Protein metabolism: proteins and protein requirements

Protein is one of the main and vital products. It has now become obvious that using protein for energy expenditure is irrational, since the breakdown of amino acids produces many acid radicals and ammonia, which are not indifferent to the child's body.

What is protein?

There are no protein reserves in the human body. Only when tissues disintegrate do proteins break down in them, releasing amino acids that are used to maintain the protein composition of other, more vital tissues and cells. Therefore, normal growth of the body without sufficient protein is impossible, since fats and carbohydrates cannot replace them. In addition, proteins contain essential amino acids that are necessary for the construction of newly formed tissues or for their self-renewal. Proteins are a component of various enzymes (digestive, tissue, etc.), hormones, hemoglobin, and antibodies. It is estimated that about 2% of muscle tissue proteins are enzymes that are constantly renewed. Proteins act as buffers, participating in maintaining a constant reaction of the environment in various fluids (blood plasma, cerebrospinal fluid, intestinal secretions, etc.). Finally, proteins are a source of energy: 1 g of protein, when completely broken down, produces 16.7 kJ (4 kcal).

The nitrogen balance criterion has been used for many years to study protein metabolism. This is done by determining the amount of nitrogen coming from food and the amount of nitrogen lost with feces and excreted with urine. The loss of nitrogenous substances with feces is used to judge the degree of protein digestion and its resorption in the small intestine. The difference between the nitrogen in food and its excretion with feces and urine is used to judge the degree of its consumption for the formation of new tissues or their self-renewal. In children immediately after birth or in low-weight and immature children, the very imperfection of the system of assimilation of any food protein, especially if it is not the protein of mother's milk, can lead to the impossibility of nitrogen utilization.

Timing of the development of gastrointestinal tract functions

Age, months	FAO/WHO (1985)	UN (1996)
0-1	124	107
1-2	116	109
2-3	109	111
3^	103	101
4-10	95-99	100
10-12	100-104	109
12-24	105	90

In adults, the amount of nitrogen excreted is usually equal to the amount of nitrogen ingested with food. In contrast, children have a positive nitrogen balance, i.e. the amount of nitrogen ingested with food always exceeds its loss with feces and urine.

The retention of dietary nitrogen, and therefore its utilization by the body, depends on age. Although the ability to retain nitrogen from food is maintained throughout life, it is greatest in children. The level of nitrogen retention corresponds to the growth constant and the rate of protein synthesis.

Rate of protein synthesis at different age periods

Age periods	Age	Synthesis rate, g/(kg • day)
Low birth weight newborn	1-45 days	17.46
A child in his second year of life	10-20 months	6.9
Adult	20-23 years old	3.0
An elderly man	69-91 years	1.9

Properties of food proteins taken into account when setting nutrition standards

Bioavailability (absorption):

• 100 (Npost - Nout) / Npost,

Where Npost is the nitrogen received; Next is the nitrogen excreted with feces.

Net Utilization (NPU %):

• (Nпш-100 (Nсn + Nvч)) / Nпш,

Where Nпш is food nitrogen;

Nst - fecal nitrogen;

Nmch - urine nitrogen.

Protein efficiency ratio:

• Weight gain per 1 g of protein consumed in a standardized experiment on rat pups.

Amino acid "score":

• 100 AKB / AKE,

Where Akb is the content of a given amino acid in a given protein, mg;

AKE - the content of a given amino acid in the reference protein, mg.

To illustrate the concept of “score” and the concept of “ideal protein”, we present data on the characteristics of “score” and the utilization of several food proteins.

"Amino acid score" and "net utilization" values of some food proteins

Protein	Skor	Disposal
Maize	49	36
Millet	63	43
Rice	67	63
Wheat	53	40
Soybeans	74	67
Whole egg	100	87
Breast milk	100	94
Cow's milk	95	81

Recommended Protein Intake

Considering the significant differences in the composition and nutritional value of proteins, protein provision calculations at an early age are made only and exclusively for proteins of the highest biological value, quite comparable in nutritional value to the protein of human milk. This also applies to the recommendations given below (WHO and MZ of Russia). In older age groups, where the overall need for protein is somewhat lower, and in relation to adults, the problem of protein quality is satisfactorily solved by enriching the diet with several types of vegetable proteins. In the intestinal chyme, where amino acids of various proteins and blood serum albumins are mixed, an amino acid ratio close to the optimal one is formed. The problem of protein quality is very acute when eating almost exclusively one type of vegetable protein.

