You’ve heard the benefits of extended fasts, but what actually happens?


I decided it’s time. Time to do an extended fast. It’s been on my list all year. Number 9. Do a 3-day fast and make a video about it.

I’m scared. I don’t know why. I know I’ll be fine. I’ve heard the stories, listened to the podcasts, know the science (sort of). It makes sense logically. Years ago, we didn’t always have food on hand, going a few days without eating would be normal.

But I was still scared. I ask myself, what if I get hungry? What if I can’t do the work I need to do? What if a good mealtime comes up? Think, oh there was that thing I had to do. I’ll probably need food for that.

All this happened within the space of a 20-minute afternoon bike ride. Before I get home I’ve decided. I’m doing it.

I have dinner knowing I’m not going to be eating for the next 3-days. 72-hours. Easy. I tell my family. I’m not eating for the next 3-days. They ask why. I tell them it’s for an experiment. My mum asks if I’m going to be alright. I say yes. She says are you sure? I say I’m sure.

Dinner is steak, potatoes and greens with a few spoonfuls of plain probiotic yoghurt afterwards. I better prepare my gut bacteria. It’s a meal worthy of being a last meal. Simple and delicious. It finishes around 7:30 pm. The clock starts.

I tell the world. Decide I’m going to document what happens to me. Let’s see.

I decided to document my first extended fast as it happened on Twitter. View the thread here.

Why fast for 3 days?

I learn things best when I’m in them. One of my rules is I can’t talk about anything where I don’t have skin in the game. I’d never done an extended fast but being a health nerd, I’d heard of their endless list of benefits. And if you’ve listened to a Tim Ferriss, Joe Rogan, Peter Attia or Aubrey Marcus podcast, you have too.

I’d done 24-hours, dinner to dinner and do 16/8 (not eating for 16-hours, eating for 8-hours) basically every day but never longer than a day.

To find out more, I had to stop being an outside observer and become a practitioner.

I could always stop. At any given time I’m within 20 meters of food.

Day 1 (Friday) — I’ve been here before

74.7 kgs (164.7 lb) this morning. Nothing out of the ordinary. I’m meeting a friend at gym. We do a bodyweight workout, lots of crawling, lots of mobility work and then some kettlebells at the end.

After the workout, we sit in the sauna for 25-minutes. I tell him about my plan to fast for the next 3-days. He asks why. I say it’s for an experiment and tell him about a few things I plan to measure.

I tell him I’ve realised most diseases are diseases of abundance. Heart disease is excess debris in the arteries, Alzheimer’s is excess protein in the brain, Diabetes is excess glucose, Cancer is excess cells. And if excess kills us all eventually, spending some time with less may prevent it.

By this stage, I haven’t researched fasting enough to know if what I’m saying holds its ground. But my friend agrees with me and says it makes sense.

I buy two blood glucose monitors after our workout. I plan on tracking my blood sugar levels throughout and beyond as a longer-term personalized nutrition experiment.

Why blood sugar?

I read The Personalized Diet and learned blood glucose response to food can be an indicator of potential health issues. Too big a spike in blood glucose, from eating the wrong food or eating too much can lead to too much insulin being released. Insulin is a growth hormone. Too much growth leads to excess and as I realised earlier, excess leads to disease.

I won’t be eating for the next three days but I decide to start quantifying my body anyway.

To track everything, I create an Airtable. I log my first entries along with the coffee I had at the cafe. Some would argue black coffee isn’t a true fast. I’d agree but decide it’s allowed for mine.

By the end of the fast, I plan on having a good baseline understanding of what my blood glucose levels are throughout various times of the day.

It’s mid-afternoon. I’ve been reading and digesting the work of Dr. Jason Fung. It could be confirmation bias but his posts on fasting are all making sense (whilst I’m fasting).

I haven’t eaten in 20-hours and I feel fine. I’ve been reading, taking notes and running errands as normal. This is chartered waters for me though. I’ve done 24-hour fasts before.

Dave sends a message through to the Jiu-Jitsu chat.

Training? I reply.

In.

