The main fuel for the body is glucose, which is obtained from the digestion of sugary or starchy food. People with untreated diabetes have too much glucose in their bloodstream and this glucose overflows into the urine and also cannot properly use the sugar to provide energy and build tissues. Body tissues are broken down to form glucose and ketones (see the section on Urine testing in Chapter 4), and this causes weight loss.

Someone who has uncontrolled diabetes may lose as much as 1000 g (just over 2 lb) of glucose (sugar) in their urine in 24 hours. Anyone trying to lose weight knows that sugar = calories. These calories contained in the urine are lost to the body and are a drain on its resources. The 1000 g of glucose lost are equivalent to 20 currant buns (4000 calories per day).