Exchange of carbohydrates

Last reviewed by: Aleksey Portnov , medical expert, on 01.06.2018

Carbohydrates are the main source of energy: 1 g of carbohydrates, with their complete cleavage, releases 16.7 kJ (4 kcal). In addition, carbohydrates in the form of mucopolysaccharides are part of the connective tissue, and in the form of complex compounds (glycoproteins, lipopolysaccharides) are structural elements of cells, as well as constituents of some active biological substances (enzymes, hormones, immune bodies, etc.).

Carbohydrates in the diet

The proportion of carbohydrates in the diet of children depends largely on age. In children of the first year of life, the carbohydrate content, which provides the need for energy, is 40%. After a year, it increases to 60%. In the first months of life, the need for carbohydrates is covered by milk sugar - lactose, which is a part of women's milk. With artificial feeding with milk formulas, the child also receives sucrose or maltose. After the introduction of complementary foods, polysaccharides (starch, partially glycogen) begin to enter the body, which basically cover the body's needs in carbohydrates. This type of nutrition of children contributes to both the formation of amylase by the pancreas, and the secretion of it with saliva. In the first days and weeks of life, there is almost no amylase, and salivation is insignificant, and only at 3-4 months amylase begins to develop and saliva sharply increases.

It is known that hydrolysis of starch occurs when exposed to saliva amylase and pancreatic juice; the starch is split to maltose and isomaltose.

Along with disaccharides of food - lactose and sucrose - maltose and isomaltose on the surface of intestinal villi of the intestinal mucosa under the influence of disaccharidases split into monosaccharides: glucose, fructose and galactose, which undergo resorption through the cell membrane. The process of resorption of glucose and galactose is associated with active transport, which consists in phosphorylation of monosaccharides and their conversion into glucose phosphate, and then into glucose-6-phosphate (respectively, galactosophosphates). Such activation occurs under the influence of glucose- or galactosokinases with the cost of one macroergic ATP bond. In contrast to glucose and galactose, fructose is resorbed almost passively, by simple diffusion.

Disaccharidases in the intestine of the fetus are formed depending on the gestation period.

The timing of the formation of functions of the gastrointestinal tract, the timing of detection and severity as a percentage of a similar function in adults

Assimilation of carbohydrates

First detection of enzyme, weeks

Expression,% of adult

α-Amylase pancreatic



α-Amylase of the salivary glands





More than 100

Sucrase and isomaltase






Suction of monosaccharides



It is seen that the earlier activity of maltase and saccharase (6-8 months of gestation), later (8-10 months) - lactase. The activity of various disaccharidases in cells of the intestinal mucosa has been studied. It was found that the total activity of all maltases at the time of birth corresponds to an average of 246 μmol of split disaccharide per gram of protein per minute, the total activity of sucrose is 75, the total activity of isomaltase is 45 and the total activity of lactase is 30. These data are of great interest for pediatricians , as it becomes clear why the infant is good at digesting dextrinmaltose mixtures, while lactose easily causes diarrhea. The relatively low activity of lactase in the mucosa of the small intestine is explained by the fact that lactase deficiency is observed more often than insufficiency of other disaccharidases.

Infringement vsysyvvanija carbohydrates

There are both transient malabsorption of lactose, and congenital. Its first form is due to a delay in the maturation of intestinal lactase and therefore disappears with age. The congenital form can be observed for a long time, but, as a rule, it is most pronounced from birth during breastfeeding. This is due to the fact that the content of lactose in human milk is almost 2 times higher than in cow's milk. Clinically, the child has diarrhea, which, along with a liquid stool (more than 5 times a day), is characterized by foamy feces of an acidic reaction (pH less than 6). There may also be symptoms of dehydration, manifested by a severe condition.

At a more advanced age, there is a so-called repression of lactase, when its activity is significantly reduced. This explains the fact that a significant number of people do not tolerate natural milk, while dairy products (kefir, acidophilus, curdled milk) are absorbed well. Lactase insufficiency affects about 75% of immigrants from Africa and the Indians, up to 90% of people of Asian descent and 20% of Europeans. Less common is the congenital malabsorption of sucrose and isomaltose. Usually it occurs in children with artificial feeding with sucrose-enriched milk formulas and with the introduction of juices, fruits or vegetables containing this disaccharide into the diet. Clinical manifestations of sugar deficiency are similar to those in lactose malabsorption. Disaccharidic insufficiency can be of a purely acquired character, be a consequence or complication of a wide range of child diseases. The main causes of disaccharidase insufficiency are given below.