We go to training. I tell him I haven’t eaten since dinner last night. Nearly 24-hours without a calorie. He asks why. I tell him.

Training is phenomenal. It’s one of the best sessions I’ve ever had. Sparring is hard and intense, three 5-minute sessions back-to-back. I notice no lack of performance and even a potential increase in performance since I tapped out a more advanced training partner for the first time ever. It means nothing in the long-run but it’s a boost to the ego in the short-term. A warrior goes to battle hungry.

I take my blood glucose measurements after training and they’re higher than before I started. This is thought-provoking since my intuition figured they’d be down since muscles use glucose during exercise. I turn to Google.

I find a study which involved Type I diabetics and different kinds of exercise. The researchers found glucose levels may increase and continue to increase up to 2-hours after high-intensity interval training versus steady state. The reason being an increased production of glucose during high-intensity training but a lack of the muscles ability to utilise it.¹

I don’t have Type I diabetes but Jiu-Jitsu is very high intensity.

The kitchen is usually the next battleground. My brothers and I get home from training, cook and feast. Not tonight. I know I’m not going to eat. I’m slightly hungry, as usual after training but nothing intolerable. The family have dinner and I watch on with a pinch of salt in water.

I write in my journal.

I don’t miss food as much I as I miss the ritual of food.

I go to bed for the first time I can remember without eating anything since the night before. 26-hours into my first extended fast.

Day 2 (Saturday) — Learning whilst doing

I wake up naturally, go the bathroom and weigh in at 72.4 kg (159.6 lbs). My mouth is dry. Nothing which can’t be fixed with a few cups of water. I record my sleep quality as 5/5 and measure my waking blood glucose. 4.2 mmol/L on meter 1 and 4.0 mmol/L on meter 2.

My Dad and I take the dogs for a walk. I feel fine aside from being a little snappier. Our dogs are on a dual lead. When one decides to smell something and the other doesn’t follow, the lead pulls tight and throws you off balance. This pisses me off more than usual. But I remind myself of where I am and what I’m doing. It’s not something worth being concerned about.

Dad and I go for coffee. I drink black again. I check my blood glucose levels, they remain stable.

It’s been 40-hours without food. Which is perfect timing because I’ve been reading more of Jason Fung and Peter Attia’s work on fasting and body parameters. One of Jason’s articles stands out.

Not eating for 36-hours does not make people more of less hungry than when they started the fast. Whether you eat or don’t, your hunger will be the same. Why? Because if you don’t eat, your body will simply take the food energy (calories) it needs from your stores (body fat). You are, in essence, letting your body ‘eat’ your own body fat. Perfect! Once you open up those stores of body fat, you are not hungry, because your organs have access to all the energy they need. — Dr Jason Fung in Controlling Hunger (Part 1)

I believe it. I’m feeling it. I’m not hungry at all. No cravings. Nothing. My body must be starting to burn fat (entering nutritional ketosis).

Starting the fast my body fat was already low, my abs are visible. Let’s say I’m 10% body fat. At 72.4 kg, this means I’ve got 7.24kg of fat. At 9 calories per gram, that’s 65,000 calories (9 calories x 7240 grams). So I’ve got plenty to spare.

By mid-afternoon, I decide to do some stretching. I get into it and start noticing my joint pain. Or lack of it. I got diagnosed with Ross River Virus at the start of the year and the symptoms are chronic fatigue and achy joints.

There’s no cure except time. When I first got it, I was like an arthritic 97-year-old. I couldn’t walk upstairs, couldn’t open the lid of a sauce bottle, couldn’t wear shoes. A few weeks in, most of the joint pain calmed down. But it stuck around in my right hand and elbow. Doing a pushup was painful.

Except now, I didn’t notice any pain at all. I was moving how I usually would, even trying to aggravate it. Nothing. This was surprising. I told my friend. He said placebo effect. Placebo or not my elbow feels the best it has in months.

Questions arose. Did my joint pain go away because of fasting? Or was it because of the lack of some food I was eating which was causing inflammation?

