The Ultimate Guide to Intermittent Fasting III – What Happens When You Fast?


What Happens When You Fast?

Interested in Intermittent Fasting? Grab my book on it here: Feast.Fast.Fit. – Train Your Body to Torch Fat, Build Muscle, and Never Diet Again


In case you missed Part 1 or Part 2 of this series:

The Ultimate Guide To Intermittent Fasting I – Feast vs. Famine

The Ultimate Guide To Intermittent Fasting II – Not All Fasts Are Created Equal


What Happens When You Fast? So what if you woke up one morning and didn’t immediately slam a protein shake or some lucky charms? What if you extended your “fast”? In short, you’d burn even more fat.

Assuming you aren’t eating during your sleep, your insulin levels should fall below baseline during the night. Have you ever heard people say you burn fat during your sleep? Well, technically you do. Before you shut your computer and run to take a nap in hopes of losing some fat, keep reading.

You have likely heard of insulin before, alongside carbohydrates or possibly diabetes. Unfortunately for insulin, it’s a very misunderstood and demonized hormone. I’m going to break it down very briefly before I continue.



  • Insulin is an anabolic hormone
  • Controls intermediary metabolism [1]
  • Most known for lowering blood glucose
  • Lowers blood glucose by increasing glucose uptake into fat + muscle cells through GLUT-4 (I will cover this later)
  • Insulin is “mitogenic” – this basically causes cells to multiply. This can be good or bad (growing muscle vs. growing tumors) [1]
  • Increases – glycogen synthesis, fatty acid synthesis, protein synthesis [1]
  • Decreases – proteolysis (protein breakdown), gluconeogenesis, lipolysis, glycogen breakdown [1]


As you can see insulin plays a pretty major role in the human body and it’s incorrect to label it good or bad. Instead, there are times when you want the presence of insulin and times when you don’t. When you eat your favorite candy bar, you want insulin to dispose of the excess glucose in your bloodstream. When you want your body to release fatty acids for energy usage, insulin gets in the way. As with all things in life (except Chipotle), moderation is key.

Insulin also helps to determine the fuel the body will use, this is why you may see it referred to as our “fuel selector switch.”  When insulin levels are low, the body shifts to using stored energy for fuel. Fat becomes the main energy source. High levels of insulin on the other hand, lead to the body burning more glucose or carbohydrates.

When fat loss is your goal, you want and need some stretches where insulin is low.

When muscle gain is the goal, insulin and its anabolic properties are your friends.

Finding how to blend periods of both is the key to body recomposition.


Insulin Sensitivity vs. Resistance

I’m sure you see insulin sensitivity and insulin resistance thrown around in pretty much every fitness article that discusses intermittent fasting or carbohydrate intake. I could post some fancy pub-med definitions but here’s is the easiest way to think about it.

Insulin sensitivity – Your cells are sensitive/receptive to insulin and therefore require less total insulin to get their normal job done (this is good).Insulin resistance

Insulin resistance – Your cells have become resistant to insulin which requires more and more to be released (this is not ideal) in an effort to lower blood glucose.


You may have heard people recommend that you eat carbohydrates in the morning because this is when the body is most insulin sensitive. While it sounds nice, it’s based off flawed reasoning.

You are insulin sensitive because you have been fasting for 8+ hours overnight and glycogen levels are becoming depleted. There is nothing magic about it being the morning. If you kept fasting through the morning, your insulin sensitivity would continue to increase (to a point).

This is one of many reasons that I recommend a protein + fat breakfast or no breakfast at all for those who are quickly looking to drop body fat or improve insulin sensitivity. Some recent research suggests that what you eat at breakfast may influence the fuel your body uses the rest of the day. More fat at breakfast may lead to more fat oxidation during the day vs. your carbohydrate breakfast initiating more glucose oxidation [Creating an Anabolic Breakfast].


What Happens When You Fast?

Energy status is of high importance to the body. Our body is constantly communicating with and amongst itself about our current energy levels and nutrient availability.  The body’s main goal is survival (and reproduction), so regulating our energy is top priority.

Even before we eat, just the thought of eating or smell of food can elicit a hormonal response. Studies have shown just seeing or thinking of food can result in “high acute insulin levels.” [2] The same researchers also found that high(er) levels of insulin produced “increased hunger, heightened perceived pleasantness of sweet taste and increased food intake [2].”

what happens when you fast

This is more evidence for the complexity of certain types of food and the hormonal response they create. It also helps explain why our relationship with food is so complex and often times a psychological game.

