Usually, that is. If the glucose level in the blood is too high, its capacity for reabsorption of gluocse is exceeded, and some of the glucose is lost in the urine. (Actually, this may be a good thing, since it helps to lower the blood glucose level if the latter is pathologically high.) The rough threshold for this in a (non-pregnant) person is about 10 mmol of glucose per litre. More than this, and some will end up in your urine.
But why does more glucose in your urine lead to more urine being produced? The answer is that glucose is a powerful osmolyte. Like a handful of other substances (sodium, urea, mannitol, etc.) it has the ability to induce water to follow it, by osmosis, wherever it goes. And since there is more glucose than normal in the urine, there will also therefore be more water than usual in the urine - i.e. polyuria.
[In fact, most of the acute complications of diabetes (e.g. diabetic ketoacidosis, or hyperosmolar coma) are largely due to the hyperosmolarity of the blood, secondary to high blood glucose levels. This induces water to leave cells, including those in the brain, which is the usual cause of death in these patients.]
Largely, this whole debacle accounts for the three classic symptoms of uncontrolled diabetes:
- polydypsia (excessive ingestion of water, to make up for the extra water lost in the urine), and
- polyphagia (excessive eating, to make up for the calories you keep urinating away).