This is the first 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 part of the series will 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.
The diabetes watchword of the 90’s is control. Diabetes health care professionals and patients realize that normal or near-normal blood glucose levels may prevent or slow the onset of diabetes complications. As part of this move to better diabetes control, more people with diabetes are using insulin and often taking smaller, more frequent doses of insulin. Maximizing the effectiveness of insulin requires a knowledge of insulin and its absorption, information about blood glucose and nutrition, and skill in matching these factors. In these articles I would like to share some of the tips about insulin administration that have worked best for my patients.
A number of factors alter the absorption and therefore the action of insulin.
Each of these factors is complex and together they are very intricate. The most important aspect of understanding insulin therapy, like most parts of diabetes education is understanding how each of these factors influences your therapy and how you can gain control over the factor. Keeping careful records of insulin doses, injection sites, times, food, exercise and blood glucose values will often help you and your health professional understand how to optimize your therapy.
Changes in the amount of time that it takes for insulin to be absorbed into the bloodstream can be a critical factor in obtaining diabetes control. A major challenge in diabetes therapy is to match the insulin with food and exercise. This means that both the amount of insulin and its timing are critical, yet both of these can be influenced by the factors in Table I. Insulin is normally injected into the fatty area just below the skin, called the subcutaneous tissue. This subcutaneous tissue has specific enzymes that can destroy insulin. Generally, the longer the insulin stays in this subcutaneous tissue, the more insulin is destroyed by these enzymes. Thus, these factors produce a “double whammy” effect. They alter both the timing and the amount of insulin absorbed.
The type of insulin is the most obvious change that influences insulin absorption. Since this will be the focus of a later article in this series, I will discuss it here only briefly. Regular insulin is absorbed faster than NPH or Lente insulin, which are absorbed faster than Ultralente insulin. Mixing insulin can sometimes change their absorption kinetics. Regular and NPH insulins can be mixed in virtually any proportion with little or no effect on their action. The insulins of the Lente series, Lente and Ultralente insulin, are very high in zinc and can slow the absorption of regular insulin by about fifteen minutes. Protamine zinc insulin, a rarely used insulin, will dramatically prolong the absorption time for regular insulin.
The species of insulin can also be important. For many patients human insulin works faster than pork or beef insulin and may not last as long. For regular insulin, this is often an advantage, as we want this insulin to work fast, but for NPH insulin, it may mean that the insulin will not last through the night, especially if you are taking only one shot each day.
The site of injection can be critical. The time for absorption of insulin varies with the site and so does the total amount absorbed. Regular insulin peaks in the blood stream about 90 minutes after it is injected into the thigh. Because of the subcutaneous enzymes, about 60-65% of the insulin is absorbed. After injection into the arm, it peaks in about 75 minutes whereas an injection into the abdomen (stomach area) takes only 60 minutes. Looking at this another way, it takes 50% longer for insulin to be absorbed from the leg than from the abdomen. These three sites also vary in the total amount of insulin absorbed. Because the insulin is absorbed faster from the abdomen, less is destroyed by the subcutaneous enzymes. Thus, regular insulin injected into the abdomen may be up to 25% more powerful than the same amount of insulin injected into a thigh.
Health care workers have always told patients to rotate their injection sites. This minimizes some of the traumatic and biochemical effects of insulin in the local areas. Recently, most diabetes health professionals have modified this policy to suggest that patients should rotate their injections to the same type of site at the same time each day. For example, you may chose to give your morning injection in the abdomen and the evening injection in the thigh, but if you do, you should give your shot every morning into the abdomen and every evening into either the left or right thigh. Because of it significant area and the rapidity of insulin absorption from this site, some patients choose to give their injections only in the abdomen. This is particularly popular among patients performing intensive insulin therapy (three or more injections per day or an insulin pump). Patients who use exclusively their abdomen are generally careful to rotate among the many possible sites in this region.