Insulin Absorption: Factors, Best Sites, and Ways to Improve [2023]

For some people, insulin is a superhero. Whether you've eaten too much or have poor glucose control, insulin swoops in and saves you before your blood sugar gets too high. People with type 1 diabetes, in particular, know how important insulin is throughout the day.

However, insulin only works if it's absorbed properly. There are tons of factors that can affect the rate of insulin absorption in your body, which in turn, affects your blood glucose levels.

In this article, we're going to cover all these factors, the best sites for injecting insulin, and how to improve your insulin absorption.

Insulin absorption ultimately determines how well you manage your diabetes.

No matter how much insulin you take, your blood glucose levels will remain high if it's not reaching your bloodstream. 

On the other hand, if it's being absorbed but too slowly, it won't be able to control or regulate huge spikes in your blood sugar.

You might think you're following your diabetes care plan and taking the right doses on time, but in reality, your insulin injections aren't working because of:

• Poor insulin absorption (part of the dose is lost)

• Slow insulin absorption

This could cause symptoms and complications ranging from mild discomfort to a hyperglycemic coma.

This is why people with diabetes need to understand insulin absorption rates and the different factors affecting it. From there, you can optimize these factors to improve your insulin therapy.

In addition, regarding absorption rates, the UK FIT guidelines state the following for insulin injection into subcutaneous tissue:

1. The thigh and buttocks are the preferred injection sites when using NPH (intermediate-acting) as the basal insulin, since absorption is slowest from these sites.

2. The abdomen is the preferred site for soluble human insulin, since absorption is fastest there.

3. Premixed insulin (human or analogue) should be given in the abdomen in the morning to increase the speed of absorption of the short-acting insulin to cover post-breakfast glycaemic excursions

4. Premixed insulin should be given in the thigh or buttock before evening meal as this leads to slower absorption and decreases the risk of nocturnal hypoglycaemia.

Insulin absorption is equally important for type 1 and type 2 diabetes since it regulates blood glucose control. However, people with type 1 diabetes are much more dependent on insulin throughout their life and often take higher, more frequent doses than type 2 diabetics.

Type 1 diabetes is caused by an autoimmune condition where your body's immune system attacks the insulin-secreting beta cells of the pancreas. This leaves your pancreas unable to produce insulin, so you're stuck taking insulin injections for life.

Some people with type 1 diabetes need to take insulin up to five times a day, so you can imagine the importance of insulin absorption.

With type 2 diabetes, on the other hand, the body produces insulin but in small amounts, so even if insulin absorption isn't perfect a few times, your body might be able to manage.

Insulin absorption differs from one person to another. However, a few common factors affect absorption in all people, such as injection site, technique, formulation, temperature, and injection device.

Where you inject your insulin is one of the most important factors affecting the absorption rate.

Insulin is usually formulated as a subcutaneous injection, which means it's supposed to be injected into the subcutaneous layer of your skin. When choosing an injection site, you want subcutaneous areas full of connective tissue and some adipose tissue (fatty tissue) rather than muscles.

Muscles have a much higher insulin absorption rate than subcutaneous tissue. This means the insulin reaches your bloodstream much faster and starts working too quickly. This could lead to severe hypoglycemia if you haven't eaten.

On the other hand, subcutaneously administered insulin works at a moderate pace that doesn't cause drastic changes in your blood glucose.

Much research has been done on diabetics and healthy subjects to determine the best sites for insulin injection. Some of the most popular subcutaneous injection sites include the abdomen, thighs, upper arms, lower back, buttocks, and hips.

Most people with diabetes prefer the abdomen because it's easier to access and less painful than other sites.

The abdomen also provides the fastest insulin absorption rate because it has a great blood supply, fewer muscles, and more subcutaneous tissue.

Aim for the best absorption rate for the area between your waist and hip bones and about two inches away from your belly button.

The upper arms have the second fastest absorption rate of all the injection sites, but self-administration is difficult. Ideally, you want to inject insulin into the triceps at the back of your arm, midway between the elbow and the shoulder.

It can be hard for some people to reach this area with their non-dominant hand, pinch it, and inject insulin simultaneously.

The thighs are a much easier injection site for self-administration but have a slower absorption rate than the abdomen and upper arm.

This is due to a larger muscle mass in the thighs, which means a longer needle length might reach muscle tissue instead of subcutaneous tissue. To help lower the chances of hitting muscle, pinch 1-2 inches of skin when doing a thigh injection.

The injection should be about 4 inches above the knee, toward the top of the leg. Stay away from the inner thigh region.

The lower back, upper buttocks, and hips have the slowest absorption rates of all the injection sites, so keep them as a last resort. These can be difficult to administer on your own, as well.

Differences in absorption speed are generally related to subcutaneous blood flow (SBF) at injection site.

