Understanding Insulin

Although genetically engineered human insulin is identical to the natural product, insulin that is injected into the fatty tissue under the skin does not act the same as insulin secreted from the pancreas directly into the bloodstream. Injected insulin reaches the bloodstream more slowly, so there’s a delay in when it starts lowering blood glucose levels. Because these differences make it difficult to control blood glucose levels with injected insulin, much research has gone into altering synthetic insulin so that it behaves more like the insulin that is secreted by a pancreas. The rapid-acting insulin analogs are one of the results of this research.

Insulin is now available in a variety of types that are categorized according to action time. These types include rapid-acting insulin, short-acting insulin, intermediate-acting insulin, and long-acting insulin. Insulin can also be purchased in mixtures of intermediate-acting and either rapid-acting or short-acting insulins. (For a list of the insulins currently approved for marketing in the United States, see “Types of Insulins.”)

Rapid-acting insulin. The rapid-acting insulin analogs currently available include insulin aspart (brand name NovoLog), insulin lispro (Humalog), and insulin glulisine (Apidra).

Rapid-acting insulin typically starts working in 5 to 15 minutes, is strongest (peaks) in 45 to 90 minutes, and diminishes in activity 3 to 5 hours after injection. Because it starts working so quickly, rapid-acting insulin is generally taken within 15 minutes of eating—either within the 15 minutes before a meal or as much as 15 minutes after starting to eat. Rapid-acting insulin comes close to mimicking the pancreas’s first-phase insulin release in response to food. If timed correctly and accurately matched to the amount of carbohydrate in the meal, a dose of rapid-acting insulin before a meal can help keep blood glucose levels in target range after the meal.

All of the rapid-acting insulin preparations are approved for use in insulin pumps. In the case of pump therapy, rapid-acting insulin is used not just for bolus doses at mealtimes but also as basal insulin around the clock.

Short-acting insulin. Regular, or short-acting, human insulin usually starts working about 30 minutes after injection, is strongest (peaks) 2 to 4 hours after injection, and decreases in activity 5 to 7 hours after injection. People who use Regular insulin are typically advised to take it approximately 30 minutes before eating a meal so that the rise in the level of insulin in the bloodstream matches the rise in blood glucose level.

Intermediate-acting insulin. Intermediate-acting human insulin, or NPH, generally starts working in 1 to 2 hours. It peaks in 6 to 14 hours, and its activity decreases 16 to 24 hours after injection. Intermediate-acting insulin is commonly prescribed once or twice daily, usually before breakfast and/or supper, to enhance overall blood glucose control. In some cases, intermediate-acting insulin may be recommended at bedtime to help control overnight and early morning blood glucose levels. In either situation, the insulin would be providing a basal-type effect.

Long-acting insulin. Long-acting insulins, sometimes called basal insulins, are typically given once daily and include the insulin analogs glargine (Lantus) and detemir (Levemir). Glargine and detemir, which are clear rather than cloudy or milky in appearance, are considered to be “peakless” insulins. Their effects last for up to 24 hours. Glargine and detemir should never be mixed with another type of insulin.

Premixed insulins. Premixed insulin preparations contain intermediate-acting insulin that is mixed with either rapid- or short-acting insulin in varying percentages. The advantage to using a premixed preparation is that you don’t have to mix the insulins yourself when drawing up an injection or take two injections rather than one.