Let’s start from the beginning: I’m not for fasting, real fasting. More or less I associate the term fasting with the end of life, because I identify food with life. Those who are hungry are fine, in a certain sense, because being hungry is always a signal from our body, it is a request, and a body that asks, which makes explicit a need with a signal, is a body that aims for health, its subsistence. There is nothing worse than not feeling hungry, having no interest in food. In my opinion, it is also bad to use fasting to lose weight: it is a bit like admitting that we do not know how to manage our relationship with food in such a serious way that our last resort is to deprive ourselves of it.
I almost always advise against fasting to lose weight, with some exceptions: intermittent fasting, for example, I do not find a bad practice, but a sort of fasting discipline. You don’t eat at certain intervals of time (drink water, herbal tea, etc.), you eat at others, and this fasting / eating interval increases metabolism and strengthens the immune system. I don’t like true, total, prolonged fasting and I won’t talk about it well. The pros: fasting for a few days (24-48 hours) with lots of fluids and some nutrients in liquid form does not lower the metabolism and makes you lose a lot of weight quickly, as well as having a sort of detox effect, which some deny . Also the patient who carries out fasting therapy with a doctor

The cons: personally I see a lot of them. Starting from the dangers of a prolonged fast that causes a state of ketosis, with the risk of loss of lean mass (other than kilos of fat, in a few days you lose fluids and glycogen).
Find the explanations  here,  here  and here , but the references are many. In Italian, the opinions of Dr. Andrea Ghiselli on fasting prolonged beyond 24 hours: “With fasting, moreover, the lean mass is likely to suffer some damage, if we use this strategy to lose weight by not associating it with physical activity, but if we do physical activity we do not need the fasting “( source); the opinion of prof. Pietro Migliaccio, nutritionist and dietician “Fasting is best if followed for one day because already on the second day it produces ketone bodies that strain the liver, red blood cells and the brain”, the opinion of prof. Pier Luigi Rossi ”Fasting can be a healthy practice to reduce the accumulated fat mass. The critical point to avoid is the transition from fasting to famine. FASTING becomes FAMINE when ketone bodies exceed the value of 3 mg / 100 ml in the blood.In this condition the brain registers the presence of food famine and activates a different hormonal profile. There is an increase in the hormone cortisol, hormone of food famine and metabolic and psychic stress. Cortisol attacks muscle proteins and obtains from 2 grams of muscle protein, with loss of muscle, the formation of one gram of glucose to be sent to the brain. The organism enters ketosis, a condition that characterizes food famine. Fasting is a healthy way of life when it does not fall into ketosis ”.
Finally, I invite you to read this biologist’s article on the effects of fasting and lean body mass reduction here .

Fasting in these diets is indeed prolonged, not intermittent nor does it last only 24-48 hours, but it is equivalent to not touching solid food for more than two days, even five or six. A bit like the tube diet , which I included among the diets to avoid. The second problem, in addition to the reduction of lean mass and the side effects of ketosis, is stress: the reaction to lack of food is subjective and can be more or less stressful for those who fast, but I’m not talking about perceptible stress. or not only (that is, it is not that we get nervous!), I am talking about a stress of the body that suddenly sees itself denied food. Certainly diet is also a stress, every deprivation is stressful. Our body is made to survive, mother nature is exceptional: when we get stressed with a forced weight loss,we also change the way we epigenetically re-fatten . In other words, mutations occur due to the fact that with the previous “moment of famine” the body reacts to its hunger for starchy and fatty foods, and assimilates twice as much.

Not to mention the “side effects” in the short and long term : physically we go from dizziness to arrhythmias up to the compromise of some functions of our vital organs if, for example, we fast for more than 5 days, while maybe we are not at home. lazing around but doing all our normal activities. Furthermore, a body, even when overweight, can have nutritional deficiencies that fasting would aggravate . Of course, in many cases it is not a total fast but a fast made with nutrients in liquid form (which, however, are only proteins and mineral salts: in order for there to be no problems, a proportion of fats should be guaranteed.), but the body does not seem to recognize liquids as food (the sense of satiety felt by chewing is lessened), and this is another big problem of fasting, even if it is “protein” or modified or assisted.
As you can see, I don’t even dare to tell you what fasting can trigger on a psychological level.
My advice is therefore to be wary.