Insulin: An Overview

Insulin is a hormone, which is a type of chemical. In the body, insulin is produced in the pancreas. It is needed to allow sugars, or glucose, from food to enter cells. There, glucose is broken down (metabolized) to produce the energy needed for the cells to work properly.

How is insulin made? 

At first, pancreases from cows and pigs were used to produce insulin. In some parts of the world, pork insulin is still on the market. However, since 1983 human insulin has been available. It is called biosynthetic human insulin. This is produced in a laboratory by introducing a synthetic (man-made) human gene into bacteria or yeast. This produces insulin that is exactly the same as that created in the human pancreas. Through further changes, people can now prepare different insulins -- called insulin analogs -- with different action times. All children and 99.9% of adults now receive human insulin products and insulin analogs.

What does insulin do? 

How does insulin work? It acts as a key opening the door to the body's cells, allowing glucose to enter and to be used to make energy. 

What Does Insulin Normally Do?
Get Adobe Flash player
In a nutshell, here’s what happens. When we eat, food goes to the stomach and the small intestines. There it is digested or broken down into nutrients. These nutrients are small enough to be absorbed into the bloodstream and carried to every part of the body.

Nutrients from food

Food consists of 3 main nutrients: carbohydrate, protein, and fat. Our bodies get energy from all 3 nutrients through digestion:

  • Carbohydrates (such as those in bread, potatoes, and fruit) are broken down into sugar or glucose.
  • Proteins (in foods such as meat and cheese) are converted into amino acids.
  • Fats, including butter and oils, turn into fatty acids.

Glucose is a very important source of energy, for two reasons:

  • It can be converted quickly into energy when we need it, such as during work or sports.
  • The brain and nerves rely on a constant supply of glucose to function.

When glucose enters the bloodstream, the pancreas responds by releasing insulin into the blood.

Insulin’s actions

Insulin does the following things:

  • It stimulates the cells of the body to take up glucose, so that they have the energy to do their work.
  • It allows extra sugar to be stored as energy for future use. The sugar can be stored in the liver as glycogen, or deposited in fat cells.

The insulin released in response to a meal is just the right amount to keep the blood sugar from going too high. The pancreas secretes less insulin into the blood after most of the nutrients from the meal have been taken up by the cells, and the blood glucose levels once again approach “fasting” (pre-meal) levels.

What Happens Without Insulin?
Get Adobe Flash player
Without enough insulin, sugar cannot be taken up and used by most of the body cells. The cells starve, even though the blood glucose concentration may be very high. As a result, the body seeks alternate sources of energy. This leads eventually to the breakdown of fat and the production of harmful byproducts called ketones.

Basal amount of insulin 

Our bodies are almost always making a little bit of insulin, called a basal amount. We need this because, between meals and while we sleep, the liver continues to release some of its stored sugar into the blood. This way our brain and nerves continue to get the constant supply of glucose needed to survive. This basal amount of insulin ensures a perfect balance between the amount of glucose being produced by the liver and that being used by the cells of the body. This prevents either low or high glucose levels.

Later, when we eat again, another burst of insulin is secreted from the pancreas into the blood. The liver stops releasing stored sugar and begins to build up its stores for later use.

Normal blood sugar regulation 

If someone without diabetes measured his blood sugar level before breakfast and after eating, he would have the following levels.

Before breakfast (fasting blood sugar)

After eating (sugar levels will rise)

3.3 to 6 millimoles of sugar per liter of blood (short form: 3.3 to 6 mmol/L)

Rarely above 8.9 mmol/L and never above 11.1 mmol/L

60 to 110 milligrams per deciliter (mg/dL), which is another way of measuring blood sugar

Rarely above 160 mg/dL and never above 200 mg/dL

The healthy pancreas is always releasing very small amounts of insulin. As soon as the blood sugar rises, a message goes to the beta cells in the pancreas. It tells them that more insulin is needed. The beta cells have sensors on their surface that detect the blood sugar level. This lets them give out just the right amount of insulin to handle the higher blood sugar.

When the blood sugar level comes down again, the burst of extra insulin shuts off. The beta cells keep extra insulin on hand. As that is used up, they produce more. So the next time the person eats, the pancreas responds with just the right amount of insulin to bring the blood sugar level back into the fasting range. 

This system works well in people without diabetes. Their blood sugar stays within the normal range even if they eat a lot or go without food for a long time. If a person doesn’t eat at all, the extra insulin just doesn’t turn on. Other hormones such as glucagon or epinephrine kick in to keep the blood sugar from going too low. Glucagon is also made in the pancreas. Epinephrine is made in the adrenal glands, near the kidneys.

What is insulin resistance?

Insulin resistance happens with type 2 diabetes. The pancreas still makes insulin, but either it doesn’t make enough to keep blood sugar levels stable, or the body’s cells and tissues do not properly use the insulin.

Insulin normally acts as a key, opening the door to the body’s cells to let glucose enter. With insulin resistance, this key does not work very well. As a result, some of the glucose is locked out of the cells and builds up in the blood. At the same time, the cells feel the need for more energy, so the pancreas pumps out more insulin to try and move the glucose into the cells. Over time, the pancreas may become exhausted and start producing less insulin. When there isn’t enough insulin, the sugar builds up in the blood.

Marcia Frank, RN, MHSc, CDE

Denis Daneman, MB, BCh, FRCPC