Completed 48-Hour Fast: Notes and Insights

by Dan Walsh


I went 48 hours without food and it wasn’t that hard.

My second attempt at a 48-hour fast was successful! It lasted from 8pm on August 17th to 8pm on August 19th. I changed a few things based on my first attempt which helped get me to the finish line. The biggest change was preventing boredom. I realized fasting is like long-distance running: 20% physical and 80% mental. I kept my mind busy so I couldn’t dwell on my hunger and talk myself into quitting.

This longer fast also had some interesting similarities to the sleep deprivation experiments I did in college. Mainly that fatigue and hunger come in waves. I could be optimistic and energetic one moment, and then lethargic and suffering 10 minutes later. Hunger would come and go, instead it hit a certain level and never got worse. Prior to these experiments I would have expected hunger to get progressively worse, but that wasn’t the case at all, even during the 48-hour fast.

So why am I doing this anyway?

Partly for the discipline, partly out of curiosity, and mostly because a recent study showed that fasting can trigger stem cell regeneration of the immune system.

So, maybe the real question is: why would I NOT try this?!

The following are the stream of conscious notes I took during my 2nd day.

Notes from Day 2

I forgot I was fasting! I’m almost at the 36 hour mark and I don’t feel hungry. I don’t feel weak. I slept normally last night (should check Basis stats). I woke up this morning and was less hungry than I was when I went to bed last night. I’m in deep ketosis despite zero fat intake. This makes me think I burned through all my glucose and am now just chewing on body fat. I wonder how long this would last.

Fasting for 4 days would be an interesting personal data point for the calorie in / calorie out theory…


Lots of energy feeling good… even though I’m hungry. It’s such a subtle feeling…


Thinking about sweet potato nachos is helping me hang on! Good to have a finish line meal to really look forward to.


Starting to feel the calorie fade. Getting kind of lethargic. Hunger is still the same.


Getting harder to think / concentrate. Could be a result of the afternoon coffee? Caffeine rush on an empty stomach.


Can’t think about anything but NACHOS!!!


Hunger is an odd feeling… it’s not really localized anywhere, but sort of comes from everywhere. I feel it in my stomach most, but also taste it in my mouth, sense it in my brain.. and kind of yearn for food in my muscles… like how a dried out sponge yearns for water. I feel ready to absorb.


I rode my bike to and from work on both days. At the end of the second day I became winded very easily. I was unable to maintain my usual pace and I could acutely feel rapid muscle fatigue in my quads. There was no energy.

I thought I would wake up starving and tired on the second day. But I felt normal. I wasn’t hungry. In fact, it actually looks like I slept better than usual in my fasted state. Two points of data is hardly a trend, but it will be worth paying attention to this next time.


Despite a large “finish line meal” of sweet potato nachos, I didn’t have any ill effects commonly associated with eating too much too fast. I didn’t throw up and I didn’t get diarrhea. My meal was easily 2,000 calories of starch, carb, protein, vegetable, and dairy. That would be hard to handle on a normal day, let alone after shutting down my digestion for 2 days.

On a similar thread, it took a few days for my digestion to get back to normal. My stomach was uncomfortable for a few days. It never hurt, but felt kind of like an old car trying to get started in the dead of winter.

My hunger / satiety signaling was out of whack for a few days. I ate a ton the next few meals and easily recouped the calorie deficit I created during the 48-hour fast. I have a hunch my leptin levels (hormone that tells the brain you’re full) were already a little off, this adds another bit of evidence. I could feel my stomach expanding to the point that I was lethargic and uncomfortable but my mouth and brain still wanted more. I was felt full in my stomach without feeling full in my brain. Odd.

I typically weigh in around 147 these days, but I dropped to 145 by the end of the fast. Fat % fell from 13.8 into the 12% range, but I didn’t do a good job recording so I don’t have specific numbers. It’s interesting to note that I only dropped 2 lbs. The fast created a minimum 4,000 calorie deficit, which is almost 2 lbs of fat by itself. But I also didn’t have any food in my digestive tract, which is almost another 2 lbs. I expected to drop 4 lbs, not 2.

I used sesame seeds this time to track my digestion. Most of them passed during the first day of the fast, but they didn’t all pass until after my refeed.

Data from BASIS doesn’t reveal much. My heart rate increased from 66 beats per minute to 69, but that could be normally fluctuation. Strangely, I burned 400 fewer calories on my 2nd day of fasting even though my activity level was much higher. I’m not sure if this is legit or a quirk with Basis.

48hourfast_basis_stats_Aug_17 48hourfast_basis_stats_Aug_18 48hourfast_basis_stats_Aug_19

Lessons Learned

I pretty much followed all the changes I outlined from my previous attempt. I was not better about measuring my attempt though. But this time around was really geared toward getting the mental component of this challenge squared away. Which I did.

Consuming warm beverages gave my stomach something to do and helped quiet the psychological noise it was otherwise creating. Coffee and tea were the best way to get through the day.

Keeping my mind busy and going to bed a little earlier than usual at the end of the first day made it easy not to break. This was the best way to get through the night.

Next Attempts

So now that I can finish the fast, I need to get better about measuring it. Aside from the psychological challenge, the real reason for the fast is to create changes in my immune system. I need to measure more vitals including blood pressure, blood glucose, blood oxygen levels, weight / fat / muscle, and regulate my sleeping patterns before the fast so I have a solid baseline for comparison. Ideally I’d also find a way to get some blood work done before, during, and after the fast. I’d also like to check out how my microbiome changes throughout the fast.

I expect the next attempt to be easier now that I’ve done it once. I probably won’t binge quite as heavily on my refeed meal though. Even though sweet potato nachos TOTALLY LIVED UP TO THE HYPE!