What are the benefits of fasting and what is fasting really?
When is it good to eat and how much, and what happens if we skip meals?
This article answers these questions based on the latest scientific evidence.
How much and when to eat, and the benefits of fasting
In a hurry? Here is all you need to know about the benefits of fasting in a nutshell:
WHAT TO EAT:
WHAT you eat is more important than WHEN you eat and, relatively, also than HOW MUCH you eat.
HOW MUCH TO EAT:
Don’t eat with the scale, listen to your body and stop a little before you feel full.
WHEN TO EAT:
Eating a certain number of times a day is an acquired behavior. The number of meals to eat per day and when to eat them depends on personal needs and preferences. However:
- Not eating for 12-16 hours each day can have health benefits
- The simplest thing for most people is to arrange to always have at least 12 hours between dinner and breakfast. But there are also other options.
FASTING:
Fasting means not eating for more than 16 hours and drinking only water.
- Fasting OCCASIONALLY for more than 16 hours (up to 36) can have positive health effects
- For example, OCCASIONALLY skipping one or two consecutive meals so as not to eat for more than 16 hours (and less than 36), can be beneficial to health
- Fasting systematically, that is, “intermittently”, is not advisable. For example:
- Eating only one meal a day always: NO
- One or two days of complete fasting every week: NO (but better than the point before)
- Fasting for more than 36 hours (therapeutic fasting) can be beneficial for specific diseases, but can only be practiced under the prescription and supervision of a specialized doctor.
Continue reading to discover all the details about the benefits of fasting.
WHAT TO EAT: The quality of your meals remains the most important thing
Before talking about the benefits of fasting, let’s always remember that what we eat is much more important than when we eat and, relatively, how much we eat.
Even if we scrupulously adhere to a certain number of fixed meals a day, or if we even fast for many hours between one meal and another, this discipline will be useless if, when we eat, we feed ourselves with the wrong foods.
The optimal diet for health is not complicated, on the contrary, it is based on a few simple recommendations: focus on the quality of food, eliminate added sugars and refined carbohydrates, eat plenty of fruit and vegetables, and choose sources of healthy protein and healthy fats.
Assuming that our diet is scrupulously based on these healthy principles, how much can we eat and when should we eat for optimal health? Does it make sense to occasionally fast, and what are the benefits of fasting?
HOW MUCH TO EAT: Stop a little before you feel full
Recent studies suggest that a slight constant caloric reduction compared to your daily caloric needs is beneficial for health and longevity. [6]
At the moment, the scientific evidence on the benefits of caloric restriction is more solid than that on the benefits of fasting.
The data that can be found in the scientific literature refer to caloric reduction values that range between 10 and 30%. This means that if, for example, your caloric requirement is 2,200 calories per day, then you should introduce no more than 1,980 calories per day with food (10% reduction), or even no more than 1,540 calories per day (30% reduction).
There is a consensus among experts that a constant caloric restriction of around 10% is actually beneficial for health, while at the moment there is no solid evidence to support a greater reduction in calories on a continuous basis.
Don’t count calories or weigh your food
In any case, in order to develop healthy and sustainable eating habits over time, except in exceptional cases, it is not advisable to have to weigh your food and calculate your daily calorie intake.
A much more sustainable practice is to develop more attention to your satiety signals and stop a little before you feel completely full.
This is a concept similar to the Japanese custom Hara hachi bu which literally means “80% full.” It is an ancient Confucian precept that encourages people to stop eating when they feel about 80% full.
This practice is not only a way to control weight, but is also considered beneficial for overall health and longevity.
In Japan, Hara Hachi Bu is often taught from a young age and is part of the country’s food culture. It is a practice that requires awareness and self-control, but can lead to significant benefits for long-term health and well-being. [7]
WHEN TO EAT: It is best to respect a period of 12 hours a day in which you do not eat
In ancient times, people ate a lot when food was available and also fasted for a prolonged period if food was not available.
For example, in the Paleolithic period, people ate when a hunting trip went well and fasted when it went badly.
