How many hours of fasting should I do?

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How many hours of fasting are just right when you are doing Intermittent fasting. There is no straight answer to this. Intermittent fasting is all the rage. It is an intervention, so cheap, without side effects, and, at the same time, with so many benefits that it is almost inevitable that you want to do it. However, it has a serious problem: there are many fasting types (12/12, 8/16, alternate days, long fast). Faced with so many options, it is normal for you to despair and wonder how many fasting hours you should do.

The answer is it depends on how you look at it. It all depends.

What does it depend on? … well, as the Jarabe de Palo song says … depending.

Depending on what your goal is, you will have to do fasts of different durations.

And this is precisely what you will discover in this article: how many hours of fasting you must do to get the results you want.

Origin of voluntary fasting

Starting this article, I told you that fasting is in fashion, and this is only half true.

For most of our history, human beings have fasted because their food intake was intermittent. They depended on hunting and gathering, so they spent specific periods without finding food.

Once the human began to settle, to practice agriculture and livestock, involuntary famine periods disappeared. Still, fasting periods were maintained in ancient cultures for what is known as voluntary fasting.

These were often called “cleansing,” “detoxification,” or “purification” times.

The earliest recorded stories of ancient Greece show a firm belief in its power.

Fasting has been the most respected healing tradition throughout the world throughout history. Over time, fasting lasted linked to religions.

👉 Fasting, for example, is widely practiced for spiritual reasons and remains a part of most of the world’s major religions. Three of the most influential men in the history of the world, Jesus Christ, Buddha, and the Prophet Muhammad, shared a belief in the healing power of fasting.

It is often called cleansing or purification in spiritual terms, but it practically refers to the same thing.

Benefits of fasting

how many hours of fasting you must do to get the results you want?

Unless you like historical data, indeed, so much history will not seem so relevant to you.

But there is something you should know, since these ancient times, our body has maintained the health effects of fasting.

👉 These benefits you will get from having a prolonged food restriction are:

  • Increases SIRT3, the youth protein, and reduces mortality.
  • Favors autophagy, another of our anti-aging systems, also in the brain.
  • Reduces indicators of inflammation.
  • Reduces triglycerides and improves the lipid profile.
  • Improves neuronal plasticity.
  • Improves insulin sensitivity and blood pressure.
  • Limits the growth of cancer cells and makes chemotherapy more tolerable.
  • Promotes weight loss while retaining muscle mass.

As you can see, there are many benefits, but not all of them appear with a 16/8 fast. For example, some of them require more than 24 hours without eating to take effect.

However, depending on which of these benefits you want to achieve, it may not be necessary to spend as much time fasting.

The optimal fasting dose is regulated by the desired benefits and the continuity of these intervals without eating food.

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👉 For this reason, we have divided periods of intermittent fasting into three large groups and have allowed me to understand how long I should help. These 3 groups are:

  • How many hours of fasting do I need to lose weight?
  • or, how many hours of fasting do I need to improve my metabolic disorder?
  • or even, how many hours of fasting should I do to improve the quality of aging and improve inflammation?

How many hours of fasting should I do to lose weight?

How many hours of fasting should I do to lose weight?

If there is something that seems like common sense, our body understands that the best time to use reserves is when no food enters, and in fact, it is.

Surprisingly, the last 20 years’ weight loss recommendations have not gone this way, but quite the opposite and have been based mainly on lowering calories and eating many times a day.

Although this type of intervention can be useful in the short term when it comes to losing weight than others, they show an inexorable stagnation and a subsequent recovery of lost weight in the medium term. This relentless recovery occurs despite strict and continued adherence to the diet and regardless of the dietary strategy followed.

As this study shows, after 5 years of dieting, almost 100% of people have regained their weight, and 40% are heavier than before.

Ultimately there are two clear facts:

  1. For the past twenty years, the usual recommendation for weight loss has been to eat less and exercise more.
  2. Over the last twenty years, the obesity rate has skyrocketed.
Obesity rate has skyrocketed

👉 Over time we have been learning more about the mechanisms behind obesity, which, in summary, would be:

Processed products that make us eat more than we need: The food industry designs products so that my body wants to consume them even though it does not require the calories at the time

Excess carbohydrates: Associated with the first point, an excess of carbohydrates makes the body cells “saturate with so much sugar arrival” they feel too full and close their floodgates to glucose entry. (They decide to fast until the pantries are empty).

The problem is that insulin and trying to force glucose into cells prevent us from using fat as fuel

This causes excess food to go more easily towards fat, and an increase in insulin appears to try to “push the sugar into the cells”). The problem is that insulin and trying to force glucose into cells prevent us from using fat as fuel. So we are filling the fat deposits, and we cannot use them.

Inflammatory context: Foods to which my body responds by inflaming: pollution, stress, lack of movement, or lack of sunlight are some of the situations that lead the body to be in an inflammatory state. Inflammation hinders the function of insulin, an essential hormone in the obesity processes.

And what does fasting have to do with all this?

When I fast, stop eating, stop confusing my brain with processed products, my insulin levels go down, and I create a context where my body has no choice but to use reserves to function.

