Fuel Metabolism in Prolonged Fasting – What Happens when You Stop Eating

Fuel Metabolism in Prolonged Fasting - What Happens when You Stop Eating - 1


Much controversy/mysticism has gathered around the idea of fasting, especially for prolonged fasting. Folks seem to regurgitate the same obsolete messages that have been promoted by fitness paranoids for decades (i.e. fasting makes you lose lean mass). Yet, the science is still there. It has always been. But it’s not been available to the ignorant eye.

George Cahill was one of the leading fasting/starvation researchers. His experiments have been widely admired and appreciated in the literature. I’m one of his fans. In the following lines you’re about to read an excerpt from my book on Periodic Fasting. It’s about energy metabolism as you stop consuming food.

From the book (Chapter 4):

So, for one to start efficiently doing prolonged fasting and intermittent fasting, I will briefly talk about their physiologic implications (in simple terms) as well as some strategies to reduce the first negative symptoms that may appear. This will serve as a basis to understand the upcoming chapters, where I will go deeper into the molecular mechanisms that occur as one stops consuming anything but water for at least 18 hours. Intermittent fasting can be so much fun, especially when you add different rewarding mechanisms to it (such as black coffee unsweetened or sweetened with some stevia, tea, and others).

Prolonged fasting – Some Basic, very important, Information

The scientific literature has mostly focused, so far, on prolonged fasting. The possible benefits of this intervention can be:

– detoxification, as the body shifts to healing and repair instead of digestion, which is a great consumer of metabolic energy
– curing or alleviating acute illnesses
– curing or alleviating chronic illnesses
– rejuvenation and/or attempts to increase life span
– etc.

From my personal perspective, the benefits of prolonged fasting can be experienced after a person stops consuming food for more than 72 hours, given they have to deplete glycogen stores (given they have a background of moderate or high-carbohydrate consumption), and more than 46-48 hours for people coming from a very-low-carb and/or ketogenic nutritional background.

For people wanting to detoxify (to a certain extent), cleanse, and/or rejuvenate their body, 3-10 days water-only fast can be optimal. I would not find it necessary to go beyond this unless a serious medical condition implies such strategy; and if one attempts doing prolonged fasting for more than that, they should be under medical supervision. Even for the less experienced folks, a 3-10 days water-only fast should not occur without supervision.

After all, complications may appear as I assume that the starting point for most wanna-be-fasters is not an optimal state of health. Plus, from a psychological perspective, I would rather shoot for 3-5 days fasts and once I reach the target I can decide if I want to go further and expand the fast to 7-10 days. Many success stories have used this technique because the mental burden (“OMG, I’m not gonna eat for 10 days“) is reduced. And it may be more challenging for folks using a high-carb template; but it’s far from unattainable.

When I considered doing my first prolonged fasting, I initially proposed it to last 50+ hours. I knew it was very attainable, especially because I was far from feeling hungry with my many previous 24+ hours shorter fasts, and possibly because of my keto-adaptation status. At that time I was consuming a well-formulated-strict-ketogenic-diet (daily) for more than 6 months.

However, 50 hours turned to 5 days. And I would have gone even longer, but I was quite lean and I did not want to lose more body fat. I suspect, from my research and from analyzing my physical appearance, that muscle catabolism was close to minimum because I did not have to go through glycogen depletion; and gluconeogenesis and muscle catabolism may have been significantly reduced (as described by Cahill [66]). I was already in efficient ketosis. Plus, I exercised with heavy weights 3 days out of the 5.

Assuming that a person consuming a high-carbohydrate template attempts to do prolonged fasting for 3-10 days or more, I will describe the adaptation phases as pointed out by George Cahill and other researchers.

In phase I (first couple of hours after the last meal), the body fuel is glucose, largely from exogenous sources. Glucose is burned at ~40g/h. From 4-16 hours (phase II), glucose is burned at ~7-8g/h and most of it comes from hepatic glycogen. At this point the body has already started to increase the production of glucose in the liver through the process of gluconeogenesis.