The whole day I’ve been pissing on ketone strips and never noticed a change off the baseline (0 mmol/L). If my body has truly switched to ketosis, I should see an increase in ketone levels in the blood or urine.

Five trips to the toilet with a container of strips later, I notice a change.

A urine strip showing blood ketone levels of between 1.5 and 4 mmol/L.
An exciting moment. My urine revealing I’m in a state of ketosis.

The bottom tile, ketone levels (top is glucose), moves from a brown colour (neutral) to a pinky-red. Which by my eye means my ketone levels are between 1.5 mmol/L and 4.0 mmol/L.

According to the blood ketone levels required to be in ketosis by Diabetes UK, thanks to fasting, I’m officially in a state of ketosis.

I keep reading about fasting. Another article by Jason Fung which provides evidence for fasting not burning muscle. It makes sense to me. Why would fasting start to burn muscle immediately?

Before food was within a couple of minutes for all of us, if we hunted an animal, had a feast and then couldn’t find another one for a couple of days, what use would we be if we’d burned all our muscle whilst waiting? We’d need it to hunt another animal.

There was also the study on fasting and ghrelin levelsGhrelin is a hormone which triggers hunger. And after fasting for a consecutive number of days, ghrelin levels of participants went down. This again lined up with what I was feeling. My body knew deep down it had to eat at some point but at no stage yet have I been overwhelmingly hungry.

A graph from a study showing how the hunger hormone Ghrelin decreases as fasting goes on.
As fasting time increases, levels of Ghrelin decrease. Source: https://www.ncbi.nlm.nih.gov/pubmed/15522942

After a couple of hours of programming study and responding to emails (note: the study and email responding was done with a state of mental clarity I don’t usually achieve, another tidbit), it hits 4:00 pm.

4:00 pm is a trigger in my brain to eat an apple covered in almond butter as fuel for training at 6:00 pm.

Since I’m not eating, I break the habit with kettlebells. The first 5-minutes are hell. My previous blood glucose measurements are the lowest they’ve been, 3.4 mmol/L and 2.8 mmol/L. But once the blood starts flowing, I feel much better.

Two blood glucose meters, one showing 3.4 mmol/L and the other showing 2.8 mmol/L.
Day 2, 4:48 pm, the lowest levels my blood glucose levels have been so far.

The workout finishes and my blood glucose levels are up again, 4.8 mmol/L and 4.2 mmol/L. I still don’t quite understand why but it’s cool to see the changes in real-time.

I’m downstairs but it hits me. I run upstairs and it gets stronger as I go.

Closer.

Stronger.

I see it. Mum has baked brownies. Steam is coming off them. I speak. They smell amazing. She speaks.

I just pulled them out of the oven. I say I can’t have any. She says bad luck. I laugh.

I tell her my sense of smell has increased 10-fold. Next door is cooking dinner. Some kind of stir fry.

The rest of the family have dinner, I stay downstairs and do some writing, measure my blood glucose and record how I’m feeling. I find an app to track fasting, Zero. I enter my details. 49-hours without food. More than two days. Other than missing out on mealtime, I feel great.

Tracking an extended fast with the Zero fasting app. The app shows 49-hours and 23 minutes spent fasting.
Tracking an extended fast with the Zero fasting app. The app shows 49-hours and 23 minutes spent fasting.

I go to bed with a new superpower. Super smell.

Day 3 (Sunday) — Intermittent sleeping

6:45 am wide awake.

I woke up three times last night. The first time I asked Alexa what the time was. 2:17 am. I’m a sleep fiend. It had been a while since I woke up in the middle of the night. The other two times were about an hour apart.

Each time I felt alert enough to get out of bed, even though I’d only slept a few hours. No hunger but I still had a deep-down feeling. Something inside was starting to signal food might be a good idea soon.

When I get out of bed I’m more dehydrated than usual. I drink a litre of water with a pinch of salt. It doesn’t feel like enough so I drink some more.

I take a blood glucose sample, 3.3 mmol/L and 2.8 mmol/L, the lowest yet. And I feel it. This morning I’m moving slow.

I’m meet a friend at gym. The same one as Day 1. He’s coming off a 12-hour night shift straight there, I’m coming off 6-hours broken sleep and 60-hours without food. Walking through the shopping mall I smell everything. It’s Sunday morning. There’s bacon. Coffee. Bread toast breakfast eggs. I smell it all.

We laugh when we see each other. He speaks. How you feeling? Slow. Me too.

We spend the next 40-minutes stretching and doing bending exercises, once the blood starts flowing I feel great. Movement is medicine. Sitting in the sauna afterwards I put another pinch of salt in my water bottle. I know I’m losing water faster than usual and the salt helps retain it.

I tell my friend I’ve been thinking about extending the fast and going to 4-days. He says why not finish today as a trial and do 4-days next time. I don’t argue with him.

After gym, I buy a blood ketone meter and use it. 3.6 mmol/L. I’m officially in nutritional ketosis. I wish I had this from the start, it would’ve been cool to see how my ketone levels increased as I started getting deeper into ketosis.

My first official blood ketone measurement with a lifesmart meter, it shows 3.6 mmol/L.
Day 3, 12:42 pm, my first blood ketone reading.

On Sunday’s I edit videos. I set a timer for 2-hours to take another measurement and start editing.

5-hours later the video is done. Usually I’d eat as a reward for finishing a video. I don’t. I’m not hungry but I do miss the ritual.

I replace the ritual with a small movement session and afternoon walk.

It’s 5:00 pm and the sun is setting. I haven’t eaten in 3-days but I feel full. I look at the water, smile at people walking by, watch the sun sinking. I can feel an aurora of balance. This fast has been physical, mental and spiritual. Some things you can’t explain. But it’s there. I decide to do an extra day.

Mum has made dinner. I tell her I’m going an extra day. She asks me if I’m alright, I say yes, I feel amazing. I tell her I really do feel amazing.

I explain some of the things I’ve learned over the past few days to her. My body is helping me. The hunger hormone, ghrelin tapers off over a few days, I’m in ketosis now so my body is using its fat stores for energy. She listens, says ok, as long as you’re alright.

I cross the 3-day mark. 72-hours without food.

Tracking an extended fast with the Zero fasting app. The app shows 72-hours and 20 minutes spent fasting.
Officially crossing the 3-day mark. 3-days was the goal from the start but by late afternoon of day 3, I decided to do 4.

Before bed, I take a glucose measurement, 3.6 mmol/L and 3.3 mmol/L. And a ketone measurement, 3.4 mmol/L. Sleep.

Day 4 (Monday) — Business as usual

8:30 am. I’ve been awake for half an hour reading in bed sipping water. No wake-ups last night but I woke up feeling as if my sleep wasn’t ideal. I rate it 2/5 in my journal.

My glucose levels are the lowest they’ve been so far, 3.0 mmol/L on meter 1 and 2.7 mmol/L on meter 2. Ketones are at 3.5 mmol/L.

Two blood glucose meters, one showing 3.0 mmol/L and the other 2.7 mmol/L, one ketone meter showing 3.5 mmol/L.
Day 4, 8:54 am, still in ketosis (3.5 mmol/L) and my lowest blood glucose readings so far.

I sip salted water in the sun and then go to the cafe and order black coffee and plan out the rest of the day. It’s Monday and I’ve got some work to do.

I get what I need to get done done. My concentration levels are at an all-time high. It’s like my body realises its got nothing else to do except work. The energy I would’ve used hunting for food gets used mashing letters into a keyboard, writing code, writing words.

I’m getting excited. I’ve said to myself I’ll break the fast after Jiu-Jitsu training tonight. I’m not hungry and have no cravings but I’m ready for food again. I’m ready to be rewarded after a hard session.

5:00 pm hits and I realise I’ve been sitting in a chair in front of a screen for 6-hours straight without moving. Not acceptable. I take a ketone measurement, 5.7 mmol/L, the highest yet. Maybe that’s where the concentration and unbroken stream of work came from. I get up put my mat out front and move around in the sunset. Stretching, yoga, bending, some squats.

