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Fasting for Four Days? Yes, Really

A curious nutrition scientist embarks on a self-experiment with a four-day fast while tracking the effects. Discover the potential of human resilience through the eyes of Kenny Westerman.

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By Kenneth Westerman
Jovan Mijailovic
Edited by Jovan Mijailovic

Updated April 23, 2024.

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Every once in a while, someone on the InsideTracker team decides to take on an exciting challenge. These challenges have really run the gamut from nutrition to exercise or a lack of both.

So, when our Data Science Intern Kenny Westerman decided he wanted to take on a challenge of his own, who were we to stop him? After all, it's all in the name of science!

Key takeaways

  • Allow some food and drink consumption during the four-day fast. Some evidence suggests that significant caloric restriction rather than strict lack of eating can produce many of the health benefits and physical changes.
  • Even without food, taking a multivitamin supplement and allowing coffee or tea could make the fasting process a bit easier to deal with mentally.
  • Plan, plan, plan. Although I really enjoyed the tracking that I completed, I could step up my game even further.
  • I'd definitely get a pre-test to establish more reliable baseline levels to which I could compare my fasting results.
  • I should've been more deliberate about tracking my weight. I would've got objective measures of physical performance and cognition and potentially even tracked ketone levels.

Here's Kenny's story

You haven’t eaten anything? In three days?!

This was the response I got from my astonished—and skeptical—coworkers at InsideTracker when they heard about my dietary experiment. We're all major foodies, and it's hard to imagine not eating for even one entire day.

So why did I choose to do what ultimately became a four-day fast? I was curious. The human body can do some amazing feats: run ultramarathons, climb at high altitudes with low oxygen, and eat 50 hot dogs in 10 minutes.

Even on this small scale, I had to see what my body could do! It came on as a spur-of-the-moment idea after I finished most of a tub of peanut butter on a Monday night. I always wanted to find out how I felt during a longer fast, so why not now?

I didn't think this plan through, which could've psyched me out if I had thought about it too much beforehand. But, after talking to my coworkers, I realized this was an opportunity to peek into my biology and see what was happening under the surface.

If I was going to continue with a fast, I had to do it right. That's how I found myself getting a blood draw on a Friday afternoon—after about 85 hours of fasting. And again the following Tuesday morning, four days after continuing my regular diet.

» Want to lower the risks of cancer and diabetes? Try autophagy fasting

The specifics of a four-day fast


One of the most exciting aspects of this four-day fast was that after the first day, I didn’t feel hungry. Did I want food? Absolutely. But I realized that my desire was based on the pleasure of eating instead of a physiological need.

Physical performance

I worked out the very first morning of the four-day fast but decided to take it easy to avoid further stress on my body. As time went on, I definitely started to feel the effects of the caloric restriction. I'd get dizzy when standing up, and my leg muscles also had a lack of "oomph" when riding my bike. 

Plus, everything felt… slow. My reaction time had slowed by a fraction, and everything I did had to be more deliberate. I'm also a notorious fidgeter—tapping my feet, flipping pens—and this went away entirely during the four-day fast.

» Check out Emily Wei's expert dietitian insights on the 24-hour fast


Sleeping was tough the first night. I could tell that my body wasn't thrilled about not getting any calories for another eight hours. But it got much easier for the second two. The quality and duration were the same level as on a regular night.

» Learn how to eat to sync with your circadian rhythm


I was paying close attention to the state of my mind since I was curious whether the onset of ketosis might give me a spike in mental performance. As my glucose levels dropped on Tuesday and Wednesday, I started to feel more “foggy," and had trouble concentrating to some extent.

That said, I didn’t feel that the four-day fast completely compromised my productivity. I could get work done, and social interactions weren't affected, though I may have been a bit moody while watching a friend eat pizza.

On Friday afternoon—84 hours into the fast—I felt an almost sudden shift as if someone had flipped a switch in my brain. Suddenly, my mood improved, and I felt back to 100% mentally.

Could this have been some effect of reaching or adapting to ketosis? Maybe so, though I'd expected ketones to rise steadily throughout the period.


Nothing too crazy here. I lost about five pounds during the process. Most of it was likely water weight since I gained all of it back within a week of continuing with my regular diet.

» Get fitter through time-restricted fasting


The good stuff. InsideTracker Ultimate Plan rescued me from the mire of subjectivity. The three points on each chart correspond to my first test in January 2016, which happened months earlier. It acts as a baseline for my fasting test about 84 hours into the fast and the one four days afterward.


Glucose went way down, which probably reflects the extremely low glycogen stores I had at that point in time. My body shifted to generating much of my energy from ketones. Although lower blood glucose is typically beneficial, it's hard to say whether levels during a four-day fast are diagnostic.

I stopped eating after polishing off a midnight snack on Monday evening. I only ate a couple of handfuls of lettuce on Tuesday night before deciding that I'd even cut out non-caloric food consumption.

I didn’t drink any coffee or tea but did have copious amounts of water, sometimes with salt sprinkled in, to keep my electrolytes at reasonable levels.
a screen shot of the glucosse website


Sodium and magnesium dropped slightly during the 84-hour fast, probably reflecting my lack of intake of these nutrients. This effect could have contributed to some of the dizziness and symptoms of low blood pressure that I had.

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» Keep your body in restorative mode longer through intermittent fasting

Iron status

Though most of my iron markers stayed relatively constant, ferritin levels almost tripled. This effect was quite puzzling to me because I couldn't think of a particular mechanism that might cause it.

I consulted a professor at Tufts' Friedman School of Nutrition. He suggested that the rise could be due to ferritin's role as an acute-phase reactant that tends to rise in response to stress. He also noted that an increase of only 100 ng/mL isn't actually that large of an absolute change.

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Hormone levels

There was definitely some crazy stuff going on here. Testosterone dropped like a rock, and cortisol shot up in four days. This effect makes some sense, as a long fast imposes a pretty big stress on the body. During this time, reproduction is probably not its most significant priority.

I was definitely relieved to see both hormones return to normal once my caloric intake returned. SHBG also increased, which usually happens with caloric restriction.

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White blood cell count

I was very interested to see what would happen to my white blood cell levels. Valter Longo’s lab at USC has really intriguing evidence about immune regeneration during fasting protocols.

I expected to see lower white blood cell levels during the fast that recovered afterwards. But according to my results, they stayed pretty constant over the entire period.
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» Discover how intermittent fasting impacts lognevity

The verdict: Is a four-day fast good for you?

Kenny's four-day fasting experiment gave us a glimpse into the body's ability to adapt. While hunger pangs subsided after the first day, fatigue and mental fogginess crept soon after. Biomarker levels also changed, with some like testosterone dropping and others like ferritin spiking.

Overall, the experience highlighted the importance of planning and tracking during fasting. While Kenny observed some potential benefits, a pre-fast baseline is needed to draw definitive conclusions.

And the best way to establish it is through InsideTracker. The Ultimate Plan offers blood tests that analyze various biomarkers related to metabolism, hormones, and other factors that can be impacted by fasting. This provides a personalized baseline before you decide if you're going for it.

Disclaimer: InsideTracker doesn't diagnose or treat medical conditions. Consult your physician for any health concern.