Normally, the pancreas secretes a hormone called insulin in response to rising levels of glucose, a type of sugar, in the blood. Insulin causes cells to take up glucose, which lowers the blood sugar. In Type I diabetes, an autoimmune process destroys the pancreas, and it is unable to secrete insulin, leading to high blood glucose. Type I diabetes doesn't have distinct phases, since the cells of the pancreas are rapidly and steadily destroyed. But Type II diabetes, which is much more common than Type I, does have definite stages.
Impaired Glucose Tolerance
Type II diabetes begins when cells become less responsive to insulin's signal to take up glucose, a condition known as insulin resistance. At first, the pancreas can compensate for insulin resistance simply by secreting more insulin. Unfortunately, high levels of insulin themselves cause insulin resistance, so insulin resistance tends to get worse over time. Eventually, the insulin resistance becomes so great that the pancreas cannot secrete enough insulin to keep blood glucose at normal levels. In the one to two hours immediately after eating, when glucose is being adsorbed into the blood from the intestines, blood glucose rises above normal levels, although it eventually gets back down to normal again. This state is known as impaired glucose tolerance.
Early Type II Diabetes
As insulin resistance continues to increase, eventually the pancreas can no longer secrete enough insulin to keep blood glucose within normal levels at any time. At this point, diabetes can be diagnosed by measuring blood glucose after an overnight fast. If the glucose is elevated in this test, the patient is considered a Type II diabetic. The early stage of Type II diabetes can be treated by oral medications that reduce insulin resistance or stimulate the pancreas to secrete more insulin.
Late Type II Diabetes
As insulin resistance increases still further, the pancreas is less able to secrete enough insulin to keep blood glucose under control, even with the help of oral medications. Unfortunately, high levels of blood glucose and a high rate of insulin secretion cause the cells in the pancreas to become exhausted and eventually die. When enough cells die, the pancreas is no longer able to secrete any insulin at all, and a type II diabetic becomes medically identical to a type I diabetic. Type II diabetics who reach this stage have to rely upon insulin injections to manage their blood sugar and they can go into diabetic ketoacidosis, just like Type I diabetics.
References
- "Harrison's Principles of Internal Medicine"; Dennis L. Kasper
- The Mayo Clinic: Type II Diabetes
