I Did a 5 day 121.5 Hour Water-Only Fast: Here’s How It Went and What I Learned

About a month ago I did a 121.5 hour (5+ days) water-only fast and it was such an intense, insane, comprehensive and unique experience, that I decided to write an article about it. I monitored a lot of my metrics closely, did a blood test right before the fast was about to end and then did a control blood test 2 weeks later and I found out some interesting things that I thought were worth sharing.

Reasons

I’ve been doing intermittent fasting (16:8) everyday for 5 years now, main reason being that by doing so, one can achieve a lot of the benefits a prolonged fast (2+ days) can give you, without doing the actual prolonged fast itself. 

So naturally, I thought I would like to do a prolonged fast at some point too, as intermittent fasting or supplements mimicking benefits of prolonged fasting like resveratrol, fucoidan, metformin, spermidine etc., cannot compare in result intensity with what a real prolonged fast can do.

I did several 1-day fasts in the past and two 2-day fasts as well and wanted to try an even longer one. I decided to go for a 5 day fast mainly because of these reasons:

• Autophagy (cellular cleanup): during a prolonged fast, our bodies recycle all sorts of cellular ‘junk’ which isn’t really touched in times of nutritional abundance, like misfolded proteins or disfunctional mitochondria. These can cause all sorts of problems like inflammatory responses from our immune system etc. 
• Sirtuin activation leading to DNA “repair”: Sirtuins regulate what a given cell is producing by switching the production of a given ‘thing’ on or off. They are basically little cellular molecular machines which physically turn genes in our cells on and off. The information theory of ageing suggests, that as we grow older Sirtuins do not perform this function as well as they should and our bodies globally decline. A skin cell that is supposed to produce only skin cell proteins now maybe produces proteins that a bone cell should produce and vice versa. When our bodies are in ‘repair mode’ due to the lack of nutrients, Sirtuins are especially active.
• Stem cell activation, immune system reset: During a prolonged fast, the body also gets rid of dysfunctional immune cells, boosts immune stem cell production and replaces these cells with new well-functioning ones, effectively leading to an immune system reset.
• Reduced inflammation: due to ketosis (energy production from fat) and autophagy body inflammation is reduced
• Surge in human growth hormone levels (HGH): during the fast, HGH is overproduced up to 5 - 10 times more compared to our baseline values. HGH is a hormone which mobilizes our body’s resources to repair all kinds of tissues. While we fast, not much is going on in this regard, because we lack nutrients to do the repair, but after we reintroduce food into our system, adding supplements like protein isolate and collagen peptides, the lingering overproduced HGH then utilizes these to supercharge the repair process.
• To give my digestive system a break: just in general, as I will write later, giving the digestive system a chemical and physical break is a return to normalcy.

I originally wanted to go for a 3 day fast but two days in, after reading a lot of articles and studies and talking to some LLMs, I decided to go for a 5 day fast because in order to get the benefits of a GH boost and immune system reset, you need to make it to days 4 and 5.

Table showing when the goals I was looking for would be met:

To make the above table more straightforward, I made a little chart showing it in more understandable way:

As can be seen from the chart above, the benefits of having an HGH spike and achieving an immune system reset are greatest on day 4 and 5. This is why I decided I should go for a 5 day fast. It was especially painful since I had originally decided to go only for a 3 day fast and I had changed my decision on day 2 reading research papers on fasting, when I felt like my fast was almost over adding 3 more days to it.

Some theory

The human body (not only the human body: it seems this feature is shared by almost all living organisms) has two main metabolic modes:

• growth mode - when the resources are plentiful
• repair mode - when the resources are scarce

Whenever we do not have enough nutrients, our bodies have to work harder, prepare for possible bad times ahead, and utilize resources from within they can reuse or recycle like misfolded proteins, malfunctioning mitochondira etc.

Another thing to consider is that, as humans, we are actually built to not eat food for prolonged periods of time by default as food was scarce (even a few hundred years ago, not to mentione a few thousand years ago) and not to eat food regularly 3-4 times per day. Looking at it this way, not eating is natural and eating 3-4 times a day is not. Overloading our metabolism this way is like like pressing the gas pedal in our cars to the floor all the time.

My background

I’ve been an active athlete since I was 6 years old. I started with competitive gymnastics, then played volleyball in the national Slovak volleyball league as a teenager for 4 years and then switched to triathlon for about 5 years. Currently I do climbing (outdoor and indoor both), calisthenics and run. I’ve been working out regularly or going to the gym on my own all this time. 