The consequence of the impact of damaging factors:

  • after enteritis of viral or bacterial etiology;
  • a particular significance of rotavirus infection;
  • malnutrition;
  • giardiasis;
  • after necrotic enterocolitis;
  • Immunological insufficiency;
  • celiac disease;
  • cytostatic therapy;
  • intolerance to cow's milk proteins;
  • hypoxic conditions of the perinatal period;
  • jaundice and its phototherapy.

Brittle Edge Brim:

  • prematurity;
  • immaturity at birth.

Consequence of surgical interventions:

  • gastrostomy;
  • ileostomy;
  • colostomy;
  • resection of the small intestine;
  • anastomosis of the small intestine.

Similar clinical manifestations are also described when the activation of monosaccharides - glucose and galactose - is disturbed. They should be distinguished from cases when the diet contains too much of these monosaccharides, which, having high osmotic activity, cause the entering of water into the intestine. Since the absorption of monosaccharides comes from the small intestine in the V. Portae basin, they primarily come to the liver cells. Depending on the conditions, which are determined mainly by the content of glucose in the blood, they undergo transformation into glycogen or remain in the form of monosaccharides and are carried with blood flow.

In the blood of adults, the glycogen content is somewhat less (0.075-0.117 g / l) than in children (0.117-0.206 g / l).

Synthesis of the reserve carbohydrate of the organism-glycogen-is carried out by a group of various enzymes, resulting in the formation of highly branched molecules consisting of glucose residues that are bound by 1,4- or 1,6-bonds (glycogen side chains are formed by 1,6-bonds). If necessary, glycogen can again be broken down to glucose.

Synthesis of glycogen begins at the 9th week of intrauterine development in the liver. However, its rapid accumulation occurs only before birth (20 mg / g of liver per day). Therefore, the concentration of glycogen in the fetal liver tissue to the birth is somewhat larger than that of the adult. Approximately 90% of the accumulated glycogen is used in the first 2-3 hours after birth, and the remaining glycogen is consumed within 48 hours.

This, in fact, provides the energy need of newborns in the first days of life, when a child receives little milk. From the 2nd week of life, the accumulation of glycogen begins again, and already by the 3rd week of life its concentration in the liver tissue reaches the level of an adult. However, the weight of the liver in children is much less than that of an adult (in children aged 1 year, the weight of the liver is 10% of the adult liver mass), so glycogen stores in children are consumed faster, and they must be replenished to prevent hypoglycemia.

The ratio of the intensity of the processes of glycogenesis and glycogenolysis largely determines the content of blood sugar - glycemia. This quantity is very constant. Glycemia is regulated by a complex system. The central link of this regulation is the so-called sugar center, which should be considered as a functional association of nerve centers located in different parts of the central nervous system - the cerebral cortex, the subcortex (lenticular nucleus, the striatum), the hypothalamic region, the medulla oblongata. Along with this, many endocrine glands (pancreas, adrenal glands, thyroid) take part in the regulation of carbohydrate metabolism.

Disturbance of carbohydrate metabolism: accumulation diseases

However, congenital disorders of enzyme systems can occur, in which the synthesis or decomposition of glycogen in the liver or muscles can be disturbed. These disorders include the disease lack of glycogen reserves. It is based on a deficiency of the enzyme glycogen synthetase. The rarity of this disease is probably due to the difficulty of diagnosis and a quick unfavorable outcome. In newborns very early, hypoglycemia (even in breaks between feedings) with seizures and ketosis is observed. More often describe cases of glycogen disease, when glycogen accumulates in the body of normal structure or glycogen is formed of an irregular structure resembling cellulose (amylopectin). This group, as a rule, is genetically determined. Depending on the deficiency of these or other enzymes involved in the metabolism of glycogen, different forms or types of glycogenoses are isolated.

In the first type, which includes hepatorenal glycogenosis, or Girke's disease, lies the insufficiency of glucose-6-phosphatase. This is the most severe variant of glycogenoses without structural disorders of glycogen. The disease has a recessive transmission; clinically manifested immediately after birth or in infancy. Characterized by hepatomegaly, which is accompanied by hypoglycemic convulsions and coma, ketosis. The spleen never increases. In the future, there is a lag in growth, a disproportion in the physique (the abdomen is enlarged, the trunk is elongated, the legs are short, the head is large). In the breaks between the feeding, paleness, sweating, loss of consciousness as a result of hypoglycemia are noted.

II type of glycogenosis is Pompe disease, which is based on the deficiency of acid maltase. Clinically manifested soon after birth, and such children quickly die. There are hepato- and cardiomegaly, hypotonia of the muscles (the child can not keep his head, suck). Heart failure develops.

