What is insulin?
Insulin is the internal secretion of the pancreas formed by groups of cells called the islets of Langerhans. It is the hormone needed to enable glucose to enter the cells and provide energy. Insulin is also important in keeping blood glucose levels within the acceptable limits.

Insulin is injected into the body by people with type 1 diabetes in whom the cells that produce insulin have been destroyed. This is the most common form of diabetes in children and young adults, and they depend on insulin for survival. Insulin may also be used by people with type 2 diabetes, where the body needs more insulin than it can produce.

How is insulin prepared?
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.

Types of Insulin
Manufactured insulin comes in several types that differ in the way in which they act inside the body. Each type differs in three ways:

• Onset: the length of time after injection that the insulin begins to work
• Peak: the length of time after injection that the insulin takes to reach its maximum effectiveness
• Duration: the length of time that the insulin remains effective

The four basic types of insulin and their respective onset, peak and duration are as follows:

• Rapid-acting: begins to work after 15 minutes, peaks in 30 to 90 minutes, and has a duration of three to four hours.
• Short-acting: begins to work in 30 to 60 minutes, peaks in two to three hours, and has a duration of three to six hours.
• Intermediate-acting: begins to work in 90 minutes to six hours, peaks in four to 14 hours, and has a duration of up to 24 hours.
• Long-acting: begins to work in six to 14 hours and remains effective for 24 to 36 hours.

Many people with diabetes use combinations of theses different types of insulin to better control and manage their condition.

Sources: Diabetes Insight (www.diabetes-insight.info); Diabetes UK (www.diabetes.org.uk); Juvenile Diabetes Research Foundation (www.jdrf.org)

Source: King H. Insulin: availability, affordability, and harmonization. WHO Drug 1998: 4: 219-223

Insulin use in the world
Insulin is currently underutilized in developing countries as shown in the table.

Some 65% of people with diabetes live in developing countries and account for only 30% of the world's total insulin usage each year. This is in sharp contrast to the industrialized world that has 35% of the world's diabetic population but account for about 70% of the world's total insulin usage each year.

This difference cannot be entirely due to the low incidence and prevalence of type 1 in the developing countries. Moreover, only a small percentage of people with type 2 diabetes in developing countries may require insulin for survival. The consequences are that in some parts of Africa many people with type 1 diabetes under the age of 15 can expect to live for just one year after diagnosis¹.

References:
1. Makame M for the Diabetes Epidemiology Research International Study Group. Childhood diabetes, insulin, and Africa. Diabetic Medicine 1992; 9: 571-573.

Source: International Diabetes Federation