General protein standardization in Russia differs somewhat from sanitary standardization abroad and in WHO committees. This is due to some differences in the criteria for optimal provision. Over the years, these positions and different scientific schools have come closer together. The differences are illustrated by the following tables of recommendations adopted in Russia and in WHO scientific committees.

Recommended Protein Intake for Children Under 10 Years

Indicator	0-2 months	3-5 months	6-11 months	1-3 years	3-7 years	7-10 years
Total proteins, g	-	-	-	53	68	79
Proteins, g/kg	2,2	2.6	2.9	-	-	-

Safe levels of protein intake in young children, g/(kg • day)

Age, months	FAO/WHO (1985)	UN (1996)
0-1	-	2.69
1-2	2.64	2.04
2-3	2.12	1.53
3^	1.71	1.37
4-5	1.55	1.25
5-6	1.51	1.19
6-9	1.49	1.09
9-12	1.48	1.02
12-18	1.26	1.00
18-24	1.17	0.94

Taking into account the different biological value of plant and animal proteins, it is customary to implement standardization both by the amount of protein used and by animal protein or its share in the total amount of protein consumed per day. An example is the table on standardization of protein M3 of Russia (1991) for children of older age groups.

Ratio of plant and animal protein in the recommendations for consumption

Squirrels	11-13 years old		14-17 years old
	Boys	Girls	Boys	Girls
Total proteins, g	93	85	100	90
Including animals	56	51	60	54

The Joint FAO/WHO Expert Group (1971) considered that the safe level of protein intake, in terms of cow's milk protein or egg white, is 0.57 g/kg body weight per day for an adult male and 0.52 g/kg for a woman. The safe level is the amount necessary to meet the physiological needs and maintain the health of almost all members of a given population group. For children, the safe level of protein intake is higher than for adults. This is explained by the fact that tissue self-renewal occurs more vigorously in children.

It has been established that the absorption of nitrogen by the body depends on both the quantity and quality of protein. The latter is more correctly understood as the amino acid composition of the protein, especially the presence of essential amino acids. Children's needs for both protein and amino acids are significantly higher than those of adults. It has been calculated that a child needs approximately 6 times more amino acids than an adult.

Essential Amino Acid Requirements (mg per 1 g protein)

Amino acids	Children			Adults
	Up to 2 years	2-5 years	10-12 years
Histidine	26	19	19	16
Isoleucine	46	28	28	13
Leucine	93	66	44	19
Lysine	66	58	44	16
Methionine + cystine	42	25	22	17
Phenylalanine + tyrosine	72	63	22	19
Threonine	43	34	28	9
Tryptophan	17	11	9	5
Valin	55	35	25	13

The table shows that children's need for amino acids is not only higher, but also that their ratio of need for vital amino acids is different from that of adults. The concentrations of free amino acids in plasma and whole blood also differ.

The need for leucine, phenylalanine, lysine, valine and threonine is especially high. If we take into account that 8 amino acids are vital for an adult (leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine), then for children under 5 years of age, histidine is also an essential amino acid. For children in the first 3 months of life, cystine, arginine, taurine are added to them, and for premature babies, glycine is also added, i.e. 13 amino acids are vital for them. This must be taken into account when planning the nutrition of children, especially at an early age. Only due to the gradual maturation of enzyme systems during growth, the need for essential amino acids in children gradually decreases. At the same time, with excessive protein overload, aminoacidemia occurs more easily in children than in adults, which can manifest itself in developmental delays, especially neuropsychic ones.

