Body 💪

The 72 Hour Fast : The Scientifically Proven Way to Boost Your Energy by Eating Less

I wanted to test how much energy I’d have without eating: how would my body react when if I stopped consuming food?

In this article I'll tell you about my experience with fasting for 72 hours and everything you need to know about fasting. I'll talk about the change in my metabolism, and go over important body changes like glycolysis and ketogenesis.

What's in it for you?

Well, you'll get a detailed roadmap from my experience and you'll have all you need to know to see the benefits for yourself.

Experiments just like this one will help you gain a better understanding of your body, your mind and your lifestyle. You get to know your limits, so that you can transcend them. There are times when you discover there aren’t any.

"Note: if you need any tips and have any question regarding the keto diet, 👉 hit me up on Messenger and I'll help you out (I respond pretty quickly)!"

Why Fasting?

Nutrition and therapy researchers such as Dominic d’Agostino recommend 5-day long fasts two to three times per year to reboot your immune system due to stem cell-based regeneration. As your body has no more incoming glucose (from carbs), it needs to break down your body fat and create ketones, an alternative form of energy.

Even though ketones are often portrayed in a negative light and associated with diabetes, it is actually a natural and harmless substance produced by your body.

Running on ketones instead of glucose has many benefits, including purging your body of cancerous cells, which are unable to process ketones (according to Agostino’s research). Those benefits can be  extended through ketogenic diets in order to remain in the state of ketosis (More on Agostino’s talk on how ketosis can kill cancer).

Cancerous cells started dying out when put on a ketogenic diet (see research)

On Ketosis

You are in a state of light ketosis when your blood level of beta-hydroxybutyrate (BHB), a major ketone, exceeds 0.5 mmol.

Increased positive effects (explained below) are often felt at 1 mmol, and deep ketosis starts at around 3 to 5 mmol. You can measure your ketosis level with ketone strips (available in most pharmacies such as the Ketostix ones) or more advanced products such as the Precision Xtra by Abbott (recommended by Tim Ferriss).

After two days, I was at around 1.5 mmol, and ended up at 4 mmol on the last day

The Experiment

During the fast, I allowed myself water, coffee and tea (without sugar) but no sweet or juicy beverages. No snacks either. Why? Consuming carbohydrates will reactivate your glycolytic pathway, dampening the effects of ketosis.

You can reward yourself with exogenous ketones or fat, such as an iced coffee with coconut oil in the evening. And don’t worry if you accidentally added sugar in your coffee or took a bite of a cake without realizing you’re fasting - but remind yourself that the goal is to change your primary energy molecule from glucose to ketone--which in this case is through eliminating carb intake.

While conducting the experiment, I continued my work and personal life without a change. I did not work less, nor did I rest more.

On my 72h, I did not have any desire to eat, probably because being on ketosis (or “starvation mode”) reduces your appetite. I thought that having people eating around me would give me a hard time, but it was the opposite - I enjoyed smelling the food, and it was even a pleasure to cook for friends.

You quickly realize that food is dispensable, at least for as long as your fast lasts.

Three days of fasting makes you feel like this dude: warm, energetic, and fooling around without food.

Key Benefits of Fasting

Contrary to popular beliefs of negative effects due to fasting, your body changes are more likely to result in the following experience:

  • High energy: I experienced high physical energy throughout the day compared to no-fasting days. That means that I didn’t experience times of low energy feelings such as after digesting. Energy and wakefulness remained high energy until very late at night. One explanation is that BHB, a major ketone, is a more efficient energy resource than glucose, providing more energy per unit oxygen used. Another explanation is that ketosis increases your mitochondrial function, which is responsible for intaking food and oxygen and transforming it into energy. It does so by increasing mitochondrial glutathione, an important antioxidant which works directly within the mitochondria.

  • Moreover, being on ketosis reduces the amount of oxidative damage caused by reducing the production of free radicals, which are formed when oxygen interacts with certain molecules in the body and which can damage our mitochondria. Lastly, ketones don’t have to rely on insulin to get the ketones into the cells, which is the case for glucose consumption, and can thus be processed more quickly.

  • Feeling warm: Although my body did not consume food, it created a lot of energy through fat consumption and I felt warm throughout the whole fast. Especially on my the third day, I could only walk in t-shirt and open coat at 4 degrees celsius (40° Fahrenheit). It felt so good!

  • Longer breath holding: I tested it every night, including after the last meal, after ten minutes of meditation and just before going to sleep (pulse was steady). Simply inhale once fully and retain the air. The first night, while still using glycolysis,  I could hold my breath for 130 seconds (just over 2 mins). Two days after, I could raise it up to 210s when my body was running on ketones.  One explanation for that is ketogenesis, in comparison to glycolysis, does not create carbon dioxide as a byproduct, which is responsible for our perceived need to breathe. Another explanation can be that BHB is a more efficient energy resource and therefore needs less oxygen per unit to run the body. Nevertheless, the latter needs to be confirmed by further studies.

