The sugar that the diabetic eats does not get inside cells because there is a lack of effective insulin. Energy in cells comes primarily from sugar. If it can't get into the cell for lack of insulin, it builds up in the blood and spills out in the urine. The high concentration of sugar in the urine pulls out fluid from the rest of the body. That is why untreated diabetics are thirsty so much of the time.
Diabetes comes in two forms.
Type I, where the islets are dead and there is little or no circulating insulin, and Type II, where the insulin, for some unknown reason, does not interact with the cell. Type I is found most often in younger, usually thin, people and often needs injected insulin for treatment. Coma with accumulation of acid-ketones in the blood can occur in Type I. Type II usually occurs over age 40 in people who are often obese and is usually insulin insensitive. Exercise improves the sensitivity of cells to insulin and is particularly helpful in Type II diabetes. It is terribly important to control diabetes.