Approximately 100 g of glucose is normally filtered from the blood at the glomeruli of the kidneys each day, more than 99% of which is reabsorbed by the kidney tubules. As a result, healthy people lose less than 150 mg of glucose in their urine each day, an amount too small to be detected by most simple screening procedures for glycosuria. When, for any reason—the most common cause of which is a blood glucose concentration of 10mmoll_1 or higher—the amount of glucose filtered at a glomerulus is more than can be reabsorbed by its tubule, glucose appears at high concentration in the urine. The osmotic diuresis so produced is associated with an increased excretion of water, sodium, chloride, and potassium and is often the first clue to the existence of hyperglycemia, the characteristic hallmark of diabetes. Moreover, it is their loss and not that of glucose that leads to the fatal outcome of diabetic ketoacidosis in patients with untreated type 1 diabetes.
Because their concentration in blood is ordinarily extremely low, except in certain rare diseases, neither fructose nor galactose occur in the urine of healthy people.
See also: Diabetes Mellitus: Etiology and Epidemiology; Classification and Chemical Pathology; Dietary Management. Fructose. Galactose. Glucose: Chemistry and Dietary Sources; Metabolism and Maintenance of Blood Glucose Level; Glucose Tolerance.
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