And what dose is effective to achieve this:

Several studies in humans have already shown that generating an intake window of about 10 hours without any dietary prescription already causes a significant weight loss maintained after one year if the intervention is continued. And not only that, but they also had more energy throughout the day, they rested better and were less hungry at night.

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👉 Therefore a window of 10/14 already seems to be useful for losing weight.

Of course, lengthening the fasting window will facilitate the adaptation of the appropriate metabolic context and accelerate the results.

If we observe this other study where we apply strategic fasting, 1 day of fasting and other ad libitum food alternate.

  1. Fasting days are total. Can not eat.
  2. The patients’ glucose levels were monitored to verify adherence to the program.

The results are as expected: Fat reduction, improvement of fat to lean mass, improvement of cardiovascular markers.

What is very interesting about this study is the incredible adherence they achieved:

  • 90 people followed a 1-month intermittent fasting program, and only 2 people failed (or 97% adherence).
  • From this group, 30 people continued until 6 months and ALL finished.

This level of adherence is unheard of.

Therefore we could conclude that if your goal is to lose weight, we will move in a range of:

  • Minimum effective dose: 10/14
  • Maximum dose: Fasting every other day.
  • Frequency: daily

How many hours of fasting should I do to improve metabolic disorders

The most common metabolic disorders can be:

Metabolic disorders: Insulin resistance and type 2 diabetes, and fatty liver

In this situation, we already find ourselves with an organism exposed to the aggressions of the modern world. The cells of the organs, mainly the liver, are full of fat. The body has been in a context where it cannot burn fat for so long that it will be difficult to convince it to do it again.

👉 In this situation, we have taken a step further to mere weight gain.

Our physiology has begun to fail: Mitochondria and other cellular organelles start to make mistakes. Our DNA may have suffered some damage. It can make small errors in the manufacture of proteins or slightly change the patterns of gene expression.

To improve, we need to help our body mobilize the reserves again. However, we also have to regenerate our organs’ cells and empty those of the fat that they have accumulated.

The primary mechanism of cellular recovery is known as autophagy. This is a cellular recycling process where parts of the cell that have been damaged are digested to obtain energy or to manufacture these cellular fragments again without errors.

Therefore, it is essential to counteract the age-related accumulation of damaged organelles and proteins and improve cells’ metabolic fitness.


Autophagy even digests useless bits of DNA that are preventing the proper expression of genes.

In short, it is a highly conserved evolutionary process that cells activate to protect themselves from stress, learn from it, and improve.

Without a doubt, the most physiological way to induce autophagy that is phylogenetically conserved from yeast to primitive animals is nutrient deprivation, fasting. A cell that is deprived of its energy supply, which may be the absence of Nutrients or the absence of growth factors. These are required for these nutrients that will destroy their bioenergy active. These are macromolecules, proteins, lipids and ribonucleic acids for energy production.

In other words, the lack of energy in a cell is what activates it to make itself better.

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But at the same time, the amount of fasting we need to activate autophagy is one of the great unknowns.

Τhe energy distribution in the organism is hierarchical

The key with all this is that the energy distribution in the organism is highly hierarchical. The more critical a system is to survival, the longer it takes to be sensitive to nutrient deficiencies.

The perhaps unfortunate and sad example, but more visual, is the image we all have in mind of a child with malnutrition with a small body and a head that seems enormous. This is because the brain is the last organ to run out of energy.

In the case where we find ourselves where the liver is one of the most affected, we can be guided by the following data:

  1. Hepatic glucose stores last at least 24 hours
  2. The liver is one of the great controllers of our energy state, and what type of energy substrate is more coherent to use at all times.
  3. Given its antigenic exposure and its role in detoxification, the liver must always repair itself.

All this leads us to extrapolate together with some preliminary data on the induction time of autophagy in this organ:

👉 Minimum effective dose: Fasting 16 / 8- fasting every other day. Daily frequency

👉 Maximum Dose: Between 3-5 days of complete fasting or Fasting Mimicking diet. Monthly/bi-monthly frequency.

How many hours of fasting should I do to improve inflammation, neurodegenerative pathologies and improve the quality of aging

Autophagy has also been shown to improve inflammation, stimulate the nervous system’s regeneration, and release neuronal growth factors. It is associated with increased life expectancy and quality of life.

In this situation, we are faced with prioritized systems at the energy level, and therefore for them to be in deficits, we must “stress the body” a little more.

Although there are still no conclusive studies, it could be extrapolated from studies in mice that they would find signs of autophagy in immune cells 3-5 days after starting fasting. In turn, diets that simulate a 5-day fast have also seen significant modulating effects on the immune system.

Therefore, to generate effective changes at the systemic level that affect our body’s most relevant organs, we need higher doses.

  • Minimum effective dose: 3 days
  • Maximum dose: 5 days.
  • Frequency 1 time every 1-2-3 months

As you have indeed seen, we have put the barrier in the 5 days. From the little data we have, it seems the moment where we have already obtained all the desired effects, it does not involve high body wear and tear and is “relatively affordable to perform.

I do not want to emphasize that more extended fast studies are being carried out, but for the moment, it seems that 5 days is a reasonable limit.

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