Now at 42, I'm aware that I'm past my peak, but I try my best to push myself to achieve the best results as a hobbyist athlete with the limited time that I can invest into sports. With my mesomorph constitution, I am aware I will never be able to beat endurance athletes or heavy lifters, but I'm proud and happy to be able to achieve these results at my age in strength, endurance and agility. 

Age: 42.5

Height: 182,3 cm

Avg. weight: 75,5 kg

Body fat: 5-10%

Body type: mesomorph

Fitness age (Garmin): 32.5

Avg. resting heart rate: 47 bpm

Max heart rate: 183 bpm

VO2 Max: 52 (measured indirectly)

Last year's fitness maximums:

75 pushups*

22 pullups*

1k in 3m 47s

5k in 21m 20s

10k in 45m52s

Half-marathon in 1h 38m 51s

Climbing Kilter board: 7a

Outdoor lead climbing: 7a

*consecutive, proper form, without breaking tempo. Due to injury couldn’t do more.

Overall maximums:

Added weight pull-up max: 45 kg/1 rep

Sprint triathlon: 1h 14m 08s

5k in 19m27s

Outdoor top rope c
limbing: 7a+/VIII+/5.12a

Bench press: 125 kg

Squat: 185 kg

Vertical jump: 81 cm

Preparation for the fast

I did my last intense workout the evening before starting my fast. I don’t remember exactly what exercises I did, but I think I did an intense full body workout and ran for about 45 minutes (with 8 HIIT sessions in between: 8 X 400m @ 18km/h with 8 X 400m rest @7.5 km/h). 

Looking back at this and seeing my blood test results, maybe this wasn’t the best idea as I will discuss later, but my reason for doing so was that I knew I would not be working out for the next 5 days, plus I would probably have to take it easy after ending the fast as well.

My dry weight before starting the fast was 75,9 kg. I’ve done a 3 day fast 3 weeks prior to starting this 5 day fast. I ate my last big meal - breakfast, at 10:30 am on the day I started the fast. 

I planned the fast for an extended weekend (2 days + 1 holiday day) because I knew doing work would be difficult and I also did some medical procedures that I knew would require some downtime so that I could make best use of these 5 days that would require me to do absolute nothing.

Fasting technique

I drank only water with electrolytes and tried to minimize physical activity to a minimum. I originally thought I would try to meditate or do body relaxation exercises or mild stretches whenever I could, but as I will explain later this was almost impossible. 

I drank 2-3 liters of mineral water every day during the fast which had a good mix of electrolytes but not enough, so I had to supplement: 

• 400-800 mg sodium (about 1-2 g of salt or 1/4 to 1/2 of a teaspoon)
• 1000-2000 mg potassium
• 300-400 mg magnesium

Fasting process

I wasn’t hungry at all on the first day and everything went well. 

On the morning of the second day however, I took some supplements (NMN, Ca-AKG, Fucoidan) and I believe this might have tricked my digestive system into thinking that food would be reintroduced soon and from that point on I started to feel extremely hungry almost until the end of the fast. 

Even with my 5 years of intermittent fasting, this feeling of hunger was really very hard to manage, especially when I had a full fridge of food within reach. 

My sleep was virtually non-existent. I am not a food person at all, but during these four days, I either thought about food and when I managed to fall asleep, I was dreaming about food. 

I tried to drink as much water to fill my stomach, to have a sense of ‘fullness’. It helped but hunger and food cravings were very difficult to manage. I developed a new sense of understanding and compassion for all animals that have to hunt their food in this state of hunger and cannot just go to a supermarket to buy bread.

By day 4, a simple bowl of steamed rice seemed like the best thing ever invented in human history. 

Interestingly, during the last few remaining hours of the fast (after waking up at 6:00 am and waiting for my first meal at around 11:30 am) the feeling of hunger went away and I felt like I didn’t have to eat to live ever. Possibly because my body knew I would eat soon. 

Also at the peak of my delirium, what in retrospect seems like a very illogical move, I started watching youtube videos with top Japanese chefs preparing the best Japanese dishes that anyone can cook at home.

My relationship to food as a whole started to change. I decided I wanted to learn how to cook well, effectively and minimally, and equip my kitchen with quality utensils and learn how to streamline my cooking process so that it was quick, elegant and produced healthy meals tailored to my needs. I also decided to buy nice table cloths, spoons, plates etc. and cleaned and washed every kitchen utensil I had at home. In short, the typical things you do when you haven’t eaten in 4 days. 

