Your pancreas has small islands of endocrine cells called pancreatic islets. These charmingly named islets have beta cells, which both create and secrete insulin. Your body needs insulin to absorb nutrients from your food. After you eat, your food goes through your digestive system so your body can absorb the nutrients from the glucose, fatty acids and amino acids within your food. Once these substances make it into your intestines, they act as a trigger for your body to start secreting insulin. What's really happening is that they stimulate your pancreatic islets' beta cells. The insulin levels in your blood begin to rise, which allows your body to now absorb the glucose, fatty acids and amino acids.
Almost every cell in your body needs insulin, but it works mostly on the liver, fat and muscle cells. Insulin stimulates your liver and muscle cells to store glucose, while also inhibiting your liver and kidneys from making glucose. It also stimulates your liver and muscle cells to make needed proteins from amino acids. Lastly, insulin helps fat cells form fat.
For people with type I diabetes, the body's pancreas has no beta cells, thus no means to secrete insulin. The pancreatic islets' beta cells could have been destroyed by the immune system, or by genetic or environmental factors. People with type 2 diabetes can secrete insulin, but their bodies don't process it well. As a result, Type 2 diabetics generally have too much insulin in their blood, which leads to insulin resistance.
Gestational diabetes is similar to Type 2 diabetes and also leads to higher than normal levels of insulin in your blood. Only pregnant women are susceptible to gestational diabetes, which is thought to occur because hormones present during pregnancy can the woman’s body from properly using its own insulin.