Please take a look at Factors Affecting the Absorption of Subcutaneously Administered Insulin: Effect on Variability - PMC (nih.gov).

Important factors are also obesity and lipodystrophy (common side effect of repeated insulin injection). 

Both factors result in poor blood flow at the injection site causing delayed absorption.

Insulin formulations are classified into five types according to how fast they work and how long they stay in your body. They range from rapid-acting to long-acting insulin, each with a unique way of managing blood sugar levels.

For example, rapid-acting insulin helps control your blood glucose around mealtimes, preventing it from rising too high. This means that rapid-acting insulins need to have a fast absorption rate.

On the other hand, long-acting insulins help keep your blood glucose at a steady, optimal level throughout the day. They usually have a much slower absorption rate, providing a steady insulin release over a longer period.

Most people require a combination of different insulin types to manage diabetes.

Here are the types of insulin:

• Rapid-Acting Insulin: It has the fastest absorption rate and works within 2.5 to 20 minutes. When you use rapid-acting insulin, eat right away to avoid low blood sugar. Popular examples include insulin aspart, insulin lispro, and insulin glulisine.

• Short-Acting Insulin: This type has a slower absorption rate than rapid-acting insulin and takes about 30 minutes to work. Short-acting insulins should be taken half an hour before food.

• Intermediate-Acting Insulin: It takes about 60 to 90 minutes to start working and has an intermediate absorption rate. It tends to be cloudy in nature, so make sure to mix it well before injection. NPH insulin is the most common type of intermediate-acting insulin.

• Long-Acting Insulin: This one has the slowest absorption rate and gets released steadily over 18 to 24 hours. It's usually clear and doesn't need mixing. Popular examples include insulin detemir and insulin glargine.

• Mixed Insulin: Three types of mixed insulins are rapid + intermediate, rapid + long, and short + intermediate. Each type has a different insulin absorption rate depending on the formulation ratios.

Note: Intermediate-acting and long-acting insulins are often called background or basal insulins.

Both insulin temperature and skin temperature can affect the rate of insulin absorption.

For example, after a hot bath or sauna, your blood vessels are much more dilated, which means insulin gets absorbed much faster. On the other hand, after a cold shower or an ice bath, your insulin absorption rate decreases.

As for insulin itself, you should never inject insulin right after taking it out of the fridge. 

Let it reach as close to room temperature as possible, or rub the vial in your hands to speed up the process before injecting.

Cold insulin injections can be painful or unpleasant and have a poor insulin absorption rate.

Physical activity is another factor that affects the absorption of insulin. When you exercise, your blood vessels become wider to pump more blood into your muscles. This leads to better insulin absorption.

Exercise also makes your body cells more sensitive to insulin, making insulin injections more effective.

However, remember that your body burns glucose for energy during physical activity, and insulin helps lower your glucose levels. If you take insulin and do high-intensity workouts, your blood glucose levels could drop dangerously low.

Another thing to keep in mind if you take insulin before exercising is the injection site. 

For example, if you're going to play basketball, you might not want to inject insulin into your upper arm. It might be unpleasant during the game and can affect insulin absorption.

Insulin absorption is also affected by the type of insulin delivery device you use.

There are many different insulin delivery devices, but the most common are insulin pumps, syringes, and pens.

The InsuJet is an innovative needle-free insulin jet administration system designed to revolutionize how insulin is delivered to individuals with diabetes. This cutting-edge device offers a pain-free and convenient alternative to traditional insulin injections, enhancing the quality of life for millions of patients worldwide.

The InsuJet operates on a simple yet ingenious principle: utilizing high-velocity liquid insulin to penetrate the skin and deliver insulin into the subcutaneous tissue without using needles. This groundbreaking approach eliminates the discomfort associated with needles and minimizes the anxiety often linked with insulin injections, making it particularly advantageous for pediatric patients and individuals with needle phobias.

The InsuJet system consists of a handheld device that houses a replaceable insulin cartridge. Patients can easily load the cartridge into the device, adjust the dosage, and place it against their skin.

Upon activation, a controlled burst of pressurized air propels a fine stream of insulin through a microscopic orifice, creating a virtually painless entry point through the skin. The device's adjustable settings facilitate the precision of insulin delivery, ensuring accurate dosing tailored to the patient's needs.

By providing an efficient, needle-free, and user-friendly solution, the InsuJet represents a remarkable advancement in diabetes care, potentially promoting better adherence to insulin regimens and improving patients' overall health and well-being.

Insulin syringes are common one-time-use insulin injection devices. They come in three different sizes according to the maximum units of insulin they can hold: 30-unit, 50-unit, and 100-unit syringes.

