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I survived a three-day water fast

I am in the home stretch of a 3-day water fast, I stopped eating 68 hours ago (I only have 4 hours to go!) and did not have any food since then. I stayed hydrated by drinking 3-4 liters of water per day and only had one black coffee every day (zero calories). Was it easy? No, but it was a very good experience.

I have been doing intermittent fasting (IF) on and off for years and it starts to feel normal to me. I now generally eat all my food within a 6-8 hour window every day, meaning I only consume calories between 12:30 pm and 7:30 pm pm the next day. Sometimes I even only eat between 6-8 pm, but I also have regular cheat days (mainly on weekends and holidays, or when traveling) where I eat a light breakfast and when I drink a few glasses of wine at night.

I feel great doing it and I can now easily run 15 km in the morning without eating anything first. The main reason I started intermittent fasting is because it’s healthy and it has been neglected too long. Big pharma wants to sell you medicine to heal you, instead of making sure you don’t get sick in the first place. I did tons of research before doing my first experiments with time restricted eating, and I found that not having 3 meals a day can actually improve your health significantly. Evolutionary, 3 meals a day certainly was not the standard, so it’s not surprising that our bodies are not fully adjusted to our new nutritional standards.

For example, fasting for 16-18 hours per day significantly reduces your blood sugar levels, especially when you combine it with exercise. By lowering your insulin resistance you decrease your risk of getting diabetes, which is more and more important with our regular sugar, carb and highly processed food filled diets. IF may actually reverse Type 2 Diabetes. It also reduces inflammation in the body, which is very important given that many diseases (including some cancers) are caused by chronic inflammation.

For me IF works well, it’s an easy diet to follow and it actually makes you more aware of the foods that you eat. I don’t mindlessly snack anymore on days that I follow IF and I appreciate my first meal of the day a lot more than I used to. Drinking tea or coffee in the morning actually tastes a lot better when you are fasting than when you had food already.

Because I read so much about health and nutrition I often came across water fasts. To be honest, I dismissed these fasting diets for a long time. I do not go on a diet to lose weight and I always assumed that that was the main reason people did them. It turns out that’s not correct, although weight loss certainly plays a role (I lost 2 kg in the past 2 days simply because I didn’t consume any calories) the main reason to do multi-day water fast is to reset your immune system and to put your body into autophagy.

What does that mean? Basically, when your body does not get any nutrition it starts to clean out damaged cells, in order to regenerate newer, healthier cells. It also puts your body into deep ketosis (the state that you are in when you don’t eat any carbs for an extended period of time), so it starts to burn its own fat. Your glucose levels go down rapidly and after about 72 hours all the glucose stored in your body has been consumed. A 3-day (or longer) water fast basically resets your immune system. Studies have shown that cancer patients who do a 3-day water fast before chemotherapy reported a higher tolerance to chemotherapy, fewer chemotherapy-related side effects, and higher energy levels. This is a significant finding that convinced me that water fasts may be hard to do, but that the long term gains are so important that they are worth the short term pain.

Autophagy may actually prevent cancers by destroying the cells before they can grow and spread. Even though I am quite healthy (knock wood, praise Murphy) I want to stay that way as well, and preventive medicine is very important to me. Three day fasting also activates stem cells of the immune system to renew and repair themselves. It not only reduces damage to cells, it also replenishes white blood cells and replaces damaged ones. All in all sufficient reason for me to give this a try.

On Tuesday this week (it’s Friday when I am writing this) I had a regular intermittent fasting day. I had a light lunch followed by a small dinner (salmon and asparagus, no carbs), and after that I decided to stop eating until the weekend would start. The first evening was easy of course, because I normally don’t eat or drink much at night. The next morning was business as usual as well, because I normally don’t eat breakfast. Around 3 pm I started to feel a bit hungry however, so I decided to go for a 30 minute run. That went well and my hunger was gone. I had a slight headache though so I decided to have one cup of coffee and that did the trick. Although I drink a lot of coffee, I normally can go without caffeine for a day or 2 without any problems, but in combination with long fasting I think my body was craving for coffee.

At night I only drank water and I was worried that I would get so hungry that I would not be able to sleep. But that was not the case. Of course I did feel hungry, but I was more tired than usual (likely because I did not eat). So when I went to bed I did not even read for 30-60 minutes like I usually do but fell asleep almost right away. The next morning I woke up full of energy. I had a great, deep sleep, much better than usual. It was full of dreams, more than usual I feel. And I did not wake up once during the night, which surprised me (I tend to wake up at least once or twice every night and then stay awake for a while).

Because of a blizzard I had to clean both my boat and my driveway, which kept me busy during most of the morning. After that I worked for a couple of hours and I felt I could concentrate much better than before. I did not really feel hungry anymore, but I started to miss eating. I was thinking about food literally all the time, something I never really do. I noticed food when reading a book or when watching TV and I felt a bit jealous that other people could eat but I could not.

