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Yesterday on Twitter, a lot of people were interested in the multiday fast I'm currently doing. Since I'm awake at 4am (more on that below), now seems like a great time to explain a bit about the fasting. I will drop the major caveat at the start: I am in no way an expert on fasting. For that, you'd want Dr. Jason Fung, and likely his book The Complete Guide to Fasting. This will be my personal anecdotes version of that information.

There are many reasons to consider fasting, including:

  • Religious reasons. As many know, I'm a religious Jew, so this concept is not at all foreign to me, and probably helped me more easily get into fasting. However, that's not my motivation today, or related to the rest of this blog post.
  • Losing weight. It's pretty hard to put on weight when you're not eating anything. (But you probably wanted to burn fat, not muscle, right?)
  • Mental clarity. Many people—myself included—report feelings of clear-mindedness and focus during a fast. I find I get some of my best writing and coding done when fasting.
  • Extra energy. This may seem counter-intuitive: where's the energy coming from? We'll get to that.
  • General health benefits. Fasting has been well demonstrated to help treat epilepsy, but more recently also have claims of anti-cancer benefits. It can also be a highly effective treatment for type 2 diabetes.

Alright, let's get into the details of how to do this.


I'll get this out of the way right now. I am not a doctor. I cannot give you health advice. I'm telling you about what works for me, a decently healthy person without preexisting conditions. If you are taking medication, especially insulin or anything that affects your blood sugar, talk with a doctor before doing this, or at the very least read one of Dr. Fung's books.

How to fast

The process is simple: stop eating. This isn't a "bread and water" fast. This isn't a "maple syrup and cayenne pepper" fast (worst TED talk I've ever seen by the way, I refuse to link to it). You are consuming virtually no calories. (We'll get to virtually in a second.)

You should continue drinking water, and lots of it. Drinking green tea is helpful too, and black and herbal teas are fine. There's also no problem with coffee. In fact: the caffeine in coffee can help you burn fat, which will help the fast progress more easily.

You should not drink any sugary drinks. (That's not just advice for a fast, that's advice for life too.) You should probably avoid artificial sweeteners as well, since they can cause an insulin spike, and some contain calories.

There are three other things you can include in your fast if desired:

  • Salts, including sodium (normal table salt), potassium, and magnesium. Personally, I've used a bit of normal table salt, himalayan salt, or high-potassium salt.
    • If you're wondering: how do you consume the salt? Yup: I drink salt water sometimes. Delicious.
  • Some cream in your coffee. Don't overdo it: you're not making bulletproof coffee here. This is intended to just take some of the edge off of your coffee if you can't stand black coffee.
  • Bone broth, essentially beef (or other) bones cooking in a crockpot with salt for 24 hours straight. This is a nice way to get in some salts, a few other trace minerals, and give you a satisfied feeling before going to sleep.

The latter two do contribute some calories, almost entirely from fat. Don't overdo this, you'll be counteracting the point of the fast. And definitely avoid getting protein or any carbs mixed in; do not eat some of the meat off the bones, or throw in carrots or other starchy vegetables.

If you're doing a fast for anti-cancer reasons, I've read that you should eliminate the cream and broth as well, and maybe consider leaving out the coffee and even tea. Again: I'm not an expert here, and I've never tried that kind of fast.

When to fast

Fasting does not require any preparation. Just decide to do it, and stop eating! I typically like to stop eating around 6pm one day, and see how many days I can go before I cave in and eat. I'll mention some reasons to cave in below.

My maximum fasting time is currently four days. It's doubtful I'll ever get beyond six days due to the weekly Jewish Sabbath, but that's currently my goal. If you're doing a multiday fast, I'd recommend targeting at least three days, since the second day is by far the hardest (from personal experience, and what I've read), and you should see it through till it gets easier.

By the way, the world record longest fast is 382 days. The rules for fasting change a bit at that point, and you would need to take vitamins and minerals to make it through.


The biggest fear people have is hunger. This is a normal, natural reaction. As someone who has done fasts plenty of times, I still sometimes feel the fear of being hungry. Allow me to share some personal experience.