The benefits of fasting are linked to the fact that our body has evolved around this natural alternation between periods of food abundance during which to accumulate energy stores, and periods of food scarcity during which to use these stores.
Eating at certain times is an acquired behavior
Nowadays we are used to eating every day, several times a day, and at the same time having a life that is not sufficiently active, if not completely sedentary.
For these reasons, many people find themselves constantly in the phase of accumulating energy and almost never in that of using their reserves.
The habit of having 3 main meals a day has developed around the need to adapt our meals to the rhythms of working life. That is, having breakfast before starting work, having lunch during the break, and having dinner after the end of work.
This is an acquired habit and not dictated by a physiological need of our body, which rather has developed by adapting to the natural alternation between periods of abundance and scarcity of food. [4]
How many times a day should you eat?
The habit we have of eating an average of 3 times a day, however, is fine. Just as it is fine to eat 2 times a day, or to eat more than 3 times a day by inserting additional snacks.
This depends on the lifestyle of each of us and individual preferences.
In general, it is considered a healthy habit to consume 3 meals a day, including one main meal and two snacks (or lighter meals). When to place the main meal depends on individual preferences.
However, by eating every day, several times a day, our digestive system risks always being in operation and can never truly enter the “maintenance” phase. With a simple trick we can let it rest a little every day and feel better.
How much time should pass between meals?
Again, there is no precise rule. However, there are some precautions to take into consideration.
Recently, some practices that we can define as time-restricted eating have become quite fashionable, which we could translate as “eating in a restricted time window.”
In practice, it is a matter of establishing a time window in the 24 hours during which you eat and consequently one during which you do not eat. The most popular protocols are:
- 16:8 i.e. you don’t eat for 16 hours and concentrate all meals in the other 8 hours
- 14:10 i.e. you don’t eat for 14 hours and concentrate all meals in the other 10 hours
- 12:12 i.e. you don’t eat for 12 hours and concentrate all meals in the other 12 hours
In all cases, there is a period in which you do not eat for 12-16 hours every day.
In other words: in the 24 hours, the time in which you do not eat must be (slightly) longer than the time in which you eat.
Give your body a break and don’t eat for at least 12 hours each day (e.g., between dinner and breakfast)
Organizing your daily meals to have a window of at least 12 hours of “digestive rest” per day may be enough, without having to push yourself to windows of 14 or 16 hours that are more difficult to integrate into our daily routine.
For most people, the most natural option to integrate into their routine is 12:12 as it is normal not to eat between breakfast and dinner anyway.
For many, this may mean getting used to having dinner a little earlier, for example finishing dinner by 8:00 PM, and then having breakfast after 8:00 AM the next day.
Obviously, everyone can organize their 12-hour (or more) window in a personalized way. There are many other possible variations.
As we will see better shortly, in this case we cannot properly speak of “fasting” but precisely of time-restricted eating. In any case, the benefits of not eating for at least 12 hours every day consist of giving our digestive system and our metabolism a break, allowing them to rest a little.
Occasionally, does it make sense to give a longer break, i.e., over 12 hours, to our digestive system and our metabolism?
Let’s see what the benefits of fasting are, the real one.
FASTING: How to do it and what triggers the benefits of fasting
People practice fasting for a variety of reasons. For example, for religious reasons such as Lenten fasting in the Catholic Church or Ramadan in Islam. Or for health reasons to lose weight or detoxify. Or even for political reasons, for example as a gesture of protest.
Animals also fast spontaneously when they are sick, in order to conserve energy for healing rather than using it for digestion.
So what are the benefits of fasting?
Imagine your body as a car and food as its fuel and understand the benefits of fasting
Our body is an extraordinary machine. But what happens if you continuously fill up with fuel without having traveled even a little? The fuel overflows from the tank and damages the components of the car.
Speaking of nutrition, many of us never even get close to empty, continuing to put fuel (food) in when the tank is still full, thus creating enormous health problems.
