The discovery of insulin in 1921 by Banting and Best, was one of the greatest advances ever to be made in the history of medicine. Prior to this, deaths due to ‘diabetic coma’ were unavoidable. Insulin treatment has evolved greatly since the early crude extracts which were derived from animal pancreas extracts as the insulin had to be extracted and purified from beef and pork pancreas. Modern day insulins are manufactured in the laboratory, some of which are copies of the exact molecule that is found in the human body. Methods of insulin delivery have likewise evolved and there are now a range of pens and devices to enable users to administer their doses relatively painlessly and without great inconvenience.
Insulin is a hormone which is secreted by a specialised group of cells within the pancreas called islet cells. It is a protein, and works as a messenger carrying information from the pancreas to the various cells of the body. Cells have receptors on the surface to which insulin attaches rather like a key to a lock. The 'key' opens the door allowing glucose to enter the cell. Insulin is essential to this process as without insulin, glucose cannot get into the cells. Without insulin, the energy contained in sugar (glucose) cannot be utilised and the body would 'starve'.
Insulin is a protein. If swallowed in tablet form it would be digested (broken down) by the acid in the stomach and enzymes in the gut. It is therefore administered as an injection.
Insulin was initially extracted from the pancreas of cows and pigs hence the early 'beef' and 'pork' insulin preparations. The extracted hormone was purified and placed in vials. Animal insulin differs from human insulin slightly but is still very effective. It does have the drawback of being different to the 'native' insulin found within the body and is therefore 'foreign' to the body and can causes allergies. More recently technology has developed further and we can now make human insulin in the laboratory on a large scale.
There is essentially only one type of native or natural human insulin which is identical in composition to Human Actrapid or Humulin S. These preparations are quickly absorbed when injected subcutaneously and tend to last no more than 6-8 hours after injection.
The longer acting insulins such as Human Insulatard or Humulin I are basically the same insulin which is complexed with a binding protein which slows down the absorption from the subcutaneous fat. This makes the duration of action longer. These preparations were prescribed for use at bed time so that the effect would last overnight.
Mixtures of insulin such as Human Mixtard 30 or Humulin M3 are simply mixtures of the long acting and short acting insulin which are combined in one vial for reasons of convenience. These mixtures take away the need for mixing doses or having two separate injections. They can be prescribed on a twice daily basis and are therefore popular with patients who do not want the inconvenience of several injections.
The new insulin analogues are 'man-made' insulins which do not exist normally in nature. These will be discussed on a separate page.
Some preparations of insulin can be mixed in the same syringe. However, it is dangerous to assume that all types of insulin can be mixed and it is best to confirm this with your doctor or diabetes nurse. Insulin mixtures are available for reasons of convenience and safety.
Unless there are good reasons to mix the short and long acting insulin manually, it is now usual practise to use these mixtures of insulin wherever possible.
Insulin is a protein and when proteins are heated they tend to suffer damage rather like what happens to the albumin in egg white when you boil an egg. If insulin overheats it loses its effectiveness. For this reason it is recommmended that insulin is refrigerated whenever possible.
A vial of insulin may be left at room temperature (less than 30° C) for up to a month as long as it is kept away from direct sunlight or heat. If the vial has been left out of the fridge for longer than 4 weeks it should be discarded. Conversely Insulin vials should NEVER be frozen. Always check on the appearance of the insulin before using it. If it looks different, don't use it!
If the vial is not opened you may keep the insulin up to the expiry date stated on the vial. Once opened, contamination with micro-organisms or 'bugs' that cause infections may occur. It is therefore recommended that you discard the vial after 28 days or one calendar month.
Dr Nishan Wijenaike, MD, FRCP
West Suffolk Hospitals Diabetes Service
Updated November 2007