Notes from an Extended 4-day, Water-Only Fast

My mom’s been diabetic since her late 40’s/early 50’s so my whole running and diet (vegan/pescatarian) journey, in a small way, has been motivated by “running” from the inevitable – assuming it’s all genetic. While 12+ years of distance running certainly got my health in a lot better place, I’ve been curious about other options, as it relates to healthspan (as opposed to lifespan). The one thing that’s consistently popped up in all my reading & research is fasting. As a minimalist of sorts, I was really drawn to fasting – incredibly simple, available any time, not to mention free, as in beer. This blog is about my first 4-day, water-only fast and what my experience was.

Disclaimer: This blog shares my personal experience and is NOT intended as medical advice. If you want to attempt fasting, I suggest you check with your physician to see if it’s appropriate for you.

Why Fast, Why Not Run More?

For a long while there, I assumed (incorrectly) I can burn off anything I ate by simply running ultramarathons. There’s some truth to it, but I enjoy eating and unfortunately don’t have the realtime GI -> brain signaling that makes me stop eating before I’m full. It can take about 20 minutes before the “satiety” signal (leptin hormone) gets to the brain. Well, that explains why caloric restriction (CR) doesn’t work for me. By the time my brain gets this signal, I’ve already downed more curly fries and beer! Besides CR is a PITA, because it’s just too much work counting the calories of every meal.

All that said, I don’t think I’ve ever missed a single meal in my 52 years! For a very long time, watching my mom go through hypoglycemia (low sugar), it’s been ingrained in me that if one skips a meal, one’s gonna die! So fasting has been a bit of a taboo. The first time I did a 24h fast this past January, I was keeping notes every hour and to my surprise, I didn’t actually die! 🙂 More importantly, the steady pulse of short-lived, hunger sensations (ghrelin hormone) didn’t seem to get qualitatively worse over the 24h period. Since Jan, I’ve been doing 24h fasting one day of every week. Even during training for Boston Marathon, I timed my fasting day with my rest day and everything went ok. It was time to up the game a little.

There were a few things in my mind to try the 4-day fast – curiosity (of course), to experience what ketosis would feel like, push my body to unchartered territory (for me), maybe fix a benign, auto-immune niggle I’ve been dealing with for a few months, and lastly put to practice what I’ve spent hours researching and reading up on.

Energy Production: Backgrounder on Metabolic Pathways

Imagine your body has a rather fixed-sized fridge and a very large, expandable freezer. Whatever you eat (carbs, fat, sugar) gets converted into glucose and is used immediately by various parts of your body. Glucose is the preferred energy molecule in every cell as this metabolic pathway is simple & efficient. Reality is, we all eat a lot more than the immediate needs of the body. So the first excess goes to the fridge – these are the glycogen stores in your liver and your muscles. But this fridge has limited space, so the liver shuttles the second excess out into adipose tissues (fat) all over the body. The visceral fat (as opposed to subcutaneous fat), the one in & around your internal organs, is the nasty one as it leads to all sorts health issues. Anyways, here’s the kicker, until you really (like really!) empty out the fridge, your freezer hardly gets touched. But when you deplete the fridge (glycogen stores), something amazing happens – your body switches over almost completely to lipolysis (fat stores from the freezer) producing what’s called ketone bodies that the cells can also use to keep you going. This is a completely different metabolic pathway each and everyone of us was born with, but we hardly exercise it. Mind blown!

Even an elite marathoner has a sizable freezer that can last for days/weeks. You really only hit starvation and start eating furniture around the house (breaking down muscle proteins for energy) once both the fridge and the freezer are completely empty. So most of us have huge reserves of energy that we can tap into during fasting. But you gotta bonk first and empty the fridge out. This is the way!

4-day, Water-Only Fast

I’d been thinking about this for a while and really wanted to do this quantitatively – blood pressure monitor, glucometer, heart rate monitor, ketone monitors, etc. during the fast, followed by a full blood test and lipid panel at the end. But just like every other stupid thing I’ve done, I saw a window of opportunity where I could pull this off and jumped into the deep end. All I managed was to run to Walgreens on the 2nd day of the fast and buy myself some ketone test strips. Here’s roughly what happened:

Day 1

  • Weight: 140lbs
  • Started the fast after eating a light breakfast
  • Usual fasting hunger pangs throughout the day, they come and go, no biggy, used to it
  • Did my weights:push repertoire (relaxed 2 hours of lifting with rest between sets)

Day 2

  • Weight: 139 lbs
  • Ketone strip showed traces, longest I’ve gone without food
  • Did my weights:pull repertoire (relaxed 1.5 hours of lifting with rest between sets)
  • Watched Spiderman: Across the Spider-Verse with my son – holy crap the smell of popcorn!
  • No other qualitative change

Day 3

  • Weight: 136 lbs
  • Ketone strip now showing moderate, hunger pangs still there, but no worse than day 1
  • Very sluggish afternoon for a few hours – likely the fridge was getting emptied out
  • Early evening, felt energy levels perk up significantly – yay freezer is open!

Day 4

  • Weight: 134 lbs
  • Yeah, ketone strips definitely indicate full-blown ketosis
  • Did my weights:push repertoire, can still bench the same pre-fasting weights
  • Feeling completely normal, in fact ketones suppress Ghrelin hormone, so no more hunger pangs!

Day 5

  • Weight: 132 lbs
  • Made it! Broke the fast with a few nuts and fruits and the strawberries tasted like they were coated with pixie dust! Just absolutely delish. I almost felt dirty pushing food down my “clean” GI track 🙂

Honestly, I was feeling so good at this point, I felt like keeping it going for another 2 or 3 days to “enjoy” ketosis. This being the first time, I didn’t want to run during fasting, because this was a water-only fast and I didn’t want to compensate sweating with electrolyte intake. But boy-oh-boy, I do dream of running a 50K powered by nothing but my fat. Look ma, no gels! Wouldn’t that be something?!!!

Maintenance Mode: Autophagy

I’d read a lot about the power of Autophagy during extended fasting – when Phagocytes (certain kinds of immune cells) that can eat other dead/zombie/malfunctioning cells in your body go into overtime, repairing all sorts of things in your body. They do this all the time as a background task, but get upregulated during fasting because all that energy to digest food can now be put to other use. Earlier this year, during a routine dentist visit, they found a small white patch on my gum. After a biopsy and three different periodontist assessments, it turned out to be benign. However, this is an autoimmune thing that nobody knows what triggers it nor what fixes it. I was very pleasantly surprised that my lichen planus had all but disappeared during these 4-days of fasting. Go figure. One of the periodontists wanted to zap me with a co2 laser and charge me $5,000! 🙄

Closing Notes

I get that my sample size is n=1, but this was such an amazing experience. Regardless of if fasting really helps in the long run, I feel like I’ve managed to unlock an entire metabolic pathway that I was born with that doesn’t seem to want to kill me. More importantly, I’ve found a very simple way to compensate feasts with famine without pills, supplements or a special diet. Psychologically, I’m now perfectly ok ignoring the stomach growling and skipping a meal or two knowing there are more upsides than downsides. Last but not the least, a key learning is that beyond necessity, habit and social play such a big part in when/how/what we eat.

There were two really hard things about the water-only, extended fasting:

  • Switching from the empty fridge to the freezer on day 3, but a nap helped with the transition
  • Not cooking with my wife and sharing a meal together 😢

Here are a subset of books and papers that I poured over to read up on fasting, if you want to dig deep into this yourself:

Happy trails!

Absolutely N.O. spam. No more than two emails each week. Learn about injury-free running, race reports, new trail routes, awesome recipes and amazing interviews.