Insulin is a hormone made by your pancreas that acts like a key to let blood sugar into the cells in your body for use as energy. If you have type 2 diabetes, cells don’t respond normally to insulin; this is called insulin resistance.
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If you have type 1 diabetes, your pancreas doesn’t make insulin or makes very little insulin. Insulin helps blood sugar enter the cells in your body for use as energy. Without insulin, blood sugar can’t get into cells and builds up in the bloodstream. High blood sugar is damaging to the body and causes many of the symptoms and complications of diabetes.
The warning signs can be so mild that you don't notice them. That's especially true of type 2 diabetes. Some people don't find out they have it until they get problems from long-term damage caused by the disease. With type 1 diabetes, the symptoms usually happen quickly, in a matter of days or a few weeks. They're much more severe, too.