Updated June 06, 2005

Rapid-Acting Insulin
Timing It Just Right

by Hope Warshaw, M.M.Sc., R.D., B.C.-A.D.M., C.D.E.

Matching insulin and food
In addition to understanding an insulin’s action curve, it’s important to understand how the food you eat affects your blood glucose level so that you can match your insulin’s action to the expected rise in blood glucose level following a meal. In general, people with normal stomach emptying can expect some glucose from the carbohydrate they’ve eaten to start raising their blood glucose level within minutes of starting to eat. Blood glucose level tends to peak about one to two hours after the start of a meal and gradually drops over the next three hours.

When to inject. If rapid-acting insulin always started working almost immediately and peaked one to two hours later, injecting it anytime within 15 minutes of starting to eat would work well. But newer observations suggest that rapid-acting insulin doesn’t get absorbed and start working that quickly in all people. John Walsh, P.A., C.D.E., coauthor of the book Using Insulin, for example, believes the maximum blood-glucose-lowering effect of rapid-acting insulin may occur much closer to two hours after an injection rather than 45–90 minutes. If this is the case, the optimal time to take rapid-acting insulin is on the earlier end of the spectrum—about 10 to 15 minutes before eating—rather than with the first bite or 15 minutes after starting a meal. Walsh’s belief is based on research suggesting that insulin’s pharmacodynamics (when, how much, and how long insulin seems to be acting on cells, working to reduce blood glucose levels) may be different from its pharmacokinetics (when, how much, and how long insulin is detectable in the bloodstream). In other words, insulin may be measurable in the bloodstream before it begins actively lowering blood glucose.

Some other factors that may cause insulin action to differ from the action curve given in product literature or to vary from person to person include thickness of the subcutaneous fatty layer at an injection site, temperature, blood flow, exercise, and dose size. (The choice of injection site—abdomen, thigh, arm, buttock—does not seem to affect the absorption rate of rapid-acting insulin as it does for slower-acting insulins.) Injecting into areas that have more subcutaneous fat tends to slow insulin absorption. Widened blood vessels (caused by higher temperatures or exercise) allow insulin to be absorbed more quickly; constricted blood vessels (caused by colder temperatures or smoking) can cause slower absorption. Large doses of insulin may also be absorbed somewhat more slowly than smaller doses.

The importance of staying ahead. Fine-tuning the timing of your premeal boluses or injections is important, but it should not overshadow one of the basics of blood glucose control: counting the carbohydrates in a meal or snack and using enough insulin to cover the anticipated rise in blood glucose. (If you don’t know how to count carbohydrates or to match your insulin dose to the amount of carbohydrate you plan to eat, speak to your health-care provider.) Many people find themselves in a reactive mode when it comes to dosing insulin, taking it in response to high blood glucose rather than using enough of it before a meal to cover the rise of blood glucose in the hours after a meal or snack. Experts agree that it’s much harder to bring high blood glucose back down than to control blood glucose levels with sufficient insulin in the first place.

Variable impact of food
To achieve a good match between the amount of insulin you take and an anticipated rise in your blood glucose level, you need to know how much carbohydrate you intend to eat, because carbohydrate has the greatest effect on blood glucose level. Some experts also encourage people to take into account the type of carbohydrate consumed as well as the amount.

Walsh, Wolpert, and Gary Scheiner, a certified diabetes educator in private practice in Pennsylvania, agree that the glycemic index of the foods in a meal or snack as well as the fiber and fat content can dramatically affect how quickly or slowly blood glucose level rises. (The glycemic index ranks foods based on how quickly they raise a person’s blood glucose.) "Using this knowledge is especially helpful at breakfast," adds Wolpert, because "some people are more insulin-resistant in the morning and therefore have more of a problem controlling blood glucose around the breakfast hours." For these reasons, both Wolpert and Scheiner suggest that people have foods with a lower glycemic index such as yogurt or a bowl of oatmeal with a piece of fruit for breakfast rather than foods with a higher glycemic index such as some cold cereals, pancakes, or muffins.

Also in this article:
Insulin Resources

Hope Warshaw is a dietitian and diabetes educator in Northern Virginia. She is the author of numerous books about diabetes nutrition management including Complete Guide to Carb Counting, 2nd ed. (2004) and Guide to Healthy Restaurant Eating, 3rd ed. (March 2005). Both books are published by the American Diabetes Association.

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