Concentration of free amino acids in blood plasma and whole blood of children and adults, mol/l

Amino acids	Blood plasma		Whole blood
	Newborns	Adults	Children 1-3 years old	Adults
Alanine	0.236-0.410	0.282-0.620	0.34-0.54	0.26-0.40
A-Aminobutyric acid	0.006-0.029	0.008-0.035	0.02-0.039	0.02-0.03
Arginine	0.022-0.88	0.094-0.131	0.05-0.08	0.06-0.14
Asparagine	0.006-0.033	0.030-0.069	-	-
Aspartic acid	0.00-0.016	0.005-0.022	0.08-0.15	0.004-0.02
Valin	0.080-0.246	0.165-0.315	0.17-0.26	0.20-0.28
Histidine	0.049-0.114	0.053-0.167	0.07-0.11	0.08-0.10
Glycine	0.224-0.514	0.189-0.372	0.13-0.27	0.24-0.29
Glutamine	0.486-0.806	0.527	-	-
Glutamic acid	0.020-0.107	0.037-0.168	0.07-0.10	0.04-0.09
Isoleucine	0.027-0.053	0.053-0.110	0.06-0.12	0.05-0.07
Leucine	0.047-0.109	0.101-0.182	0.12-0.22	0.09-0.13
Lysine	0.144-0.269	0.166-0.337	0.10-0.16	0.14-0.17
Methionine	0.009-0.041	0.009-0.049	0.02-0.04	0.01-0.05
Ornithine	0.049-0.151	0.053-0.098	0.04-0.06	0.05-0.09
Proline	0.107-0.277	0.119-0.484	0.13-0.26	0.16-0.23
Serene	0.094-0.234	0.065-0.193	0.12-0.21	0.11-0.30
Taurine	0.074-0.216	0.032-0.143	0.07-0.14	0.06-0.10
Tyrosine	0.088-0.204	0.032-0.149	0.08-0.13	0.04-0.05
Threonine	0.114-0.335	0.072-0.240	0.10-0.14	0.11-0.17
Tryptophan	0.00-0.067	0.025-0.073	-	-
Phenylalanine	0.073-0.206	0.053-0.082	0.06-0.10	0.05-0.06
Cystine	0.036-0.084	0.058-0.059	0.04-0.06	0.01-0.06

Children are more sensitive to starvation than adults. In countries where there is a sharp protein deficiency in children's diet, mortality at an early age increases by 8-20 times. Since protein is also necessary for the synthesis of antibodies, then, as a rule, with its deficiency in the diet of children, various infections often occur, which, in turn, increase the need for protein. A vicious circle is created. In recent years, it has been established that protein deficiency in the diet of children in the first 3 years of life, especially long-term, can cause irreversible changes that persist for life.

A number of indicators are used to judge protein metabolism. Thus, the determination of the content of protein and its fractions in blood (plasma) is a summary expression of the processes of protein synthesis and breakdown.

Content of total protein and its fractions (in g/l) in blood serum

Indicator	At mother's	Umbilical cord blood	In children aged
			0-14 days	2-4 weeks	5-9 weeks	9 weeks - 6 months	6-15 months
Total protein	59.31	54.81	51.3	50.78	53.37	56.5	60.56
Albumins	27.46	32.16	30.06	29.71	35.1	35.02	36.09
Α1-globulin	3.97	2.31	2.33	2.59	2.6	2.01	2.19
Α1-lipoprotein	2.36	0.28	0.65	0.4	0.33	0.61	0.89
A2-globulin	7.30	4.55	4.89	4.86	5.13	6.78	7.55
Α2-macroglobulin	4.33	4.54	5.17	4.55	3.46	5.44	5.60
Α2-haptoglobin	1.44	0.26	0.15	0.41	0.25	0.73	1.17
Α2-ceruloplasmin	0.89	0.11	0.17	0.2	0.24	0.25	0.39
Β-globulin	10.85	4.66	4.32	5.01	5.25	6.75	7.81
B2-lipoprotein	4.89	1.16	2.5	1.38	1.42	2.36	3.26
Β1-siderophilin	4.8	3.33	2.7	2.74	3.03	3.59	3.94
B2-A-globulin, U	42	1	1	3.7	18	19.9	27.6
Β2-M-globulin, U	10.7	1	2.50	3.0	2.9	3.9	6.2
Γ-Globulin	10.9	12.50	9.90	9.5	6.3	5.8	7.5