Since no food is provided, gluconeogenesis substrates come from muscle catabolism, recycled pyruvate and lactate, glycerol (not so efficiently, yet), and to a certain extent from

BOHB -> AcAc -> Acetone -> Propanediol -> Pyruvate -> Glucose [66].

Gluconeogenesis (GNG) increases until the end of phase III, where glycogen stores are depleted. After phase III, GNG decreases significantly and glucose is used at a rate of 3.5-4 g/h. This is where the body starts efficiently burning fat and ketones, while holding tight of the muscle mass [66].

Fuel Metabolism in Prolonged Fasting - What Happens when You Stop Eating

During the first three phases is when the average person would find their most difficult time to cope with. As the body’s sugar levels drop and as the metabolism switches to the fat burning mode, the panicking brain screams for the fuel it’s been widely used to (especially in the first 0-16 hours).

Since this is the most crucial time, I’d rather recommend one to start a prolonged fasting experiment consuming the last meal sometime in the evening, 3-4 hours before going to bed. Assuming an 8 hour sleep, the subject will be ~12 hours into fasting by wake-up time. There still remains a few more painful hours with respect to hunger.

Serving some black coffee, tea, and a ton of water will definitely help. After 24-30 hours most of the (false) hunger symptoms are gone, glycogen is depleted, GNG is still undergoing but at a lower pace, insulin secretion is low and the body can tap into using fats and ketones efficiently. Occupying oneself with work around the house or other errands can serve as means of distraction from hunger.

Some of the possible negative symptoms as these adaptations occur can be: increased hunger, light-headedness, fatigue (increased fatigue in many folks), higher irritability, headaches, higher acidity in the stomach (much reduced after a couple of hours), drowsiness, and others. If the faster has a fairly decent health condition, most of these symptoms will be reduced. However, they are uncomfortable and they are the reason why many people will not push through, to reach to the promised land :).

Many of them may not be present if the faster comes from a very-low-carb or ketogenic nutritional background. I, for example, have not felt any hunger and/or the rest of the symptoms when I did my 5-day water-only fast. Though I had a headache by the end of the second day, it was gone when I woke up the third day.

I would recommend increasing the consumption of water (purified) during the first day, and the consumption of decent amounts of water in the rest of the days. The kidneys may excrete minerals as the adaptation undergoes and this can be the reason for some of the negative symptoms.

To mitigate this, as well as to fuel the reward mechanism in the brain, one can use a small vegetable bouillon cube (6-10g) dissolved in some hot water. It’s ~10-15 kcals and cannot be considered to breaking the fast as it contains very little nutritional value.

Coffee consumption is permitted (from my perspective), but it should be black and unsweetened or sweetened with a tiny stevia tablet, which contains 0-1 kcals. It may help as the blood pressure and pulse go down, as seen in many fasting experiments.

In my case, I also used 1 multivitamin-multimineral pill/day because I was not supervised and I wanted to know I have an additional measure of precaution. But, nothing else; no diet soda and no workout supplements, no oil/butter enriched coffee, no nothing.

In my future prolonged fasting experiments I will most likely use coffee and tea as companions and also as rewarding mechanisms, strategically. For example, in the second day of the fast I could have a cup of coffee in the morning and another one in late afternoon, and a nice big cup of tea in the evening. This will most likely help me.

I would not recommend exercising if one feels the negative symptoms I mentioned above. And I would strongly discourage exercising for the first 2 days if one comes from a high-carb nutritional background. Thereafter, one could start exercising. I will provide more details on workouts and I will add more to the prolonged fasting strategy as we progress through the book.

To help the detoxification and rejuvenation processes, one can adopt different strategies, as we have seen in prior examples. Prolonged fasters can use coffee enemas (may be a bit uncomfortable) to efficiently eliminate waste products from the bowels. The details of a good protocol are provided by Amanda Brocket [94].

