As part of the “Dom D’Agostino” chapter in Tools of Titans, Tim Ferriss discusses his foray into fasting.

He discusses fasting in the context of a “purge fast”. To quote Dom:

“If you don’t have cancer and you do a therapeutic fast 1 to 3 times per year, you could help your body to purge any precancerous cells that may be living in your body.”

There is evidence to suggest that fasts of 3 days or longer can effectively “reboot” the immune system via stem cell-based regeneration (Tim skips over the scientific detail here, but read the paper linked below for details1).

Tim Ferriss on the left, and Dom D’Agostino on the right. Dom helped create this fasting process discussed below.

Tim came into fasting as method to tackle his long-standing Lyme disease. He began with a 7 day fast at a fasting clinic – and whilst he didn’t like their style and rules; he found his long standing skin issues cleared up, as well as chronic joint pain.

Over time Tim optimised his fasting methodology. The longest fast he has done was a 10 day fast, during which he managed to maintain all his muscle, compared with his first fast where he lost 12 pounds.

He had two “tricks” to prevent muscle loss whilst fasting:

  1. Consume trace amounts of BCAAs and 300 to 500 calories of pure fat each day. These provide the body small amounts of protein that can help with muscle sparing. Specifically Tim consumed roughly 1.5 g of BCAAs upon waking and roughly 3g of BCAAs during a workout.
    BCAAs are “branch chain amino acids”, namely leucine, isoleucine, proline, and valine. In Tools of Titans Tim doesn’t name the BCAA brand he uses, but on his blog Tim mentions that Dom D’agostino uses Scivation Xtend BCAAs, which is a flavoured powder version. Peter Attia, who you may know, has noted that he’s only aware of two legitimate BCAA sources on the market; Ajimoto & Biosteel. Ajimoto sell their BCAA formula to other companies under the name “Ajipure”, of which Scivation Xtend BCAAs are using.
  2. Get into ketosis as quickly as possible, because ketosis itself is a muscle preserving state. Below we go into detail about how to get into ketosis ASAP.

Tim explains that the more you get into fasting, the faster the transition to ketosis is. Due to a biological muscle memory related to monocarboxylate transporters and other things “above his pay grade”.

Currently Tim aims to do a 3-day fast once per month and a 5 to 7 day fast once per quarter.

3-Day Fast Summary:

  • Have your last meal by 6pm Thursday night
  • Friday morning, take a long walk (ideally 3-4 hours) to accelerate your transition into ketosis
  • Friday, Saturday & Sunday consume moderate amounts of exogenous ketones to “fill the gap” of food, while you’re moving into ketosis
  • Sunday evening break the fast

3-Day Fast in Detail:

Thursday Evening

  • Eat a low-carb dinner around 6pm, once that meal is finish, the fast begins.

Friday Morning

  • Upon waking, consume exogenous ketones and (optionally) caffeine. Then WALK. Your goal is to walk for 3-4 hours so that you use up your liver and muscle glycogen stores. This hastens the switch from burning glucose to burning ketones for energy.
  • Tim suggests you bring water with you and add a little salt to it. You want to stay well hydrated throughout the walk, and the salt is to prevent the misery of headaches/cramping.
  • To keep you busy during the walk (and avoid boredom), Tim recommends scheduling in phone calls during the few days prior. Alternatively, podcasts are a good time fill.

Saturday Morning

  • After your long walk on Friday, you should expect to be in ketosis. Using a ketone meter like the Precision Xtra, test your blood ketones to verify. You’re aiming to be at 0.7mmol or greater by this point. Urine testing strips are inaccurate, and should not be used for this. You may think that testing your blood ketones is unncessary, but don’t underestimate the value of having a measurable parameter, that shows you are heading in the right direction.
  • Exogenous ketones…
    Throughout the fast you will be using exogenous ketones as a tool to plug a gap during the fast that carbohydrates would usually fill. Once you get into deep ketosis the fats can be omitted, because your body will be effectively using its own stores.
  • What is “deep ketosis” you might ask? One way to measure it is when your blood glucose and blood ketones are at a 1:1 ratio, or greater. Greater means your blood ketones are higher than your blood glucose. When comparing these, ensure both measurements are in mmol/L or mg/dL – such that you’re comparing apples with apples.
  • Specifically you’ll be taking exogenous ketones upon waking and 2 more times throughout the day at 3-4 hour intervals. Tim primarily uses KetoCaNa and Brain Octane. KetoCaNA provides exogenous ketones directly, whereas Brain Octane is made of C8 caprylic acid, which can then be used by the body to increase endogenous ketone levels. Note that whilst Brain Octane is “MCT oil”, it’s specifically the C8 chain of MCTs which is found to be most effective at raising ketone levels. If you opt for another brand of MCT oil, check the % of C8 oil that it’s using to make sure it’s significant.

