Protein metabolism – the use and conversion of amino acids to proteins in the human body.
As a result of the oxidation of 1 g of protein, 17.2 kJ (4.1 kcal) of energy is released.
But proteins are rarely used in the body for energy, since they are needed to perform more important functions (the main function is building). The human body does not need food proteins by themselves, but the amino acids of which they are composed.
In the process of digestion, food proteins are broken down by the action of digestive enzymes to amino acids. Amino acids are absorbed by the villi of the small intestine and enter the bloodstream, which delivers them to the cells. In cells, new proteins characteristic of the human body are synthesized from amino acids.
The liver controls the content of individual amino acids in the blood. Amino acids break down to form water, carbon dioxide and toxic ammonia. In the liver cells, urea is synthesized from the formed ammonia (which is then excreted together with water by the kidneys as part of urine and partly by the skin), and carbon dioxide is exhaled through the lungs.
Remains of amino acids are used as energy material (converted into glucose, the excess of which is converted into glycogen).
Carbohydrate metabolism is a set of processes for the conversion and use of carbohydrates.
Carbohydrates are the main source of energy in the body. When 1 g of carbohydrates (glucose) is oxidized, 17.2 kJ (4.1 kcal) of energy is released.
Carbohydrates enter the human body in the form of various compounds: starch, glycogen, sucrose or fructose, etc. All these substances break down during digestion to glucose, which is absorbed by the walls of the small intestine and enters the bloodstream.
Glucose is the main energetic substance of the body. It is necessary for the work of all organs.
The main part of glucose is oxidized in cells to carbon dioxide and water, which are removed with exhaled air or urine.
Part of the glucose is converted into the polysaccharide glycogen and is deposited in the liver (up to 300 g of glycogen can be deposited) and muscles (glycogen is the main supplier of energy for muscle contraction).
The blood glucose level is constant (0.10–0.15%) and is regulated by thyroid hormones, including insulin. With a lack of insulin, the level of glucose in the blood rises, which leads to a serious illness – diabetes mellitus.
Insulin also inhibits the breakdown of glycogen and increases its content in the liver.
Another pancreatic hormone – glucagon – promotes the conversion of glycogen into glucose, thereby increasing its content in the blood (i.e., it has an effect opposite to insulin).
With a large amount of carbohydrates in food, their excess turns into fats and is deposited in the human body.
1 g of carbohydrates contains significantly less energy than 1 g of fat. But carbohydrates can be oxidized quickly and quickly to get energy.
Fat metabolism is a set of processes for the transformation and use of fats (lipids).
When 1 g of fat breaks down, 38.9 kJ (9.3 kcal) of energy is released (2 times more than when 1 g of proteins or carbohydrates are broken down).
Fats are compounds that include fatty acids and glycerin. Fatty acids under the action of enzymes of the pancreas and small intestine, as well as with the participation of bile, are absorbed into the lymph in the villi of the small intestine. Further, with the flow of lymph, lipids enter the bloodstream, and then into the cells.
When oxidized, fats are converted to carbon dioxide and water, and metabolic products are removed from the body.
The endocrine glands and their hormones are involved in the humoral regulation of fat levels.
The value of fats
- Oxidation of fats provides energy for the work of internal organs.
- Lipids are structural elements of cell membranes, are part of mediators, hormones, form subcutaneous fat deposits and omentum.
- Being deposited in a reserve in the connective tissue membranes, fats prevent displacement and mechanical damage to organs.
- Subcutaneous adipose tissue does not conduct heat well, which contributes to maintaining a constant body temperature.
It is recommended to consume 80-100 g of different fats daily. Excess fat is deposited under the skin, in the tissues of some organs (such as the liver), and also on the walls of blood vessels.
If the body lacks some substances, then they can be formed from others. Proteins can be converted to fats and carbohydrates, and some carbohydrates can be converted to fats. In turn, fats can become a source of carbohydrates, and the lack of carbohydrates can be replenished at the expense of fats and proteins. But neither fats nor carbohydrates can be converted into proteins.
It has been established that an adult spends at least 1500-1700 kcal per day. Moreover, 15–35% of the energy received is spent on the body’s own needs, and the rest is spent on generating heat and maintaining body temperature.