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[Fr., fr. Gr. gleukos, sweet (new wine)]
A simple sugar or monosaccharide, C6H12O6, that is the end product of carbohydrate digestion. Its right-handed (dextrorotatory) isomer (d-glucose) serves as a primary energy source for living organisms. Glucose is found naturally in fruits and other plants. It is also formed during digestion from the hydrolysis of disaccharides and polysaccharides. After absorption by the small intestine, glucose is carried by the portal vein, where it may be stored as glycogen. Within cells, glucose is used to synthesize the pentose sugars, ribose and deoxyribose, for RNA and DNA, respectively.
SYN: SEE: dextrose
In healthy people, normal fasting blood glucose levels are maintained at about 70 to 100 mg/dL. (To convert glucose measurements from mg/dL to SI units, multiply by 0.0555.) Lower blood glucose levels (hypoglycemia) may cause confusion, anxiety, or other neurological complications. Higher blood glucose levels (hyperglycemia) may result in the glycosylation (sugarcoating) of body tissues. Hyperglycemia is characteristic of diabetes mellitus (diagnosed when a fasting patient has two blood glucose measurements exceeding 126 mg/dL); and of prediabetes (in which the fasting blood sugar levels are 100 to 125 mg/dL). Hypoglycemia may result from starvation, the treatment of diabetes mellitus, or, rarely, insulin-secreting tumors of the pancreas.
Within most cells, glucose is the primary energy source and is oxidized in cell respiration to carbon dioxide and water to produce energy in the form of adenosine triphosphate. There are three stages of cellular respiration: glycolysis, the Krebs (citric acid) cycle, and the electron transport chain. Stage 1 (glycolysis) takes place in the cytoplasm; stages 2 and 3 (the Krebs cycle and the electron transport chain) take place in the mitochondria. Insulin facilitates the entry of glucose into cells, fueling cellular respiration. Excess glucose may be converted to glycogen and stored in the liver and muscles; insulin and cortisol facilitate this process. The hormone glucagon (and epinephrine in stress situations) stimulates the liver to change glycogen back to glucose when the blood glucose level decreases. Any further excess glucose is converted to fat and stored in adipose tissue.
When the glucose level is below normal, fat stores are metabolized. Incomplete metabolism of fats leads to the formation of ketone bodies, a finding in poorly controlled diabetes. Blood glucose acts as a protein sparer. Neurons are esp. dependent on glucose as their source of energy; the brain oxidizes glucose directly.
Blood glucose monitoring is used by patients with diabetes mellitus to provide immediate glucose readings that guide decisions on dietary intake, medications, and exercise. Self-monitoring data (along with hemoglobin A1C values) give the patient and the health care providers information on the effectiveness of treatment.
MAINTENANCE OF BLOOD GLUCOSE LEVELS (A) Normal physiology: foods (especially carbohydrates) are broken down into glucose, which is absorbed into the bloodstream for transport to the cells. Insulin, produced by beta cells of the islets of Langerhans in the pancreas, is needed to “open the door” to the cells, allowing the glucose to enter. (B) In type 1 diabetes mellitus, the pancreas does not produce insulin. Because glucose is unable to enter the cells, it builds up in the bloodstream, causing hyperglycemia. (C) In type 2 diabetes mellitus, insulin production is reduced and/or cells are resistant to insulin. Less glucose enters the cell, and hyperglycemia results. Key: G = Glucose; I = Insulin.GLUCOSE (B) In type 1 diabetes mellitus, the pancreas does not produce insulin. Because glucose is unable to enter the cells, it builds up in the bloodstream, causing hyperglycemia.