Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Carbohydrates
Blood Glucose Level and Its Regulation

Normally, Blood glucose concentration in healthy adults is maintained within 4.4–6.0 mmol ∙ l^-1, or 80–120 mg% (per 100 ml of blood), despite significant fluctuations in its intake and consumption throughout the day (Fig. 61). This constant blood glucose level is regulated primarily by the Liver, which can either take up or release glucose depending on its concentration in the bloodstream and in response to hormonal signals. An elevation in blood glucose following a carbohydrate-rich meal stimulates the enzymatic synthesis of Glycogen in the liver, whereas a drop in blood glucose enhances glycogen breakdown into glucose, which is then released into the blood.

Hormones play a vital role in maintaining constant blood glucose levels, most notably Insulin and Glucagon, which exert opposite physiological effects. Insulin is actively secreted by The Pancreas in response to postprandial blood glucose elevations; it stimulates glucose uptake by Skeletal Muscle, the liver, and adipose tissue, thereby promoting the synthesis of glycogen or fat (in adipose tissue). Conversely, glucagon is secreted when blood glucose levels fall, triggering Glycogenolysis (the breakdown and mobilization of glycogen) in The Liver and the release of glucose into the blood. When blood glucose concentrations drop, skeletal muscle and the liver begin utilizing Fatty acids as an alternative energy source, which also helps sustain stable blood glucose levels.

Following a heavy carbohydrate meal or during intensive hepatic glycogenolysis, blood glucose can exceed the normal upper limit and reach 10 mmol ∙ l^-1 or higher, a condition known as hyperglycemia. Hyperglycemia may also occur when tissue glucose utilization is impaired, as seen in severe cases of Diabetes Mellitus. This condition is associated either with a deficiency in pancreatic insulin production (hypofunction)—a hormone that facilitates glucose entry into Tissues—or with a loss of sensitivity in insulin receptors to the hormone. A temporary spike in blood glucose immediately following a carbohydrate-rich meal is referred to as alimentary or postprandial hyperglycemia. Blood glucose levels typically return to normal within 2–3 hours after eating. Hyperglycemia can also be observed in certain athletes prior to competition: while it may enhance short-term high-intensity performance, it impairs endurance capacity. An increase in blood glucose concentration to 8.8–10 mmol ∙ l^-1 (the renal threshold for glucose) leads to glucosuria, the appearance of glucose in the urine.

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Fig. 61. Diagram of Blood Glucose Regulation

A drop in blood glucose to 3 mmol ∙ l^-1 or lower (hypoglycemia) is rare, as the body is capable of synthesizing glucose from Amino Acids and Lipids via Gluconeogenesis. Hypoglycemia may develop when hepatic glycogen stores are depleted due to prolonged, strenuous physical exertion, such as marathon running, or during extended fasting. A decrease in blood glucose concentration to 2 mmol ∙ l^-1 impairs the function of the Brain, erythrocytes, and Kidneys, for which Glucose serves as the primary energy substrate. This can lead to loss of consciousness—hypoglycemic Shock—or even death. To prevent this condition, sports practice involves supplementing with CARBOHYDRATES during prolonged exercise.

Blood glucose is predominantly utilized by tissues as an energy source (about 70%), with a smaller fraction (30%) serving plastic (structural) Functions. More than 5% of dietary glucose is stored by the liver through glycogen synthesis. In a sedentary lifestyle accompanied by high carbohydrate intake, up to 40% of glucose is converted into lipids, including Cholesterol. Approximately 90% of blood glucose is consumed by the brain, where it acts as the primary energy substrate. During muscular activity, particularly prolonged exercise, glucose uptake increases significantly in skeletal Muscles, where carbohydrate reserves become depleted.