The syringe size, more specifically the needle length, and angle of injection, can affect the rate of insulin absorption. Needle length is typically between 6 to 8 mm.

Depending on your body fat percentage, the needle reaches either subcutaneous tissue or the muscle, which have different absorption rates.

Your doctor should decide which syringe size and needle length are best for you, depending on your insulin dosage. For example, if you take 40 units of insulin per dose, the 30-unit syringe won't work for you. Also, keep in mind that syringes are to be used once and then disposed of.

In the UK, U100 single use insulin syringes are the only ones used for insulin injection.

Also, the Forum for Injection technique (FIT4Diabetes :: United Kingdom ) has guidelines for insulin syringes inthe UK with regards to needle length:

• 4mm needles with the smallest diameter (higher gauge number) are suitable for patients with insulin doses less than 50 units

• Patients with insulin doses above 50 units may require insulin needles of 6mm

Insulin pens are another method of injecting insulin without the hassle of measuring your dose. They allow for precise dosing each time, which improves your insulin absorption rates and keeps your insulin concentration steady with each dose.

There are generally two types of insulin pens: disposable and reusable pens. Disposable ones are pre-packed with an insulin cartridge, and when it's empty, you throw away the whole pen. Reusable pens have a replaceable cartridge or penfill.

Both types of pens require a disposable needle that screws onto the top of the pen for injection. These needles can vary from 4 to 12.7 mm in length and also have different thickness grades. 

You'll probably be comfortable with 4 or 5-mm lengths, but still, consult your doctor.

An insulin pump is a small device you wear outside the body that delivers small, programmed insulin doses throughout the day. Only rapid-acting insulin is used with insulin pumps, and the doses are typically delivered to the body's fatty tissue, usually in the abdomen.

These pumps provide a more precise and hands-free approach to insulin administration, which gives a more predictable insulin absorption rate than syringes and pens.

Now that you know the different factors affecting insulin absorption, you can better control your blood glucose levels.

Here are a few ways to improve insulin absorption:

Site selection and rotation are the best ways to improve insulin absorption.

When you repeatedly use the same injection site, the tissue becomes irritated and often develops scar tissue. This makes it even more uncomfortable and harder for insulin to be absorbed.

Could you make sure to rotate the injection site between doses to avoid these complications? 

For example, if you take your nightly dose of long-acting insulin and prefer injecting it in the thigh, alternate between the left and right thighs each night.

If you always take your regular insulin (short-acting insulin) in the morning and prefer abdomen injections, use different areas of the abdomen around the belly button.

It's important to choose the injection site based on how fast you want the insulin to be absorbed. Long-acting insulin typically covers 24 hours, so it doesn't need a fast-absorption injection site. The thigh or hips will do.

On the other hand, rapid-acting insulins injected into the abdomen have the fastest insulin absorption rate. It's perfect for when your blood glucose levels are too high or are about to spike post-meal.

Remember, the order of insulin absorption from fastest to slowest is: abdomen, upper arm, thighs, and then upper buttocks, lower back, and hips.

Improving your injection technique can enhance subcutaneous insulin absorption. Here are a few tips to keep in mind:

• Use a clean needle or syringe, inject at the correct depth by pinching your skin, and pick the right injection site

• Avoid scar tissue or areas with too much adipose tissue (fat deposits)

• Make sure the insulin isn't too cold before injection

• Warm up the insulin vial or pen by rubbing it between your hands before injecting it

• Avoid injecting insulin into body parts that you'll heavily rely on during exercise, such as the upper arms

If you're interested, the American Diabetes Association has a quick article on how to take insulin safely and with little to no pain.

If you are in the United Kingdom, Diabetes UK has an article on Injecting insulin

Sometimes the reason insulin absorption is poor is that the insulin has gone bad. It might be due to improper storage or simply because it's been out of the fridge for too long.

Here are a few tips to keep in mind when storing insulin:

• Keep unopened insulin in the fridge, on its side, and at a temperature of 2-8 °C

• Never freeze insulin

• Keep opened insulin at room temperature (less than 25 °C) and away from direct sunlight

• Once you've used an insulin vial or pen, it should be thrown out one month from the date of opening, even if you haven't finished it

• If you need to take insulin with you and the weather is too hot, pack it in an insulated insulin carry bag or cooler

Please always look over your insulin injections before taking them. You should never use insulin injections if any of the following is true:

• Normally clear insulin has turned cloudy

• Normally cloudy insulin has formed clumps or flakes that don't go away when you gently rotate the vial

• The insulin is past its expiry date

• The opened insulin pen or vial has been out of the fridge for over a month

Now that you know how insulin absorption works, your insulin injections are no longer a shot in the dark. You can get the most out of each insulin dose with the proper technique and tips.

With this, you're one step closer to stable, steady blood glucose levels.