Protein and amino acid levels in the body

As can be seen from the table, the total protein content in the newborn's blood serum is lower than that of its mother, which is explained by active synthesis, rather than simple filtration of protein molecules through the placenta from the mother. During the first year of life, the total protein content in the blood serum decreases. Especially low indicators are observed in children aged 2-6 weeks, and starting from 6 months, a gradual increase is noted. However, in primary school age, the protein content is somewhat lower than the average in adults, and these deviations are more pronounced in boys.

Along with the lower content of total protein, a lower content of some of its fractions is also noted. It is known that albumin synthesis occurring in the liver is 0.4 g / (kg-day). With normal synthesis and elimination (albumin partially enters the intestinal lumen and is again utilized; a small amount of albumin is excreted in the urine), the albumin content in the blood serum, determined by electrophoresis, is about 60% of the serum proteins. In a newborn, the percentage of albumin is even relatively higher (about 58%) than in his mother (54%). This is obviously explained not only by the synthesis of albumin by the fetus, but also by its partial transplacental transfer from the mother. Then, in the first year of life, there is a decrease in the albumin content, parallel to the content of total protein. The dynamics of the γ-globulin content is similar to that of albumin. Particularly low values of γ-globulins are observed during the first half of life.

This is explained by the breakdown of γ-globulins received transplacentally from the mother (mainly immunoglobulins related to β-globulin). 

The synthesis of the child's own globulins matures gradually, which is explained by their slow increase with age. The content of α1, α2- and β-globulins differs relatively little from that of adults.

The main function of albumins is nutritional and plastic. Due to the low molecular weight of albumins (less than 60,000), they have a significant effect on colloid-osmotic pressure. Albumins play a significant role in the transport of bilirubin, hormones, minerals (calcium, magnesium, zinc, mercury), fats, etc. These theoretical premises are used in the clinic in the treatment of hyperbilirubinemia, characteristic of the neonatal period. To reduce bilirubinemia, the introduction of a pure albumin preparation is indicated to prevent toxic effects on the central nervous system - the development of encephalopathy.

Globulins with a high molecular weight (90,000-150,000) are complex proteins that include various complexes. α1- and α2-globulins include muco- and glycoproteins, which is reflected in inflammatory diseases. The main part of antibodies is γ-globulins. A more detailed study of γ-globulins showed that they consist of different fractions, the change of which is characteristic of a number of diseases, i.e. they also have diagnostic value.

The study of protein content and the so-called spectrum, or protein formula of blood, has found wide application in the clinic.

In a healthy person, albumins predominate (about 60% of protein). The ratio of globulin fractions is easy to remember: α1- 1, α2-2, β-3, y-4 parts. In acute inflammatory diseases, changes in the protein formula of the blood are characterized by an increase in the content of α-globulins, especially due to α2, with a normal or slightly increased content of y-globulins and a reduced amount of albumins. In chronic inflammation, an increase in the content of y-globulin is noted with a normal or slightly increased content of α-globulin, a decrease in the concentration of albumin. Subacute inflammation is characterized by a simultaneous increase in the concentration of α- and γ-globulins with a decrease in the content of albumins.

The appearance of hypergammaglobulinemia indicates a chronic period of the disease, hyperalphaglobulinemia - an exacerbation. In the human body, proteins are hydrolytically broken down by peptidases into amino acids, which, depending on the need, are used to synthesize new proteins or are converted into keto acids and ammonia by deamination. In children, the content of amino acids in the blood serum approaches the values typical of adults. Only in the first days of life is an increase in the content of some amino acids observed, which depends on the type of feeding and the relatively low activity of enzymes involved in their metabolism. In this regard, aminoaciduria in children is higher than in adults.

