The normal human body maintains control of blood glucose levels by releasing insulin in response to high glucose levels in the blood. After food is digested, glucose is released into the bloodstream. Certain cells in the pancreas, called beta cells, have proteins that act as glucose sensors on the surface of the cells. High levels of glucose trigger these cells to release high levels of insulin into the bloodstream. As the level of glucose in the bloodstream drops, the beta cells in the pancreas dial back the release of insulin.
In diabetes, the beta cells in the pancreas are destroyed. A diabetes patient must overcome two deficiencies: first, the lack of insulin and second, the inability to control when insulin is released. The lack of insulin was overcome by isolating insulin from beef or pigs. This insulin could be injected into the body. However, the lack of control is still a problem. The control problem was first addressed by strictly controlling diet and exercise.
One problem noted in the 1930’s was that when injected interstitially, under the skin, insulin take a few hours before it gets into the blood and is active. This injected insulin remains at a high level for 8 to 10 hours, before fully entering the bloodstream. This particular activity profile limits patients with diabetes. For instance, these patients needed to take a shot of insulin an hour or so before eating a meal to insure that the glucose from the food in their bloodstream coincides with the arrival of the insulin from their shot of insulin. Then, for several hours after eating, they had to worry about whether the insulin contained within the shot was in excess of the glucose provided by their meal. If so, they could slip into a hypoglycemic state.