CausesBy Mayo Clinic staff
To understand diabetes, first you must understand how glucose is normally processed in the body.
How insulin works
Insulin is a hormone that comes from the pancreas, a gland situated behind and below the stomach.
- The pancreas secretes insulin into the bloodstream.
- The insulin circulates, enabling sugar to enter your cells.
- Insulin lowers the amount of sugar in your bloodstream.
- As your blood sugar level drops, so does the secretion of insulin from your pancreas.
The role of glucose
Glucose — a sugar — is a main source of energy for the cells that make up muscles and other tissues.
- Glucose comes from two major sources: food and your liver.
- Sugar is absorbed into the bloodstream, where it enters cells with the help of insulin.
- Your liver stores and makes glucose.
- When your insulin levels are low, such as when you haven't eaten in a while, the liver metabolizes stored glycogen into glucose to keep your glucose level within a normal range.
Causes of type 1 diabetes
In type 1 diabetes, your immune system — which normally fights harmful bacteria or viruses — attacks and destroys your insulin-producing cells in the pancreas. This leaves you with little or no insulin. Instead of being transported into your cells, sugar builds up in your bloodstream. Type 1 is thought to be caused by a combination of genetic susceptibility and environmental factors, though exactly what many of those factors are is still unclear.
Causes of prediabetes and type 2 diabetes
In prediabetes — which can lead to type 2 diabetes — and in type 2 diabetes, your cells become resistant to the action of insulin, and your pancreas is unable to make enough insulin to overcome this resistance. Instead of moving into your cells where it's needed for energy, sugar builds up in your bloodstream. Exactly why this happens is uncertain, although as in type 1 diabetes, it's believed that genetic and environmental factors play a role in the development of type 2. Being overweight is strongly linked to the development of type 2 diabetes, but not everyone with type 2 is overweight.
Causes of gestational diabetes
During pregnancy, the placenta produces hormones to sustain your pregnancy. These hormones make your cells more resistant to insulin. As your placenta grows larger in the second and third trimesters, it secretes more of these hormones — making it even harder for insulin to do its job.
Normally, your pancreas responds by producing enough extra insulin to overcome this resistance. But sometimes your pancreas can't keep up. When this happens, too little glucose gets into your cells and too much stays in your blood. This is gestational diabetes.
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