How does intermittent fasting work (and what is it)?

How does intermittent fasting work (and what is it)?

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How does intermittent fasting work (and what is it)?

Picture of intermittent fasting clock, food, Dr.ludidi logo

1. How does intermittent fasting work?

Fasting has been around for centuries and recent scientific interest has gone to the health benefits of what is called intermittent fasting. Simply stated,intermittent fasting means that you do not eat for a relatively long period of time (at least 14-16 hours) on a regular basis. The time frame within which you don’t eat nor ingest any type of calories is called the fasting period or fasting window. When you interrupt your fast and start eating again, your eating window (or feeding window) begins, i.e., the period in which you are allowed to eat and ingest calories. The fasting window follows the eating window in a recurring cycle over days or weeks.

Different forms of fasting: things that are and aren’t allowed

Not eating has different interpretations. Strictly speaking, it means that you should not eat energy-containing food. In that case only water would be allowed (water fast). Some milder varieties also allow black coffee and tea, others allow broths and even less-strict varieties allow you to take low-energy, but sweetened soft drinks like diet cokes etcetera.

Because fasting affects the metabolism and energy-containing food has a direct effect on the metabolism and on autophagy (more on this later), the strict variants strive for a complete energy restriction. Water fasting therefore only allows water during the fasting period, because water contains no energy and therefore does not affect autophagy. Even more extreme variants take into account the possible interaction between caffeine and the circadian hormones (related to the day and night rhythm) and the interaction between coffee or sweeteners and metabolic processes. As such, it has been proposed that frequent consumption of sweeteners not only misleads your mind, but also your body as it may affect gut-health and metabolic health (1–3). In health-terms, merely focusing on energy, or more precisely on calories, is a facade. It may therefore be clear why certain, natural forms of fasting only allow water for intake.

Important effects of fasting: metabolic switch and autophagy explained

In fact, water fasts in its purest form could be seen as the most complete form of fasting. Because it stimulates two important mechanisms of intermittent fasting at the biological-metabolic level:

  1. The metabolic switch, which happens when your body switches from using carbohydrates for fuel to fat (4). This is the primary principle via which fasting exerts its effect and helps improve insulin sensitivity and improve blood markers related to cardiovascular disease, such as triglycerides.
  2. Autophagy, which refers to the "recycling" of cellular substances and molecules, which extends the life of the cells. It is a process that takes place in your body after you have abstained from energy intake for a longer period of time (typically after 20 hours of fasting). In the absence of new building materials from food - for example through fasting - your body has to be able to recycle existing molecules as residual materials in order to stay alive (5). The additional advantage of autophagy is that it reduces aging by preserving cells and structures (6). You see, if we can preserve cells, it means that cells do not need to divide. And cell division is what makes us age. Mother nature has put a limit on how often our cells can divide. At some point, errors will occur and you age as a result.

So both of these mechanisms are stimulated with water fasts, which promotes fat burning and inhibits cell aging. As a consequence, fasting reduces the risk of developing non-communicable diseases (metabolic syndrome, obesity, cardiovascular disease, diabetes). Next to this, your mental health and wellbeing might improve as well (better alertness and a lower risk of Alzheimer's)(7).

Bulletproof Coffee & Broth and fasting

However, there are also fasting forms in which you can drink natural, self-drawn broths in addition to water (and black coffee and tea) during the fasting window. And there is one - now very popular - item called bulletproof coffee. Bulletproof coffee is black coffee with a teaspoon of full "real" butter or coconut oil dissolved in it. Because both the stock and the bulletproof coffee contain energy from amino acids and fats respectively, both drinks do not stimulate the autophagic processes during the fasting window. The other benefits of the metabolic switch however, are retained. The above drinks are therefore excellent alternatives for people who feel that they have to get some 'nutrition' in.

2. Forms and Protocols of Fasting

The 16:8 fasting and intermittent fasting for women

There are many different forms of intermittent fasting. For some, the fasting period repeats every day. During such a protocol, people generally don’t eat for about 14-16 consecutive hours each day and will then eat 3-4 meals during the  remaining hours. The amount of of food eaten during the 8-10 hour eating window is usually the same amount that you would normally eat throughout the day; i.e., it would equal your normal consumption between 7 a.m. and 9 p.m. This way of fasting, 16 hours on a daily basis, is typically referred to as the 16:8 protocol.

Also, there is 14:10, which refers to a fasting block of 14 hours and an eating window of 10 hours, respectively, which is often more suitable for women, due to differences in hormonal regulation and energy metabolism.

The most popular way of executing these protocols, for instance  the 16:8 variant, is by setting the eating window between 12 midday and 8 p.m. in the evening, but one could adjust that to personal preference. Some would prefer to start eating around 10, whereas others would rather start eating a bit later, depending on their schedule or biorhythm.

Other forms of fasting

Alternatively, there are protocols in which you don’t eat for as long as a whole day. For example, the “Every other day” or “Alternate day” fasting protocol dictates that you do not eat for the whole day every second day. These fasting days are alternated by eating days: on those days you would eat your regular 3 main meals, as you normally would.

The 5:2 variant, finally,  requires two days of not eating, followed by 5 days of eating as you normally would.

To wrap it up, from a practical point of view, intermittent fasting essentially consists of shifting your meals, so that you stay fasted for longer than you normally would when eating at standard times (breakfast, lunch and dinner). The result, as we will now dive a bit deeper into, is a wide range of health benefits, such as improved fat burning capacity and longevity.

3. Health Benefits of fasting

The wide range of health benefits makes fasting a strong means to reduce the risk of lifestyle-related disorders, amongst which  type 2 diabetes, obesity, cardiovascular disease and metabolic syndrome. On top, intermittent fasting can help strengthen your natural biorhythm.

