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This is the second part of a six part series on "How to Understand and Use Insulin." The goal of this series is to promote a better understanding of insulin for those readers who already take insulin, including the many people with Type 2 diabetes who have switched from pills to insulin to treat their diabetes. The first and second parts of the series discuss the technical factors involved in minimizing variations in insulin absorption. Parts three and four will focus on adjusting insulin, and parts five and six will focus on insulin research.
Dr. Barry H Ginsberg, MD, PhD, and Endocrinologist, is currently the Medical Director for Becton-Dickinson, the largest manufacturer of syringes in the United States. Becton-Dickinson provides 95% of the syringes used in this country.
The temperature of the injection site can strongly influence the rate of absorption: the warmer the area the faster the absorption, the colder, the slower. For example, regular insulin injected into a thigh peaks in the bloodstream at 90 minutes. If the leg is immersed in cold water, this time increases to 120 minutes, but when immersed in warm water, it falls to less than 40 minutes. The amount of insulin absorbed can also vary substantially. A warm thigh may absorb almost twice as much insulin as a cold thigh.
The practical implications of this are obvious. Taking your insulin injection and then taking a hot shower, sauna or going into a hot tub will increase the rate of absorption of insulin and may lead to an insulin reaction. Alternatively, going out into the cold with only thin slacks or hosiery can lead to the absorption or less insulin and serious high blood sugars. For most patients, the best advice is to avoid extremes of temperature at injection sites. Occasionally (and only after a thorough discussion with a physician), patients may use a hot shower to speed and increase the power of insulin.
A similar effect is seen with exercise, although the full effect of exercise is much more complex than just its effect on insulin absorption (this will be discussed in subsequent articles). Exercising a region will lead to a dramatic increase in the rate of absorption of insulin and the amount absorbed, similar to that seen when warming the region. Although the clinical implications are complex, as a simple rule, people with diabetes should generally avoid injecting into an area that will be heavily exercised in the next 4 hours and should check their blood glucose before exercise. If their blood glucose is less than 150 mg/dl, they should eat some simple carbohydrate, such as glucose tablets before exercising (the amount will vary with the type and duration of the exercise and the initial blood glucose). If it is above 300 mg/dl they should probably postpone the exercise.
Injection technique can make a major difference in the rate and amount of insulin absorbed. As seen in Figure 1, we normally inject insulin into the subcutaneous fat. Under varying conditions, however, insulin might be injected into the subdermal space (between the skin and the subcutaneous fat), the subcutaneous fat, the epimuscular space (between the subcutaneous fat and the muscle) or intramuscularly (into the muscle). Absorption of insulin from each of these sites is different and all are faster than absorption from the subcutaneous tissue.
Injection of insulin into the sub-dermal space is an unusual event. It can occur if you go at too great an angle or use a very short needle. Insulin absorption from this area is not very well-studied, but seems to be very fast. It is difficult to reproducibly inject into this space and few try.
Injection into the epimuscular space can occur with some degree of regularity and some Dutch physicians train their patients to inject into this area. To do so requires considerable skill and an area with only a small amount of subcutaneous tissue (a thin area). Insulin absorption from this area seems to be very fast, both because the insulin spreads out as a thin layer between the muscle and the subcutaneous fat and because there is a very high blood supply to this area.
Intramuscular injections have been of some concern, particularly in Europe. Some areas, such as the outside of the thigh, may have very little subcutaneous tissue. When you inject into one of these areas without pinching up the skin, you may go through the subcutaneous tissue and directly into the muscle. This creates two problems. Injection of insulin into the muscle often hurts and insulin is very rapidly and completely absorbed from the muscle. Insulin injected into a muscle may peak in the bloodstream in as little as 15 minutes. This is much faster than the 90 minutes normally required for injection into a thigh and up to twice the amount of insulin may be absorbed. Some health professionals have suggested shorter needles to overcome this, but to be truly certain that you would avoid intramuscular injection, the needles would have to be only 1/8 of an inch long and then there would be problems of leaking insulin out of the injection site and subdermal injections. A far better approach is to make sure that you pinch up the skin and therefore thicken the subcutaneous tissue before injecting.
The type of device may alter insulin absorption. Although most people with diabetes use syringes to deliver insulin, there are the alternatives of jet injectors, insulin pens and insulin pumps. In addition, there are some experimental techniques such as the use of "sprinkler" needles. Insulin injected by syringes or pens is deposited as a single large drop (Figure 2). Since this minimizes the area for absorption of insulin, movement of insulin to the blood is slow. In contrast, the insulin delivered from a jet injector disperses upon hitting the skin and forms multiple small droplets. Since the surface area of these multiple droplets is much larger, insulin is absorbed about 15 minutes earlier after jet injection than after a syringe or pen. One type of experimental needle delivers insulin through minute holes in the wall. Insulin is also absorbed faster from this "sprinkler" needle.
These various factors can work for or against you in your quest for better blood glucose control. Understanding the components described above will help you avoid some of the major problems that cause low and high blood sugar and may allow you and your doctor to work out the best possible diabetes regimen for you.
Next Issue: Part 3-Adjusting Insulin.
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