Insulin is a hormone that is usually produced by cells within the pancreas. In patients with diabetes, the pancreas is either not producing insulin at all--type 1 diabetes--or is producing some insulin but not enough to prevent dangerous increases in the level of sugar within the blood--type 2 diabetes. All patients with type 1 diabetes, and some with type 2 diabetes, need exogenous, or external, sources of insulin in order to be healthy. Commonly, insulin is delivered by an injection under the skin; this is called a subcutaneous injection. Several types of subcutaneous insulin may be prescribed.
Human Insulin
Human insulin formulations are considered "human," as opposed to "analog" insulin, because the protein structure is the same as that of insulin produced by the human pancreas. Both regular insulin and insulin NPH--which stands for neutral protamine Hagedorn--when injected subcutaneously, take several hours to start producing glucose-lowering effects.
Depending on factors such as the site of injection, the amount of fat at the site of injection, and the way in which the insulin is injected--a shallow compared to deeper angle of injection--NPH insulin may take up to two hours to begin lowering the level of sugar in a patient's blood. Furthermore, according to the medical reference UpToDate, the glucose-lowering actions of NPH insulin are not consistent, reaching peak levels of activity six to 10 hours after the injection is given and lasting anywhere from 18 to 28 hours.
Rapid-Acting Insulin Analogs
Insulin aspart is one example of a subcutaneous insulin that falls within the analog category; it is nearly identical to human insulin.
Similarly, insulin lispro has had a reversal of two amino acids in the protein structure--the positions of the lysine and proline amino acids have been reversed. These seemingly minor changes result in significant differences in the amount of time it takes for the insulin to begin working, the amount of time to reach peak level of activity, and the duration of action of the insulin.
According to pharmacologist Sara Noble and colleagues, writing in the "American Family Physician," insulin lispro takes less than 15 minutes to begin working; sometimes the blood sugar level can begin coming down within 5 minutes of subcutaneous injection. Peak action is within about 45 to 75 minutes, and overall, lispro activity lasts around 5 hours.
According to McCullogh, insulin aspart shows a similar pattern of activity. Because of this, patients who are using either of these rapid-acting insulin types should be certain to eat shortly after injection, to prevent the development of an overly-low blood sugar level. In addition, the rapid onset of action makes either of these insulins good choices to use for pre-meal injections, to cover the amount of sugar that the person will be eating in the meal.
Insulin Glargine
Insulin glargine is another analog of insulin that has had several small modifications to the original insulin structure made. These changes cause certain chemical changes in the insulin, with the result that absorption of the insulin into the bloodstream is delayed. In addition, the insulin has a much longer duration of action: up to 24 hours or greater. This aspect, combined with the fact that insulin glargine has no "peak effectiveness" to consider, makes glargine good option for providing a basal, or "steady state" level of insulin. Such an insulin would not be good for mealtimes, because it takes too long to absorb and would not be effective in covering the amount of sugar consumed within a meal. For providing a "background level" of insulin--similar to what the pancreas of a non-diabetic patient secretes--a long-acting insulin such as insulin glargine is a good choice.
References
- "UpToDate;" General Principles of Insulin Therapy in Diabetes Mellitus; David McCullogh M.D.; May 2010
- "American Family Physician; "Insulin Lispro: A Fast-Acting Insulin Analog; S. Noble et. al.; January 1998
- "UpToDate;" Insulin Therapy in Type 1 Diabetes Mellitus; David McCullogh M.D.; March 2010