When I'm in the middle of eating food, the thought of purposely witholding from eating is terrifying. Once I've stopped eating for half a day or so, that terror seems in retrospect to be completely irrational. I strongly believe that I, like many others, can react with an almost chemical addiction-like response to food. It's nice to occassionally break that dependence.

Many people assume that they'll be overcome with hunger once they've gone somewhere around 8 hours without eating. I'd recommend thinking of it as a challenge: can you handle not eating? Your body is built to withstand times of no food, you'll be fine. This is a straight mind-over-matter moment.

Another concern many have is memories of trying to diet, and feeling miserable the whole time. Let me relate that it is significantly easier to spend a day not eating anything, and another day eating normally, than it is to spend two days eating less. The simple decision to not eat at all is massively simplifying.

Relationship to ketosis

When most people hear "keto," they're thinking of a diet plan that involves eating an absurd amount of bacon with an occassional avocado. In reality, ketosis is a state in the body where your liver is generating ketone bodies for energy. Here's the gist of it:

  • The two primary fuel sources for the body are fat and carbs (sugar/glucose)
  • Most of your body can happily use either of these
  • However, fat cannot cross the blood-brain barrier, so your brain typically runs exclusively on glucose
  • When you dramatically restrict your carb intake (and to some extent your protein intake), your body has insufficient glucose to run your brain
  • Instead: your liver will convert fat into ketones, which is an alternate energy source your body, and in particular your brain, can use

I'm honestly a bit unsure of the whole topic of proteins in ketosis, as I've read conflicting information. But the basic idea of restricting carbs is straightforward. One way to do this is to eat a very-high-fat diet. But another approach is to eat nothing at all. The same process will take place in your body, and you'll end up in ketosis, likely faster than with a ketogenic diet.

At this point, your whole body will be running off of a combination of ketones and fats, with a small amount of sugar circulating for some cells which can only run on glucose. This explains some of the purported benefits I mentioned above:

  • Running your brain on a different energy source can give you mental focus. This is not universal: some people report no such feeling.
  • Once you switch over to ketosis on a fast, you're burning your (likely ample) body fat stores, providing a nearly-unlimited supply of raw calories. Compare this to having 3 meals a day: you are typically dependent on each meal to provide your energy, and are just waiting for your next infusion of food for a pick-me-up.
  • Using your body fat as a fuel source obviously means you'll be burning fat, which is the right way to lose weight! But fasting+ketosis does something even better: it trains your body to be better at burning fat, making it easier to continue burning fat in the future.

One final note: people familiar with Type 1 diabetes may be terrified of the term "keto" due to ketoacidosis. Ketoacidosis is a dangerous medical condition, and does involve ketones. However, it is not the same state as ketosis, and being in ketosis is not dangerous. (But remember my caveats about not being a doctor above, especially if you are a type 1 diabetic.)


My personal approach to exercise on a fast is:

  • Follow my normal lifting routine
  • Try to train as hard and heavy as I can, accepting that the lack of incoming food will make me a bit weaker.
  • Schedule a lifting session at the very beginning of the fast, followed by a High Internsity Interval Training (HIIT) session. I still have plenty of energy and glycogen stores to get me through this session on the first day, and the session helps drain those glycogen stores and kick me into ketosis faster.

For weight lifting, I follow an RPE-based program, which essentially adjusts the weight on the bar to how hard it feels. Therefore, as I progress through the fast, I can typically continue following the same program, but the weight I lift will naturally go down a bit.

Side note: at the time of writing this blog post, I'm recovering from wrist tendonitis, and was already deloaded in my lifting, which is why I'm not lifting hard. It's not because of the fast, but it is affecting how I'm responding to the fast most likely.