In short, the benefits of fasting arise from waiting a little longer to refuel (eat), ensuring that our metabolic tank (we will see later what it is) occasionally empties. In this way, our body can finally do some maintenance for the benefit of our health.
To understand the benefits of fasting for our health, below you can find a timeline of what happens to our body during fasting.
Remember that fasting means not eating, and drinking only water.
The times indicated below are average times and may vary significantly depending on the level of physical activity during the fast and the diet prior to the start of the fast.
From 0 to 16 hours after the last meal: GLYCOGEN is the main fuel for our body
FROM 0 TO 16 HOURS
To function, our body derives the necessary energy primarily from glucose (a sugar) that we obtain from the food we eat and from the glucose stores we have in the form of glycogen, which is stored primarily in our muscles and liver.
After consuming a meal, under the action of insulin, the glucose derived from the food we eat is pushed into the cells to be used as an energy source, while the excess compared to the current need is stored in the form of glycogen in the muscles and liver, and then converted into reserve fat.
We can imagine that in our body we have a “tank” full of glycogen, which our body fills and then uses later to function properly, just as a car uses the fuel that is in the tank to function.
Once digestion and absorption are complete, after about 6-12 hours from the last meal, blood sugar (the concentration of glucose in the blood) and insulinemia (the concentration of insulin in the blood) begin to fall. Once they drop below a certain level, you begin to feel the stimulus of hunger.
At this point, if we continue not to eat, blood sugar continues to fall.
Since it is important that blood sugar remains constant, our body begins, under the drive of the hormone glucagon, to draw on glycogen reserves in the liver to release glucose to be put into circulation in the blood in order to maintain its levels constant.
Glucagon in a sense serves to draw fuel from our tank.
Like the fuel in a car, however, even our glycogen reserves eventually run out.
If we don’t put gas in our car, sooner or later the car will stop. But if we continue not to eat, our body cannot stop. It must continue to function even when the glycogen tank is empty.
This is precisely the moment when we begin to experience the benefits of fasting.
From 16 to 48 hours after the last meal: PROTEINS are used to produce glucose
FROM 16 TO 48 HOURS (over 36 hours only under medical prescription and supervision)
When our glycogen stores are depleted, the body begins to use a stratagem to manufacture new glucose from the proteins found in our body: gluconeogenesis.
In a sense, it is as if our car were able to create fuel on its own by consuming some pieces of the bodywork or the engine. Thus, gluconeogenesis creates glucose from our proteins, such as those in our muscles.
It takes 24 to 36 hours before our glycogen tank is completely empty (in the absence of intense anaerobic physical activity which, on the other hand, could use it all up even in a few hours). In any case, our body begins to activate gluconeogenesis after about 16 hours from the last meal, therefore before glycogen is completely exhausted.
This moment is called by some the “metabolic switch“: this is the moment in which the glucose circulating in the blood is manufactured more by gluconeogenesis from proteins than from glycogen reserves.
Fasting is technically the process that begins on average about 16 hours after the last meal consumed, when the metabolic switch is activated.
If we continue not to eat, after about 24-36 hours of fasting all glycogen reserves are completely exhausted and it is only gluconeogenesis that creates glucose from proteins.
Gluconeogenesis continues to increase for the first two days of fasting or so and then the levels of glucose created from proteins drop until it stabilizes at a more or less constant level.
So far we have focused on glucose as the main fuel for our body.
In addition to glucose, however, almost all the cells in our body can also use fats to obtain energy and already in this first phase of fasting our body also uses fats for energy purposes in addition to glucose.
The use of fats for energy purposes increases significantly already in the first 12 hours from the last meal.
From 2 to 4 days after the last meal: FATS become our body’s main fuel
FROM 2 TO 4 DAYS (only under medical prescription and supervision!)
As mentioned above, almost all the cells in our body can use fats to obtain energy, with some important exceptions. For example:
- Glucose is essential for the cells of some organs in our body, including: the brain and the cells of the nervous system, some blood cells such as red blood cells, and the kidneys (in the renal medulla).