I go to Jiu-Jitsu training. I’m excited. I know I’m going to eat after training. The ritual will return.

Dave’s there. I tell him I haven’t eaten in four days. He asks me how I’m feeling. I say fine. Ready to fight. He laughs. We go through the motions practice takedowns. When it’s time to roll, I taunt him, remind him I’ve had no food in four days, he pats hits stomach says I’ve had plenty. We laugh. Get into it.

The roll goes as usual. Hard. Two egos battling it out. No one gets a tap. Dave would’ve won on points though. I last the distance even after 4-days without food. We complement each other afterwards say it was a good roll. The only thing missing was my high-end explosive energy. Dave says he didn’t notice anything different.

I get home with the biggest smile on my face. I know what’s coming. It’s time. I’m still not hungry but it’s time. I consider extending it another day. I don’t. It’s time.

One of my brothers has cooked steaks. Thank you, Will. It’s ready to go. I could eat it right now. But I don’t. I have a shower, take it slow. It’s been 4-days, I’m not in a rush.

The counter has ticked over. 96-hours without food. I’m not in a rush.

Screenshot of the Zero fasting app showing a time counter of 96 hours and 22 minutes, my four day fast is complete.
The Zero timer officially ticking over. My first extended fast complete.

The family gathers around the table. I prepare vegetables to go with the steak. Mix up a fresh tub of my favourite food. My favourite food I eat every day. My favourite food I haven’t had in four days. Almond butter. I mix the oil into the crushed almonds watch it create a beautiful brown creamy goodness. My favourite food.

I put the food on the plate, steak, greens, olives, pickles, cauliflower, almond butter, lots of almond butter. I plan on staying in ketosis for the next week so I make sure there’s plenty of fat on the plate.

My first meal to break the fast. Steak, greens, olives, cauliflower, pickles and my favourite food, almond butter.
My first meal in four days. Although I’m not hungry, I’m ready to break to my fast.

I sit down. Look at my food. Smile. Look at my family. Smile. Mum asks if I’m excited. I say yes. I tell them what I felt this afternoon. I still can’t explain it. I feel great. Connected with the world. Excited to sit around and share a meal with loved ones. This is what I’ve missed.

The first bite reignites my desire for food. It’s not overwhelming but it’s back. Not hungry the entire time but the minute food hits my mouth, I start thinking about what else I could eat.

I take my blood glucose after dinner. 6.0 mmol/L and 5.4 mmol/L, slightly higher than what they were after training. Double what they were this morning. This makes sense. I’ve just eaten my first meal in four days. Maybe it was too high in protein or carbohydrates. I hope it doesn’t kick me out of ketosis.

I go to bed proud. Excited for food the next day.

Takeaways and findings

Tracking blood glucose and ketones every couple of hours is tedious. The hardware could be improved. There were times I forgot and times where I didn’t take a measurement because I was doing something else.

I’m not stopping though. I treated the fast as the start of a longer-term personalised nutrition experiment. My goal is to get data behind how different foods and activities influence my different body parameters.

Below is an insight into how my body parameters changed over the 4-day fast.

Blood glucose (mmol/L)

I used two meters to compare different values. One of them was consistently lower than the other. The consistency of one being lower than the other was a good thing. If they alternated between one being higher or lower, the accuracy of each would be in question. My blood glucose levels were usually lowest in the morning and generally increased with physical activity. The more intense the activity, the heavier the increase in blood glucose.

Graph showing my blood glucose changes over the four day fast measured with the Accu-Chek guide.
Blood glucose measurements (mmol/L) with the Accu-Chek guide blood glucose meter, coloured dots indicate a measurement taken after physical activity.
Graph showing my blood glucose changes measured over the four day fast, measured with the Contour Next blood glucose meter.
Blood glucose measurements (mmol/L) with the Contour Next blood glucose meter, coloured dots indicate a measurement taken after physical activity.

Blood ketones (mmol/L)

I didn’t get ketone measurements until day 3. By then I was already in ketosis. But by the urine strips (I didn’t record these), my ketone levels were flatlining up until just before this time anyway.

