Carbohydrates are sugars and the purpose of their homeostasis (i.e. equilibrium) is to supply the nervous tissue (brain), in conditions of lack of food intake, the quantity of glucose sufficient for its functioning. In fact, in order to function properly, the nervous tissue is strictly glucose-dependent. A further purpose of glucose homeostasis is to store in some organs the excess of energetic substances, in particular glucose, introduced with food, preventing an excessive increase in glycaemia (ie the concentration of glucose in the blood).
After a night of fasting, the glucose present in the blood is used for the most part by the brain, to a lesser extent by red blood cells, intestines and tissues sensitive to insulin (muscle and adipose tissue), which is the hormone that allows these same tissues to take advantage of the glucose and store it inside them. The liver is able to store glucose in the form of glycogen (many glucose molecules "packed" together) and to release it in the form of glucose. a fundamental role in the homeostasis of sugars. The production of glucose by the liver, in fact, is regulated by two hormones, insulin and glucagon. In the absence of insulin there is a release of glucose from the liver into the blood, which leads to an increase in blood sugar (hyperglycemia) in the blood itself. In the absence of glucagon, the hepatic breakdown of glucose is blocked with consequent reduction of the same in the blood (hypoglycemia). The utilization of glucose by other organs, called peripheral, is also reflected in a reduction in glycaemia; it follows a reduction of insulinemia (quantity of insulin in circulation), an increase of glucagonemia (quantity of glucagon in circulation) and a readjustment of the system through an "increased hepatic dismissal of glucose.
Alongside and in equilibrium with the insulin-glucagon system, there is the so-called counter-regulator or counterinsular system, represented by the pituitary and adrenal glands. Through the secretion of hormones such as GH, ACTH, cortisol and catecholamines (adrenaline and noradrenaline), this system exerts a hyperglycemic effect, that is, it increases the release of glucose into the circulation.
Following a meal, glucose absorbed from the intestinal tract causes an increase in blood sugar. Carbohydrates (which are polysaccharides, or formed by different types of sugars put together), once arrived in the intestine, are reduced to monosaccharides, which are glucose (80%), fructose (15%) and galactose (5%). They are then absorbed by the cells of the intestinal mucosa and, from there, are transported to the blood. Generally, after a mixed meal (50% carbohydrates, 35% fat, 15% protein) the blood sugar returns to pre-meal levels (those before lunch) after about 2-3 hours.
The passage and energy absorption of sugars (but also proteins and fats) through the alimentary tract trigger a series of signals that allow the storage of nutrients in various organs. At the same time, the secretion of insulin, the main blood sugar regulating hormone, is stimulated. The increase in plasma levels of this hormone causes a decrease in the levels of glucagon, its antagonist, and causes a decrease in hepatic glucose clearance because it inhibits the breakdown of glycogen into glucose (glycogenolysis) and the synthesis of new glucose from amino acids (gluconeogenesis). The liver, which is freely permeable to glucose, sequestrates about 50% of glucose to convert it to glycogen (action controlled by insulin). Glucose not sequestered by the liver is distributed to muscle and adipose tissue. When blood glucose tends to drop, there is a gradual increase in hepatic glucose production, along with a decrease in plasma insulin levels and an increase in counterinsular hormones, especially glucagon.