Basically, our body initiates the release of certain hormones to handle the “stress” of no food and low insulin, this is also referred to as our fight or flight response. Increases in growth hormone, epinephrine, ghrelin and nor-epinephrine are some of the most notable.

The catecholamines, particularly (epinephrine, nor-epinephrine), activate something called hormone sensitive lipase (HSL) and from there, fat will be moved from the cell (mobilized) and burned. All of these are working in conjunction to supply our body with the necessary energy it needs. No matter how cool the IPhone gets nothing compares to the intricacy and complexity of the human body.

Meanwhile, growth hormone’s presence will help spare muscle tissue by limiting amino acid oxidation and results in an increase in lipolysis (breakdown of fat for energy) as well. Ghrelin, on the other hand, is one of our main “hunger hormones” and is part of the reason you will experience those “hunger pangs” during stretches of limited food intake.

But part of what follows increases in ghrelin is growth hormone, so we can’t complain too much. Other hormones like leptin, neuropeptide-y, PYY, GLPP-1, etc. all play a role here and are influenced by current energy status, so you can start to understand how complex human metabolism is.

These hormonal changes were useful during times of famine because the catecholamines would give us the energy to get up and go find our food while the growth hormone made sure we had enough strength (muscle tissue) to effectively do this as often as needed. This feast and famine approach is a lost art in today’s society. Our constant feasting and lack of famine has led to the outbreak of metabolic diseases that continue to plague our country.


The Master Metabolic Switch

But let’s say you continue to ignore the feelings of hunger and look past all of the food you are constantly surrounded by for a few hours, what else happens? Your body starts to look inward for energy; something must be used as fuel for you to survive. Enter AMPK or AMP-activated protein kinase; see why it’s abbreviated?

AMPK has been called the master metabolic switch, as it is basically the energy messenger for the body. If energy levels are low, you begin catabolic processes to break things down and get the body some energy (ATP).  And while you normally think of catabolism negatively, it’s not all bad.

When you want to reduce body fat you need to make sure fat is being broken down and used for energy enough to lower body fat stores. This is a catabolic process. It’s naïve to think that catabolism only happens to muscle tissue and therefore we want to avoid it at all costs, this is simply not true.

During this “catabolic” state of a fast, AMPK activates glucose uptake (translocation of GLUT-4) as well as fatty acid oxidation (fatty acids being mobilized to then be used for energy), two things that we want for an improved physique [3].  Alright, well what the hell is that?

GLUT-4 is one of many glucose transporters in the body. It does exactly what it sounds like, it transports glucose (sugar) into your cells. When GLUT-4 is “translocated” these gatekeepers rush to the front of the cell where they essentially open the floodgates and allow glucose into tissue.


The more glucose your cells can suck up or take in, the less will stay floating around in the blood and eventually stored as triglycerides (or fat). So essentially when AMPK is initiated you improve your body’s response to insulin and work to decrease stored fat in the body.

At the other end of the spectrum, AMPK will also inhibit protein synthesis and glycogen synthesis, as these will be viewed as energy-requiring processes that aren’t needed at that moment. Think of it like this, there are anabolic (growth) and catabolic (breakdown) pathways or processes. Clearly, when one is initiated the other must be inhibited; energy is either being stored or utilized.

This idea that we want to remain in a constant state of growth is troubling and unhealthy. Catabolism (AMPK) and anabolism (mTOR) have often been described in a yin and yang or seesaw comparison. Both need to occur and both need to be controlled. We live in a society where anabolism (growth) now far outweighs our catabolism and the result is a host of overweight, unhealthy people with severe metabolic diseases.


Fasting for Muscle?

But if you want muscle you’re still probably wondering why we would ever want to stop protein synthesis, right? In order for us to gain muscle, protein synthesis (the creation of new proteins) must be greater than protein degradation (the breakdown of proteins). Luckily for us, the body undergoes some unique changes during the halt of nutrients that actually protect muscle quite effectively if it’s planned properly.

One example is the increase in growth hormone that is seen during a fast period [4, 5]. Growth hormone decreases amino acids being broken down for energy (oxidation), which preserves amino acids and allows them to continue to build new proteins. Because of this, growth hormone has a positive impact on protein synthesis. There are also increases in SIRT-1 and mitochondrial biogenesis that have a positive effect on muscle growth.

