Glucose, the body's fuel, is produced by digestion and launched into the blood. At this point, the body tissues can only reach this glucose to feed on it if there is insulin present. In diabetics, insulin may be inadequate or not properly used, and the glucose continues to build up in the blood (see hyperglycemia) until the renal threshold for glucose is reached. At this point, the kidneys start to filter excess glucose from the blood and excrete it in the urine, a condition known as glycosuria.
Glycosuria makes the urine "sweet" (the origin of the term diabetes mellitus), and can promote urinary tract infections and nephropathy. It also leads to the kidneys demanding more water from the tissues, leading to the classic diabetic symptoms of polyuria and polydipsia, as well as possible dehydration.