How to fast
A friend of mine is embarking on his first 4-day water only fast, so I sent him a voice memo with all of my lessons learned from fasting over the last few years. Then, I realized it’d be useful to write this up.
First, my bonafides.
In 2024, I didn’t eat for a total of 46 days. I did those fasts for between 1 and 6 days throughout the year, and learned a lot in the process.
This is an article about how to do a long term fast, not about why you should. But first I’ll articulate a few of the benefits I’ve found.
Caveats: I’m not a doctor and don’t play one on the Internet. This isn’t medical advice. Please consult with your medical provider. And please don’t sue me.
Why I fast
Autophagy
Autophagy is the state in the body where the body recycles cells. This happens during any fast – even just not eating for twelve hours overnight results in mild autophagy.
We’re built this way – to break down the most unhealthy cells in the body so that healthy cells predominate and to decrease the chance of them turning into cancer or causing other harm.
Cancer prevention
I first came to learn about fasting because my best friend, having been diagnosed with breast cancer, was doing multiple water fasts every month.
By creating a context in the body inhospitable to cancer cells, the theory – and a great deal of evidence – suggests you decrease the chance of cancer growing or metastasizing.
It is, of course, a much longer conversation, but three books I recommend about cancer are:
The absolute maximum that most bodies can sustain is not eating one day for every two days of eating. I wouldn’t recommend even that much for anyone not combating cancer.
A caffeine reset
I’m a lifelong caffeine drinker.
I love nothing more than green tea or pu-erh first thing in the morning. (I also love coffee, but gave it up when I sold Robin's Cafe.)
I’ve tried many times over the years, to cut caffeine entirely, and suffered caffeine headaches and even nausea.
During a 5 day water-only fast – during which I don’t drink anything but water – not only don’t I suffer caffeine withdrawal, but I come through the experience feeling as if I haven’t had my regular green tea or coffee for many months.
Fasting provides a great reset.
Our bodies are made to fast
I don’t believe much about "paleo" or the paleolithic diet, but I do think it is useful to consider how our ancient ancestors ate. And it is abundantly clear that humans did not have food as readily available as we do today.
Our bodies, it turns out, are built to be able to fast. Physically, we can do a couple of days without food without adverse effects. (Psychologically, of course, is another matter.)
If we can fast with ease, it makes intuitive sense to me that there might be some benefits to doing so every so often.
Get comfortable with heightened adrenaline
The psychology of fasting is difficult. And managing the heightened adrenaline that comes with a fasted state is my least favorite element.
After the first day or two, the body kicks into a state of heightened energy and lethargy. You’re either on or off!
But intense adrenaline, which most of us don’t experience outside of extreme experiences like competition or a car crash, is also a useful state to get familiar with.
Getting comfortable with high adrenaline is good practice for when the world gives you something really worth freaking out about.
Reevaluate your relationship with food
I love food! And to my detriment, I’ve been known to eat, for flavor, even when I’m not hungry.
The most useful element of fasting I’ve discovered is the forcing function of having to re-evaluate my relationship with food.
Practice being hungry, wanting to eat, and not eating. Being hungry, wanting food, and not eating. The definition of delayed gratification.
Tactics for fasting
Electrolytes
The worst moments during my longer fasts have come from not having enough water and electrolytes.
“Drink plenty of water” is the most common advice articles and YouTube videos give. And it is true: during a long fast, you have to drink more water than during your normal life.
Because you aren’t absorbing any liquid through food and to get into a fasted state, the body dumps a lot of water, it is really important to stay hydrated. But the advice of “drink water” falls flat when I’m pumping full of adrenaline, have a splitting headache, am cold, and – in short – feel miserable.
The secret, in addition to drinking water before you feel like that is consuming enough electrolytes.
The advice I was given is to eat pinches of salt through my fast. That’s terrible advice. Salt on the tongue isn’t great at the best of times, but when you haven’t tasted flavor for days, straight salt is the last thing I’d recommend.
Similarly, LMNT, while a useful tool, is much too strongly flavored and contains Stevia, both of which interfere with a fast. Chalk full of sugar, you absolutely must avoid Gatorade. Anything with sugar negates a fast, and consumed during a fast can result in refeeding syndrome, which is quite serious.