  • Productivity in peaks: Although I did not quantifiably measure my productivity, I had the feeling that my productivity wasn’t stable and can be categorized in ups and downs compared to when I was eating. Studies suggest that fasting results in increased production of brain-derived neurotrophic factor (BDNF), which increases the resistance of neurons in the brain to dysfunction and degeneration. Nevertheless, I couldn’t say that I had a tremendous increase in concentration - I suggest you to keep track of your productivity level when fasting.

  • No appetite: The most surprising thing is that I didn’t feel hungry. This was normal because hunger is triggered by neutral signals with high concentrations of ghrelin, a hormone not released during ketosis. Be careful though, as there is a discrepancy between what your body needs and what it wants. Things like cravings for example, are usually unhealthy and not needed by the body.

Negative Side-effects of Fasting

The only downsides I experienced where the following:

  • Problems falling asleep: Very high energy until late at night led to problems falling asleep (up to 90 minutes). After 30 minutes of lying in bed, I decided to read for another half an hour to adapt to my new body clock and let the sleep come over me. One explanation can be that carbohydrates improve the entry of the amino acid L-tryptophan into the brain, which contributes to the creation of serotonin, a monoamine neurotransmitter which among others calms the body and helps you sleep. As I cut off carbs, my source of L-tryptophan was reduced, and can thus explain troubles falling asleep.

  • Sweating more: The downside of feeling warm is that you sweat more, at least that was the case for the third day for the region around my axillas, although I was only in t-shirt and not practising any strong physical activity. This is probably due to the fact that sweating, just like urinating and breathing, is a major method on getting rid of excess ketones. It is therefore important to maintain a high level of hydration.

  • Times of low focus: You might have times of low focus, as it seemed to occur to me. This can occur at the beginning of the fast, when ketones aren’t yet present to process an excess of glutamate into GABA molecules, which are necessary to reduce stimulation,  Nevertheless, everyone reacts differently, and measuring your mental focus should be on your list when fasting.

Fasting (especially for periods such as 20 or 30 days) can also result in other problems:

  • Weight loss: As your body consumes your own stored fat resources while on ketosis, you might lose between 1-2 pounds (up to 1kg) per day during your first days. This will nevertheless vary on your metabolism and body composition.

  • Muscle loss: As your body consumes your body fat to create ketones, it might also start using your muscles to cope with your lack of protein consumption. Studies have shown that while your body will mostly consume fat, muscle destruction can occur during long-term fasting.

  • Detox symptoms: Another potential downside of long-term fasting is the rate of detox. Fat is your body’s main place of storage, including for any malignous toxins. When consuming your fat reserves, those toxins are processed through your bloodstream and it can lead to feeling sick or nauseous.

  • Potential side effects might also include headaches, dizziness, lightheadedness, fatigue, low blood pressure and abnormal heart rhythms. But I have personally never experienced any of them. You might feel these symptoms mildly during the first 24h and then it gets much much better.

  • There are claims that long-term ketosis (not short-term fasting) can negatively affect some organs such as the liver and kidneys as they are less used than while running on glucose.

Breaking the fast with a big meal put me straight back to sleep!

Fast Like a Pro - My Fasting Tips:

Don’t think about food: the less think about eating, the less you’ll need to eat!

Water is essential: not drinking water while fasting gave me headaches, dry throat, and inability to focus. Make sure you’re hydrated!

Don’t cheat: eating carbs will not make the experience worth it, but you can eat exogenous ketones and fat. I rewarded myself with an iced coffee with coconut oil in the evening.

Define what you will track beforehand: are you fasting to feel more energized, work out better, feel warmer, lose weight, and what will be your key performance indicator? Use the tracking spreadsheet - it will help you track the right KPIs.

Measure your BHB level: buy yourself ketone strips or designed devices to measure your ketone blood level and correlate effects.

Right timing: Your body fasts naturally while asleep, so having your last meal before sleeping is the easiest way to start fasting, and feel free to sleep in as long as you want and let the body do the work. Starting your fast with a final low-carb meal is a great to get going--I recommend starting your fast after dinner, as you will go through three days of fasting and will be able to break the fast on the third night.

Don’t break your fast with a heavy meal: It won’t harm you, but will put you straight back to sleep - at least that’s what it did to me ;)

Your Turn

I now challenge you to fast and test your own body reaction!

For a specific period of time - whether 24h, 48h or 72h - try it out. Test your productivity level, your desire to breathe, your desire to eat, your energy level and your emotional mood.

  1. Set a date to commit to it : from when to when will you do your fast ?
  2. Tell your friends in order to have accountability. If you want to take it even further, do it together.
  3. Track your experience with the tracking spreadsheet. Also note down specific events such as exercise, amount of drinking, or stressful situations that might affect your KPIs.
  4. Capture your experience afterwards : what did you learn from your experience ?

Need help or tips?

👉Write me on Messenger and tell me what you're up, I'd love to help


This is not medical advice. Although I did base my fast on studies and had a scientific approach, your body might react differently, so be cautious -do not fast if you are pregnant, sick, psychologically stressed, having eating disorders, just adopted a new diet, or have diseases that might jeopardize your health even more.

Gui Perdrix

You can find Gui dancing in the streets of Buenos Aires, sipping coffee in Paris, recording raps in underground studios, and at major real estate events, where he talks about communal living and the need for community facilitation.

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