Intermittent fasting and biorhythm

Fasting can give you a "reset" from the outside to get back in balance, purely by taking into account the timing of your meal intake. Regularity and rhythm are required in life, they help your body to recognize certain patterns, get hungry, get tired, wake up. Many of these patterns are regulated hormonally, and research has shown that fasting can have a positive effect on your hormone balance (8). In a practical sense,  taking into account rhythm and hormonal balance also means that, once you’ve chosen a protocol, you have to stick to the rhythm. Changing the onset of your first eating moment after the fast, or even changing meal times is not recommended as it disturbs your bio clock.

How abstinence of food and shifting your eating window affects your health

Key to realising health benefits are metabolic adaptations that occur in your body, following fasting. These metabolic adaptations are both the result of 1) the absence of food as well as 2) the shift in energy usage that is, the shift from using carbohydrates for fuel, to fat for fuel, which is a direct consequence of fasting. Both induce certain biochemical effects in our cells, resulting in improved cell function, leading to different energy production. This affects muscle & heart, gut health and brain health, to mention some. Altogether it results in increased fat-burning capacity, improved insulin sensitivity (9) and neuroplasticity (10).

4. Practical benefits of intermittent fasting diet

Furthermore, intermittent fasting can offer more than a few practical benefits like:

  1. The clarity intermittent fasting gives you in terms of how to deal with food choices comes through the little amount of rules you have to adhere to. In fact, you just have to take into account when to eat and when to stop eating.
  2. Because you eat less frequently and at the same time, this ‘eat on’ vs ‘eat off’ rule offers you a sense of grip when it comes to making food choices.
  3. Experimental studies indicate that extended periods of not eating can improve brain function and can help reduce the risk of brain diseases like Alzheimer's disease (7).
  4. Fasting also creates extra time, which you would normally spend cooking or eating, and helps the body rest and recover. This is an often overlooked, yet fundamental aspect, when pursuing a healthy lifestyle.
  5. Not in the last place, one of the many benefits of intermittent fasting diet is that it doesn't really matter how you eat. It offers the possibility to connect from many different eating styles, making the method suitable for almost every lifestyle. You don't have to count calories, omit carbohydrates, bread, lactose, or whatever, and there's even room for candy moments.

5. Research and intermittent fasting weight loss

Although there is no real consensus yet and research on fasting is an ongoing journey, intermittent fasting has huge potential to improve people’s health. Some studies have shown that research subjects who follow an intermittent fasting protocol are capable of losing more weight versus subjects that follow a classic slimming diet (with a daily restriction on energy intake) (11,12). Intermittent fasting even shows positive effects on health outcomes when there are no restrictions on the amount of food during the feeding window (13),(14). Even if people can choose what they eat, this effect persists (13). What’s more, the type of fasting protocol you go through doesn't really matter, studies show. It once again shows that merely relying on calorie counting is obsolete, but I do believe it is a good idea to choose a protocol that is especially suitable for you when you start intermittent fasting. Never force yourself into a certain protocol or paradigm; because in the end, you are looking for something that is sustainable due to its ease, not a struggle.

I am convinced that the personalised approach, combined with the relative freedom I gave to my clients was one of the reasons for the success they booked with me, through the Dr. Ludidi method of fasting.

6. Conclusion

When applied correctly and becoming part of your lifestyle, intermittent fasting offers many benefits. It is not without reason that its popularity has risen enormously in the past decade. It is often touted as a holy grail for anything and everything, to lose weight faster, as a rejuvenation cure, or to get a sexy and slender figure. Although intermittent fasting can indeed help with this, such quick fix claims do not suffice for the method. I would like to emphasize that working on obtaining and (especially) maintaining good health is a lifelong process. This also applies if you want to realize certain aesthetic ideals, because you prefer to keep them throughout the year. When you are aware of this, you can get the maximum return from intermittent fasting.                                      

I have been doing intermittent fasting diet for about 2 years now. I usually do it 2 or 3 times a year, every 2 months. Although I started it because I wanted to get leaner, I quickly realized it brought me more. The protocol that suits me is based on a fast that starts in the early evening and continues into the afternoon the day after. And I must say, the fast during the first part of the day has greatly increased my productivity. Since I have started intermittent fasting, my life and work rhythm have become clearer and I know when to do mentally demanding work, such as writing blogs like this one and writing client programs (first part of the day), and when to plan meetings (second part of the day). Food does not keep me busy for the first 4 to 5 hours of the working day, so I can be extremely productive.

In addition, intermittent fasting weightloss has been helping me keep my fat percentage around 10% for years, or even lower (during my college years, about a year ago, I was at 15%). Because of intermittent fasting weightloss, my body weight is now at a stable 90kg. Incidentally, do not compare yourself to me, because these figures are relative and they only reflect how my personal situation changed over time. And they depend on many factors, but they do show what intermittent fasting - as part of a lifestyle in which healthy eating and exercise play a central role, but where you also enjoy regularly - can yield you in the long term.


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    2. Debras C, Chazelas E, Sellem L, Porcher R, Druesne-Pecollo N, Esseddik Y, et al. Artificial sweeteners and risk of cardiovascular diseases: results from the prospective NutriNet-Santé cohort. Bmj. 2022;378:e071204.
    3. Suez J, Cohen Y, Valdés-Mas R, Mor U, Dori-Bachash M, Federici S, et al. Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell. 2022;185(18):3307-3328.e19.
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    10. Cabo R de, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. New Engl J Med. 2019;381(26):2541–51.
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