Insulin Definition

Insulin Definition

Insulin is a protein secreted by the islet cells of the pancreas.  It is used to allow glucose to enter the cells of the body to be used for cellular metabolism.  People who have type 1 diabetes have antibodies that have destroyed the islet cells so they no longer make insulin.  Those with type 2 diabetes produce insulin but have insulin resistance so the glucose cannot get in the cells to be used for fuel.

There are also insulin preparations, most often used by type 1 diabetics.  The insulin levels vary according to the length of time that they are active in the body. The most common type of exogenous (manmade) insulin is U-100.  Most insulin is made in a factory; however, some people use insulin that has been harvested from animals.


Insulin is normally released by the islet cells (beta cells) of the pancreas in response to a decrease in blood sugar levels.  In normal individuals, the insulin facilitates the transfer of glucose from the bloodstream to the cells, where it is used as part of the metabolic process within the cells.

Exogenous insulin must be injected into the fatty tissues because it cannot survive the acidic environment of the stomach.  In some cases, a person can become allergic to the insulin they are taking for their diabetes.  This is more common among those taking animal insulins.

Types of Injectable Insulin

Type 1 diabetics and some type 2 diabetics take exogenous insulin as part of their management of diabetes.  The different types of insulin are as follows:

Injectable Insulin

  • Rapid Acting Insulin. This is insulin that begins to take effect within 15 minutes and peaks after about an hour. Its total duration of action is between 2 and 4 hours.
  • Regular or Short Acting Insulin. This is insulin that gets into the blood within a half hour of injection and peaks at about 3-6 hours.
  • Intermediate Acting Insulin. This is insulin that gets into the blood in about 2-4 hours and peaks at about 4-12 hours following its injection.
  • Long Acting Insulin. This is a type of insulin that takes several hours following its injection and works over about 24 hours.

They also make premixed insulin that is used by people who need more than one type of insulin and cannot draw up the insulin properly from more than one bottle.  In diabetics with poor vision or clumsiness, premixed insulin can also be used.

An inhaled insulin called Afrezza was developed in 2015.  It is used before each meal and begins working within 12 to 15 minutes.  It is out of the bloodstream after 180 minutes and peaks at 30 minutes.  It is used by patients with both type 1 and type 2 diabetes mellitus along with injectable insulin types that are longer acting.


  1. Insulin Basics. Accessed 5/16/16.
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