A day-by-day account of what it’s like to do a 5 day water fast with exercise

There’s plenty of evidence to suggest fasting is important in increasing longevity. Dr Valter Longo’s excellent book The Longevity Diet and Dr Jason Fung and Jimmy Moore’s book The Complete Guide to Fasting both explore how fasting promotes autophagy.

Autophagy is a regular process where your cells are broken down and recycled, starting with the damaged parts first. There are many other benefits to fasting too including increased body fat loss, increased insulin sensitivity, lower blood cholesterol and more.

With all these benefits I decided to try a 5 day water fast. Before we get any further, let’s not mince words: fasting for five days was quite difficult. It wasn’t intense like lifting a very heavy weight or getting through a 3-minute sparring round, but it was a monotonous, unpleasurable and hazy five days.

By the end of the fast I definitely saw the bright side: I’m now much more grateful for the food I shovel down my mouth each day.

Note: This is a reader supported blog, so some of the links to books in this article will earn us a commission.

Table of contents

Before you do your own fast: Do plenty of research from trusted sources and speak to a health professional if unsure. I’ve included a list of videos, books and podcasts at the bottom of this page to help. Note that according to The Complete Guide to Fasting, pregnant or breastfeeding mothers, children under 18, those who are severely malnourished or underweight, and people suffering from some conditions should not fast. I’m not a health professional and this is just a recount of my own experience.

Before and after results from my 5 day water fast

  • Bodyweight went from 77.4kg to 72.4 kg
  • Ketones went from 0.1mM to 3.6mM
  • Strength and endurance stayed the same
  • Mental clarity decreased during the fast
  • Mood dropped significantly during the fast

5 day water fast before and after: weight

5 day water fast before and after: ketones

How I structured my 5 day water fast

  • 5 days of zero calories
  • Unlimited water including sparkling water
  • No food or supplements including coffee, tea and bone broth
  • Zero-calorie unflavoured electrolyte sachets containing sodium, potassium and magnesium were allowed to help with workouts (I used unflavoured LMNT sachets)
  • To preserve muscle, I would attempt to workout as normal (2x weight sessions and 1x 45 minute street run)
  • Photos and weigh-in each morning after waking, ketones taken throughout the day
  • Fast started at Sunday 4pm and ended Friday 4pm

Day 0 – Pre-fast – Sunday

My pre-fast day started with some bodyweight and ketone measurements to get a control to compare against during the fast.

I ate normally until my fast began at 4pm and by 9pm I was already feeling a little hungry. Doubt started to gnaw at me and I wondered how I would fare during the crucial first 48 hours where hunger hormones rise and then plateau according to Dr Fung.

  • Morning mood: 7.5/10 – Excited, a bit anxious, and a little hungry (already!), but ready to rise to the challenge.
  • Weight: 77.4kg (170.6lbs)
  • Ketones: 0.1mM

Day 1 – Monday

I woke feeling refreshed after a great night of sleep. I’m not usually hungry in the mornings so I couldn’t tell a major difference yet.

By mid-morning the reality of the fast kicked in: time goes by slowly when you have no food or coffee to look forward to!

My coffee break was instead replaced with a morning walk watching people around the city devour fresh pastries and hot drinks. My sense of smell had already started picking out food from what seemed like a mile away.

Later in the day I started to feel a little less sharp in my reactions and a bit more sleepy, but chalked it up to the lack of caffeine.

At 4pm I realised I had completed the first 24 hour period of fasting. It was funny seeing my colleagues’ reactions to my fast, with reactions generally being supportive and amused.

By the end of the day I started to feel more tired and mentally foggy. I was easily distracted and sleepier than usual. I decided to pass on my regular Brazilian Jiu Jitsu class to see how I would feel the next day.

  • Morning mood: 5/10 – Tired and unfocused, and not bothered for intense exercise.
  • Weight: 75.6kg (166.6lbs)
  • Ketones: 0.6mM

Day 2 – Tuesday

I woke up again feeling good. My morning weigh-in showed that I had lost a little more weight and my ketone levels were steadily rising.

I was already sick of only drinking water. I got some slight taste relief by drinking my morning sachet of electrolytes though.

My trusty water bottle and electrolytes.

Today was my first fasted workout. I was anxious to see if I would be able to squat or if I would just buckle. My workout went surprisingly well, and I hit my target weight and rep range. I did feel a little dull during the workout, but the results were better than I expected.

According to The Complete Guide to Fasting, growth hormone rises during a fast, which helps to preserve muscle and help with workouts.

After the workout my mood and energy improved somewhat. I felt a little bit more energised and less hungry.

The biggest realisation I had on day two was how much pleasure I derive from eating. I enjoy eating a big meal after training, buying a hot coffee on a cold winter morning, and coming home after work to cook a fresh dinner. We take these rituals for granted and this fast was reminding me of that.

