Glucose tolerance test

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Introduction

Carbohydrate forms the principle source of energy. Usually polysaccharide (starch and glycogen) which are glucose units joined by a-glucosidic links and disaccharides (sucrose and lactose) the main dietary carbohydrate. Carbohydrate absorption must be presented to the intestinal epithelium in monosaccharide from mainly glucose and therefore digestion must precede absorption. Glucose gained a significant importance because brain cells are very dependent on it as it is sole source of energy supply. Red blood cells also depend on glucose to carry out their functions. Therefore the blood glucose concentration must be maintained within relative narrow range. After a carbohydrate-containing meal, glucose is transported in the portal blood to the liver, which takes up 60% of the glucose load. Consequently, a rise in the blood glucose concentration causes the release of insulin which will increase the entry of excess glucose into the liver where it is stored in form of glycogen. The normal plasma glucose concentration remains between 4.5 and 11 mmol/L, despite the intermittent load entering the body from the gastrointestinal tract. The maintenance of plasma glucose concentration below 11 mmol/L minimizes loss from the body as well as providing the optimal supply to the brain. Mayne, (1994). All the filtered glucose through glomeruli is reabsorbed in the proximal tubules. Therefore no glucose should be detected in urine; significant glycosuria occurs if the plasma glucose concentration exceeds 11 mmol/L. The two most important hormones in glucose homoeostasis are insulin and glucagon. Insulin is a 53 amino acid polypeptide, secreted by the ?-cells in the islet langerhans of the pancreas in response to a rise in the blood glucose concentration. Insulin stimulates glycogen synthesis and inhibits glycogenolysis through interaction with an exquisitely coordinated control mechanism that is central to the regulation of blood glucose concentration. Glucagon is a 29 amino acid polypeptide secreted by the ?-cells of the pancreatic islet. Its secretion is decreased by a rise in the blood glucose concentration. The action of glucagon is opposite those of insulin. It stimulates hepatic glycogenolysis through activation of glycogen phosphorylase, gluconeogenesis, lipolysis and ketogenesis. Marshell, (2000). The world health organization (WHO) defined diabetes on the basis of laboratory findings as a fasting venous plasma glucose concentration greater than 7.8 mmol/L and greater than 11.1 mmol/L two hours after the oral ingestion of the equivalent of 75g of glucose even the fasting concentration is normal. Mayne, (1994). Diabetes mellitus classified in two types; insulin dependent diabetes (IDDM type-1) where there is a defective insulin secretion. This condition presents in childhood or early adulthood (less than 20 years). Because of insulin deficiency, hyperglycaemia is very likely to occur. As a result glucose will leak to urine (glycosuria) because the plasma glucose concentration exceeds the renal threshold (10 mmol/l). Other consequences related to this condition are polyuria (frequent urination), glucose lost in urine draw water with it by osmosis producing osmotic diuresis characterized by polyuria. The excess fluid lost from the body leads to dehydration and thirst which is a compensatory mechanism to counteract the dehydration.

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