Graph showing my blood ketone measurements over day 2 and day 3 of the fast.
My blood ketone levels (mmol/L) throughout day 3 and 4 of the fast.

Weight changes

My weight didn’t change as much as I expected. It even went up on Day 3. This is probably because it was the morning I drank 2-litres of water after waking.

Graph showing weight changes during my four day fast.
My weight changes during the 4-day fast. I finished 2kg lighter than when I started, from my eyes, most of this seems like water weight.

Numbers at a glance

It was interesting to see how the two blood sugar meters differed. Meter 2 was consistently lower than meter 1 but they both finished with the same standard deviation (amount of variation in measurements). Note how my hunger level stayed an average of just over 1/5.

Mean, min, max, standard deviation of my blood glucose, weight, blood ketones, hunger levels and energy levels throughout.
Mean, min, max values of some of the different parameters I was tracking during my first extended fast.

Progress pictures

These aren’t perfect. I took them as I was recording a clip for the video version of this article each morning. Each has slightly different lighting and were done at slightly different times. Day 5 also uses an iPhone instead of my camera.

Progress photos in mirror showing changes from day 1 to day 5, after the fast. Day 5 taken with different camera.
Photos I took every morning. Should be taken with a grain of salt since not every photo is the same lighting or exact same time. Day 5 is the morning after my first meal.

Extras

Despite the trackable parameters, I learned eating habits kick in even when you’re not eating. My hunger increased (not anything dramatic) around mealtimes like clockwork. Remember the 4:00 pm apple and almond butter? Every day at 4:00 pm a switch went off reminding me of this.

As for the health benefits of fasting, it’s hard to tell if I caused any immediately. But what I did learn by reading the works of Jason Fung, Peter Attia and various others, eating less often with periods of extended fasting here and there is likely beneficial.

And this makes sense. If you’re eating constantly, you’re going to be triggering insulin releases. Insulin leads to growth, more growth leads to a higher chance of unnecessary growth, unnecessary growth leads to a higher chance of developing unwanted diseases.

What would I change?

  • Water only — If you count the black coffee, I didn’t really do a true water-only fast. This is something I’d change for next time. Water and electrolytes only.
  • Low carbohydrates beforehand — The hardest part of the fast was when I was switching from burning glucose to burning fat. It was nothing unbearable but eating low carbohydrates or being in ketosis a week beforehand would probably rule out almost all discomfort.
  • Ketone measurements from the start — This ties into the previous point. It would have been helpful (and cool) to know my blood ketones from the start and all the way through to see how they changed.
  • Measurements — My method of taking measurements is still imperfect. In an ideal world, everything would be at the same time each day, weight, blood glucose, blood ketones. But most of the time reality isn’t ideal. I’m currently looking into a continuous blood glucose monitor to help fix this.

What next?

This was an experiment to learn more. And I did. I learn best when I’m in the middle of something.

I don’t know why I was scared to begin. There was never a single time where I felt any discomfort higher than a 4/10. And after getting into ketosis, I achieved states of concentration like I’ve never felt before.

I plan on doing more extended fasts at least once every 6–12 months. I’ve been playing with the timeline of 3-days every quarter and a 5-day once per year.

I’m going to read Jason Fung’s book, The Obesity Code, in the near future. I used to be the person who ate six times per day and tracked every macronutrient they ate to maintain a healthy weight level. Now I’m starting to realise doing so was taking my hormones on a rollercoaster ride. I could’ve done away with that and eaten less often instead.

If you’re thinking about doing an extended fast, try it out. Go from dinner to dinner to start with. If things go bad, you can always eat.*


*This is not medical advice, consult your doctor before doing anything drastic.

You can find a video version of this article on YouTube.

¹ Effect of intermittent high-intensity compared with continuous moderate exercise on glucose production and utilization in individuals with type 1 diabetes.

² Fasting unmasks a strong inverse association between ghrelin and cortisol in serum: studies in obese and normal-weight subjects.