Side effects

There are some side effects to fasting worth mentioning, including why I'm writing this blog post now:

  • Insomnia. For me, the ketone rush seems to disrupt my sleep cycle. I don't actually feel tired during the day, so maybe this is a good thing.
  • I won't do it justice, but Dr. Fung describes many hormonal changes that occur when you're in a fasted state. These include things like increased human growth hormone and insulin-like growth factor 1. The upshot of this is that you should experience less protein and muscle wasting during a fast than when following a calorite-restricted diet.
    • This is the primary reason I'm doing the multiday fast right now: I've been on a "cut" for a few months and unhappy with the amount of muscle loss I've had. I want to finish up the cut quickly and get back into maintenance mode eating, without incurring greater muscle loss. I intend to continue experimenting into the future with using multiday fasts as mini-cuts.
  • Nausea. The first time I did a multiday fast, I got so nauseous that by the end of the third day I had to break the fast. It was bad enough that I couldn't stand the taste of coffee for the next two months. However, that first fast included no cream or bone broth. Since I've added those two things, I haven't had any more nausea when fasting.
  • Dizziness. Especially when first adapting to the fasted state, you may feel lightheaded. Just be careful, and be sure to drink enough water, and supplement salts as needed. More generally: make sure you get enough fluids.

There are certainly others, these are just the ones that have most affected me.

Ending the fast

I've ended the fast in the past because I either felt sick, or because of some personal obligation where I didn't want to be "compromised" in a fasted state. (This was likely just a cop-out on my part, going on a 4 hour trip with a group of fourth graders isn't that taxing.) I would recommend setting a goal for how long you want to try to fast for, and be ready to be flexible with that goal (either longer or shorter) depending on how you're responding.

I don't really have any great recommendations on what to break your fast with. However, you may want to continue eating a low carb or ketogenic diet. You've just put yourself into a deep state of ketosis, which many people struggle to do. May as well take advantage of it!

This may not be universal, and I won't go into details, but expect some GI distress after the fast ends. Your digestive system has been off duty for a few days, it needs to reboot.


Alright, so that was a lot of information all over the place. You're probably wondering: what should I do? Frankly, you can do whatever you want! You can choose to dive in at the deep end and start off immediately with a multiday fast. Dr. Fung mentions this in his book. However, in my own experience, I started much more gradually:

  • Low carb and ketogenic diets
  • Intermittent fasting: only eating between 12pm and 8pm, for example
  • Experience with religious fasts
  • Randomly skipping meals
  • A 24-hour fast (stop eating at 6pm, eat the next day at 6pm)
  • A multiday fast

Keep in mind: many people in the world today have never skipped a meal. Consider starting with something as easy as not having breakfast to prove to yourself that you don't need to consume calories all waking hours of the day.

If you have questions you'd like me to answer in a follow up post, either on fasting, or other related health topics, let me know in the comments below or on Twitter.

Extra: muscle loss

I'm not going to claim that fasting is the miracle that allows us to gain muscle and lose fat. I don't have enough personal experience to vouch for that even in my own body, and the research I've seen is conflicted on the topic. As I love doing n=1 experiments on myself, I intend to do more tests with fasting following periods of moderate over eating to see what happens.

Extra: some calorie math

For anyone in a healthy or above weight range, your body is carrying more than enough calories in fat to sustain you for days, likely weeks. If you do some quick math: there are 3,500 calories in 1 pound (0.45 kg) of fat. If you burn about 2,000 calories a day, you can burn roughly half a pound (.22kg) of fat a day. A 165 pound (70kg) person at 20% bodyfat would have 115,500 calories in fat storage, enough to survive over 55 days. Point being: once you've entered the ketosis stage of your fast, you essentially have unlimited energy stores available to use, as opposed to needing another hit from a meal to recharge.

Extra: anti-cancer

To be a broken record: I'm not an expert on this topic. But the basic idea of anti-cancer claims for fasting come down to starving out the cancer cells. Cancer cells require lots of glucose (and a bit of protein) for energy. Fasting starves out the cancer cells. I'm not telling anyone to skip chemo or radiation. But I do personally know someone who defeated breast cancer with an (I believe) 11 day fast.

Extra: relevant tweets

Here are some of the relevant tweets from the discussion yesterday.

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