- Muscles can use fats but only for aerobic activity, while for anaerobic activity they need glucose.
When glucose is lacking, our body can use an alternative strategy to provide energy to these cells, converting fats into a particular category of fatty acids called ketones (or ketone bodies) which can be used by many of these cells for energy purposes instead of glucose.
Therefore, fats can directly power our muscles for anaerobic work and indirectly also our brain through ketone bodies.
When gluconeogenesis begins to decrease, this new metabolic process called ketosis is activated, which creates ketone bodies from fats.
Therapeutic fasting
OVER 36 HOURS (only under medical prescription and supervision)
Prolonged fasting beyond 36 hours is defined as therapeutic fasting and can only be practiced under medical supervision and when not contraindicated. It may be indicated in cases of diseases such as metabolic syndrome, pre-diabetes, type 2 diabetes, ongoing cancer therapy, persistent gastrointestinal disorders and dysbiosis, autoimmune diseases with an inflammatory component, and chronic pain. [5]
From 4 days of fasting onwards: protein conservation phase
OVER 4 DAYS (only under medical prescription and supervision)
If fasting continues, our body will use less and less glucose made from proteins through gluconeogenesis and more and more fats.
As we said before, however, the use of fats begins already in the early stages of fasting, increasing significantly already during the first 12 hours, but it only takes over after about 48 hours and reaches its maximum after about 4 days of fasting.
At this point our body settles on a concentration of ketone bodies in the blood of about 70 times higher than normal levels and 90% of the total energy expenditure in our body occurs through fats (directly or through ketone bodies).
10% of our body’s energy needs, however, continue to be covered by glucose, because some cells in our body, such as some brain cells, cannot do without it (from the fourth day of fasting onwards, the brain consumes energy 75% from ketones and about 25% from glucose).
Since blood sugar must always be kept constant, the amount of glucose created by gluconeogenesis continues to remain constant.
When a lot of fat is used to produce energy, our body also produces a waste product called glycerol which can be used to make glucose to continue to maintain blood sugar.
Therefore, in this phase of fasting, our body can also create glucose using glycerol in addition to proteins.
Already from the third day of fasting onwards, and then more and more from the fourth and fifth day, a significant amount of glucose is manufactured from glycerol.
This phase is called the “protein conservation phase”, because precisely there is less and less need to manufacture glucose from proteins with gluconeogenesis.
Autophagy: one of the benefits of fasting when prolonged
So far we have talked about the use of proteins for energy purposes. In any case, our body will continue to need proteins for structural and functional purposes, for example to maintain tissues. But since our body does not have access to new proteins because we are not eating and because it cannot manufacture them on its own, it begins to recover them from within the body itself.
To do this, our body in a sense begins to clean up, that is, to recover proteins by recycling them from where they are less needed to where they are most needed. This process is called autophagy and is one of the therapeutic benefits of fasting.
Autophagy recycles, for example, protein waste, old tissues, defective or precancerous cells, enzymes that are not immediately necessary such as those of digestion (since you are not eating) or those responsible for reproduction, those of the immune system, in addition to muscle proteins.
Therefore, old and damaged cells, including tumor cells, are destroyed, and a deep detoxification takes place with the elimination of viruses, toxins and bacteria.
Once the fast is complete, our body quickly rebuilds the healthy tissues that have decreased during the fast, such as muscle mass, by building new cells, driven by growth hormone and a process of stem cell division and proliferation. Therefore, fasting ultimately makes our body renewed, strengthened, cleansed and regenerated. [5]
The Japanese scientist Yoshinori Ōsumi won the Nobel Prize precisely thanks to his studies on autophagy.
“The human body lives through this process of self-decomposition, which is a form of cannibalism. It tries to maintain a delicate balance between construction and destruction. And this is what ultimately characterizes life.”
Yoshinori Ōsumi
In Summary: all the benefits of fasting
Benefits of Time-Restricted Eating (technically not fasting)
Not eating for 12-16 hours every day: YES
The practices of time-restricted eating (i.e., not eating for 16, 14, or 12 hours every day) are often mistakenly referred to as “intermittent fasting.”
