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I warn you, I will write a lot, but I will try to make the text as readable as possible. If you've been skewering tuevy heaps of explanations on the buoy, it's better to immediately scroll to the final. From a biochemical and physiological point of view, it looks like this:
The fact is that glucose in our body, for the most part, is needed for the brain. (Glucose – – – > the chain of biochemical reactions – – – – – > > Energy (ATP) – – – – – > > the work of the brain, as well as other organs and tissues). Feeding the brain is its main function. Of course, there are others whose importance should not be belittled, but the question is not about them. The brain plays the main role in the body, works the most, respectively, has the greatest need for energy and, accordingly, has the most intensive blood supply of all organs. In addition, nerve tissue is one of the so-called glucose-dependent tissues, along with red blood cells and kidney medulla. That is, the brain can burn only glucose for the sake of obtaining energy on which it can function. And not to use the organ in order to save what is intended and necessary for its operation is simply meaningless.
Of course, with increased mental activity, the blood supply to the brain increases, and the consumption of glucose and oxygen increases. But this is a natural physiological process.
Glucose is contained in our blood in a certain concentration (~ 3.3-5.5 mmol / l), and in addition it is stored in the form of glycogen in the brain itself (in a very small amount, which is enough for 5-6 minutes after the supply of glucose with blood is stopped), in the muscles and, most of all, in the liver.
When the concentration of glucose in the blood decreases or increases, a series of chain reactions are triggered, leading to its increase or decrease in the blood, respectively. That is, everything is kept at a certain level, not just glucose. And this self-regulating balance is called homeostasis.
Carbohydrates cover no more than 60% of all energy expenditure. In many tissues of the body, it is possible to use other intermediate metabolic products of proteins, fats and other carbohydrates ( that is, in addition to glucose) for the sake of energy. And glucose can also be synthesized from all this (proteins, fats, carbohydrates) through the Krebs cycle (Google it if you're interested). Some of them are used to a greater extent, others to a lesser extent, with different nuances, different additional conditions and different energy output.
(For example: the brain works mainly 90% on oxygen-dependent glucose breakdown, that is, on aerobic glycolysis, with an energy output of 38 ATP molecules from one glucose molecule, and 10% on anaerobic-only 2 ATP molecules from the same glucose molecule. For example, muscles are capable of anaerobic glycolysis, resulting in lactic acid, which causes muscles to ache after intense physical activity. As was correctly written by Denis Tsaregorodtsev, muscles spend a lot of energy at the time of work, especially intense. In general, yes, they spend more energy, but it's a slightly different story with glucose. Muscles can also receive energy in several other ways than the way the brain receives it. Athletes and workers of heavy physical labor burn mostly fat for energy. And glucose continues to feed their brains. The resulting energy (ATP) is used further in chemical reactions, depending on the tissue in which it was formed. In muscle – ATP goes to the contraction of this muscle.)
Why does the brain need aerobic oxidation? Such oxidation has an important advantage – the brain does not swell, the person does not fall into a coma and does not die. This is already a separate topic, which will require the same canvas of text.
“Do not think” will help, in view of the decrease in the intensity of blood supply to the brain, imperceptibly for you, to slow down glucose consumption by microscopic amounts.
The brain can't function without glucose. Don't think to save glucose = don't breathe to save air. In principle, it is possible, but why? (if you allow a complete shutdown of consciousness, as in the situation described by Dmitry, with hibernation-then you will benefit).
All metabolic pathways in the body intersect. They spiral away and intersect again. Therefore, the body will find glucose from where to get out no matter how hard you try to spend it with persistent thoughts or physical exertion. And it doesn't make any sense to “protect” it in the way you suggested.
Not thinking is UNPROFITABLE! Survival in nature is provided primarily by the power of the mind, and not at all by physical abilities. Both the strong and the weak can escape from the enemy, but a stupid person who misjudged their capabilities and chose the wrong line of behavior will not be able to survive.
And the “intracranial muscle” needs constant training, just like any other. Therefore, using energy for the thought process is completely normal. That's what energy is for.
By the way, according to physiologists, 80% of the energy absorbed with food, a living being spends on mental activity!
The brain, of course, consumes a lot of energy and, accordingly, glucose for its activities. But the muscles (and especially the muscles involved in physical work or sports) consume it in much larger quantities when they work. From this, to conclude that it is profitable to lie down and not move, and athletes are just crazy spenders of valuable energy-this is some kind of wrong conclusion. It seems to me that there are no benefits from not thinking twice. And there are no problems with eating and getting carbohydrates either.
Well, except to imagine a fantastic situation that a spaceship has crashed, and the crew is carrying it is not known where, and food is limited, but there is a chance that rescuers will arrive – well, then yes. It is better to go into hibernation – the chances of survival will increase.