Fasting to lose weight and get more benefits as compared to the same practice carried out on a full stomach. Usually this activity, such as running in the morning, it is practised, following an overnight fast, on average, lasts 8-10 hours and provides in most cases the performance of programs, as just cardio, and then the aerobic, which would favour the oxidation of fat during the exercise itself.
This method of training is at the heart of many debates: those who want to inquire are often conflicting opinions that generate confusion and doubts.
Beyond the information provided by several websites and magazines, where each claims basically his idea citing scientific articles to convince you that they are on the side of reason, it is good to know that the results that emerged from the numerous studies can not and must not be taken for liquid gold.
Why, you ask yourself, since they are the result of a study conducted by doctors and scientists?
The answer is quite simple: the results must always be CONTEXTUALIZED or we always need to keep in mind the conditions in which they were obtained, as for example the physical state of the people who took part in the research (they were athletes or not?), what were they fed before physical activity (carbohydrates? Protein? A mix?), the level of intensity at which they trained, and so on talking.
In essence, different conditions give different results.
The aim of this article is to try to make some clarity and to get involved in the different studies carried out.
Let’S start with the theory that underpins this training methodology.:
The nocturnal fast, which lasts 8-12 hours, leads to a reduction in blood sugar levels, which the body faces by using stocks of hepatic glycogen (the form in which the glucose is stored).
The reduction of such stocks causes the body to prioritize fat as an energy source rather than glucose, not only in the hours of fasting but also during the aerobic workout, especially if low-intensity (< 50% VO2max).
In addition, since insulin is stimulated by ingestion of carbohydrates (also present in mixed meals) exerts an inhibitory action on the expenditure of fat, promoting the use of carbohydrates, both at rest and during exercise in an aerobic intensity mild, if its levels are low, such as those arising from the overnight fast, the situation reverses, and therefore during the year will be mobilised fat will be used as a source of energy.
In addition, it is never considered to be a fundamental aspect of power: the different macronutrients (carbohydrates, proteins, and fats) are capable of stimulating different metabolic reactions on the basis of their nature that some foods inhibit the oxidation of fats, while others have an effect permissive on this process.
Having said that, let us now address the main factors that determine the effects of training on our body.
As a general rule, when glucose levels fall below a certain threshold, so-called anti-regulatory catabolic hormones are released, which can be divided as follows::
fast-acting hormones (catecholamines and glucagon));
slow acting hormones (GH and cortisol).
In short-term fasting, such as nocturnal fasting, fast-acting hormones play a more important role.
The lack of food intake during the night leads to affect the balance of insulin to glucagon, two hormones that regulate sugar levels in the blood, increasing plasma concentrations of glucagon (which stimulates the degradation of glycogen and release of glucose) and reducing concentrations of insulin (which lowers blood sugar and promotes the storage of glucose in form of glycogen).
This situation promotes catabolic processes, i.e. all processes that determine the mobilization of substrates and the production of energy, at the expense of those of anabolic, or build up processes: in other words, since during fasting we do not take nutrients the body uses up its reserves to maintain blood glucose constant, or produce glucose from other substrates, such as:
Lactate (or lactic acid) which is converted to glucose from the liver through the cycle of choirs.
The various glucogenetic amino acids (alanine, threonine, glycine, glutamic acid), resulting from the catabolism of proteins in skeletal muscle.
glycerol, which comes from triglyceride splitting into adipose tissue. At this stage, free fatty acids (FFA) from the hydrolysis of triglycerides deposited in adipose tissue become the main fuel to support energy demands. Some tissues, such as skeletal muscle and the liver, are able to increase the utilization of FFA at the expense of glucose, to increase the availability to the central nervous system (CNS) and other tissues are strictly dependent on glucose tissues glucose-dependent).
Thus, during the early stages of fasting, hepatic glycogen is the main source of glucose available.
Glycaemic stability is maintained as skeletal muscle and other tissues begin oxidising substrates of lipid origin instead of glucose, and hepatic gluconeogenesis (i.e. glucose from non-glucidic substrates) replaces hepatic glycogenolysis (i. e. glycogen degradation to glucose).
Two factors can stimulate catabolism of triglycerides deposited in fat tissue during fasting:
the reduction in circulating insulin concentration results in the suppression of triglyceride synthesis (lipogenesis) and gives way to lipolysis (mobilisation of the same) noradrenaline is released from nerve endings by directly stimulating lipolysis.
Different conditions are catabolic of prolonged fasting after 12-16 hours, in which the reserves of hepatic glycogen are exhausted: the process of gluconeogenesis becomes dominant, for which the glucose must be derived from the conversion of other molecules that are not glucidiche, which, as mentioned, are lactate, glycerol and amino acids arising from breakdown of skeletal muscle (muscle catabolism). It is at this stage that the slow acting counter-regulators such as GH and cortisol intervene. cortisol stimulates hepatic gluconeogenesis as well as lipolysis, resulting in increased levels of free fatty acids and glycerol, and also causes muscle catabolism GH has similar effects on lipolysis and gluconeogenesis, while simultaneously suppresses peripheral glucose utilisation.
Now that we have a general picture of the physiology, it seems clear that practicing aerobic activity, fasting is an extremely effective method for losing fat mass; however, is missing an essential element!
The physiology of the metabolism in fasting that we have considered is valid for a resting person; the metabolic framework, in fact, undergoes a considerable variation if physical activity is carried out, especially if prolonged and intense: glycogen reserves are consumed first the hormonal framework changes: an organism under stress produces cortisol, which at rest would be secreted in greater quantities only after a prolonged fast, which stimulates the demolition of proteins and inhibits resynthesis;
These are just two of the processes we name, as it would certainly not be enough to describe all the changes and adaptations that take place.
The concept that really is important to understand is that our organism is endowed with intelligence, and all it does is to safeguard itself.:
Glycogen stocks are sufficient to cope with even 12/16 hours of fasting, but, of course, time is reduced if glucose is to be used to support training;
also, no less important, our body will not wait to exhaust its reserves before starting to use other ways to produce energy substrates:
Muscle catabolism begins significantly when the reserves were reduced by a quarter (well, sooner than you think) as there are tissues and cells, such as the nervous tissue, that is the brain, and red blood cells in the blood that are defined glucose dependent, or function dependent on glucose;
It follows that our organism prefers to preserve the glycogen reserves in order to allocate them to these fundamental tissues, and in the meantime to produce new glucose for the others, sacrificing less important tissues.