However, you only want periods of AMPK activation, not necessarily constant activation. These are periods you are dedicating to mobilizing fat and using it as energy. These periods are used to give the body a chance to regulate itself and get rid of unnecessary tissue.

You aren’t trying to build muscle during a fast, your body isn’t building muscle 24 hours a day anyway. Unfortunately, it just doesn’t work like that. If it did, you’d just hook yourself up to an IV of non-stop amino acids so you could grow all day long.

This goes for anabolism as well. If you stay in a constant flux of excess nutrients, your sensitivity to nutrients and the hormones they engender can be impaired. Bill Willis (Phd.), refers to this as “anabolic resistance” and makes quite a case for watching the excess bulk you chase (Is Your Fat Making You Suck?).

Other noted benefits of AMPK activation:

  • Autophagy – Cellular degradation process (will present more research later on).
  • Life extension – Through a host of mechanisms but mainly making cells more resilient to stress.
  • Mitochondrial biogenesis – the formation of new mitochondria, the energy powerhouse of the cell. This has implications in helping support muscle tissue.
  • Shuts down fatty acid synthesis and cholesterol synthesis [3] – Less fatty acids, less cholesterol formation? Sounds good.
  • Improved blood glucose levels – through a regulation of insulin secretion and synthesis [3].
  • Regulation of satiety centers in the brain [3] – Your ability to feel full which is essential when attempting to stay compliant with your diet.
  • Metabolic flexibility – the body’s ability to switch fuel sources. You want to be able to run efficiently on your own stored fat, also vital in health.
  • Increase SIRT1 – plays a role in protection from metabolic disorders, muscle growth, and life extension [6]
  • Inhibit hepatic glucose production [3]
  • Increased vasculature [7] – helps supply oxygen and nutrients to the body as well as waste product removal.
  • Improvements in body fat distribution and adiponectin levels [8]
  • Reduced cancer formation, kidney disease, and increased resistance of neurons in the brain to excitoxtic stress [9]

So now the not so scary catabolic effects of AMPK sound cool because it leads to a decrease in fat stores since we are using mainly lipids (fats) for energy, improved insulin sensitivity, more years added to your life and increased resting glycogen stores. The next question then is how you fully benefit from PERIODS of AMPK activation. You don’t want to always be in a catabolic state because we still need protein synthesis and other growth processes in order to gain or retain muscle tissue. This means you can balance out your periods of anabolism with catabolism.

In the final two segments of this series I will go deeper into fasting and muscle loss as well as present a case by case analysis of research studies on intermittent fasting.

The last installment of this series is below:

Ultimate Guide to Intermittent Fasting IV – What The Research Says



[1] Beardsall, K., (2008), Insulin and carbohydrate metabolism, Best Pract. Res. Clin. Endocrinol. Metab.

[2] Rodin, J. “Insulin Levels, Hunger, and Food Intake: An Example of Feedback Loops in Body Weight Regulation.” National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 01 Oct. 2015.

[3] Rutter, Guy. “Roles of 5 -AMP-activated Protein Kinase (AMPK) in Mammalian Glucose Homoeostasis.” Henry Wellcome Laboratories of Integrated Cell Signalling and Department of Biochemistry (2003): n. pag. Web.

[4] Groschi, M. “Endocrine Responses to the Oral Ingestion of a Physiological Dose of Essential Amino Acids in Humans.” National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 01 Oct. 2015.

[5] Muller, A., S. Lamberts, J. Janssen, L. Hofland, P. Koetsveld, M. Bidlingmaier, C. Strasburger, E. Ghigo, and A. Van Der Lely. “Ghrelin Drives GH Secretion during Fasting in Man.” European Journal of Endocrinology 146.2 (2002): 203-07. Web.

[6] Tonkin, J. “SIRT1 Signaling as Potential Modulator of Skeletal Muscle Diseases.” National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 01 Oct. 2015.

[7] Ouchi, Noriyuki. “AMP-Activated Protein Kinase Signaling Stimulates VEGF Expression and Angiogenesis in Skeletal Muscle.” American Heart Association, n.d. Web.

[8] Improvements in body fat distribution and circulating adiponectin by alternate-day fasting versus calorie restriction, Krista A. Varady, Journal of Nutritional Biochemistry 21 (2010) 188–195

[9] Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake, R. Michael Anson, et al., PNAS May 13, 2003 vol. 100 no. 10 6216–6220


  1.  The Ultimate Guide to Intermittent Fasting I - Fred Duncan

Leave a Reply

Your email address will not be published. Required fields are marked *