What is Insulin?



Insulin is a hormone produced by beta cells in the pancreas. It has three important functions:1

  1. Allow glucose to pass into cells, where it is used for energy.
  2. Suppress excess production of sugar in the liver and kidneys.
  3. Suppress breakdown of fat for energy.

In the absence of insulin, blood sugar levels rise because muscle and fat cells aren't able to utilize glucose for energy. They signal the body that they're "hungry." The liver then releases glycogen, a form of stored glucose. This further increases the blood sugar level. When the blood sugar level rises above 180 mg/dl, glucose begins to spill into the urine. Large amounts of water are needed to dissolve the excess sugar, resulting in excessive thirst and urination.

Without glucose for energy, the body begins to metabolize protein and fat. Fat metabolism results in the production of ketones in the liver. Ketones are excreted in the urine along with sodium bicarbonate, which results in a decrease in the pH of the blood. This condition is called acidosis. To correct the acidosis, the body begins a deep, labored respiration, called Kussmaul's respiration. Left unchecked, a person in this situation will fall into a coma and die.

insulin cwd

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Production of Insulin in the Body

The creation of insulin in the beta cells of the pancreas is a two step procedure. Beta cells first produce preproinsulin. Preproinsulin is cleaved to create proinsulin, which is further cleaved to produce equal amounts of insulin and C-peptide. Endogenous (self-produced) insulin has a half-life of about four minutes in the bloodstream. C-peptide lasts about 30 minutes. By measuring the amount of C-peptide in the blood, scientists can determine the amount of insulin produced by the pancreas.2

In people who do not have diabetes, the pancreas stores about 200 units of insulin. The average basal rate for adults is one to two units per hour. After meals, insulin secretion increases to four-to-six units per hour3

Common Questions

Where does insulin come from?

Insulin used by people with diabetes can come from three sources: human (created via recombinant DNA methods), pork, or beef. Beef insulin has been discontinued in the US, and essentially all people who are newly diagnosed are placed on human insulin.

Why do I have to inject insulin?

Insulin is a small protein which would be destroyed by the stomach’s acidic juices if taken orally. It has been used by an injection under the skin since its introduction in 1922. However, it could possibly be taken any other way which bypasses the digestive system, such as by inhalation into the lungs, inhalation through the nose, by a skin patch, or even as a rectal suppository. Currently, the only alternative way available on the U.S. market (for adults only, with study in children in progress) is the preparation of insulin which can be inhaled by mouth slated for absorption into blood stream through one’s lungs. There are companies working on oral insulin preparation but it’s uncertain when this might become available.

What about inhaled insulin?

One company makes an insulin that can be inhaled instead of injected. That insulin is called Afrezza and it is very rapid acting.

Where should I store insulin?

Unopened insulin vials or pens should be kept cool. Storing them in the refrigerator will help them last as long as possible. Never freeze insulin or expose it to extreme heat, however, as freezing can destroy it. Open insulin, whether vials or pens, should be kept at room temperature for a month or longer, depending on the brand of insulin.

What kinds of insulin are there?

Insulin is classified according to how long the insulin works. There are several types of insulin, listed here in order of rate of action. Be aware that duration of insulin action varies by individual, activity level, and location of injection (or inhalation).

Types and Duration of Action of Insulins Available in the United States

Rapid Acting
Afrezza (Regular insulin) MannKind Inhaled 12 Minutes 30-45 Minutes 2 Hours
Fiasp® (insulin aspart) Novo Nordisk Clear 16 - 20 minutes 90 - 120 minutes 5 - 6 hours
NovoLog® / NovoRapid® (insulin aspart) Novo Nordisk Clear 15 - 20 minutes 60 - 180 minutes 3- 5 hours
Apidra® (insulin glulisine) Sanofi Clear 15 - 20 minutes 60 - 120 minutes 4 - 5 hours
Humalog® (insulin lispro) Eli Lilly Clear 20 - 45 minutes 60 - 120 minutes 4 - 5 hours
Admelog® (insulin lispro) Sanofi Clear 20 - 45 minutes 45 - 150 minutes 3.5 - 4.75 hours
Short Acting
(Humulin, Actrapid, Velosulin®)
Eli Lilly and Novo Nordisk Clear 30 minutes 2 - 4 hours 5 - 8 hours
Intermediate Acting
Eli Lilly and Novo Nordisk Cloudy 2 - 4 hours 6 - 8 hours 12 - 15 hours
Human Regular U-500 Eli Lilly Clear 30 minutes 5 - 6 hours 18 - 20 hours
Long Acting
LANTUS® (insulin glargine U-100) Sanofi Clear 4 - 6 hours No pronounced peak Up to 24 hours (depends on injected dose)
Toujeo® (insulin glargine U-300) Sanofi Clear 4 - 6 hours No pronounced peak Up to 24 hours (depends on injected dose)
Basaglar® (insulin glargine U-100) Eli Lilly Clear 4 - 6 hours No pronounced peak Up to 24 hours (depends on injected dose)
SEMGLEE™ (insulin glargine U100) Viatris Clear 4 - 6 hours No pronounced peak Up to 24 hours (depends on injected dose)
Levemir® (insulin detemir) Novo Nordisk Clear 1 - 2 hours 2 - 12 hours (mild, varies by dose) Up to 24 hours (depends on injected dose)
Tresiba (insulin degluec U-100 or U-200) Novo Nordisk Clear About 1 hour About 12 hours 42+ hours
Pre-Mixed (Action Varies)
70/30 or 50/50 are common mixes
Eli Lilly and Novo Nordisk Cloudy 30 minutes Varies 18 - 24 hours
75/25 and 50/50
Eli Lilly Cloudy 10 - 15 minutes Varies 12 - 15 hours
NovoLog Mix 70/30 Novo Nordisk Cloudy 10 - 15 minutes Varies 10 - 12 hours

Injecting Insulin



Insulin should be injected into the subcutaneous (Sub-Q for short) tissue which lies between the muscle and the skin. When injected here, insulin is absorbed at a steady rate. This helps you maintain good control. There is sub-q tissue all over your body, but some places are better than others for giving insulin, because they are away from large blood vessels and nerves. The best places for injecting insulin are:

  • Upper arms
  • The front and sides of your thighs
  • Buttocks
  • Abdomen (at least two inches away from the navel)

Rotation and Absorption

It’s important to change the site of your injections. Injecting in the same spot can cause lipohypertrophy, a buildup of fat under the skin which can slow absorption.

Insulin is absorbed fastest from the abdomen, followed by the upper arms, the thighs, and the buttocks. Because the rates of absorption vary considerably from one body region to another, the American Diabetes Association currently recommends rotating injection sites within body regions rather than rotation to a different region with each injection.


Financial and Educational Resources

These links will take you to online financial and educational resources for patients.  Please consult with your clinician for additional information.

  1. Understanding Insulin-Dependent Diabetes, 12th Edition by H. Peter Chase, MD. & David Maahs, MD, PhD, 2012.
  2. Management of Diabetes Mellitus: Perspectives of Care Across the Life Span edited by Debra Haire-Joshu, MSEd, MSN, PhD, RN., St. Louis, 1992, pp. 120-121.
  3. Ibid.