Results

Weight:

Start: 75,9 kg 

End: 72,8 kg

Back to normal: 75,6 kg (6 days later)

Difference: 3,1 kg

Weight loss breakup:

Water: ~2.1 kg (68%)

Glycogen: ~0.5 kg (16%)

Fat: ~0.35 kg (11%)

Protein: ~0.15 kg (5%)

Gut Contents: ~0.15 kg (5%)

This shows a minimal loss of protein which I was worried about. Additionally, the 0,15 kg of protein lost was easily recovered within a few days/weeks with proper nutrition and workout. 

Blood tests

I took a blood test about an hour before ending my fast (so around the 120 hour mark into my fast) and then did a control test 14 days later. Unfortunatley I didn’t do any test right before starting the fast, but I did have a pretty comprehensive test done around the same time of the year from one year ago. 

Hormones

My hormones were all over the place which all returned to normal after my fast has ended:

Suppressed: Testosterone, free testosterone, thyroid hormones (TSH, T3), insulin—all dropped as my body conserved energy and prioritized survival.

Increased: Growth hormone, cortisol, noradrenalin, SHBG—reflecting a stress response to mobilize energy (fat, glucose) and preserve muscle.

Stable: T4, adrenalin, dopamine—less affected by fasting.

Cholesterol

I have naturally quite high cholesterol levels, but these seem to be related to the fact that I generally do not drink too much water (I know I should, I just don’t feel thirsty and often simply forget to drink) and due to my higher protein intake (2g per 1kg of bodyweight so about 150 g/day).

Whenever I drink a lot of water several days before my blood test and on the day of the test itself, my cholesterol levels drop signifficantly (biggest drop was 31%). 

During this 5 day fast, total cholesterol, LDL, and triglycerides spiked (15%, 13.9%, 69.1%), while HDL dropped (-11.8%), due to fat mobilization for energy. 

Vitamin B12

I experienced an enormous spike in Vitamin b12 levels. B12 surged from 302.00 pmol/l to 969.00 pmol/l (+220%) during fasting, likely from liver release, then dropped to 364.00 pmol/l (-62.4%) post-fast as my body cleared excess and stabilized.

These changes align with fasting physiology—cholesterol reflects fat metabolism shifts, and B12 highlights metabolic adjustments, both normalizing post-fast.

Glucose and related metrics

In general my glucose metabolism and related hormones are very good so I didn’t expect any problems during fasting either.

Glucose: Dropped slightly from 4.41 mmol/l to 4.20 mmol/l during fasting, staying normal, then stabilized at 4.16 mmol/l post-fast. My body maintained glucose via gluconeogenesis and ketosis, supported by cortisol and noradrenalin.

Insulin: Fell to 2.01 mIU/l during fasting (low due to no food intake), then doubled to 4.09 mIU/l post-fast as I refed, reflecting normal glucose management.

Related Changes: Cortisol and noradrenalin spiked to support glucose production, triglycerides rose with fat breakdown, and insulin resistance doubled but stayed optimal, showing a temporary refeeding adjustment.

Fasting shifted my body to fat-burning mode, lowering insulin to allow lipolysis and ketosis, while stress hormones ensured glucose stability. Post-fast, refeeding restored glycogen and raised insulin, normalizing metabolism. My athletic profile ensured efficient glucose regulation throughout.

Human growth hormone

According to the literature I read, growth hormone levels should spike greatly during a prolonged fast: 500% to 1000%. 

My blood test results at the end of my fast showed a very low level of HGH at only 0.67 µg/l (normal range is 0-3 µg/l for adult males my age) which almost left me quistion the test results. I had never measured my HGH levels before so I didn’t have any point of reference, but my contol measurement 14 days after ending my fast showed an even lower HGH level at 0,06 µg/l, which was a 91% drop from my fasting values. 

If values around 0,06 µg/l are my baseline, this suggests an almost 1000% increase in HGH levels during fasting which is great news and in line with literature, but the puzzling part is why my baseline HGH levels are so low. I will do follow up checks with an endocryonologist. 

Specifically, for a 42.5-year-old, athletic male with my health and body type, normal baseline GH levels should be around 0.2-0.5 µg/l, with peaks of 5-10 µg/l during pulses (e.g., sleep, exercise). My baseline of 0.06 µg/l is below this range, suggesting my GH levels are lower than expected for my profile. 

Inflammation markers

As mentioned, one of the reasons I wanted to do this 5 day fast was because I read it would lower inflammation. However my inflammation markers showed the exact opposite. 

In hindsight, this was possibly due to the fact that I did heavy exercise the day before starting the fast.