III type of glycogenosis - Cory's disease, caused by a congenital defect of amylo-1,6-glucosidase. Transmission is recessive-autosomal. Clinical manifestations are similar to type I - Girke's disease, but less severe. Unlike Girke's disease, it is limited glycogenosis, not accompanied by ketosis and severe hypoglycemia. Glycogen is deposited either in the liver (hepatomegaly), or in the liver and simultaneously in the muscles.

IV type - Andersen's disease - is caused by a deficiency of 1,4-1,6-transglucosidase, as a result of which glycogen is formed of an irregular structure, reminiscent of cellulose (amylopectin). It is like a foreign body. There is jaundice, hepatomegaly. Cirrhosis of the liver with portal hypertension is being formed. As a result, varicose veins of the stomach and esophagus develop, the rupture of which causes profuse gastric bleeding.

V type - muscle glycogenosis, Mc-Ardl's disease - develops due to a deficiency in muscle phosphorylase. The disease can occur during the 3rd month of life, when it is noted that children are not able to suckle their breasts for a long time, quickly become fatigued. In connection with the gradual accumulation of glycogen in the striated muscle, its false hypertrophy is observed.

VI type of glycogenosis - Hertz's disease - is caused by a deficiency of hepatic phosphorylase. Clinically, hepatomegaly is detected, and hypoglycemia occurs less often. There is a lag in growth. The flow is more favorable than other forms. This is the most common form of glycogenesis.

There are other forms of accumulation diseases, when mono- or polyenzymatic disorders are detected.

Sugar in the blood as an indicator of carbohydrate metabolism

One of the indicators of carbohydrate metabolism is the sugar content in the blood. At the moment of birth, the level of glycemia in a child corresponds to that of his mother, which is explained by free transplacental diffusion. However, since the first hours of life, a drop in sugar content has been observed, which is due to two reasons. One of them, more significant, is the lack of counterinsulant hormones. This is proved by the fact that adrenaline and gliczhagon are able to increase the sugar content in the blood in this period. Another cause of hypoglycemia in newborns is that the reserves of glycogen in the body are very limited, and the newborn, who is applied to the breast a few hours after birth, consumes them. By the 5 th -6 th day of life, the sugar content rises, but in children it remains relatively lower than in adults. The increase in sugar concentration in children after the first year of life goes undulating (the first wave - by 6 years, the second - by 12 years), which coincides with the increase in their growth and a higher concentration of growth hormone. The physiological limit of glucose oxidation in the body is 4 mg / (kg • min). Therefore, the daily dose of glucose should be from 2 to 4 g / kg of body weight.

It should be emphasized that the utilization of glucose with its intravenous administration occurs in children faster than in adults (it is known that intravenously administered glucose is utilized by the body, usually within 20 minutes). Therefore, the tolerance of children to carbohydrate loading is higher, which should be taken into account when studying glycemic curves. For example, for the study of the glycemic curve, the load is applied on average 1.75 g / kg.

At the same time, children have a more severe course of diabetes, for which it is usually necessary to use insulin. Diabetes mellitus in children is most often detected during periods of particularly intense growth (the first and second physiological stretching), when the correlation of endocrine glands is more often observed (the activity of the growth hormone of the pituitary gland increases). Clinically, diabetes in children is manifested by thirst (polydipsia), polyuria, weight loss and often increased appetite (polyphagia). An increase in the sugar content in the blood (hyperglycemia) and the appearance of sugar in the urine (glucosuria) are found. The phenomena of ketoacidosis are frequent.

At the heart of the disease is insulin insufficiency, which makes it difficult to penetrate glucose through the cell membranes. This causes an increase in its content in the extracellular fluid and blood, and also enhances the breakdown of glycogen.

In the body, the cleavage of glucose can occur in several ways. The most important of these are the glycolytic chain and the pentose cycle. Splitting along the glycolytic chain can occur both in aerobic and anaerobic conditions. Under aerobic conditions, it leads to the formation of pyruvic acid, and for anaerobic acid - lactic acid.

In the liver and myocardium, the processes take place aerobically, in erythrocytes - anaerobically, in the muscles of the skeleton with increased work - mainly anaerobic, during rest - mainly aerobic. For the body, the aerobic path is more economical, as a result of it, more ATP is produced, which carries a large energy reserve. Anaerobic glycolysis is less economical. In general, cells can quickly, albeit uneconomically, supply energy, regardless of the "supply" of oxygen. Aerobic cleavage in combination glycolytic chain - the Krebs cycle is the main source of energy for the body.