Technically, fasting only begins when you continue not to eat beyond 16 hours after the last meal, and it is precisely after 16 hours that the most important benefits of fasting begin.
In any case, time-restricted eating can also bring health benefits, mainly because it can keep insulin low for several hours.
However, it is precisely after about 16 hours that the metabolic switch is triggered. Therefore, the famous “intermittent fasting” 16:8 restarts the process every time, leaving us in that phase where we have to break down some protein to maintain blood sugar. Whereas, true fasting (that which exceeds 16 hours) passes through this phase only once, after which our body begins to burn mostly fat.
For this reason, and also for practical reasons, it is therefore sufficient to focus on time-restricted eating of 12 hours, for example, simply by getting into the habit of scheduling dinner and breakfast the next day so that at least 12 hours pass between these two meals.
Benefits of fasting intermittently (intermittent fasting)
As explained earlier, we only speak of fasting when you do not eat and drink only water for more than 16 hours (fasting) on a regular (intermittent) basis.
Consuming only one meal a day every day (fasting for about 24 hours every day): NO
If practiced every day and it becomes a lifestyle, it is not advisable because in this way every day the body would spend a certain amount of time (between the 16th and 24th hour approximately) in which gluconeogenesis consumes proteins. Muscles are the most abundant protein reserves in our body, so every day this type of fasting would consume some of our muscles (the so-called “lean mass”).
Practicing one or two days of complete fasting regularly every week: NO, but a little better than just one meal a day always
For the same reasons as a single meal a day, fasting for one or two days every week is also not advisable. This practice, however, in theory can be relatively better because you go through the protein consumption phase (gluconeogenesis) only once or twice a week instead of every day.
Benefits of fasting occasionally
Fasting occasionally for more than 16 and less than 36 hours: YES
A true and feasible fast that can be practiced occasionally in a safe way (if you are in good health) consists of not eating, and drinking only water, for more than 16 hours and less than 36 from the last meal.
This type of occasional fasting, unlike intermittent fasting which is practiced regularly, minimizes the problem of protein consumption and can bring the following benefits to our health:
- Lower blood sugar and improve insulin sensitivity [1] [3]
- Reduce chronic inflammation [1] [3]
- Modulate the intestinal microbiota [3]
- Boost the immune system once the fast ends (but temporarily weakens it during the fast) [1]
- Help to resynchronize biological rhythms [3]
Occasional fasting may also provide the following benefits, although it is not clear whether these depend on the practice of fasting, or on the caloric restriction that often accompanies fasting*:
- Lower blood pressure, cholesterol and triglycerides [1] [3]
- Reduce abdominal fat and help with weight control [1] [3]
* A fast of even 36 hours could in theory not be accompanied by a caloric deficit. For example, if the caloric balance of the entire week in which the fast is practiced is higher than the personal caloric requirement: in other words, if you binge before or after the fast.
Benefits of fasting beyond 36 hours (therapeutic fasting)
As explained above, prolonged fasting beyond 36 hours is defined as therapeutic fasting. In particular cases, the benefits of therapeutic fasting can be significant, but it can only be practiced under medical prescription and supervision. [5]
References
This article is based on a series of 5 videos dedicated to fasting on the YouTube channel of Stefano Vendrame, a nutritionist biologist with a PhD in Food and Nutrition Sciences from the University of Maine and author of the book Trappole Alimentari (Food Traps).
[1] Il digiuno: tipologie, fisiologia e benefici
[2] Restrizione calorica: mangiare meno per vivere di più?
[3] Digiuno intermittente: la strada della salute?
[4] Tempi, frequenza e numero dei pasti
[5] Il digiuno terapeutico: nozioni pratiche
[6] Calorie restriction in humans builds strong muscle and stimulates healthy aging genes
[8] Yoshinori Ōsumi
Versione in Italiano: Benefici del digiuno intermittente e occasionale: come fare
