In the seventh century the physician and bureaucrat Li Xuan wrote a monograph on diabetes and attempted to explain the reason for the sweetness of the urine in diabetic patients. He observed:

This disease is due to weakness of the renal and urinogenital system. In such cases the urine is always sweet. Many physicians do not recognize this symptom . . . the cereal foods of the farmers are the precursors of sweetness . . . the methods of making cakes and sweetmeats . . . mean that they all very soon turn to sweetness. . . . It is the nature of the saline quality to be excreted. But since the renal and urinogenital system at the reins is weak it cannot distill the nutrient essentials, so that all is excreted as urine. Therefore the sweetness in the urine comes forth, and the latter does not acquire its normal color.

Source: Robert Temple...