Then around 5 pm my energy was suddenly completely gone, I literally had to sit down. I had planned to go for a run, but I realized I could not do it. For a minute I thought I should just push through and get on the treadmill (too much snow to run outside) but because this is my first 3-day fast I wasn’t sure what would happen. I actually felt a bit dizzy and did not want to pass out on the treadmill. So I put myself in front of the fire place with a great book (How To Change Your Mind, What the new science of psychedelics teaches us about consciousness, dying, addiction, depression and transcendence by Micheal Pollan) and a litre of carbonated water. That helped a bit, but I felt tired for the rest of the night. I did not feel hungry though although sometimes I had a short (1 minute) craving for food. I watched a show on Netflix in bed but was too tired to really enjoy it and fell asleep quickly.

I again had a pretty good sleep. I woke up once but instead of thinking about work or other things, I could easily relax and fell asleep right away again. But in the morning I realized I still had no energy. I could hardly get myself out of bed. I also felt cold, something that had started the day before already when I suddenly had very cold feet. But because it was the last day of the fast I did not care too much. I did some emails and realized I could not concentrate as well as I usually can. I then decided to study Chinese characters for an hour, but I gave up after 10 minutes. I was not able to focus on it and I seemed to forget characters that I would have remembered in a normal situation.

So around lunch time I decided to just write a blog post about my experiences – and that’s what I am doing now. I still have a few hours to go before I can slowly start eating again and my plan was to just spend those hours with a book in front of the fireplace. However, while writing this post I start to feel my energy coming back. It’s a strange experience, as if my body is coming out of a deep sleep and all of a sudden it is awake. I read that in 7-day fasts (which you should not do without medical supervision), day 4-6 are generally much easier than the first 3 days, so maybe I am getting into that stage right now.

Although I am not hungry at all anymore I still look forward to eating something again later. I actually brush my teeth a few times per day just to feel toothpaste in my mouth. I will likely start with a light chicken broth, a salad and maybe an egg. If my stomach can handle that I plan to grill a steak and some veggies later at night. If I feel well tomorrow morning I plan a light breakfast (no intermittent fasting for me for the next couple of days), maybe a quick lunch (I have a 4 hour meeting Saturday afternoon) and then a dinner party with friends tomorrow night.

Generally I feel happy that I did this experiment. It’s interesting to get to know your body better. And it’s great to know that if needed I can quite easily go without food for a while (I hope I never need it though!). I have to say that it was harder than I had expected. Because of IF I knew I can easily set aside thoughts of food, but I had not expected to hit a wall after 48 hours of fasting. Maybe my glucose ran out by then already and I started to be in full ketosis? I guess I should have measured it, but it’s too late now. I do know that my current ketone level is 8 mmo/l, which is MUCH higher than most people who are in full ketosis (they are between 1.5 and 3 mmo/l).

Will I do this again? Yes, I probably will. I like a bit of a challenge and this is a relatively easy one (a full marathon is much harder). But because I had no energy left over the past 24 hours I am thinking of doing a 5-day fasting mimicking diet first. It should lead to similar results, but because you still eat about 700 calories a day you will likely feel a lot more energized. If I do so I will write another blog about it.

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  1. Hi Marc. I think you read Lifespan, or are at least familiar with the author and his work. Where are you on taking supplements? I think I read somewhere you said you don’t take medicine unless you are sick. Does it mean you are not taking any supplements like David Sinclair?

  2. No, I actually take a lot of (highly personalized) supplements. I think it is essential to do that.

  3. Would you be willing to write a blog post on your supplement intake?

  4. The main things I take are multivitamins (Metagenic Phytomulti), vitamin B12, Zinc, Magnesium, milk thristle extract, fish oil, vitamin d3, and of course brain octane in my coffee. Make sure to only use supplements from high end brands.

  5. No NMN, Resveratrol, Metformin for you?

    Have you tried insidetracker or recommend other service(s)?

  6. If those who want to lose weight have your perseverance, they will not worry about losing weight.

  7. “If my stomach can handle that I plan to grill a steak and some veggies later at night.” Ever studied the benefits of a vegetarian or vegan diet?

  8. Vincent, I looked at that in depth and my conclusion is that unless you eat tons of supplements it’s not a good idea to not eat any meat. We have been eating meat for over 2 million years and it made us into what we are right now, so there is no convincing reason for me to eat no meat at all (I do eat a lot less meat than a few years ago). Just saw this BBC article on how eating a vegan diet can have a negative effect on your intelligence. No thank you…

  9. Hi Marc
    think I might have mentioned it on your blog before about the book – The China Study by T Colin Campbell. It’s probably one of the books I’ve come across regarding science and nutrition. The book was grounded in solid scientific research and more so than any other books on the same subject. The author – the professor who wrote it has done decades of research in nutrition and science. I remember I was underlining every page when I was reading it for the first time. The book has not a lot to do with China despite the name.

  10. oh .. I forgot mentioning I did 21 days water fast over ten years ago. During the period I ate nothing at all and only drank water everyday. I rested mostly and spent time reading and walking around in the neighborhood. I lost almost 10kgs by the end of it. Looking back I think I was young, crazy and extreme. But one can definitely go quite far in water fasting. Many people have reported incredible mental clarity and emotional high during long water fast.