Friday, Saturday & Sunday

  • Friday, Saturday & Sunday sleep as late as possible. Letting the sleep do some of the work for you
  • You only need to do the long walk on your first day of fasting (Friday), with the goal being to kick you into ketosis faster. Saturday and Sunday you will be proceeding with your regular affairs – assisted by some exogenous ketones as necessary.
  • Each day of fasting, feel free to consume exogenous ketones or fat (e.g., coconut oil in tea or coffee) as you like, up to 4 tablespoons. Tim often rewards himself at the end of each fasting afternoon with an iced coffee with a bit of coconut cream in it. Sometimes he even allows himself a SeaSnax packet of nori sheets as a treat.

Sunday Night

  • Break your fast Sunday night and enjoy it. For longer fasts, you need to think carefully about how you refeed, but for a 3 day fast, you don’t need to be so careful.

Staying in Ketosis Post Fast

After Tim’s 3 day fast you will be deep into ketosis. For most people, this will be enough fasting, and you’ll want to start eating again. One possibility is to stay in ketosis for a while, before transitioning back to “normal” meals. This could certainly help if weight loss is your goal.

How to stay in ketosis:

  • Your goal will be to eat lots of fat (~1.5 to 2.5g per kg of bodyweight), little to no carbs and moderate protein (1 to 1.5g per kg of bodyweight) each day.
  • High protein and low fat is no good – the excess protein gets converted by the liver into glucose, which kicks you out of ketosis. You need to be consuming 70 to 85% of calories in the form of fat.
  • For example, a chicken breast could kick you out of ketosis, but a chicken breast cut up into a leafy green salad with a lot of olive oil, feta cheese and some bulletproof coffee (for example) can keep you in ketosis.

Closing Words

Hopefully the above has helped you on your fasting journey. If you’d like to discuss Tim Ferriss’ 3 day fasting method, or indeed longer fasts (water only?), then you’ll find like-minded people on this Facebook group.

See Post Sources Below:

  1. Prolonged Fasting Reduces IGF-1/PKA to Promote Hematopoietic-Stem-Cell-Based Regeneration and Reverse Immunosuppression – Cheng et al. (2014) –

Posted by Alex


  1. Avatar

    Hi Alex,

    Thanks for the great article. I’m looking into a regular monthly 3-day fast for cancer prevention (to purge pre-cancerous cells). Would it be counter-productive to take the BCAAs/Black coffee/fat/exogenous ketones if I’m only doing this for cancer-prevention rather than weightloss? I was thinking I would do this water only for the greatest benefit.


    1. Alex

      Hi Scott. I tend to agree, that if you can do your 3-day fast with water only – that’s likely optimal.

      The idea with using exogneous ketones/MCT is to make the fast easier – but if you can manage without – great!

      And the idea with using BCAAs is to ensure muscle mass maintenance. But if the primary goal is weight loss, this can easily be skipped.


  2. Avatar

    Thanks for the awesome info man, this is very well organized. A Tim Ferriss supplement page would be amazing but probably way too complex to attempt.


    1. Alex

      Thanks Son! Will consider it, if there’s time.


  3. Avatar
    Ayman Haboubi May 16, 2019 at 7:25 pm

    Amazing article, thanks. Anyone know how often we should exercise during 3 day fast? If we are fasting 3 days we should exercise every day? I usually lift moderate weights 3 times a week for about ah hour (upper and lower body) so I can time things so that I exercise on day 2 only I guess.


    1. Alex

      In terms of exercise, as the post suggests, there is value in doing low heart rate, steady cardio (such as walking) to deplete the glycogen reserves. Then in terms of exercise beyond that, such as weight lifting, that’s really down to the individual. I find by about day 3 of a fast I’m feeling a lot weaker than normal, so if I lift weights, it’s low intensity, and not as heavy as I might go otherwise.

      It’s a balance, because you don’t want to wear yourself out, when you can’t eat to refuel and re-energize.

      All that being said, there are some people who can still work out pretty hard whilst fasting. So we’re all different in that respect.