High sensitivity C-reactive protein, a blood marker that measures inflammation, was normal pre-fast, spiked dramatically during fasting (exceeding the limit by ~165% if using 3 mg/l as the upper threshold), and returned to baseline post-fast.

Ferritin is related to inflammation because it’s an acute-phase reactant, increasing in response to inflammatory cytokines (e.g., IL-6) during stress, injury, or infection.

White blood cells

My white blood cell levels are either on the lower end or below recommended limits, but this is something that seems to run in the family as several family members have similar levels to mine. 

I usually fall sick only once or twice a year with the seasonal flu, sometimes going several years without falling sick at all. 

I tried all sorts of things to increase my WBC count: vitamin D, C supplementation, work on my sleep, take ice baths, intermittent fasting etc.. Nothing seems to be increasing their levels but as I wrote I am in pretty good health and my WBC levels seem to be genetically conditioned so I don’t mind. 

The only thing that was interesting was that I had a 400% increase in Eosinophils (a type of a White blood cell), which I don’t know too much about.

One of the reasons for this fast, as I wrote above was to do an ‘immune system reset’ and I was also curious to see whether there would be any change to my WBC count. This goal shows mixed success. The fasting table predicts immune depletion ending at 72 hours with regeneration peaking by 96-120 hours. My low WBC and lymphocyte stability with eosinophil rebound suggest a partial reset—temporary suppression followed by recovery—but the lack of dramatic lymphocytosis or immune marker shifts (e.g., cytokines) makes this inconclusive.

Sleep

My sleep basically went out the window. I either felt like I didn’t sleep at all or had very light sleep here and there throughout the night with very little REM sleep. Interestingly, I did not feel tired as I probably should have.

Refeeding

Refeeding after a 5 day fast is no joke. It has to be done very sensitively and it took me about 3 days to get back to my normal metabolic and digestive state and food intake.

I cooked a chicken soup (800 g of chicken breast, bak choy, parsley, carrots, celery, kohlrabi, ginger, 1/2 tbsp of salt, brown mushrooms, 200 ml of olive oil) and drank the broth from it as my first fast-breaking meal.

I would greatly suggest buying ingredients and cooking the soup before starting your fast and then freezing it. Going to the grocery store after 117 hours of fasting, and cooking food really just isn’t fun. 

Also, warm up the soup ahead of time on the day you want to end your fast, because hunger will push you to eat very quickly even if you try eating slowly and that can cause all sorts of problems (soup is too hot, too much soup too quickly will shock your digestive tract etc.). 

Actually, one of the biggest insults to injury during this whole experiment was that even if I could decide to end the fast and eat food at any point I wanted, I couldn’t just eat anything and eat like there was no tomorrow. It is absolutely essential to start very gently and very slowly. 

I started with just drinking the broth, sipping it from a teaspoon, so that I wouldn’t drink it too quickly. I first drank two soup plates slowly, which took me about 50 minutes. 

I then gradually added small amounts of rice, meat from the soup, veggies from the soup etc. making sure I would chew everything properly and eat slowly. 

Later in the day I would make a small protein shake made of lactose free milk (to be easy on the digestive system), protein isolate (1/4 of the usual amount) and a little bit of chia seeds (fiber). 

I read several times, that fiber needs to be introduced very gently and I can confirm this. Even though I added only a small amount of chia seeds to my protein shake (half of a teaspoon maybe) I could feel it expanding my intestines and I had this feeling for about 2-3 days every time I added Chia seeds to my protein shake.

In general, after doing some research, the order of difficulty for the digestive system to deal with stuff it has to process is the following (from easiest to the most difficult):

• carbohydrates
• protein
• fat
• fiber

One medium mistake I did, that I would like to avoid next time is that I added about 200-300 ml of olive oil to the soup. This was too much. I did feel slight discomfort, nothing too serious, but next time I would like to be more careful with adding so much fat.

Main discoveries

• my surprisingly low baseline Human Growth Hormone levels
• quite an intense Vitamin B12 spike during fasting
• high inflammation (I expected inflammation to be low. High levels probably due to my high intensity exercise session the day before)
• very clear mind
• lots of evergy that in theory I should not have been having
• both of the above probably due to ketosis which is a very efficient energy pathway and very gentle on the brain.