My preferred form of fasting supplement is Trace Minerals tablets. While they aren’t small, and a serving size is 6(!), you can take these with just a sip of water, down a lot in a short period, and they contain the sodium, magnesium, potassium, and trace minerals you need for a complete electrolyte balance.
Random aside: I’ve always struggled with high altitude acclimatization, which is unfortunate because I love climbing big mountains. Taking these Trace Mineral supplements has, in recent years, completely eliminated my challenges with acclimatization.
Start and end with keto
This is a little hack that I have only discovered recently and that I wish I had known prior to my earliest and most difficult fasting.
"Keto," or the ketogenic diet is, essentially, a modified fast. The body enters autophagy and burns fat for fuel like during a deeper water-only fast.
“Keto flu” is a term ascribed to the state of discomfort, and sometimes even nausea, that sometimes accompanies entering ketosis if you are unaccustomed to keto. It is much easy to deal with the symptoms of keto flu before enter a full fast with it’s heightened adrenaline, sleep deprivation, and other challenges.
Like fasting, the body gets accustomed to ketosis. With practice, you can enter ketosis more readily and more gently over time. When you begin a longer fast with a few days of eating a keto diet, you short circuit that challenged state by entering it prior to fuller demands of a full fast.
Start with 24 hours
As with most things worth doing, start small.
My friend is considering a four day fast, never having done even as much as 24 hours previously. That’s going to be difficult because there is a lot to learn about your own body through the process of fasting.
Instead, start small. Fast for 24 hours before trying 36 hours, 48 hours days, or longer.
During a 24 hour fast you won’t enter a state of deep ketosis, but you will get a sense of your relationship with food and hunger. For me it was a revelation that you can go to bed hungry and wake up the following morning feeling just fine.
I did my first 24 hour fast in early 2023, and by the end of the year had done multiple 5 and 6 day fasts.
Start small. Your gains will compound.
Beware the witching hour
I call the hours of 6-9pm each evening my witching hour. This is the period during which the human body is most able to put on calories and retain weight.
If we trace the time back to prehistory, this is when humans were most likely to eat large meals and then be able to rest, so our bodies have learned that this is the time to signal hunger, and also to store calories.
This is the most difficult time during a fast.
In the morning, I wake up – and while I might miss my caffeine rituals – generally feel fine.
During the day, there are short bouts of hunger, but if I keep busy – it helps to remain busy with relatively unimportant tasks – they are easily passed by.
But in the evening my body is ready to eat. This is the time that I don’t allow myself into the kitchen or around other people eating food. Sitting with someone at breakfast? No problem. But joining someone for dinner is a miserable experience.
The best advice I have to get through the witching hours is to distract yourself. (I suggest The Chef and other movies depicting food.)
Go to bed early. Find something that you can do to make that time pass.
The wrap
If all of this sounds difficult, it is.
Long term water-only fasting is probably the most difficult thing I do on a regular basis. (My current cadence is two 5-6 days fasts each year.)
But, like most difficult things, it is among the most rewarding.
Not only have I re-evaluated my relationship to food (among other things, cutting out sugar and alcohol from my diet), but the knowledge that I can delay gratification and take small steps into doing something that previous was impossible gives me the confidence to attempt future hard things.
I hope this is useful, and inspires you to try something new and difficult.
Start small, listen to yourself, and as ever, let me know if you ever need anything.
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3 things I’ve loved this week
This was my first water–only fast, and it was pretty miserable!
Among other things, I started and ended the fast with ice cream and pizza, which is too fast a transition into and out of a fasted state.
But it is fun to look back and recall the experience, which I might not have been able to do had I not recorded it, and realize how much I’ve learned since.
Recommended by Responsive Conference speaker Tara Viswanathan, Annie Duke is a professional gambler who writes about human psychology and decision making. I appreciated the reminder about hindsight bias and recency bias, and that tactics for training the accuracy of our decision making.
I’ve tried a lot of electrolyte formulas, including popular brands like Gatorade (read: sugar water) and LMNT (too strongly flavored for fasting and includes stevia).
These days, I only take Trace Minerals tablets or use their liquid product 40,000 Volts. I also make my own electrolyte liquid, but that’s a longer story!
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