  4. Avatar
    Linda Hochstenbach April 28, 2019 at 6:17 pm

    Hi tim,
    Just to be sure (I am Dutch), is it necessary to take exogenous ketonis? Is this also possible without and only with the BCAAs? I fasted before and sometimes I took unfermented vegetable juice to bridge it. Finally my last question, is it possible to cycle with a low heart rate instead of walking? Best regards Linda


    1. Alex

      Hi Linda. Thanks for the message.
      No, it’s not necessary to take the exongenous ketones. They are part of Tim’s method for making the fast easier, but are not necessary.
      The BCAAs are also not necessary – they are part of Tim’s method for conserving lean body mass during the fast.
      Yes, I think cycling with a low heart rate would also use up your glycogen stores, similar to a long walk.


  5. Avatar

    So when exactly am I taking the BCAAS? It says Tim would take it in the morning when he first wakes up and then during his workout. So does that mean for us during the fast to take it Friday morning and then drink more while we’re doing our 3-4 hour walk? I’m confused lol


    1. Alex

      Hi Kaylee. Tim mentions BCAAs in the context of maintaining muscle mass throughout the fast. That being said, after 3 days of fasting, the muscle loss is likely to be very minimal – especially given:

      1. How quickly we aim to get into ketosis
      2. We will be exercising, and this (from what I’ve read previously) signals to the body to try to preserve the muscle because it’s in use

      Muscle loss whilst fasting becomes a bigger issue in the longer 5+ day fasts.

      If you do still want to take BCAAs, it’s likely most effective to take them just prior to your workout.

      If we take them at other times, the BCAAs will have a small impact on our blood glucose, which could then reduce blood ketones. And this isn’t desirable.


  6. Avatar

    If I don’t have the opportunity to walk for that long on the first day will the process be prolonged to get into ketosis? Would I have to fast for 4 days instead of 3?


    1. Alex

      Hi Jada, yes, as you say, without the morning walk on day 1, you will go into ketosis slower. Ideally try to measure your blood ketones, because each individual is different in terms of their transition to ketosis, and blood ketone levels over time. As you say, extending the fast longer could compensate. However, if you’re not a seasoned faster, be careful and start slow. 3 days is more than enough to begin with. And as your familiarity and confidence with fasting grows, then that may be the time to experiment going to 4 days.


  7. Avatar

    Tim consumed roughly 1.5 g of BCAAs upon waking and roughly 3g of BCAAs during a workout.

    How much BCAAs would you suggest for non-work out days or for recovery from heavy lift/exercise session the day before I start prolonged fasting? Just 1.5g upon waking for the entire day?

    Do you think consuming trace amounts of BCAAs (1.5g) throughout the day would hinder a 3 day fast (e.g. 1.5g every 2 hours or so)? Just wondering the basis for BCAA consumption.



    1. Alex

      Hi Stan, thanks for your message.

      Regarding the first question, I shouldn’t worry about BCAAs the day before you start fasting. Assuming you’re not a vegetarian or vegan (?), then if you eat a good protein based meal (chicken breast/tuna/steak etc) that day, you should cover all your bases for amino acid requirements.

      Regarding the second question. As far as I know, BCAAs do actually cause a slight spike in blood glucose. You could test this by measuring blood glucose before and after. Our bodies can actually produce glucose from amino acids – which has its uses of course. So, I would suggest not to consume BCAAs throughout the day whilst fasting. Ideally we want to spike blood glucose as little as possible. Tim’s method of taking BCAAs twice during the day, would be more optimal than taking them 3+ times.


  8. Avatar

    Excellent article. I have a question? I am a 66 year old Male runner and wondering if you can run for an hour and a half or so instead of walking for 4 hours? Is it movement time one is trying to achieve or to burn x number of calories?


    1. Alex

      Hey. So the primary goal of the walking (as best I understand it) is for glycogen depletion. Glycogen being stored glucose in the liver and muscle. Once depleted, it causes the body to ramp up fat burning, resulting in the utilisation of ketones.

      Running (and other forms of exercise) would also delete glycogen, so could be substituted.

      I would say the main issue with exercise as vigorous as running is the risk of driving hunger. Which you can’t quench whilst fasting.

      But if you can handle the hunger that the run may bring, it should be a good substitute for the walk.


    2. Avatar

      I would advise against running. Any vigorous activity triggers gluconeogenesis, essentially your body will make glucose from other sources to meet the sudden and/or extreme demand that intense exercise creates.
      As a cool little experiment, wake up in the morning, do not eat and measure your blood glucose. Then exercise (run, lift weights, cycle) in a fasted state and measure your BG again afterward. It will have spiked significantly.
      The point of walking is to gently burn through your stored glycogen. Running or any other vigorous exercise will be counter-productive, that’s why Tim stresses WALKING.


      1. Alex

        Hi Todd, thanks for your message. Question – would your body not *first* exhaust its existing glycogen stores (when running, weight lifting or cycling), before resorting to de-novo synthesis of glucose?