Subjective feelings

• I felt quite cold (due to reduced metabolic rate, ketosis, blood circulation concentrating on internal organs rather than extremities). I generally feel cold easily, enjoy very hot weather and dislike cold in general, maybe because of my 5-10% body fat levels, but this type of ‘fasting’ cold, when the heating was up, was especially annoying. 
• I just couldn’t sleep. I am not a food-type of a person, but during this fast, I was thiking about food all the time and I just couldn’t stop myself from doing it.
• slight discomfort in my bowels because of the magnesium I was supplementing
• my skin seemed very smooth to the touch and looked very smooth too.
• surprisingly, I subjectively felt like I had less ringing in my ears. I don’t have a serious problem in this regard, my hearing is great, but when I use earplugs, I do notice a slight ringing in my ears. This noise seemed to have been reduced by about 40% (my rough estimate) during fasting, but returned to previous levels after the fast has ended.
• as expected, immodestly, I was very shredded. I wish I had measured my body fat right before ending the fast with fat measuring clippers. I do not consider devices that measure body fat by measuring electric resistance in the skin to be accurate - skin thickness itself influences the measuring result, not only the amount of subcutaneous fat. A person who has thicker skin on their palms for whatever reason (like working out in the gym) has higher body fat readings when measured by handheld devices than a person with smooth palms. I speak from personal experience. 
• my sense of smell seemed to have improved. Probably I was just more aware of smells in general since my instinct to go ‘hunt’ was raging. 
• I did have this strange ‘alkaline feel’ in my body. It’s difficult to explain and this is the best way I can describe it. Apparently it was due to ketosis. 
• clear thinking. I honestly felt like I if you gave me some difficult mathematical equations Einstein had problems solving, I would solve them. I was surprised at how well I was compressing my thoughts into simple sentences, and even my thought process seemed to have switched a few gears. 
• a lot of energy. Despite not sleeping for 4 days and not eating anything, I did feel like I was high on energy. I had the urge to constantly talk to everyone, especially about my fasting experience and also my confidence and me not worrying about other people’s opinion went through the roof.
• My digestive system calmed down. I used to have quite severe digestive problems from around 2018 to 2022 that I’ve solved for the most part. However, after this fast my digestive system calmed down completely and with another technique that I use (protein shake with 2.5 tbsp of chia seeds, lactose free milk and probiotic milk as the first meal of my day) I feel like I can drink gasoline and still be ok. I literally feel like I can go compete in the World digestion championships.
• Torture. Be it as it may.. the 5 days were very challenging, physically and mentally. The fact that even though my fridge was full of food, I just couldn’t touch it, and even if I wanted to I’d have to first cook my chicken soup and then very slowly reintroduce food into my system was quite a test of mental resilience. Also.. being home, with bad weather outside and nothing to do, time passed very slowly.

Conclusion

In general the experience was worth it. It was quite challenging at times, if not to say very hard, especially mentally, but worth it.

Of the 6 or so main goals I set out for this fast, most were met. Those that weren’t (like DNA repair) I couldn't measure and my inflammation was high rather than low probably due to my intensive workout the day before I started my fast. 

I learned a lot of things about how the human body works, about food, myself and at the peak of my hunger delirium, I probably saw the 4th, 5th and 6th dimensions. 

Of the many interesting things I’ve learned about how our bodies work, I read a few studies according to which a person with a body type and athleticism like mine can look forward to adding only 0,2 kg of lean muscle per YEAR no matter how ideal the diet, workout and rest are which is a huge surprise. All remaining gains or losses are mostly glycogen, water and fat. 

I felt very light, strong and ‘healed’ returning to the gym after my fast. In general I feel pretty good, but I am 42 and every now and then I do feel some pain here and there. Going to the gym for the first time after my fast, nothing that used to hurt or had an inflamed sort of a feel to it (lower back, joints, tendons etc.) hurt that day. I felt very light and very healthy. This however could also be due to the simple fact that I rested for 5 days, which I usually don't do. 

My sharp decline in weight concerned me a bit but after talking to some LLMs and monitoring my myself closely, everything was within normal ranges, most of the loss was water, glycogen and fat, very little protein and after refeeding, my weight, including protein, returned to my previous levels on the 6 th day after finishing my fast.

I would again suggest caution with the following:

• careful with food being too warm when refeeding
• drinking the broth from a teaspoon helps monitor your feeding pace
• chia seeds are pure magic, but need to be introduced very slowly (less than half a teaspoon per meal and maybe 12 hours after breaking the fast)
• careful with fats
• careful with too much salt
• buy food and cook the soup before you start the fast, then freeze it, otherwise you will be looking at some voluntary not ok time at the grocery store and kitchen. 

Going forward, learning from my previous experiences, I would like to do this kind of a fast once or twice a year.