  • Morning mood: 6.5/10 – Not feeling hungry, but generally feeling unsatisfied. The weights workout improved mood and energy.
  • Weight: 75.1kg (165.5lbs)
  • Ketones: 0.8mM

Day 3 – Wednesday

Today I woke up feeling drained after yesterday’s workout. My sleep was also low quality and full of tossing and turning.

On the plus side my ketone levels jumped to 2.2 millimolars (mM), which is expected on day three of a fast. This fast had put me squarely in ketosis for the first time in my life. My body had run out of fuel from the glycogen (sugar) stored in my liver and was now using my fat.

Apart from this I felt slow, mentally unfocused, dull, and more sleepy than usual. I also had a slight headache all day.

At lunch I left the office and picked probably the worst place to do some writing: a nearby food court. My mood plummeted and my feelings of weakness were made worse by the smells, sights and sounds of people eating.

My sense of gratitude for being able to eat food continued to rise. My free thoughts turned to food, especially what I was going to eat once I broke my fast!

  • Morning mood: 5/10 Almost gave up at lunch as I felt weak and cloudy.
  • Weight: 74.2kg (163.5lbs)
  • Ketones: 2.2mM

Day 4 – Thursday

I woke up on day four with even more muscle soreness from my Tuesday workout. It was interesting to see how my recovery was lagging without food.

My bodyweight had fallen by another kilogram. I now weighed 73.3kg, 4kgs lighter than what I started. My ketone levels hadn’t moved.

Today was deadlift day and I was unsure I could do it feeling so tired and dull. Surprisingly, as with Tuesday, my workout didn’t really suffer. I lifted roughly the same amount of weights for the same number of reps as usual. I felt quite slow and clumsy throughout the workout though.

After the workout my mood and energy levels improved as was the case on Tuesday.

I ended the day relieved that I’d come this far. All that was separating me from completing my fast was one night’s sleep, a full day of work and my normal morning run.

  • Morning mood: 6.5/10 My mood is lifting now that I’m almost done! But still feeling very tired.
  • Weight: 73.3kg (161.5lbs)
  • Ketones: 2.2mM

Day 5 – End of fast day! – Friday

I woke up feeling somewhat energised and fresh with the promise of food later.

My morning weigh-in showed that I’d now lost 5kg (11lbs) since starting the fast five days ago. My ketone levels had also jumped higher to 3.6mM. Overall though I was feeling tired and sluggish. I had never walked so slowly in my life.

My final weigh-in

I reluctantly started my usual friday morning run. It was more difficult than usual and I had to mentally push myself to even finish it. Somehow I only ended up being a minute or so slower than usual.

At about lunchtime I was painfully close to throwing in the towel early. The office lunch of burritos looked amazing and I still had another four hours to go!

30 minutes before I reached the 4pm finish line I prepared my first meal. Taking the instructions in The Complete Guide to Fasting by Dr Fung and Jimmy Moore on board, I chose a small bag of plain macadamia nuts to break my fast. I put the bag on my desk as I waited for the time to tick by. The book also recommends drinking a tall glass of water before you eat, so I also got this ready.

I mentally limped through to the 4pm finish line. I drunk my water and thrust my hand into the bag of nuts and greedily munched down a handful with a huge grin. The book says to chew thoroughly, which I tried at first but quickly forgot. The macadamias tasted so great.

Following the instructions in the book, I waited 30 minutes to make sure my stomach was feeling okay and then basically ran to get a sashimi salad.

The delicious sashimi salad

It was one of the best tasting meals I’d ever eaten in my life. Every little texture and flavour was magnified, from the bursting roe to the crunch of toasted shallots.

I felt so happy to finally be eating again. Once again this fast drove home the importance of food in my life and the gratitude I have to be able to eat whenever I want to.

By the end of the night after a huge tapas dinner I was already feeling like myself again.

  • Morning mood: 7/10Feeling happier since I’m on the home stretch, but quite tired
  • Weight: 72.4kg (159.6lbs)
  • Ketones: 3.6mM

What would I do differently next?

For my next fast I will:

  • Be more careful with my transition back to normal eating. I will follow Valter Longo’s suggestions of eating predominately complex carbs and minimal fats and proteins – I was visiting the bathroom numerous times after breaking my fast!
  • Get into nutritional ketosis prior to beginning a fast to help reduce the brain clouding effects
  • Try a fasting mimicking diet (FMD), which allows you to eat a small amount of food each day for five days, getting you many of the benefits of a 5 day water fast but without the strain of not being able to eat. This also means I can drink coffee!
  • Collect more data (unfortunately my replacement Oura ring was still in transit at the time of this fast) including:
    • Sleep
    • HRV
    • Resting heart rate
    • Blood pressure
    • Blood glucose
    • Body fat percentage before and after the fast
  • Think about ways to maximise the effects of my fast. Leading UK biohacker Tim Gray offered me the advice of consuming proteolytic enzymes, and there are also other foods and fluids like apple cider vinegar which may help lower blood sugar.

