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.

Drawn-out meals. Pump users who are planning to have a meal that is eaten over time, such as a cocktail party or Thanksgiving dinner or a meal that is higher in fat or lower in glycemic index and high in fiber, may want to consider using one of the optional bolus delivery tools on their insulin pump. Most insulin pumps allow you to deliver a bolus over time rather than all at once or to deliver some of the bolus immediately and the rest over the next few hours. People who inject insulin could take half their bolus at the start of a meal and the other half an hour or two later.

Snacks. Regardless of whether the carbohydrate you eat is part of a meal or snack, it has the potential to raise your blood glucose level. Alison Evert, R.D., C.D.E., a diabetes educator at Joslin Diabetes Center at Swedish Hospital in Seattle, advises people to "take rapid-acting insulin with any amount of carbohydrate over 10 grams." Although it is common to think that a few grams won’t make a big difference, the reality is that 10 grams of carbohydrate can raise many peoples’ blood glucose 30 or more points.

Unused bolus insulin
While the duration of action of rapid-acting insulin is usually given as 3–4 hours, some diabetes experts believe it may continue to lower blood glucose level for as long as 5 hours. Walsh believes that a good rule of thumb is to assume that about 20% of a dose of rapid-acting insulin is used each hour after it is given. In his book Using Insulin and on his Web site http://diabetesnet.com/diabetes_control_tips/bolus_on_board.php, he provides a table that shows insulin activity at 1, 2, 3, 4, and 5 hours after bolus doses of insulin from 1 to 10 units.

This information becomes important if you give bolus doses of rapid-acting insulin less than four to five hours apart. When two doses of rapid-acting insulin overlap, their effects overlap, too, and the result can be hypoglycemia. Therefore, when you’re considering the size of a bolus dose of insulin, it is critical that you factor in what Walsh calls "the unused insulin" or "bolus [insulin] on board." This is the amount of "active" rapid-acting insulin left from a previous injection or bolus dose from a pump that continues to lower your blood glucose.

To illustrate this idea, consider the following example. Before lunch, you take a bolus of rapid-acting insulin. Three hours later you decide to have a snack with 30 grams of carbohydrate. You check your blood glucose and find that it’s high at 195 mg/dl. Assuming your insulin sensitivity factor is 45 mg/dl, you calculate you’ll need two units of insulin to bring your blood glucose level down to your premeal target of 100 mg/dl and another two units to cover the snack you’re about to eat (assuming an insulin-to-carbohydrate ratio of 1:15). You take the insulin, and several hours later, your blood glucose has dropped to 55 mg/dl. Why? Because you didn’t factor in the hour or so of action left on the bolus you took at lunch.

To prevent hypoglycemia from unused insulin, get in the habit of thinking about when you took your last bolus dose and how much (if any) action is still left before taking another bolus to "correct" high blood glucose.

The latest generation of insulin pumps, which some people call "smart pumps," has a built-in feature that keeps track of how much of a previous bolus dose is still active. If the user attempts to administer a bolus dose while a previous bolus is still active, the pump will suggest subtracting the amount of insulin still "on board" from the requested amount.

How are you doing?
Measuring and observing your postmeal (postprandial) blood glucose values will help you to determine how well you are timing your rapid-acting insulin and figuring your doses. Walsh suggests that "much of the postprandial high blood glucose values observed are because people aren’t giving rapid-acting insulin long enough before a meal to act in tandem with their food. Most foods affect the blood glucose within two hours, while most of the effect of rapid-acting insulin is seen over five hours."

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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