Carbohydrate metabolism

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Carbohydrates are organic, water-soluble substances. They consist of carbon, hydrogen and oxygen, with the formula (CH2O)n, where 'n' can vary from 3 to 7. Carbohydrates are mainly contained in plant products (except lactose).

Based on the chemical structure, carbohydrates are divided into three groups:

Monosaccharides are the "basic units" of carbohydrates. The number of carbon atoms distinguishes these basic units from each other. The suffix "osa" is used to define these molecules in the category of sugars:

  • triosa is a monosaccharide with 3 carbon atoms

The hexose group includes glucose, galactose and fructose.

  • Glucose, also known as sugar contained in blood, is the sugar into which all other carbohydrates in the body are converted. Glucose can be obtained by digestion or formed as a result of gluconeogenesis.

Oligosaccharides consist of 2-10-linked monosaccharides. Disaccharides, or double sugars, are formed from two monosaccharides linked together.

  • Lactose (glucose + galactose) is the only kind of sugars that does not occur in plants, but is contained in milk.

Monosaccharides and disaccharides form a group of simple sugars.

Polysaccharides are formed from 3 to 1000 monosaccharides linked together.

  • Starch is a vegetable form of storing carbohydrates. Starch exists in two forms: amylose or aminopectin. Amylose is a long unbranched chain of spirally twisted glucose molecules, while amylopectin is a highly branched group of bound monosaccharides.

Most carbohydrates we consume in the form of starch. Digestion of starch begins in the mouth under the action of saliva amylase. This process of digestion with amylase continues in the upper part of the stomach, then the action of amylase is blocked by gastric acid.

The digestion process is then completed in the small intestine with the help of pancreatic amylase. As a result of the cleavage of starch with amylase, maltose disaccharide and short branched chains of glucose are formed.

These molecules, now present in the form of maltose and short branched chains of glucose, will then be split into individual glucose molecules by enzymes in the cells of the small intestine epithelium. The same processes occur when lactose or sucrose is digested. In lactose, the relationship between glucose and galactose is broken, resulting in the formation of two separate monosaccharides.

In sucrose, the bond between glucose and fructose is broken, resulting in the formation of two separate monosaccharides. Individual monosaccharides are then passed through the intestinal epithelium into the blood. When absorption of monosaccharides (such as dextrose, which is glucose), digestion is not required, and they are absorbed quickly.

Once in the blood, these carbohydrates, now in the form of monosaccharides, are used for their intended purpose. Since fructose and galactose eventually turn into glucose, I will then refer to all the digested carbohydrates, labeling them as "glucose".

Getting absorbed, glucose is the main source of energy (during or immediately after eating). This glucose is catabolized by cells to obtain energy for the formation of ATP. Glucose can also accumulate in the form of glycogen in muscle and liver cells. But before that, it is necessary that glucose enters the cells. In addition, glucose enters the cell in various ways depending on the type of cells.

To assimilate, glucose should get into the cage. In this it helps transporters (Glut-1, 2, 3, 4 and 5). In cells where glucose is the main source of energy, for example, in the brain, kidneys, liver and erythrocytes, glucose uptake takes place freely. This means that glucose can enter these cells at any time. In fat cells, heart and skeletal muscles, on the other hand, glucose uptake is regulated by the Glut-4 transporter. Their activity is controlled by the hormone insulin. Reacting to an elevated blood glucose level, insulin is released from the beta cells of the pancreas.

Insulin binds to the receptor on the cell membrane, which, through various mechanisms, results in the translocation of Glut-4 receptors from intracellular stores to the cell membrane, allowing glucose to enter the cell. Reduction of skeletal muscles also enhances translocation of the Glut-4 transporter.

When the muscles contract, calcium is released. This increase in calcium concentration stimulates the translocation of GLUT-4 receptors, contributing to the absorption of glucose in insulin deficiency.

Although the effects of insulin and exercise on the translocation of Glut-4 are additive, they are independent. Once inside the cell, glucose can be used to meet energy needs or synthesized into glycogen and stored for later use. Glucose can also be converted to fat and stored in fat cells.

Once in the liver, glucose can be used to meet the energy needs of the liver, stored as glycogen or converted to triglycerides for storage in the form of fat. Glucose is a precursor of glycerol phosphate and fatty acids. The liver converts excess glucose into glycerol phosphate and fatty acids, which then combine to synthesize triglycerides.

Some of these formed triglycerides are stored in the liver, but most of them together with proteins are converted to lipoproteins and secreted into the blood.

Lipoproteins, which contain much more fat than protein, are called very low density lipoproteins (VLDL). These VLDLs are then transported through the blood into the adipose tissue, where they will be stored as triglycerides (fats).

In the body, glucose is stored as a glycogen polysaccharide. Glycogen consists of hundreds of glucose molecules linked together and stored in muscle cells (about 300 grams) and liver (about 100 grams).

The accumulation of glucose in the form of glycogen is called glycogenesis. During glycogenesis, glucose molecules are alternately added to the existing glycogen molecule.

The amount of glycogen stored in the body is determined by the consumption of carbohydrates; a person on a low-carb diet of glycogen will be less than a person on a diet high in carbohydrates.

To use the accumulated glycogen, it must be split into individual glucose molecules during a process called glycogenolysis (lys = cleavage).

For normal functioning, glucose is necessary for the nervous system and the brain, since the brain uses it as the main source of fuel. With insufficient supply of glucose as an energy source, the brain can also use ketones (byproducts of incomplete fat breakdown), but this is more likely to be considered a reserve option.

Skeletal muscles and all other cells use glucose for their energy needs. When the necessary amount of glucose is not supplied to the body with food, glycogen is used. After the glycogen stores have been exhausted, the body is forced to find a way to get more glucose, which is achieved by gluconeogenesis.

Gluconeogenesis is the formation of a new glucose from amino acids, glycerol, lactate or pyruvate (all non-glucose sources). In order to obtain amino acids for gluconeogenesis, muscle protein can be catabolized. When providing the necessary amount of carbohydrates, glucose serves as a "protein wholesaler" and can prevent the cleavage of muscle protein. Therefore, it is so important for athletes to consume enough carbohydrates.

Although there is no defined consumption rate for carbohydrates, it is estimated that 40-50% of calories consumed should be supplied by carbohydrates. For athletes, this is the expected rate of 60%.

Adenosine triphosphate, an ATP molecule contains macroergic phosphate bonds and is used to store and release the body's energy.

As with many other issues, people continue to argue about the amount of carbohydrates needed by the body. For each person, it should be determined taking into account a variety of factors, including: type of training, intensity, duration and frequency, the total number of calories consumed, the purpose of training and the desired result, taking into account the constitution of the body.

  • Carbohydrates = (CH2O) n, where n varies from 3 to 7.

The material is presented in an accessible form, a good language. Thanks to the author!

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