The insulin I am referring to is the hormone made in the pancreas; not the injected insulin.
Insulin is a hormone produced by the pancreas; and it is the main anabolic (building) hormone in the body. Insulin controls the change of food (carbohydrates, fats and protein) to energy by supporting the absorption of glucose (sugar) from the blood into liver, fat and muscle cells. In these tissues, the absorbed glucose is converted into either glycogen (stored sugar) or fats. Glucose production and secretion by the liver is hindered by high concentrations of insulin in the blood. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism (breakdown for energy), especially of reserve body fat.
That last sentence explains why the ketogenic diet for fat loss works for most people.