, I took one last set of blood measurements:

Blood ketone levels

  • Before: 5.7 mmol/L
  • After: 1.8 mmol/L

I was expecting the meal to knock me out of ketosis entirely, so I was quite surprised to see that my ketone levels were still quite high at almost 2 mmol/L. These probably dropped further during the night, but I didn't track them after this last measurement.‍

Blood glucose:

  • Before: 2.4 mmol/L
  • After: 6.7 mmol/L.

Ah, that's better.‍

One final observation, which is likely unsurprising: I felt incredibly sleepy after the first meal on both fasts, and decided to just head to bed not long after eating. I fall asleep quickly anyway, but I was out cold about a minute after my head hit the pillow.

Summary of Day 5: Hunger / Food Cravings / Thirst

Summary of Day 5: Energy / Focus / Happiness

Conclusions and wrap-up

How do you feel about it?

I'm really glad to have done it, for all of the reasons stated above. Extended fasts are a theme that I frequently see online (I got introduced to the idea by Tim Ferriss and Kevin Rose years ago, and it's only risen in popularity since then) and I wanted to truly experience it for myself instead of just reading about it. After the first time doing a 5-day fast, I wasn't sure if I would do it again, but the second one followed just a few months later.

How did your weight fluctuate?

Here's a diagram of the fluctuation of my weight measurements, starting on Sunday and continuing for two days after the fast ended.

Fluctuation of weight over the course of the fast.

You can see that the final pre-fast meal on Day 0 actually caused me to gain some weight the following day, but it dropped rapidly again after that. In addition, you can see that some of the weight starts to return after the fast is finished, which is completely to be expected.

That said, though, I have noticed on both fasts that although you do gain some of the weight back, it does tend to stabilise at a noticeably lower bodyweight than before the fast.

Was it annoying taking the blood measurements?

Kind of. I don't love needles but I can deal with them, and given that I'm not a diabetic, I wasn't used to pricking my finger to obtain blood, so that took a little bit of getting used to.

I ordered a blood measurement kit specifically to collect data for this fast, with the stipulation that I wanted to track both blood glucose AND blood ketone levels, so I needed a device which could manage both. In the end, I went for the CareSens Dual, which was relatively inexpensive and seemed perfectly functional for my needs. I'm not recommending this one, necessarily - if tracking blood markers is something you want to try, see what's available in your region and I'm sure you can find something which is both inexpensive and reliable. (I'm in the Netherlands, so many of the US-based recommendations in "top 10" articles didn't apply to me.)

The "lab" for blood measurements.

It contained:

  • CareSens Dual for measuring the levels of blood glucose and blood ketones
  • Lancet pen for drawing blood samples
  • Individual lancet needles (I used a fresh one every time)
  • Measurement stick for blood glucose
  • Measurement stick for blood ketones
  • Carrying case
  • (additional recommendation: alcohol and a cotton swab for disinfecting your finger before taking a blood sample.)
Pretty straightforward to use.

Did your days feel longer, or shorter?

Actually, this is a really interesting question. Obviously, in many ways, I got a lot of time back from my day - not just the time spent eating, but the time spent preparing food, organising the kitchen, cleaning up afterwards, going to the supermarket, and so on.

On some of the days, I was able to maximise those additional hours to their full extent, such as on Day 1 where I got a solid 6 hours of deep work and it barely even felt like I was expending any effort.

However, on the other hand, I noticed that two or three factors actually reduced some of my available time, so it's hard to say whether I gained or lost the hours. On the evenings when I felt tired and irritable, I ended up going to bed a little earlier than I would ordinarily have done, and I also noticed on multiple days that I slept a little later than I would ordinarily have done, too. On the first fast, I was really struggling with the wait until the end of the week, and slept multiple additional hours every day just to get through the week a little faster. That wasn't so much the case on the second fast, but it's quite hard to work out how the symmetry ended up. On balance, I'd say it was more or less the same.

What did you actually consume in the end?

My general rule of thumb was this: if it has any calories at all, it's off-limits. I wanted to maintain a strict fast, properly, from start to finish, without breaking it for any reason.

The vast majority of what I consumed was simply ordinary tap water. Coffee was the next-highest consumption. In total, I had:

  • Water (13.7 litres)
  • Coffee (3.2 litres)
  • Bouillon / stock cubes (for salt and electrolytes) (1.45 litres)
  • Tea (generally fruit tea) (0.7 litres)
  • Carbonated water (0.5 litres)
Water was by far the most common thing that I drank.

In terms of quantity, I drank between 2-4 litres of liquids per day for an average of about 3.1 litres per day. This is actually more than I normally drink (I'm usually terrible at remembering to drink water) but for some reason, I was consistently thirsty throughout the fast, with an average thirst level of  5 out of 10 with frequent spikes. I'd imagine that this was because my body was losing a lot of water weight, even after drinking lots of water to replace and trying to retain some salt by having either bouillon or some water with a little added salt.‍

Fell off the wagon slightly on the first and last days - whoops.

I also took some supplements throughout the fast - the ones that I normally take on a daily basis anyway. These included:‍

  • a general multivitamin to attempt to keep some of my electrolyte levels stable
  • a magnesium supplement (specifically: magnesium bisglycinate)
  • omega-3 supplements

I also had about half a teaspoon of MCT oil, which is usually consumed in order to accelerate the onset of nutritional ketosis. This probably amounted to 20 calories in total, so we'll let it slide.‍

What did your blood glucose and ketone levels look like?

I mentioned each one individually in the daily summary, but here's a diagram of what it looked like over the course of the whole fast. I've labelled the point where I ended the fast and started eating again.‍

You can very clearly see the decrease in blood sugar and increase in ketones, and the immediate reversal upon starting to eat again.

