Rapid-Acting Insulin
Timing It Just Right

Low blood glucose before a meal. If your blood glucose is low before a meal (below about 80 mg/dl), “Wait to take your insulin,” says Shwide-Slavin. “Let the food have 15 minutes to raise your blood glucose before taking your insulin.”

Low glycemic index foods. On a related note, Wolpert advises, “If your blood glucose is less than 100 mg/dl before a meal and you plan to have a meal with a low glycemic index, wait until you start to eat to take your rapid-acting insulin.”

Uncertain carbohydrate intake. If you don’t know how much carbohydrate you will eat at a meal, consider splitting your rapid-acting insulin dose. Take enough insulin before the meal to cover the amount of carbohydrate you are sure you will eat. Then as the meal goes on and you know how much more carbohydrate you will eat, take more insulin to cover that amount. This method is easiest if you are on an insulin pump.

Large meals. Splitting your rapid-acting insulin dose can also work well for meals that are larger than normal. It has been shown that large meals can delay the rise of blood glucose regardless of the nutrient composition of the meal.

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, RD, CDE, 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.