At the same time, by reverse flow of the glycolytic chain, the body can carry out the synthesis of carbohydrates from intermediate products of carbohydrate metabolism, for example from pyruvic and lactic acids. Conversion of amino acids into pyruvic acid, α-ketoglutarate and oxalacetate can lead to the formation of carbohydrates. The processes of the glycolytic chain are localized in the cytoplasm of cells.

The study of the ratio of metabolites of the glycolytic chain and the Krebs cycle in the blood of children shows quite significant differences in comparison with adults. In the blood serum of a newborn and a child of the first year of life, a considerable amount of lactic acid is contained, which indicates the predominance of anaerobic glycolysis. The child's organism tries to compensate for the excessive accumulation of lactic acid by increasing the activity of the enzyme lactate dehydrogenase, which converts lactic acid into pyruvic acid and then incorporates it into the Krebs cycle.

There are also some differences in the content of lactate dehydrogenase isoenzymes. In children of early age, the activity of the 4th and 5th fractions is higher and the content of the first fraction is lower.

Another, no less important, pathway for the cleavage of glucose is the pentose cycle, which begins with the glycolytic chain at the level of glucose-6-phosphate. As a result of one cycle of 6 glucose molecules, one is completely cleaved to carbon dioxide and water. This is a shorter and faster way of decay, which secures the release of a large amount of energy. As a result of the pentose cycle, pentoses are also formed, which are used by the body for the biosynthesis of nucleic acids. Probably, this explains why in children the pentose cycle is of great importance. Its key enzyme is glucose-6-phosphate dehydrogenase, which provides a link between glycolysis and the pentose cycle. The activity of this enzyme in the blood in children aged 1 month - 3 years - 67-83, 4-6 years - 50-60, 7-14 years - 50-63 mmol / g hemoglobin.

The violation of the pentose glucose cleavage cycle due to deficiency of glucose-6-phosphate dehydrogenase is the basis of nonsferocytic hemolytic anemia (one of the forms of erythrocytopathies), which is manifested by anemia, jaundice, splenomegaly. As a rule, hemolytic crises are provoked by medication (quinine, quinidine, sulfonamides, some antibiotics, etc.), which strengthen the blockade of this enzyme.

A similar clinical picture of hemolytic anemia is due to the insufficiency of pyruvate kinase, which catalyzes the conversion of phosphoenolpyruvate to pyruvate. They are distinguished by a laboratory method, determining the activity of these enzymes in erythrocytes.

Violation of glycolysis in platelets is the basis of the pathogenesis of many thromboasthenia, clinically manifested by increased bleeding at a normal number of platelets, but impaired their function (aggregation) and the preserved factors of blood coagulation. It is known that the basic energy metabolism of man is based on the use of glucose. The remaining hexoses (galactose, fructose), as a rule, transform into glucose and undergo complete cleavage. The conversion of these hexoses into glucose is carried out by enzyme systems. Deficiency of enzymes that transform this transformation, lies at the heart of tectosemia and fructoseemia. These are genetically determined fermentopathies. In the case of cystactomy, there is a deficiency of galactose-1-phosphaturidyl transferase. As a result, galactose-1-phosphate is accumulated in the body. In addition, a large number of phosphates is extracted from the circuit, which causes a deficiency of ATP, which causes damage to energy processes in the cells.

The first symptoms of galactosemia appear soon after the beginning of feeding of children with milk, especially female, containing a large amount of lactose, which includes identical amounts of glucose and galactose. There is vomiting, the body weight is poor (hypotrophy is developing). Then hepatosplenomegaly with jaundice and cataracts appear. Possible development of ascites and varicose veins of the esophagus and stomach. In the study of urine, galactosuria is detected.

With galactosemia, lactose should be excluded from the diet. Specially prepared dairy mixtures are used, in which the lactose content is sharply reduced. This ensures the proper development of children.

When fructose is not converted to glucose, fructosemia develops as a result of deficiency of fructose-1-phosphataldolase. Its clinical manifestations are similar to those in galactosemia, but are more mild. The most characteristic symptoms are vomiting, a sharp decrease in appetite (before anorexia), when children are given fruit juices, sweetened porridges and mashed potatoes (sucrose contains fructose and glucose). Therefore, clinical manifestations are especially intensified when children are transferred to mixed and artificial feeding. At an older age, patients do not tolerate sweets and honey containing pure fructose. In the study of urine, fructosuria is detected. It is necessary to exclude sucrose and foods containing fructose from the diet.

It is important to know!

Since fructose has a low glycemic index (it causes a weaker response of blood glucose and insulin), athletes can mistakenly consider it to be the best source of energy ... Read more..

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