How was your sleep?

Fairly standard. The first night wasn't great - about 5 and a half hours - but the rest of the time I averaged about 7 hours of sleep per night. As I mentioned, I was supplementing here with melatonin in order to prevent any sleep-related issues, as I needed to be functional during the week. I slept pretty normally - more or less right through the night without waking, but as mentioned previously, melatonin always gives me ridiculously vivid dreams.‍

Averaging out at 7 hours.

Did you ever think about quitting before the end?

Honestly... no. Not once.

Isabela actually summed this up extremely well during a conversation we had during the week:

"My friends are on this group chat and they asked me how I was doing and I felt sheepish last night saying 'all good!' Especially since they were all so clear they could 'never do it' and were so apparently impressed with my willpower. And yet.. there's something interesting about the nonlinear decision making I do. When I decide I'll do something, I just don't stray often. My problem is deciding and starting... never going through with it when it starts."

There's a huge amount of truth to this. I don't consider myself to be somebody who has an inhuman amount of willpower. Rather, the rationale was as follows: rather than just fasting for "a while", I made sure to set extremely clear boundaries on when I would start, when I would stop, what I would consume, and what I needed to do in the meantime. In short: the specifics matter. I would start on Sunday evening and continue until the full 5 days had elapsed, no matter what.

I also posted about it on Twitter to ensure that other people would hold me accountable, discussed it at length with friends, and told several people in my personal circles well in advance. I also wanted to track the data and write it up, and knew that I would feel foolish writing about half a fast.

This is a productivity trick that actually works pretty well, and essentially amounts to turning it from a vague "maybe I'll do X" into a hyper-specific, time-limited project with clearly-defined terms and boundaries.

  • Decide the boundaries (when you'll start, when you'll stop, and define exactly what it means to "start" or "stop" this activity)
  • Make sure that you are VERY clear, well in advance, about exactly what it is that you need to do during this time. In my case, it was simple: don't put any food in my mouth for a week. Not necessarily easy, but absolutely simple. The clearer you are, the easier it'll be.
  • Boost your self-commitment by telling a few people, such that you'll look a little bit foolish if you don't go ahead with it. (Hardcore commitment folks might recommend a tool like Beeminder, which charges you actual money if you don't do what you said you were going to do.)

Post-Fasting Epilogue

Last question, but: come on. Tell us. What were the food cravings that you wrote down during the week?

(Note: If you're currently fasting, maybe quit reading now...)

First off: I deliberately wrote off the entire weekend immediately following the fast. This was not me caving into temptation - in fact, it was consciously planned that way long before I even hit start on the timer. From a nutritional point of view, this is obviously not great, and perhaps if I'd followed a healthier approach, I could have ridden the wave of nutritional ketosis right through a few more weeks of keto. (Maybe I'll try this next time.)‍

That said, a 5-day fast can take quite an intense psychological toll on you, as it's an unusually strict self-restriction. For that reason, I decided to just permit myself to have whatever I wanted without worrying about it, knowing that I would dial it back to something a little healthier on Monday. This also has the effect of allowing you to get past all of the cravings within the span of a day or two, as - once you've gorged yourself on all of the food you missed - the cravings drop away and you're quickly back to neutral territory again.‍

Also, damn it, I was basking in the luxury of being able to eat again, and it felt wonderful. It wasn't just the sensory perception of the food - the smell, the taste - or the newfound re-appreciation for playful cooking and culinary experimentation that I developed during the fast. It was bigger than that; a sense of rediscovered freedom. The lifting of limitations is something that's truly hard to describe unless you've experienced it for yourself, but if you imagine a person locked in one small room for a week until finally being released out into the wide world, basking in the fresh fresh air and sunshine and allowed to wander wherever they want - imagine that, and you might begin to get some idea of what it's like.‍

I intended to maximise my enjoyment of that experience.

I got up early on Saturday morning, slipped out of the front door, and went to a local bakery to pick up a croissant and a pain au chocolat, passing by one of my favourite cafés on the way back to pick up a cappuccino too.‍

Croissant and pain au chocolat.
Cappuccino from Anne & Max.

After that, I experimented with a protein pancake recipe that I'd discovered during the week, and I wanted to test it out. I served it with blueberries, strawberries, raspberries maple syrup, and a light dusting of powdered sugar.

Protein pancakes with strawberries, raspberries, blueberries and maple syrup.

During the afternoon, I went for a walk and went on a mission to fulfil one of the two main cravings I'd had during the week: chocolate cake. There's a local bakery that sells single slices and I went to it with something very specific in mind. I wasn't disappointed, but dear god - it was much denser than I had anticipated. Amazing, though.

"Five High" chocolate cake from Multivlaai.

I have no idea why, but the one consistent craving I'd had during both fasts - and hadn't actually tried - was a gourmet hot dog. I duly did some hunting online, found one of the few places that sold them, and promptly ordered it. Glorious.

Hot dog from Thrill Grill in Haarlem.
Probably negating the positive effects of the entire fast within a few hours, but totally worth it.

Later that week, I also tried out a Chefsteps recipe that I'd been intrigued by: a double chocolate tart. It turned out pretty well, though I need to tweak a few things if I make it again.

Chefsteps double chocolate tart.


‍I didn't quite expect to write 11,000+ words about this experience, but in the event that any of this is useful to somebody thinking about trying it, I'm glad to have done so. PLEASE take serious precautions if you choose to try this yourself, and remember, if you have any conditions (either physical or psychological) which might make it a bad idea to try an extended fast, then DO NOT try it yourself.

Otherwise, if you have any questions or comments about this, hit me up on Twitter and let me know!

Stay well.