Normally, the body's immune system fights off foreign invaders like viruses or bacteria. But for unknown reasons, in people with type 1 diabetes, the immune system attacks various cells in the body. This results in a complete deficiency of the insulin hormone.

Some people develop a type of diabetes – called secondary diabetes -- which is similar to type 1 diabetes, but the beta cells are not destroyed by the immune system but by some other factor, such as cystic fibrosis or pancreatic surgery.

Understanding Insulin and Type 1 Diabetes

Normally the hormone insulin is secreted by the pancreas in low amounts. When you eat a meal, glucose (sugar) from food stimulates the pancreas to release insulin. The amount that is released is proportional to the amount that is required by the size of that particular meal.

Insulin's main role is to help move certain nutrients -- especially glucose -- into the cells of the body's tissues. Cells use sugars and other nutrients from meals as a source of energy to function.

The amount of sugar in the blood decreases once glucose enters the cells. Normally that signals the beta cells in the pancreas to lower the amount of insulin secreted so that you don't develop low blood sugar levels (hypoglycemia). But the destruction of the beta cells that occurs with type 1 diabetes throws the entire process into disarray.

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In people with type 1 diabetes, glucose isn't moved into the cells because insulin is not available. When glucose builds up in the blood instead of going into cells, the body's cells starve for nutrients and other systems in the body must provide energy for many important bodily functions. As a result, high blood glucose develops and can cause:

Dehydration. The build up of sugar in the blood can cause an increase in urination (to try to clear the sugar from the body). When the kidneys lose the glucose through the urine, a large amount of water is also lost, causing dehydration.
Weight loss. The loss of sugar in the urine means a loss of calories which provide energy and therefore many people with high sugars lose weight. (Dehydration also contributes to weight loss.)
Diabetic ketoacidosis (DKA). Without insulin and because the cells are starved of energy, the body breaks down fat cells. Products of this fat breakdown include acidic chemicals called ketones that can be used for energy. Levels of these ketones begin to build up in the blood, causing an increased acidity. The liver continues to release the sugar it stores to help out. Since the body cannot use these sugars without insulin, more sugars piles into the blood stream. The combination of high excess sugars, dehydration and acid build up is known as "ketoacidosis" and can be life-threatening if not treated immediately.
Damage to the body. Over time, the high glucose levels in the blood may damage the nerves and small blood vessels of the eyes, kidneys, and heart and predispose a person to atherosclerosis (hardening) of the large arteries that can cause heart attack and stroke.