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Understanding Insulin: What You Need To Know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Nov 23, 2022.

What Is Insulin?

Insulin is a hormone that is produced naturally in our bodies. Its main role is to allow cells throughout the body to uptake glucose and convert it into a form that can be used by these cells for energy. Without insulin, we cannot survive, and death from diabetes was a common occurrence until insulin was discovered in the early 1900s by Frederick Banting and Charles Best.

Insulin is made by beta cells, which are only found in the Islets of Langerhans (distinct clusters of 3000 to 4000 cells) within the pancreas. When people without diabetes eat food, these beta cells sense the sugar in the blood and release just the right amount of insulin necessary to maintain normal blood sugar levels.

How Do Insulin Injections Help People With Diabetes Type 1?

Diabetes type 1 is considered an autoimmune disorder and is thought to be a result of the body's own immune system destroying the beta cells within the Islets of Langerhans, although experts are not sure exactly what triggers this attack. Genetics, viruses, and other environmental factors may be to blame.

More than two-thirds of people with type 1 diabetes produce no insulin at all. The one-third that do produce insulin, do not produce enough to satisfy the body's requirements, although this low-level residual insulin production may go on for several decades.

But regardless of whether there is some insulin production or not, everyone with type 1 diabetes needs lifelong insulin therapy. In addition, they need to eat nutritional foods, maintain a healthy body weight, and calculate how many of their daily calories come from carbohydrates, fats, or protein. Their blood sugar levels require frequent monitoring and regular physical activity is an important part of their diabetes management.

How Does Insulin Help People With Type 2 Diabetes?

Both genetics and lifestyle choices are thought to play the most important roles in the development of type 2 diabetes. Type 2 diabetes usually develops slowly over time and is more likely to develop in people with a specific type of genetic mutation who have a poor diet, excess body weight and low levels of physical activity. Most people with the disease are overweight or obese; however, it can also develop in thin people, particularly those who are older.

Most people with type 2 diabetes do not require insulin in the early stages of the disease. The beta cells in the pancreas still produce insulin and certain drugs for diabetes can be given that stimulate these beta cells to produce more insulin, or improve the action of insulin in target tissues such as the muscle, liver, and fat. Some people can even delay or reverse the progression of the disease with good diet choices and exercise.

However, beta cells in people with type 2 diabetes decline with time, with an over 50% reduction in the number of beta cells evident in people who have had type 2 diabetes for over 15 years. When other medications fail to keep blood sugar levels within the normal range in people with type 2 diabetes, a doctor will prescribe insulin.

How Is Insulin Made?

In the space of 90 years, insulin has gone from being a crude extract originally sourced from animals to a highly purified, synthetic, predictable, non-allergenic treatment for insulin-dependent diabetes.

The early commercial forms of insulin were initially extracted from beef and pork pancreases. Synthetic human insulin replaced beef and pork insulin in the early 1980s, and now insulin analogues are replacing synthetic human insulin.

Insulin analogues are a type of human insulin that has been genetically modified so that it better mimics naturally occurring insulin. Although synthetic human insulin is identical in structure to naturally produced insulin, it doesn't work as well and tends to clump together when injected under the skin in high concentrations. This clumping can affect the absorption of human insulin, making it slow and unpredictable, and out-of-sync with your body's needs. Clumping also affects how long insulin works for.

Insulin analogues have a more predictable duration of action and a more reliable absorption than synthetic human insulin which allows them to better mimic normal insulin levels.

Mimicking Natural Insulin Release: Easier Said Than Done

How insulin is released naturally by the pancreas in people without diabetes is both amazing and complicated.

Our pancreas continuously releases a low level of insulin to help keep our blood sugar levels controlled overnight, when fasting, and between meals. As soon as we first smell or chew food, a little insulin is released followed by a big surge during digestion. Natural levels of insulin peak within 45 minutes to an hour of eating a meal, before quickly falling back to the baseline level. Even though insulin takes only 5 to 6 minutes to break down once in the bloodstream, its effect on cells lasts for one to one and a half hours. Our bodies automatically turn on and turn off this insulin supply instantaneously, depending on our needs.

Trying to mimic this level of control in people with diabetes who require insulin can be difficult. For a start, people with diabetes need to know how many carbohydrates they are going to eat, before they eat them, in order to calculate what daily dose of insulin they are going to need. Once somebody has injected a dose of insulin, it will be absorbed into their blood stream whether they need it or not.

Generally speaking, the more frequently insulin is administered throughout the day, the better the control, so a continuous subcutaneous insulin infusion (an insulin pump) or frequent injections mimic natural insulin release the best.

What Are The Different Types Of Insulin?

Insulin is available as human insulin and human insulin analogues.

Insulins are usually grouped as fast-acting, intermediate-acting, or long-acting. Within these groups, they can be further classified depending on how long they take to reach their peak effect or the length of time they last for.

