Insulin is a hormone made by the beta cells in the pancreas. It is the key that opens the doors of cells and allows glucose to get into the cell.

Each cell needs glucose to function. Carbohydrates in food are broken down into glucose which enters the blood stream. As the blood circulates through the pancreas, the beta cells ‘read’ the level of glucose and secrete the appropriate amount of insulin. In people with diabetes the transport of glucose from the blood stream to the cells is impeded because the body is not making enough insulin or the cells are resistant to it.

In people without diabetes, the beta cells make and release some insulin all of the time. This is a “basal” level of insulin that helps the body to function. When a person eats and their blood glucose level rises, the body releases an extra burst or ‘bolus” of insulin to help bring the blood glucose back into the target range. The insulin taken to manage diabetes is designed to work the way the body works as closely as possible.

There is one big difference between the insulin that is taken and the insulin made by the body. The body makes insulin in response to a person’s blood glucose level while the insulin that is taken works regardless of whether a person eats or not. The body has little control over the action of the insulin delivered by injection.

People with type 1 diabetes require insulin for survival. In people with type 2 diabetes, there is a progressive loss of beta cells. When the remaining beta cells are unable to produce enough insulin to keep glucose levels in the target range, insulin is required. Approximately 50% of people with type 2 diabetes require insulin therapy within 5 to 10 years of diagnosis.

Four types of insulin are used to treat diabetes. It is common to take more than one type. The different types of insulin have different peak action times (the time when they are working hardest) and durations (how long they last).

Rapid and short-acting insulin start to work very quickly and last only a short time. They are used before meals to provide a bolus of insulin. These types of insulin have the greatest effect on post-meal blood glucose readings. Intermediate and long-acting insulins start to work more slowly and last a much longer time. They are used to provide the basal level of insulin in the body and keep glucose levels at target between meals and through the night.

Mixtures of insulin provide both rapid or short and intermediate acting insulin.

Since the landmark discovery of insulin by Frederick Banting and Charles Best in 1922, huge steps have been made in research and development regarding its preparation. Early preparations of insulin were purified quite crudely from pancreas tissue extracted from animals - either pigs or cattle. Today, insulin is mostly made biosynthetically by recombinant DNA technology or 'genetic engineering'.

Animal insulin

Until the 1980s, all insulin was extracted from the pancreases of cattle and pigs. The sequence of amino acids (the building blocks that make up the protein) is slightly different in insulins from the different species. Compared to human insulin, porcine (pork) insulin has one different amino acid and bovine (beef) insulin three different amino acids. These very slight differences do not affect the way in which the insulin works inside the human body. Pork insulin is structurally closer to human insulin than is beef insulin. These days, animal insulins are made from highly purified pancreas extracts and are marketed as 'natural' insulins.

Human insulin

Human insulin is not prepared from human pancreas tissue. Rather than being extracted from human pancreases, commercially available human insulin is manufactured through recombinant DNA technology, in which the gene for making human insulin is transferred into simple cells such as bacteria or baker’s yeast. The insulin made by those cells is identical to insulin made by the human pancreas. Unlike animal insulins, recombinant DNA human insulins can be made in unlimited supply, since they do not depend on the supply of bovine and porcine pancreases.

Analogue insulin

Analogues are altered molecular versions of a natural substance (in this case insulin). They have been used in many therapies where hormone treatment is needed. The natural hormone is changed slightly, by rearranging the position of amino acids within the molecule (rather like changing the position of beads on a necklace). The insulin molecule is modified so as to give it a more desirable activity profile, thereby making it more convenient to use. These molecularly engineered hormones more closely mimic normal insulin secretion than do traditional insulins.