In newborns, physiological azotemia (up to 70 mmol/l) is observed in the first days of life. After the maximum increase by the 2nd-3rd day of life, the nitrogen level decreases and by the 5th-12th day of life it reaches the level of an adult (28 mmol/l). In premature babies, the level of residual nitrogen is higher, the lower the child's body weight. Azotemia in this period of childhood is associated with excision and insufficient renal function.

The protein content in food significantly affects the level of residual nitrogen in the blood. Thus, with a protein content of 0.5 g/kg in food, the concentration of urea is 3.2 mmol/l, with 1.5 g/kg - 6.4 mmol/l, with 2.5 g/kg - 7.6 mmol/l. To some extent, the excretion of the end products of protein metabolism in the urine serves as an indicator reflecting the state of protein metabolism in the body. One of the important end products of protein metabolism - ammonia - is a toxic substance. It is neutralized:

• by excreting ammonium salts through the kidneys;
• conversion into non-toxic urea;
• binding with α-ketoglutaric acid to glutamate;
• binding with glutamate under the action of the enzyme glutamine synthetase to glutamine.

In adults, nitrogen metabolism products are excreted in the urine, mainly in the form of low-toxic urea, which is synthesized by liver cells. In adults, urea accounts for 80% of the total amount of excreted nitrogen. In newborns and children in the first months of life, the percentage of urea is lower (20-30% of total urine nitrogen). In children under 3 months of age, 0.14 g / (kg • day) of urea is excreted, 9-12 months - 0.25 g / (kg • day). In newborns, a significant amount of total urine nitrogen is uric acid. Children under 3 months of life excrete 28.3 mg / (kg • day), and adults - 8.7 mg / (kg • day) of this acid. Its excess content in urine is the cause of uric acid infarctions of the kidneys, which are observed in 75% of newborns. In addition, the body of a young child excretes protein nitrogen in the form of ammonia, which in urine is 10-15%, and in an adult - 2.5-4.5% of the total nitrogen. This is explained by the fact that in children in the first 3 months of life, the liver function is not sufficiently developed, so an excessive protein load can lead to the appearance of toxic metabolic products and their accumulation in the blood.

Creatinine is excreted in urine. Excretion depends on the development of the muscular system. Premature infants excrete 3 mg/kg of creatinine per day, full-term infants excrete 10-13 mg/kg, and adults excrete 1.5 g/kg.

Protein metabolism disorder

Among the various congenital diseases based on protein metabolism disorders, a significant proportion are aminoacidopathies, which are based on a deficiency of enzymes involved in their metabolism. Currently, more than 30 different forms of aminoacidopathies have been described. Their clinical manifestations are very diverse.

A relatively common manifestation of aminoacidopathies is neuropsychiatric disorders. Delay in neuropsychiatric development in the form of various degrees of oligophrenia is characteristic of many aminoacidopathies (phenylketonuria, homocystinuria, histidinemia, hyperammonemia, citrullinemia, hyperprolinemia, Hartnup disease, etc.), which is confirmed by their high prevalence, exceeding that in the general population by tens and hundreds of times.

Convulsive syndrome is often found in children suffering from aminoacidopathies, and convulsions often appear in the first weeks of life. Flexor spasms are often observed. They are especially characteristic of phenylketonuria, and also occur in cases of tryptophan and vitamin B6 (pyridoxine) metabolism disorders, glycinosis, leucinosis, prolinuria, etc.

Often, changes in muscle tone are observed in the form of hypotension (hyperlysinemia, cystinuria, glycinosis, etc.) or, conversely, hypertension (leucinosis, hyperuricemia, Hartnup disease, homocystinuria, etc.). Changes in muscle tone can periodically increase or decrease.

Delayed speech development is characteristic of histidinemia. Visual disturbances are often found in aminoacidopathies of aromatic and sulfur-containing amino acids (albinism, phenylketonuria, histidinemia), pigment deposition - in alkaptonuria, lens dislocation - in homocystinuria.

