Diabetes: What Is Insulin?

Location of the PancreasInsulin is a hormone. And like many hormones, insulin is a protein.   Insulin is secreted by groups of cells within the pancreas called islet cells. The pancreas is an organ that sits behind the stomach and has many functions in addition to insulin production. The pancreas also produces digestive enzymes and other hormones. Carbohydrates (or sugars) are absorbed from the intestines into the bloodstream after a meal. Insulin is then secreted by the pancreas in response to this detected increase in blood sugar (blood glucose). Most cells of the body have insulin receptors which bind the insulin that is in the circulation.  When a cell has insulin attached to its surface, the cell activates other receptors designed to absorb glucose (sugar) from the bloodstream into the inside of the cell.

Pancreatic Islet Cells secrete insulinWithout insulin, you can eat lots of food and actually be in a state of starvation since many of our cells cannot access the calories contained in the glucose very well without the action of insulin.

This is why people with type 1 diabetes who do not make insulin can become very ill without insulin shots. Insulin is a necessary hormone. Those who develop a deficiency of insulin must have it replaced via shots or pumps (type 1 diabetes).

Check out our type 1 diabetes slideshow, and learn about common causes, symptoms, and treatments for type 1 diabetes.

Want to learn more about how insulin is used to treat diabetes?

More commonly, people will develop insulin resistance (type 2 diabetes) rather than a true deficiency of insulin. In this case, the levels of insulin in the blood are similar or even a little higher than in people without diabetes.

However, many cells of people with type 2 diabetes respond sluggishly to the insulin they make and therefore their cells cannot absorb the sugar molecules (glucose) well. This leads to blood sugar levels which run higher than normal. Occasionally people with type 2 diabetes will need insulin shots, but most of the time other methods of treatment will work. 

Pancreas Secretes InsulinInsulin was the first hormone identified (late 1920s), which won the doctor and medical student who discovered it the Nobel Prize (Banting and Best). They discovered insulin by tying a string around the pancreatic duct of several dogs.

When they examined the pancreases of these dogs several weeks later, all of the pancreas digestive cells were gone (died and were absorbed by the immune system), and the only thing left was thousands of pancreatic islets.

They then isolated the protein from these islets, and they discovered insulin. Note that there are other hormones produced by different types of cells within pancreatic islets (glucagon, somatostatin, etc), but insulin is produced in far greater amounts under normal conditions making the simple approach used by Banting and Best quite successful.

Where Does Commercial Insulin Come From?

The first successful insulin preparations came from cows (and later pigs). The pancreatic islets and the insulin protein contained within them were isolated from animals slaughtered for food in a similar but more complex fashion than was used by our doctor and med-student duo. The bovine (cow) and porcine (pig) insulin were purified, bottled, and sold.

Bovine and porcine insulin worked very well (and still do!) for the vast majority of patients, but some could develop an allergy or other types of reactions to the foreign protein (a foreign protein is a protein which is not native to humans).

In the 1980s, technology had advanced to the point where we could make human insulin. The advantage would be that human insulin would have a much lower chance of inducing a reaction because it is not a foreign protein (all humans have the exact same insulin, so we do not "see" this as a foreign protein).

The technology which made this approach possible was the development of recombinant DNA techniques. In simple terms, the human gene which codes for the insulin protein was cloned (copied) and then put inside of bacteria. A number of tricks were performed on this gene to make the bacteria want to use it to constantly make insulin. Big vats of bacteria now make tons of human insulin. From this, pharmaceutical companies can isolate pure human insulin.