The word diabetes is an interesting one. Its origin is in the Greek language where it is derived from the word for a siphon or, more simply, a pipe or hose. This word was used to describe the disorder in ancient times (and diabetes was recognized in great antiquity) because those suffering from it produced such plentiful amounts of urine that they were reminiscent of a water pipe. The reason for the plentiful amounts of urine lies in the fact that when the sugar glucose reaches excessively high levels in our blood-stream, it is filtered into the kidney and enters the urine in large quantities.
Due to its chemical and physical properties, when large amounts of glucose are filtered by our kidneys into the urine, it cannot be fully reabsorbed and retains a large amount of water with it, thus creating very large volumes of urine. The second part of the name, mellitus, is derived from the word meaning sweet, as in mellifluous music. Mellitus was added when it was discovered that the urine in a person with diabetes and very high blood sugar is sweet.
Diabetes insipidus is a disorder with an entirely different basis, but its sufferers share the siphon-like quality of very frequent and very high volume urination. Diabetes insipidus is due to failure of production or action of another vital hormone, known as arginine vasopressin (AVP), also called antidiuretic hormone (ADH), that is responsible for maintaining the normal volume and concentration of our urine.
When AVP is deficient (usually due to damage or dis-ease of the hypothalamus or pituitary gland) or fails to work (usually due to disease of or damage to the kidney), we are unable to concentrate our urine and it becomes excessively dilute. As such, it appears pale, almost colorless and watery—in a word insipid, hence insipidus. It is not sweet, as it has negligible amounts of sugar in it.