See also: fasting training
Although in our society the cases of prolonged fasting due to the absence of food are extremely rare, voluntary food deprivation is often undertaken for political, social or religious reasons. Since man can survive absolute fasting for about 24-30 days, the physiological response of the organism to this deprivation can be distinguished in 4 phases, respectively called the post-absorption period, short fast, fasting medium and prolonged fasting.
Metabolic modifications induced by fasting
Post-absorption period. It arises a few hours after the last food intake, as soon as the foods introduced in the last meal have been completely absorbed by the small intestine. On average it lasts three or four hours, followed by food intake that breaks the temporary state of fasting under normal conditions.
Short-term fasting. In the first 24 hours of food deprivation, the metabolism is supported by the oxidation of triglycerides and glucose deposited in the liver in the form of glycogen. Over time, given the modest amount of liver glycogen stocks, most tissues (muscle, heart, kidney, etc.) adapt to using mainly fatty acids, saving glucose. The latter will be destined above all to the brain and anaerobic tissues such as red blood cells which, in order to "survive", absolutely need glucose (they cannot, in fact, use fatty acids for energy purposes). In similar conditions the cerebral glucose demands amount to 4 g / hour, while those of anaerobic tissues amount to 1.5 g / hour. Since the liver is unable to extract more than 3 g of glucose per hour from glycogenolysis, he is forced to activate an "emergency" metabolic pathway called gluconeogenesis. This process consists of the production of glucose from amino acids.
Medium duration fasting. If food deprivation lasts longer than 24 hours the action described in the adaptation phase continues with a progressive accentuation of gluconeogenesis. The amino acids necessary to satisfy this process derive from the degradation of muscle proteins. Since there are no protein deposits in the body to be used for energy purposes, the body, in order to survive fasting, is therefore forced to "cannibalize" its muscles. This process is accompanied by an inevitable reduction in muscle mass, resulting in the appearance of weakness and apathy.
In the early stages, gluconeogenesis can also produce over 100 g of glucose a day, but soon the efficiency of this process decreases to about 75 g / day. Unlike the first, this quantity is no longer sufficient to guarantee an adequate supply of glucose to the brain. This organ is therefore forced to resort in an increasingly important way to ketone bodies, three water-soluble molecules deriving from the oxidation of fats in conditions of glucose deficiency. The overproduction of ketone bodies (ketosis), while prolonging the survival of the organism for a few days, causes an important increase in blood acidity.
In medium-term fasting, which extends up to the twenty-fourth day of food deprivation, the use of other tissues to lipid oxidation is increasing, in a general perspective of maximum savings in blood glucose.
Prolonged fasting and death. This phase begins when fasting lasts beyond the 24th day. The body has now exploited all the protein resources, including plasma proteins (reduction of the concentration of albumin in the blood → passage of plasma in the intracellular spaces → dehydration and the appearance of edema). The cocktail of ketosis, reduction of immune defenses, dehydration and reduced respiratory efficiency (given by the catabolism of diaphragm proteins and intercostal muscles) condemns the individual to an inauspicious fate.
Fasting: does it do good or does it hurt?
Many people resort to fasting driven by fads, advertising or food and health beliefs, at least questionable. Voluntary abstinence from food intake is understood, in these cases, as a moment of physical purification, aimed at eliminating the toxins accumulated due to an incorrect diet.
To analyze the question, after having described the biochemical aspects in broad terms, we can start from two assumptions. The first, irrefutable, is that we have plenty of food available, a high-calorie food that is often the basis of obesity; in short, we eat too much and the consequences are there for all to see: food excesses and a sedentary lifestyle are among the first causes of death in industrialized countries, including Italy. The second point is that a moderately hypocaloric diet, which can be summarized in the Japanese saying "hara hachi bu" (raised from the table with a 80% full stomach), is one of the best strategies to live longer and healthier.
Although many people should reduce food consumption, there is no need to resort to extreme solutions like prohibitive diets or fasting. Instead, as our grandparents used to say, it is enough to get up from the table when you are still a bit hungry and keep in mind that some movement never hurts.
Fasting, similar to physical activity, is a stress for the body. The difference is that, while sport leads to an improvement in organic capacities, fasting moves in the opposite direction. The lack and prolonged intake of nutrients reduces muscle mass and basal metabolism (up to 40% in extreme cases), the mind becomes clouded and a global state of debilitation occurs, characterized by a decrease in muscle strength and the ability to concentrate. All this has nothing therapeutic or detoxifying, rather ...
The partial or attenuated fasting could instead have positive implications, provided that it is applied with rationality. After a Christmas dinner, for example, it is useful to follow a hypocaloric diet rich in liquids and vegetables for two or three days. The important thing is to associate these foods with a certain amount of protein, perhaps obtained from lean fish (usually easy to digest), and fats, for example by consuming a handful of dried fruit. In this way one avoids "cannibalizing one's muscles" and excessively depressing one's metabolism to then pay the consequences. This last point must also be clear to those who resort to fasting in extremis to lose weight in view of the costume test. A few pounds can actually be lost, but the amount of energy associated with each unit of weight lost is very low. In other words, weight loss is mainly linked to increased diuresis and muscle catabolism induced by prolonged fasting.