        And if not, then why would the body create more glucose if it already has some stored in the muscles, ready to use.


        1. Avatar

          Great question … that I don’t have a definitive answer for! My *guess* is that when faced with an extreme demand for glucose your body prepares for the worst case to ensure that it has enough glucose for whatever duration of activity is coming. Evolutionarily this makes sense, you might have to run from a predator (or chase game) for a long time so better to have more than enough.

          I have been doing intermittent fasting of various kinds for a bit over 5 years and extended fasts for over 2 years. If I walk on day 1 then I get into ketosis by that evening, if I do crossfit or cycle on day 1 then ketosis is delayed by up to an extra day and a half.

          These n=1 numbers are from an Abbott Precision Xtra ketone blood meter, a Countour BG meter, and on-and-off over the past year with a Freestyle Libre continuous BG monitor. But they are in line with what I have read from Tim and various other sources over the years.

          As a side note, I paid out of pocket for the Freestyle implantable glucose monitor and it is a fascinating exercise in understanding how/when your body digests, uses, and makes glucose. It was especially helpful to see what foods spike my insulin and what does not without having to prick your finger 10+ times a day, not to mention to see what happens when you are asleep.


  9. Avatar


    Thank you for the very detailed article! Helped me a lot in getting the motivation I need to fast for a period longer than 1 day.

    I need some advice please, I hope you can answer me in time.

    I will be moving from the UK to Samos in Greece and with the connections and everything it is approximately an 8 hour journey all together. I will be really busy with last minute shopping, packing, etc and I feel like I want to start my fast tomorrow as the 3 days will be much more bearable with my schedule and I feel like once I arrive and settle I will be happy to break the fast.

    Do you advise this with travelling etc?

    PS. I do intermittent fasting 23 / 1 and I was doing keto dieting for a few weeks so fasting is not a new concept in general.

    Thank you 🙂


    1. Alex

      Hey. So it’s do-able, and success is dependent on your level of self control – as with any fast I guess.

      Personally I’ve found most fasting success when I’ve a relaxed schedule. Perhaps in part because fasting is a stressor on the body, and you ideally don’t want too much additional external stress in addition.

      You could try it out and see how you get on.

      Any particular reason you’re in a rush to do it?


  10. Avatar

    I am hoping to start the 3 day fast tonight… I have been on a strict Keto diet for a week now, but want to use this to ensure that I am in ketosis. My biggest concern is that I will be completing this fast with nothing but water. No bulletproof coffee, no BCAAs, no mixes/vitamins whatsoever. Is this an issue, or does it just make my fasting more difficult?


    1. Alex

      Hi Luke. Actually, going from keto into a water only fast should be a big advantage. You’re already keto adapted, and thus the uncomfortable period that people usually face (~3 days) where they are switching from burning primarily glucose to primarily ketones should be skipped. Wish you luck on your fast 🙂


  11. Avatar

    Thanks for this, very helpful. Im currently traveling so it is difficult for me to get a hold of some of these more complex supplements (or a ketosis measurement device for that matter). I’d love to know if you have alternatives for some natural, easy to come-by items I might consume in small doses that could help. Also, is it all that much better or worse to simply eat nothing for the 3 days, and only drink water/occassional coffee?


    1. Alex

      Hi Jordan. Definitely recommend a ketone meter when you’re finished travelling. It’s great psychologically to see the transition in and out of ketosis, and to be able to correlate that with your state of wellbeing.

      The whole aim with Tim’s method is to ease the transition into ketosis, and get there ASAP. If you can manage it with just water, then will totally work also.

      As an easy to come by alternative to C8 oil or ketone salts, you can use coconut oil, which contains MCTs. This will give your body some ready to burn substrates for ketone production.


  12. Avatar

    What does BCAA stand for please. And what is bulletproof coffee


    1. Alex

      Hi Katherine, thanks for the comment. BCAA stands for branch chain amino acids, specifically called leucine, isoleucine, proline, and valine. When we consume complex protein sources such as meat, fish and eggs – we get lots of amino acids (including the aforementioned branch chain amino acids) – which make up what we call “protein”.

      The value in the BCAAs for a fast, is that we get some of the protein, without the accompanying calories that would come with eating something like meat. It’s almost a “cheat” for fasting.

      Bulletproof coffee was popularized by a guy called Dave Asprey. It’s where you consume coffee accompanied by a fat source. Typically this is grass fed butter, but can also be coconut oil, caprylic acid (C8 oil) such as brain octane, or some combination of the three.


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