Daily cold showers and/or alternate hot/cold showers can also help tremendously. Cold showers could last for 5-10 minutes, depending on the level of cold adaptation of the experimenter, while hot/cold showers may last for up to 15 minutes. In hot/cold showers, one can alternate hot/cold water in a ratio of 2:1 (20 seconds cold, 10 seconds hot) or they can use hot water for the first 5 minutes and cold water for the remaining 5-10 minutes. Anyway you do it, please make sure your body can tolerate such stress. Consult your personal health care practitioner first.

Luigi Cornaro, the fellow who subjected himself to life-long calorie restriction, Upton Sinclair and many others also made sure they got enough fresh air and sun exposure every day. Some of them slept with their windows wide-open at night, even in the winter time.

Do know that if your body has to increase thermogenesis (heat generation) to preserve its homeostatic core temperature, you may burn more energy in terms of kcals. In such situations it may be uncomfortable due to the mild or increased shivering thermogenesis, but once you get use to it, you know your internal engine can run smoother and cooler, with less inflammation, for a longer period of time.

If you have to stay indoor for some reason, make sure you get some fresh air and direct exposure to sun light (predominantly in the morning) by opening the windows. It will not help if you stay by the window and the window is closed because beneficial UVB (Ultraviolet B) radiation does not seem to efficiently penetrate glass [95]. Sunlight is critical for many metabolic processes as it allows the production of vitamin D3 (which is considered a hormone); this does not mean it should be abused.


If there’s anything you’re concerned about, please leave your thoughts in either of the comment sections below. If you want to read more from the book, go here.


  1. Cahill Jr, G. F. (2006). Fuel metabolism in starvation. Annu. Rev. Nutr., 26, 1-22.
  1. Brocket, A. (2012). 10 Reasons Why You Should Try a Coffee Enema.
  1. Mead, M. N. (2008). Benefits of sunlight: a bright spot for human health. Environmental health perspectives, 116(4), A160.

Photos: here

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2 Responses to Fuel Metabolism in Prolonged Fasting – What Happens when You Stop Eating

  1. Valerie says:


    Maybe you can help me with my touble with fasting. I did a 3-week fast a few years ago, and I encountered two main issues that to this day make me dread fasting again:

    1- Hunger: I did not stop being hungry after 2 or 3 days, as everyone promises. If anything, my level of hunger worsened after the first few days (it went from “waves” of hunger around usual meal times, to super hungry all the time). After persistant reading about the issue, it seems that fasting gurus know and reluctantly admit that not everyone is magically liberated from hunger during a fast, but I couldn’t find any explanation or useful advice about it. Do you know what could be responsible for unrelenting hunger, or what to do about it? (By the way, I was plenty fat enough when I fasted, so I rule out the possibility of true starvation.)

    2- Obsession with food: I kept thinking about food 24/7. Distraction quickly proved futile. I attributed it to the hunger at the time, but it seems that even people who are not hungry at all during a fast can experience obsession with food. Did you face that problem? Do you have any advice on how to minimise it?

    Thanks for the realistic write-up on fasting. It often comes with lots of mystical pseudo-science, but your piece is refreshingly down to earth.

    • Chris Chris says:

      Valerie, I cannot tell why you kept feeling hunger post glycogen depletion. I’d suspect a somewhat inefficient ketone/fatty acid metabolism; you may know better if you do a genetic test with 23andme. Not all people can fast efficiently. There’s a mutation in a gene coding for an enzyme (CPT1a) that makes people being stuck in a glucose based metabolism. And there are many other SNPs that could be the culprit for poor fatty acid/ketone oxidation.

      Hunger thoughts could be attributed to the same physiologic mechanism. Although, psychology may play a factor in it as well. I personally did not obsess with food during my 5 day fast and my hunger was absent. I had total lack of interest in food. However, every now and then, I felt like something was missing (we’re so addicted to food).

      Once again, instead of blind guessing and listening to gurus, a genetic test may shed some light on your situation (we’re all genetically, epigenetically, and metabolically different). It’s about $99 I think (the 23andme one). Hope this helps.

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