5 day fast

Last week I did a five day “water” fast, although on some days I took black coffee in the morning. This means, maybe obviously, that I didn’t consume anything for five days other than water, coffee, some salt, and a little cocktail of 0 calorie unflavored electrolyte supplements. It wasn’t bad, and it’s something I plan on doing at least once a year.

I’ve gotten a variety of responses when I tell people this. Most people have heard of intermittent fasting, a milder form of the practice, which is all the rage these days (and for good reason IMHO). IF is cool, but there are unique features of extended fasts that I think are really compelling even above and beyond some kind of possible longevity benefit.

Why did I do the fast?

Like I said, there’s plenty of research to get excited about when it comes to fasting, but if you’re in want of citations I’ll refer you to the resources section below: many other people have condensed that info more usefully and succinctly than I could, and I’m not necessarily here to sell you on fasting. Do your own research.

That said, a summary of the rationale goes like this: when you deprive your body of exogenous energy (i.e. food), whether that’s through exercise or not eating, that energy has to come from somewhere. Your body preferentially burns glucose because it burns quickly (which incidentally creates volatility in energy levels, e.g. sugar highs) but glucose runs out pretty quickly. Once the carbs stop flowing, the glucose stops burning1 and your body has to figure something else out.

It then switches to something called lipolysis (as opposed to glycolysis) which uses fat as a source of energy. Lipolysis burns more slowly and some argue more cleanly (citation needed). When people talk about ketosis, this is it: your body is being powered off fat which is slower and burns more steadily. A lot of people report mental clarity, enhanced focus, more steady energy levels, and reduced hunger.

So one of benefit of fasting is that you get into ketosis, cool. But the benefits go beyond that.

I’m not a doctor (probably should have said this earlier eh), but my understanding of cancer is that when cells go through mitosis (i.e. single cell splits to form two new cells), transcription errors in DNA caused by older, faultier cells lead to defective, “malicious” cells being created. These bad cells reproduce as often as they can which creates a tumor, and that’s cancer.

During a fast, something kind of brutal and amazing happens: to generate energy, your body starts to break down the older more defunct cells which are then fed to younger cells to keep them going. This process is called autophagy and there’s a pretty good medical narrative (yet unproven though) that it may prune out cells likely to cause cancer before they can be harmful. In my mind this is a powerful enough mechanistic explanation to get my attention in a big way.

But it doesn’t even stop there. When you fast, your insulin and IGF-1 levels drop because your body no longer has to route sugar from its bloodstream. This apparently kicks your stem cells into gear which is maybe obviously regenerative. Researchers are using this fact to see if mice undergoing chemotherapy are more resilient while fasting, and it looks like they are:

Multiple cycles of fasting abated the immunosuppression and mortality caused by chemotherapy and reversed age-dependent myeloid-bias in mice, in agreement with preliminary data on the protection of lymphocytes from chemotoxicity in fasting patients.


I won’t belabor the point, but there’re more than enough hints here of something profound from a preventative medicine standpoint to get me curious.

Beyond longevity

I’ve written before about fasting. Maybe it’s the latent Irish Catholic guilt in me, but I think there’s something to depriving yourself periodically of basic things to make sure you don’t take them for granted. A friend of mine pointed out that this makes Lent appealing even from a secular standpoint.

But anyway, New York has a way of inducing an unhealthy relationship with food. Ever since I moved back to the city, nutritional prudence has taken a decided backseat to the sheer glut of delicious food available, so I’ve developed a kind of unhealthy relationship with food - consuming it as entertainment instead of sustenance.

Beyond my city-specific woes, it’s nice to really want something. I mean really want something the way you want a slice of pizza after not eating for four days. Desire that potent kind of resets your hedonic scale and, I dunno, maybe it staves off depression or something.

I think there’re a lot of spiritual arguments that could be made for a fast, and so I guess it’s no surprise the practice occurs widely in most major religions.


So how did I do it? Beyond just, ya know, not eating.

Well first I’ve gotta give credit where credit is due. This fast as thanks almost entirely to a good friend out on the west coast, Matt Leggett, who told me he planned to do a five day fast and then I jumped on the bandwagon. It’d been on my todo for a few months, but him pulling the trigger got me to commit. Matt’s smart as hell and a wizard with spreadsheets, so he rigged a Google form that reported restuls to a Slack channel. This allowed us to not only track our bloodwork (glucose, ketones) effortlessly but also shit talk each other about our bad numbers in real time despite being on separate coasts.

We followed Peter Attia’s method to some extent: eat a low carb ketogenic diet two weeks before to get into metabolic shape. This makes adapting to no food a heck of a lot easier because as anyone who’s ever been hangry knows, being carbohydrate-deprived without preparation can be painful. It’s much easier for your body to switch to eating itself when you’ve been burning fat for a few weeks.

I was mostly compliant (ahem) but I wouldn’t say I got into nutritional ketosis. I was pricking my finger most days and doing bloodwork and I apparently never got into ketosis.

pre-fast bloodwork

My hypothesis here is that my understanding of keto involved eating too much protein, which allowed gluconeogensis to kick in and have my liver turn the extra protein into glucose.