Most insulin in the U.S. is sold as a concentration of 100 units per ml (U100). However, there are currently five concentrated products on the market:

  • Humulin R (human insulin) which has a 500 units per ml (U500) strength available
  • Humalog (insulin lispro) which has a 200 units per ml (U200) strength available
  • Lyumjev (insulin lispro-aabc) which has a 200 units per ml (U200) strength available in addition to the U100 strength
  • Toujeo Solostar which contains 300 units per ml (U300) of insulin glargine
  • Tresiba (insulin degludec) which has a 200 units per ml (U200) strength available in addition to the U100 strength.

Fast-Acting Insulin: Available As Insulin Analogues Or Human Insulin

Fast-acting insulin is absorbed quickly into the bloodstream and is used to control blood sugar levels during meals or snacks, to correct unexpectedly high blood sugar levels, or as the main insulin used in an insulin pump. Fast-acting insulins can be further divided into rapid-acting insulin analogues or short-acting human insulin (also referred to as regular insulin).

Rapid-acting insulin analogues

These work within 5 to 15 minutes and peak within 1 to 2 hours. Larger dosages of insulin analogues affect the duration of action, for example, a few units may last less than 4 hours, whereas 25 to 30 units may last 5 to 6 hours. However, a larger dose does not affect the onset of action or the time it takes to peak.

Examples include insulin lispro (Humalog, Admelog, Lyumjev), insulin aspart (Fiasp, Novolog) and insulin glulisine (Apidra).

Short-acting/regular human insulin

These work within half to one hour and peak within two to four hours. Larger doses of human insulin result in a faster onset of action, but a delayed peak effect and a longer duration of action.

Examples of regular insulin include Humulin R and Novolin R.

Intermediate Acting Insulins

Although you can still buy intermediate-acting human insulins, their use has largely been superceeded by human insulin analogues.

Intermediate-acting insulins may also be called isophane insulin or NPH (stands for Neutral Protamine Hagedorn) insulin. Examples include Humulin N and Novolin N.

Intermediate-acting insulins are absorbed more slowly and last for longer than short-acting insulins. They typically start to work within 1 to 2 hours and peak within four to six hours. Their duration of action can be anything from 12 to 24 hours and depends on the actual dose (higher dosages have a prolonged duration of effect and take longer to peak).

They cover elevations in blood glucose that occur when fast-acting insulins stop working. They are typically used to control blood sugar levels overnight or to mimic the natural basal release of insulin that occurs during fasting or between meals. Intermediate-acting insulins are usually administered twice a day but may be given as a single dose overnight. They may be premixed with a short-acting human insulin (for example, Humulin 70/30 or Novolin 70/30).

Premixes combining rapid-acting and intermediate-acting insulin analogues are also available, examples include Humalog mix 75/25 and Novolog 70/30.

Long-Acting and Ultra Long-Acting Insulins

Long-acting insulin analogues release insulin as a steady amount over most of the day, without any noticeable peak in insulin levels, mimicking natural basal insulin release.

They are preferred over intermediate-acting insulins (NPH insulin) because they have a longer duration of action, flatter concentration profile, more consistent effects, and less risk of hypoglycemia (low blood sugar levels).

Long-acting insulin analogues that are currently available are insulin detemir (Levemir) and insulin glargine (Lantus, Semglee, Toujeo Solostar), and insulin glargine-aglr (Rezvoglar Kwikpen), a biosimilar to Lantus that is interchangeable with it.

Detemir starts to work in about an hour and it reaches a plateau in five hours. The effects of glargine usually last for 24 hours. Both types are considered safe and effective; however, there may be less interindividual variability, risk of hypoglycemia, or weight gain with glargine.

Insulin degludec (Tresiba) is an ultra long-acting insulin that can last in the body for up to 42 hours, although it is generally still administered once daily. Another advantage is that once the pen is in use, it is good for 8 weeks either refrigerated or at room temperature (below 86°F).

How Is Insulin Administered?

Insulin is most commonly administered subcutaneously and there are three main delivery methods:

  • Insulin Pens: these hold a replaceable cartridge of insulin and use a replaceable needle to puncture the skin and deliver the insulin subcutaneously
  • Insulin pumps: these are small computerized devices that deliver a continuous supply of insulin under the skin. They are also known as continuous subcutaneous insulin infusion devices (CSII)
  • Via a syringe and needle using a vial of insulin.

Your doctor or diabetes nurse can give you more detailed information about which delivery method is best for you; however, you may need to take into account what type of insurance you have and the level of co-pays you can afford as diabetes treatment can be costly.

In a hospital, insulin may be administered directly into a vein or into a muscle under certain circumstances. At the moment, there is no effective oral insulin preparation available, although several pharmaceutical companies have attempted to develop one.

Unfortunately, insulin is easily broken down by enzymes in our digestive system, and despite their best attempts, no company has so far succeeded in bringing an economically viable product to market. In 2015, an inhaled insulin was produced (Afrezza), but this rapid-acting insulin can cause sudden or serious lung problems and should not be used by people who smoke or have recently quit, or by those with chronic lung disease such as COPD or asthma.