Skin changes in aminoacidopathies are not uncommon. Disorders (primary and secondary) of pigmentation are characteristic of albinism, phenylketonuria, and less commonly histidinemia and homocystinuria. Intolerance to insolation (sunburn) in the absence of tanning is observed in phenylketonuria. Pellagroid skin is characteristic of Hartnup disease, and eczema is characteristic of phenylketonuria. Hair fragility is observed in arginine-succinate aminoaciduria.

Gastrointestinal symptoms are very common in aminoacidemias. Difficulty in eating, often vomiting, are characteristic of glycinosis, phenylketonuria, tyrosinosis, citrullinemia, etc. almost from birth. Vomiting can be paroxysmal and cause rapid dehydration and a soporous state, sometimes a coma with convulsions. With a high protein content, vomiting increases and becomes more frequent. With glycinosis, it is accompanied by ketonemia and ketonuria, respiratory failure.

Often, with arginine-succinate aminoaciduria, homocystinuria, hypermethioninemia, and tyrosinosis, liver damage is observed, up to the development of cirrhosis with portal hypertension and gastrointestinal bleeding.

Hyperprolinemia is accompanied by renal symptoms (hematuria, proteinuria). Blood changes may be observed. Anemia is characteristic of hyperlysinemia, and leukopenia and thrombocytopathy are characteristic of glycinosis. Homocystinuria may increase platelet aggregation with the development of thromboembolism.

Aminoacidemia may manifest itself in the neonatal period (leucinosis, glycinosis, hyperammonemia), but the severity of the condition usually increases by 3-6 months due to significant accumulation of both amino acids and products of their impaired metabolism in patients. Therefore, this group of diseases can rightfully be classified as storage diseases, which cause irreversible changes, primarily in the central nervous system, liver and other systems.

Along with the disruption of amino acid metabolism, diseases based on the disruption of protein synthesis can be observed. It is known that in the nucleus of each cell, genetic information is located in chromosomes, where it is encoded in DNA molecules. This information is transmitted by transport RNA (tRNA), which passes into the cytoplasm, where it is translated into a linear sequence of amino acids that are part of polypeptide chains, and protein synthesis occurs. Mutations in DNA or RNA disrupt the synthesis of proteins of the correct structure. Depending on the activity of a specific enzyme, the following processes are possible:

1. Lack of formation of the final product. If this compound is vital, then a lethal outcome will follow. If the final product is a compound less important for life, then these conditions manifest themselves immediately after birth, and sometimes at a later date. An example of such a disorder is hemophilia (lack of synthesis of antihemophilic globulin or its low content) and afibrinogenemia (low content or absence of fibrinogen in the blood), which are manifested by increased bleeding.
2. Accumulation of intermediate metabolites. If they are toxic, clinical signs develop, for example, in phenylketonuria and other aminoacidopathies.
3. Minor metabolic pathways may become major and overloaded, and normally formed metabolites may accumulate and be excreted in unusually large quantities, for example, in alkaptonuria. Such diseases include hemoglobinopathies, in which the structure of polypeptide chains is altered. Currently, more than 300 abnormal hemoglobins have been described. Thus, it is known that the adult type of hemoglobin consists of 4 polypeptide chains aapp, which include amino acids in a certain sequence (in the α-chain - 141, and in the β-chain - 146 amino acids). This is encoded in the 11th and 16th chromosomes. The replacement of glutamine with valine forms hemoglobin S, which has α2-polypeptide chains, in hemoglobin C (α2β2) glycine is replaced by lysine. The entire group of hemoglobinopathies is clinically manifested by spontaneous or factor-induced hemolysis, changing affinity for oxygen transport by heme, and often an enlarged spleen.

Deficiency of vascular or platelet von Willebrand factor causes increased bleeding, which is particularly common among the Swedish population of the Åland Islands.

This group should also include various types of macroglobulinemia, as well as disorders of the synthesis of individual immunoglobulins.

Thus, protein metabolism disorders can be observed at the level of both its hydrolysis and absorption in the gastrointestinal tract, and intermediate metabolism. It is important to emphasize that protein metabolism disorders are usually accompanied by disorders of other types of metabolism, since almost all enzymes contain a protein component.

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