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Insulin - What Is It and How Does It Work?

What is insulin?

The main job of insulin is to facilitate the uptake of glucose into the body’s cells. Insulin is a hormone made by the pancreas. The beta cells of the pancreas are stimulated to make insulin when the blood glucose level rises. Insulin tells your muscle, fat, and liver cells to accept glucose from your bloodstream. When these cells take up glucose, they can use it for energy or store it for later use, causing your blood sugar levels to decrease.

What is glucose?

Glucose is a simple sugar, a type of carbohydrate found in most foods. When broken down, glucose is the main form of energy for the body; our muscles and brain could not function without it. The amount of glucose in the blood must be tightly controlled: glucose is necessary for the body to function properly, but excess glucose in the blood can lead to health complications. As glucose circulates in the blood and comes into contact with cells, insulin allows the glucose to enter those cells to be used for energy.

How does insulin work?

When we eat, our food is broken down, and glucose is absorbed through our digestive system and released into the bloodstream. This causes our blood glucose level to rise. When the body’s pancreatic beta cells sense this, insulin is released from the pancreas, telling cells in the body to absorb glucose from the blood to be used for energy. Once this happens, your blood glucose levels return to the premeal range.

Insulin also helps store glucose in the form of glycogen. Once your body has enough glucose for energy use, insulin tells your liver and muscle cells to store the extra glucose for later. In order to do this, a special process in your cells links glucose molecules together into a long-branched structure – this glycogen can then be converted back to glucose later when your blood sugar is low and your body needs energy. Glucagon, another hormone, stimulates the body to convert glycogen to glucose.

People with type 1 and type 2 diabetes do not naturally produce enough insulin in their bodies to fully regulate their blood sugar and allow regular glucose uptake by the cells. People with type 1 diabetes and many people with type 2 diabetes must inject insulin in order to keep their glucose levels in the healthy range.

Who uses insulin?

People with type 1 diabetes cannot produce insulin because their immune system (which protects the body from harmful invaders) attacks their own insulin-producing beta cells, damaging them to the point that they can no longer release adequate amounts of insulin. As a result, those with type 1 diabetes must take insulin, through an injection (syringe or pen), or a pump, or in inhaled form. 

People with type 2 diabetes have insulin resistance and do not make enough insulin. This means that the cells have a hard time responding to insulin, so those individuals need more insulin than a person without type 2 diabetes to keep their blood glucose levels in range. People with gestational diabetes may also experience insulin resistance during their pregnancy. Some people with type 2 diabetes or gestational diabetes are able to manage their glucose levels with oral medications, non-insulin injection medicines, or lifestyle adjustments alone. For other people, as time goes on and their diabetes progresses, they do not produce enough insulin to manage their glucose levels, and they’ll need to take insulin injections. Additionally, if diet alone cannot help a person with gestational diabetes keep their glucose in the desired range, their doctor may put them on insulin, which is safe for the baby.

What are the different types of insulin?

Basal vs. Prandial. Insulins are categorized by how quickly they act (onset), when they peak, and how long they act (duration). There are two main categories of insulin, based on function: basal (long-acting insulin) and prandial (rapid-acting or “mealtime” insulin). Most basal insulin is designed to be injected once or twice daily to provide a constant level of insulin action throughout the day and to provide adequate levels of insulin throughout the night. Basal insulin helps keep blood sugars at a consistent level when you are not eating – but it is not enough to cover glucose spikes after meals. Prandial insulins, on the other hand, are taken at mealtime and act rapidly in the body, serving to manage the elevation of glucose levels following meals. Prandial insulins can also be used as correction doses – between meals or during the night – if glucose levels are high and out of range on the high side.

A newer approach to basal insulin for people with type 2 diabetes is an injectable drug that combines basal insulin with GLP-1 agonist medication. Taken as one daily injection, GLP-1/basal treatments effectively lower glucose levels while reducing weight gain and risk of hypoglycemia (low blood sugar). Learn more here.

Analog vs. Human Insulin. There are two types of insulin structures: human insulin and analog insulin. Human insulins were developed first and are essentially identical in structure to the insulin produced in the human body. Human insulins qualify as short-acting and intermediate-acting insulins. Compared to rapid-acting analogs, short-acting insulin takes longer to act and lasts longer in the body. Intermediate-acting human insulin functions similarly to long-acting insulin but with a quicker and more unpredictable onset and a shorter duration. Analog insulins are similar in structure to human insulins but have minor structural modifications to make them either rapid-acting or long-acting when they enter the body. While analog insulins cost more, they generally lead to less hypoglycemia and weight gain – this is almost always true for people with greater insulin sensitivity, which is very common for those with type 1 diabetes.

Inhaled insulin works through the same mechanism as injectable insulin, but it is taken with an oral inhaler. The only currently approved inhaled insulin in the US is Afrezza, an ultra-rapid-acting mealtime insulin. Compared to injected mealtime insulin, Afrezza starts working and finishes working quickly: its blood-glucose-lowering action peaks just 12 minutes after inhalation and stops after 90 minutes to three hours. In contrast, typical rapid-acting, injectable mealtime insulins take about 30 minutes to peak and can last about four hours.

What are the different brands of insulin?

The insulins in this chart are sorted from shortest duration and fastest-acting to longest duration and slowest-acting. 

Category of insulin

Onset

Peak

Duration

Types of insulin

Prandial

Rapid-acting analog

30 minutes

1-2 hours

3-5 hours

NovoLog

(insulin aspart)

Humalog

(insulin lispro)

Admelog

(insulin lispro)

Apidra

(insulin glulisine)

Prandial

Ultra-rapid-acting analog

12-20 minutes

30 minutes - 2 hours

2-7 hours

Afrezza

(insulin human inhalation powder)

Fiasp

(faster-acting insulin aspart)

Lyumjev 

(insulin lispro)

Prandial

Short-acting human

30 minutes

2-3 hours

3-6 hours

Humulin R

Novolin R

Basal

Intermediate-acting human

2-4 hours

4-12 hours

12-18 hours

Humulin N (NPH)

Novolin N (NPH)

Basal

Long-acting analog

1-2 hours

No peak

12-24 hours

Tresiba

(insulin degludec)

Levemir

(insulin detemir)

Basaglar

(insulin glargine)

Lantus

(insulin glargine)

Semglee

(biosimilar insulin glargine)

Basal

Ultra-long-acting analog

6 hours

No peak

36 hours

Toujeo

(insulin glargine U-300)

How do you take insulin?

There are multiple ways to take insulin. Insulin is commonly injected with syringes or insulin pens. Insulin can also be administered using an insulin pump or an insulin inhaler.

Resources to help pay for insulin

Though insulin can be expensive, there are resources available in the United States to help people access and afford insulin. Depending on your health insurance coverage, income, and location, you may be eligible for financial assistance from insulin manufacturers or your state government. See our resources on ways to access affordable insulin:

Last updated: January 25, 2021