I picked up some electrolyte supplements. I’m not sure if this is strictly necessary, but I wanted to be drinking a lot of water and I heard it was easy to get deficient on potassium, magnesium, and sodium.

The fast itself

Day 1

After an ill-advised Indian feast on Sunday night, 7/29, I stopped eating at around 6:30p. I was in Virginia at the time and drove back up to New York the next morning, having a cup of black coffee on the way. Personally the combination of being fasted for ~12 hours and having a cup of coffee has an incredibly euphoric effect on me, so I was feeling grand.

When I got to my office I worked pretty normally. At one point I was feeling a little sluggish so I found an empty meeting room and did a few quick pushups and got a quick hit of energy off of that. A few minutes afterwards though, I fell into a malaise and entered a weird near-sleep state where I was nearly dreaming with my eyes open. I went to sleep easily that night.

Day 2

The next day my glucose spiked to 96 - not sure what that was about, but it didn’t feel great. Having not tasted anything in 24 hours, the coffee I made at work was remarkably delicious. The texture and detail of the cup was indescribable - it felt like unexpectedly hearing wider frequencies of sound from a favorite song you’ve heard a million times.

I was motivated to work and felt productive, though I got a few slight twingey headaches in my left temple.

Later in the afternoon I went to the gym and did moderate squats (5x5 @ 205lbs) and bent-over rows (5x8 @ 135lbs). The lifts felt heavy and I was quickly spent on the rows, but the weight got moved. Aerobically I didn’t feel taxed at all.

I came home after work and completely vegged on the couch. In general I found myself pretty useless in the evenings and allowed myself to embrace the uselessness: after all, the important thing was not to eat. This in itself was maybe kind of therapeutic.

bloodwork during fast

Day 3

I woke up at 4am on day 3 and it wasn’t pleasant but it wasn’t horrible. My stomach felt somewhat uncomfortable but certainly not painful. Getting out of bed had become an incredible chore - indeed being any kind of stationary made me want to stay stationary in almost all cases, but once I got moving I felt okay.

I found that in general early in the days I felt great, but that as the day went on I became much more tired. Interestingly, walking around felt like the most laborious thing - like walking through mud. Peter Attia also described this, so it sounds like it may not just be me.

I went to the gym midday and did a reasonable set of deadlifts (3x5 @ 325) and felt pretty good afterwards. Productivity at work was a wash, which is probably explained by the fact that I didn’t have any coffee that morning. Whether or not I consumed coffee basically determined my ability to do anything with my brain, but that may or may not be the case normally.

That night I woke up around 3AM feeling horrible, but I’d later find out that’s probably when my body finally hit a glucose+ketones low and I finally started producing ketones en masse.

Day 4

I woke up, struggling to get out of bed, and measured my blood. Ketones were bumpin’ - finally! - at 3.8 mmol/L, so I was definitely in ketosis. That morning I felt really good - exceedingly calm and focused. Who knows if that was just placebo from being happy to see the numbers finally go in the right direction.

My energy waned as the day went on, but I still felt pretty good. After work I went and saw the new Quentin Tarantino flick, doing a lot of walking to and from.

Up until that point in the fast, I hadn’t thought as much about food as you might think. But that night I started to fantasize about all the stuff I’d eat after the fast was over - stuff I definitely don’t normally eat like pizza.

Day 5

I woke up last day of the fast really dragging my wagon. I had a hard time staying awake at work after jolting awake in the middle of the night. The one saving grace of the day was a cup of coffee in the morning. By the afternoon I was basically useless at work.

Walking felt continued to feel exceedingly difficult.

Breaking the fast

As 6:30PM approached, I eagerly planned how I’d break the fast. Basically everything I’d read told me to take it slow, and I’d find out that advice was well warranted.

In at least a token attempt to be prudent, I decided that at 6:30 I’d have half an avocado and a single egg, wait a bit, and then walk over to my neighborhood barbecue place to do a reserved gorging.

I started back from the office around 5:30p and basically sat on my couch reading the manual for a drum machine until my phone’s timer went off at 6:30. I excitedly made the avocado and ate half the plate pretty quickly - delicious. Like the coffee, there were dimensions of the avocado’s taste that were new to me in that moment. I quickly made the egg scrambled, salted it, and put it down in about 30 seconds. Also amazing.

I set another timer and sat on the couch, just kind of zonked out and digesting.


  • not as bad as I thought, week went pretty quick
  • doing it with other people is super helpful
  • don’t overeat on the way out
  • don’t do carbs for a week
  • wrist better

  • didn’t enjoy coming back to food as much as i thought i would
    • made me realize how unhealthy my relationship with food is, how much i overeat

kicks in, but we can ignore that for now.


  • https://www.foundmyfitness.com/episodes
  • https://podcastnotes.org/2019/04/04/zero-fasting-attia-1/
    • https://peterattiamd.com/ama02/
  • https://peterattiamd.com/tomseyfried/
  • Mark Sisson on Fasting: https://www.marksdailyapple.com/long-fasts-worth-the-risk/
  • Pop-sci take on the metabolic theory of cancer: https://books.google.com/books/about/Tripping_Over_the_Truth.html?id=SH28oQEACAAJ&source=kp_book_description
  1. Unless you’re consuming a lot of protein in which case gluconeogensis