Protein metabolism has a special place. It provides reproduction and renewal of the body proteins, controls variety of chemical reactions in living organism. All other kinds of metabolism serve for the protein metabolism. Carbohydrate metabolism is mainly a source of carbon chains in amino acids biosynthesis. Fats metabolism supplies necessary energy. Nucleic acids metabolism provides storage and transfer of information about protein synthesis. Mineral metabolism promotes formation or disintegration of enzymatic systems by means of which protein synthesis occurs.
Protein metabolism is related to providing the body with vitamins and regulating the activity of the endocrine glands. For example, after the injection of thyroid gland hormones intensive degradation of tissue proteins is observed. Androgens and estrogens promote protein synthesis. Protein metabolism proceeds more intensively in child age, during active muscle work, pregnancy and lactation.
The main sources of proteins for humans are food of animal and vegetable origin. Protein content: soy (soya beans) - 35%, peas - 26%, meat, fish - 18-22%, potatoes - 1,5-2%, apples - 0,3-0,4%.
The intake of dietary protein is 100-120 g/day for an adult who is engaged in brainwork, 130-150 g/dayare necessary for workers who are carrying out heavy physical activity.
The qualitative composition of protein nutrition is very important. Only 10 out of 20 necessary amino acids are synthesized in a human body. These are essential amino acids. The others 10 amino acids should be got with nutrition. Essential amino acids are valine, leucine, isoleucine, lysine, methionine, threonine, tryptophan, phenylalanine. Arginineand histidine are semi-essential, because adult organism can partially synthesize them. The absence or even lack of one essential amino acid can limit biosynthesis of all proteins in an organism.
There are biologically more valuable and less valuable proteins. For human the proteins of meat, milk, eggs are biologically more valuable, as their amino acid composition is closer to the amino acid composition of human body. The composition of proteins of wool, hair, feathers is also close to human body proteins, but they are not hydrolyzed by intestine proteinases and not used as nutrients.
Proteins are not stored, but as the end products of nitrogen metabolism they are excreted with urine.
Due to insufficient intake of proteins with meal proteins of liver, muscles, blood plasma (reserve proteins) degrade (about 25g/day). The formed amino acids are used for synthesis of necessary proteins, enzymes, hormones, and also as energy sources.
More than 95% of all nitrogen in an organism is nitrogen of amino acids (as part of proteins and free). Amino acid and protein metabolism can be judged by the state of the nitrogen balance. Nitrogen balance is the measure of nitrogen output subtracted from nitrogen input. Healthy adult with normal nutrition has nitrogen equilibrium. During the period of organism growth, as well as after convalescence from exhausting diseases nitrogen is deduced less, than it arrives, it is a positive nitrogen balance. Ageing, starving and during exhausting diseases an organism deduces more nitrogen than it gets, it is a negative nitrogenbalance. A positive value is often found during periods of growth, tissue repair or pregnancy. This means that the intake of nitrogen into the body is greater than the loss of nitrogen from the body. A negative value can be associated with burns, ageing, starving and exhausting diseases. This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested.