Intermittent Fasting Explained

Siim Land

The structure of this article: (1) intermittent fasting explained, (2) how to do it and (3) the best way to do it.

The Fundamentals of IF

What is Intermittent Fasting (IF)? It’s a way of eating (or rather not eating), where you restrict the consumption of your food to a certain time window. This means that no calories whatsoever get put into our body. It’s a dietary strategy that simply times when you eat. Let me explain you how does it work.

The two governing states of metabolism are fed (anabolic) and fasted (catabolic).

  • Fed – we’ve consumed food, digested it and now there are some calories circulating the blood stream. This refers to ‘anabolism,’ or building new tissue.
  • Fasted – meaning that there are no exogenously (externally) consumed calories present but the body can still create energy from endogenous (internal) sources. This refers to ‘catabolism,’ or breaking down. Usually it happens after several hours (6-8) of not eating.

There are only two options: (1) eating and storing calories, (2) not eating and burning calories. Caloric restriction will still keep you in a fed state, a malnourished one, but still.

There are several implications to what was said above. For instance, being fed won’t necessarily mean that you’re building new tissue nor will fasting mean that you’re breaking down.

Intermittent Fasting Explained –
The Beginning Phase

In a Fasted State There Are Several Ways to Produce Energy 

The body’s default fuel source is glucose, which exogenously comes in the form of sugar and carbohydrates and is stored endogenously as glycogen. The liver can deposit only 100-150 grams and our muscles about 300-500 grams. They’re used for back-up.

Liver glycogen stores will be depleted already within the first 14 to 24 hours of not eating – almost overnight. This decreases blood sugar and insulin levels significantly, as there are no exogenous nutrients to be found.

The Role of Insulin

Insulin is a hormone released by the pancreas in response to rising blood sugar, which happens after the consumption of food. Its role is to unlock the receptors in our cells to shuttle the incoming nutrients into our muscles, or when they’re full into our adipose tissue (body fat).

Intermittent Fasting Explained
Intermittent Fasting Explained

The counterpart to insulin is glucagon and also gets produced by the pancreas. It gets released when the concentration of glucose in the blood stream gets too low. The liver then starts to convert stored glycogen into glucose.

This initial phase of fasting is characterized by a high rate of ‘gluconeogenesis’ (the creation of new glucose) with the use of amino acids (protein) from bones and muscles.

Fasting and Ketosis

Because there is no endogenous glucose to be found, the body will then seek out other ways of getting energy and begins to adapt to these new conditions.

As fasting continues, the liver starts to produce ketone bodies which are derived from our own fat cells.

‘Lipolysis’ (the conversion of fatty acids) and ‘ketogenesis’ (creation of ketone bodies) increase significantly due to fatty acid mobilization and oxidation.

Fasting Ketosis
Fasting Ketosis

After 2-3 days already, the body shifts into a state of nutritional ketosis. Triglycerides are broken down into glycerol, which is used for gluconeogenesis and broken down into three fatty acid chains. Fatty acids can be used for energy by most of the tissue in the body, but not the brain. They need to be converted into ketone bodies first.

What is Nutritional Ketosis

Ketosis is an altered, but still natural, metabolic state that occurs either over a prolonged period of fasting or by restricting carbohydrate intake significantly, usually up to less than 50 grams per day [i].

Fasting induces ketosis very rapidly and puts the body into its more efficient metabolic state. Ketone bodies may rise up to 70-fold during prolonged fasting[ii].

The more keto-adapted you become the more ketones you’ll successfully utilize. At first, the brain and muscles are quite glucose dependent. But eventually they start to prefer fat for fuel.

After several days of fasting, approximately 75% of the energy used by the brain is provided by ketones.

Is Fasting Safe?

There is no reason to think that fasting is dangerous. Instead, there are a ton of health benefits to it that can actually reverse some of the disease you might have.

Some of the Benefits of Intermittent Fasting:

  • Autophagy,’ is a metabolic pathway which removes waste material from cells[iii] and re-cycles old toxic proteins.
  • Increased fat burning – fasting makes your body burn and not store fat. In fact, metabolic rate can increase by 3.6-14% after 48-hours[iv].
  • Improved biomarkers. Almost all of metabolic disease is positively affected by fasting. Reduced cholesterol, improved insulin sensitivity, lower blood sugar levels, and protection against tumor growth.

Fasting also skyrockets human growth hormone exponentially within the first few days to maintain lean body mass and muscle tissue. After 14-18 hours of fasting it does so by an astonishing 1300-2000%[v].

Afterwards it does so less significantly because protein catabolism gets reduced to almost non-existent levels. In this state, the majority of the body’s energy demands will be met by the use of free fatty acids and ketones.

Intermittent Fasting Explained –
The Feeding Phase

Now, you can’t fast indefinitely. If you were to not starve to death and actually reap the benefits mentioned above, you have to also transition out of a fasted state into the feeding phase.

Intermittent Fasting incorporates many eating windows and schedules. The general rule of thumb is to fast for 12-14 hours a day at minimum. 16-18 hours is a good balance between autophagy and anabolism.

A few examples:

  • One Meal a Day – you basically eat once within a 22-24 hour period. From dinner to dinner.
  • Two Meals a Day – you have either breakfast/lunch; lunch/dinner; or dinner/breakfast. You would still need to fast for 12 hours overnight to actually get into a fasted state.
  • 16/8 – this is the easiest yet extremely effective option. You basically break your fast at lunch and then eat until the evening. 80% of the results you’re after can be achieved with this one.
  • 24+ Hour Fasts – there are still greater benefits to occasionally fasting for longer than a day. This will promote autophagy and allows the body to heal itself. What I recommend is to have a 48-hour fast once after a few months and to not eat anything for 3-5 days 1-3 times per year. Doing this will greatly improve your protection against cancer and tumors.

KETO plus IF

In order to make your experience with fasting easier and healthier, you would want to know how to do it in a state of nutritional ketosis. Keto and IF are like a match made in heaven because they’re physiologically almost identical.

Combining intermittent fasting and keto puts you into your prime-primal state and turns your body into a fat burning beast. This is the most self-sufficient and empowering nutritional strategy because it prevents you from almost ever getting hungry, experiencing muscle catabolism or feel tired.

Instead, you’ll have access to constant energy because of having tapped into your greatest fuel supply there is – your own body fat. Losing weight, improving your health and increasing your productivity are granted.

If you want to master the art of intermittent fasting and ketosis, then you should definitely check out my program called KETO // IF. It teaches you everything you need to know about these two miraculous states and also comes with a 21-day meal plan, including a fasting schedule, in which I tell you what to eat, in what amounts and at what time.

Keto intermittent fasting cover
Keto intermittent fasting cover


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[i] Body composition and hormonal responses to a carbohydrate-restricted diet. Metabolism. 2002 Jul;51(7):864-70.

[ii] Owen OE, Felig P, Morgan AP, et al: Liver and kidney metabolism during prolonged starvation. J Clin Invest 1969 Mar; 48:574-583

[iii] Short-term fasting induces profound neuronal autophagy. Autophagy. 2010 Aug 16; 6(6): 702–710.

[iv] Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000 Jun;71(6):1511-5.

[v] Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988 Apr; 81(4): 968–975.