What Insulin Regimen Is Best for You

The ultimate goal of an insulin regimen is to mimic how your body would naturally release insulin if you didn't have diabetes.

Intensive insulin regimens are the standard method of insulin replacement as they most closely resemble natural insulin release and have been shown to reduce the risk of complications of diabetes (such as heart disease, nerve damage and eye damage).

If you are using a pen or a vial, your doctor will usually prescribe you one or two injections of a long-acting insulin, plus a rapid acting insulin to use as a bolus dose to cover meals or as a high blood sugar correction.

If you are using an insulin pump, your doctor will prescribe a rapid acting insulin that is delivered continuously as a small amount, then boosted around meal times to cover the expected calories in your food.

Common intensive regimens include:

  • Levemir or Lantus once or twice a day combined with a rapid-acting insulin analogue (Apidra, Humulog, Novolog)
  • A rapid-acting insulin analogue (Apidra, Humulog, Novolog) delivered via an insulin pump.

Side Effects Of Insulin

One of the most common side effects of insulin is hypoglycemia (low blood sugar). Symptoms include a headache, sweating, trembling, anxiety, confusion, irritability, rapid breathing, or a fast heartbeat.

People with hypoglycemia may also faint and severe hypoglycemia that is left untreated may be fatal. Hypoglycemia is relatively common because insulin requirements can vary depending on the food you eat, exercise you do, and how well you are. Keep some glucose tablets or another form of quick sugar (such as fruit juice) handy in case you feel hypoglycemic. Let your friends and family know what to do and what symptoms to expect as well, and wear a Medical Alert bracelet or necklace that states you take insulin for diabetes.

Other common side effects of insulin include:

  • Swelling, itching or redness around the injection site
  • Lipohypertrophy (a lump caused by an accumulation of fat under the skin at the injection site) or lipoatrophy (a dent in the skin caused by a loss of fat at the injection site)
  • Weight gain. This is more likely with intensive insulin therapy and may be due to more efficient calorie use, an increase in body fat because glucose is no longer present in the urine, or a reduction in daily energy expenditure and protein metabolism
  • Electrolyte disturbances, such as hypomagnesemia and hypokalemia
  • Blurry vision - typically only occurs on insulin initiation and resolves with continued use.

Infection is also a real risk but can be minimized by always using disposable needles and syringes and sterilizing any reusable equipment.

Insulin analogues are less likely to cause weight gain and night-time hypoglycemia than the older human insulins; however, they only reduce the risk, not eliminate it.

Tips For Insulin Administration

How much insulin our body releases not only depends upon what food we eat, but also our activity levels, general and mental health, current weight, and our age. During pregnancy, insulin requirements tend to drop during the first trimester. Talk to your doctor about how you should adjust your insulin dose when you:

  • Are unwell or have an infection
  • Reduce or increase your activity levels
  • Gain or lose weight
  • Are prescribed other medications (such as prednisone) that may affect your body's sensitivity to insulin
  • Feel stressed or exceptionally tired
  • Are trying for a baby or find out you are pregnant.

You should also regularly monitor your blood glucose and be able to count your carbohydrates and calculate how much insulin to take. You should know what to do if your blood sugar levels are consistently higher or lower than the target level, and have an action plan in place so your family and friends know what signs to look for and what to do if your blood sugar levels get extremely high or extremely low.

Storage Of Insulin

Insulin is easily broken down by extreme temperatures, which means you need to be careful if you live in a part of the U.S. that gets very hot in summer, or very cold in winter.

Opened and in-use cartridges and vials are fine to keep at room temperature (59 to 77 degrees Fahrenheit [15-25 degrees Celcius]) for up to 28 days (the exception is Tresiba which may be kept for up to 8 weeks at room temperature).

However, if you are going out in the sun, use an insulated bag protected by a cool pack to ensure your insulin doesn't heat up; but avoid freezing it. During cold weather, keep your insulin supplies close to your skin so your body heat keeps them at a more even temperature. All unopened vials and cartridges should be kept in the fridge, between 36-46 degrees Fahrenheit (2-8 degrees Celcius). Discard any insulin that you think may have inadvertently got too hot or too cold. The expiry date on insulin applies to unopened, refrigerated insulin.

Final Words On Insulin

If you have been prescribed insulin then your doctor considers it vital for your health. Follow your insulin regimen exactly. Do not miss any doses. If you mistakenly do miss a dose, contact your doctor and ask what you should do.

It is important to tell other health professionals (such as pharmacists, other doctors) that you take insulin before you buy or are prescribed any other medications. This includes dietary supplements, herbs, minerals, nonprescription medicines, and vitamins.

If you can, use only one pharmacy to get your insulin and other medications from. This makes it easier to spot any harmful drug interactions or medication duplications.

And lastly, learn as much as you can about insulin and the type of diabetes you have. This will allow you to contribute to any discussions with your doctor about your insulin regimen and better identify any side effects. Also, check out the Insulin Support Group.

Finished: Understanding Insulin: What You Need To Know

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