5 day water fast resources and information

I did a lot of reading and listening to podcasts before I fasted. Here are some great resources which you should work your way through before attempting your own water fast:

Books

Videos / Podcasts

Closing thoughts: My 5 day water fast results

Measuring autophagy is impossible, so I can only hope and assume this fast got me the benefits that come with this cell cleaning process.

I didn’t realise I would lose so much weight during my 5 day water fast, but my worries decreased after I saw my weight steadily rising each day after.

The fast was a rewarding experience and something I will definitely do again, albeit with either a period of ketosis prior to fasting or using a fasting mimicking diet.

Have you done a 5 day (or even longer) water fast yourself? Tell me about your experience below!

Author

8 Comments

  1. Hey Marc, thanks for a great article. Am currently on day 3 of an attempted 5 day fast, and thought I’d learn a bit more from others who have done it!

    I’ve managed to hit 5 days once before (and failed to hit it twice!), and had a similar experience with the refeed. Went a bit heavy on the food, and ended up spending more time in the bathroom that is ideal! Hopefully this time around I’ll be able to moderate my food intake on the refeed. Certainly easier said than done.

    This is the first article I’ve read on your site, but looking forward to reading more.

    • Marc Reply

      Hi Alex, thanks for the comments!

      Firstly, good luck on your own fast! It’s difficult but worth doing. I’ve just received my first Prolon fast mimicking diet kit and am reviewing it in about a month so stay tuned!

      I definitely agree with you about being careful during the refeeding period. I didn’t mention it in the article but I also had acid reflux for most of the afternoon and night during my refeed. The next time I do a water fast I’ll give myself half a day to get back into the swing of things.

      Thanks for stopping by!

    • Marc Reply

      Hi Angela, thanks for stopping by!

      I used the Precision Xtra by Abbott, which I have had for a few years now. It’s easy to use and supposedly accurate according to reviews. Honestly though I wouldn’t recommend buying it as it can be difficult and expensive to find ketone strips online in Australia. Abbott have another device called the Optium Neo which seems cheaper, although I haven’t done any research on it / checked reviews.

      Hope this helps!
      Marc.

      • Hi Marc,

        Embarking on a 5-day tomorrow after 30 years since the last time. I’ve been in ketosis 80-90% over the last three months, and intermittent fasting over the last two. Looking forward to it.

        I’ve been using the Optium Neo since April (around $60), and ketone strips are usually available in Australian chemists for between $0.90 to a dollar a strip for a pack of 10. I buy in bulk off ebay for as low as 69¢ a strip.

        You can upload your basic data to an app, although the reports are pretty basic. I export to a more sophisticated spreadsheet, but that’s me.

        Good luck!

        • Marc Reply

          Hey Sean!
          Thanks for the comment.

          That’s awesome! I hope your fast goes well. I want to try my next water fast from ketosis too to make the transition a little bit easier.

          And also thanks for your thoughts about the Optium Neo. I think I’ll replace my Precision Xtra with it at some point in the next few months as it seems way more affordable and the app seems like a nice little extra.

          Thanks for stopping by,
          Marc.

  2. Hey Marc, 🙂
    Thanks for taking the time to put this together,
    I am off to the mountains of the south of France for my 5 days at the end of this week. Place called Le Mas. I like to be quiet and alone and defo away from the usual hustle and bustle of city life…and away from people eating food 😉
    I went to a place in Scotland called Isle of Eigg the same time last year and had a really good experience.
    I only did light activity, walking on the island and push-ups and body squats to see if strength changed significantly. I think you working out every day made it way tougher and would maybe affected your mood more. (Or maybe not)
    Anyway like you I did collect data but will collect a lot more this time. Bloods(glo + keyt), RHR, body temp, pulse, weight., hunger index, mood ect ect and then stick this all into a movie. I will do a lot of filming I did last time but think it came out a bit pants so look to improve this time LOL
    Anyway, good luck with your training and future fasting.

    • Marc Reply

      Hey Miles, thanks for the comment!
      Your upcoming trip sounds amazing, almost like a fast and retreat combined into one!

      I am gearing up to try the Fast Mimicking Diet from Prolon in a couple weeks, so it’ll be interesting to see if this improves my mood while still working out.

      Please let us know when your video is up as I would love to watch. I also want to record much more data for my next fast similar to you. What will you be using to track this all?

      Good luck and enjoy the French countryside.

      Cheers,
      Marc.

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