Insulin Absorption Facts and Myths

By Kelly Close

Blood glucose checks can be frustrating. Sometimes I think I am doing everything right - I know my target, I know the carb count of any food I'm about to eat, I know my insulin-to-carb ratio, and I understand my insulin sensitivity.

But even if I pay close attention to these factors, my blood glucose results can be mysteriously higher or lower than I expected. What people may not know is that of the many things that can influence blood glucose, insulin absorption is one of the most important. Let's look at some common misconceptions:

MYTH: My blood glucose should not be affected by where I take my shot, or where I insert the catheter for my pump. After all, insulin going in is insulin going in!

FACT: Actually, the injection site you pick can affect how quickly the insulin is absorbed. For example, insulin is usually absorbed faster in the abdomen - sometimes almost twice as fast - than in other sites such as the thigh or arm.

MYTH: Insulin will always be absorbed faster if I inject in my abdomen.

FACT: Well, not exactly. There are additional things to consider, says Dr. Howard Wolpert of the Joslin Clinic:

• If you take your shot or place your pump catheter in a spot on your abdomen where there is more fat underneath the skin, insulin may be absorbed more slowly. For example, the middle of the abdomen may be fattier than the side. Even though it might seem more comfortable to take insulin here, because there are fewer nerve endings, your insulin may not be absorbed as quickly as you want. 
  
  
• You should avoid using the same injection site continuously, because this can cause scar tissue to build up. Scarring at infusion sites is often seen in pump wearers, or those who have had surgery, but it is also seen in any people who use the same injection site constantly. Scars can cause a slowdown in absorption. That’s why site rotation is recommended, whether you use a syringe, pen, or pump.
  
  
• When you exercise, blood flow to your legs increases, which speeds insulin absorption. If you inject insulin into the thigh and then exercise, the insulin will be absorbed faster than if it was injected in the abdomen.

MYTH: Temperature does not significantly affect insulin absorption.

FACT: Actually, heat can cause blood vessels to increase in size, as Wolpert writes in his book Smart Pumping. So avoid taking a hot bath, shower or sauna after you take insulin. It could cause you to have a low blood sugar reaction. Gloria Yee, RN, CDE at USCF also suggests that you avoid cooling the injection area (using cold packs, for example). This can slow the absorption rate.

MYTH: Injection site and temperature are the only things that affect insulin absorption.

FACT: That’s not quite true … there are rarely “absolute” rules with respect to diabetes, as most of you probably know! Gloria Yee notes that other factors can affect insulin absorption, such as:

• Exercising
  
  
• Injecting into a muscle (by using a needle that’s too long, or by not pinching up)
  
  
• Dose size
  
  
• Dose errors (take your time!)
  
  
  

MYTH: As long as I use up my insulin within a month of receiving it, I don’t have to worry about how I store it.

FACT: It’s easy to forget, but insulin storage is critical to ensure its stability, which can affect absorption. Check the insulin package insert for storage guidelines. Vials that are currently in use can be kept at room temperature – avoid exposing them to extreme heat or cold. Keep all other unopened vials in the refrigerator. Never put your insulin in checked baggage when you fly: the luggage hold could get a lot colder than your fridge. Insulin that has been frozen and thawed should be thrown away, no exceptions, If you leave your opened insulin vials at room temperature, be sure the sun does not shine on them and heat up the contents while you’re out for the day. It’s a good idea to get into the habit of inspecting vials currently in use to ensure that the insulin has not crystallized. If the insulin that’s supposed to be clear has gone cloudy, or you can see what looks like salt inside the vial, do not use it! The insulin may be less effective, it might act unpredictably, or it might not work at all.

This may seem like a lot to keep track of, but remember: knowledge is power. The smarter you are about your diabetes, and insulin in particular, the better choices you can make to help you stay in good control. As always, be sure to check with your healthcare team before making any changes to your insulin injection routine.