Rapid-Acting Insulin
Timing It Just Right

Even when you think you’re doing everything right with your diabetes care regimen, it can sometimes seem like your blood glucose levels are hard to control. One potential source of difficulty that you may not have thought of is how you time your injections or boluses of rapid-acting insulin with respect to meals.

Since the first rapid-acting insulin, insulin lispro (brand name Humalog), came on the market in 1996, most diabetes experts have recommended taking it within 15 minutes of starting a meal (any time between 15 minutes before starting to eat to 15 minutes after starting to eat). This advice is based on the belief that rapid-acting insulin is absorbed quickly and begins lowering blood glucose quickly. However, several years of experience and observation suggest that this advice may not be ideal for everyone who uses rapid-acting insulin. As a result, the advice on when to take it needs updating.

Insulin basics
The goal of insulin therapy is to match the way that insulin is normally secreted in people without diabetes.

Basal insulin. Small amounts of insulin are released by the pancreas 24 hours a day. On average, adults secrete about one unit of insulin per hour regardless of food intake.

Bolus insulin. In response to food, larger amounts of insulin are secreted and released in two-phase boluses. The first phase starts within minutes of the first bite of food and lasts about 15 minutes. The second phase of insulin release is more gradual and occurs over the next hour and a half to three hours. The amount of insulin that is released matches the rise in blood glucose from the food that is eaten.

In people with normal insulin secretion, insulin production and release is a finely tuned feedback system that maintains blood glucose between about 70 mg/dl and 140 mg/dl at all times, no matter what or when a person eats or when he engages in physical activity. During illness, when insulin needs may rise, the pancreas just produces more.

People whose pancreas does not secrete insulin normally often must inject insulin or infuse it with an insulin pump. People who have Type 1 diabetes, in which the pancreas secretes no insulin or virtually no insulin, must inject or infuse insulin. But learning when to take insulin and how much to take is challenging, because injected or infused insulin does not act exactly like insulin released from the pancreas. The first step to figuring out when to take insulin and how much to take is understanding an insulin’s action curve.

Insulin action
An insulin’s action curve has the following three phases:

• Onset: when the insulin starts to lower blood glucose
• Peak: when insulin has its greatest effect on blood glucose
• Duration: how long the insulin continues to have some blood-glucose-lowering effect

Rapid-acting insulin is often called mealtime insulin because its action curve most closely resembles the body’s normal release of insulin at mealtimes. (However, most people who use an insulin pump use rapid-acting insulin as a basal insulin as well, infusing small amounts 24 hours a day.) The three rapid-acting insulins currently approved by the U.S. Food and Drug Administration—lispro, aspart (NovoLog), and glulisine (Apidra)—have similar action curves, with an onset occurring in 5–15 minutes, a peak in 45– 90 minutes, and an overall duration of about 3–4 hours.

However, Howard Wolpert, MD, editor of the book Smart Pumping and Senior Physician and Director of the Insulin Pump Program at Joslin Diabetes Center, cautions against blind-faith acceptance of insulin action curves or standard advice about when insulin works, noting that insulin can show “a lot of variability…between individuals and even within the same person from day to day.” The time ranges given for an insulin to reach its peak action are averages, so they may not fit everyone or every situation. You may find through blood glucose monitoring and experience that rapid-acting insulin typically reaches peak effectiveness within 45–90 minutes or possibly sooner or later. This information can affect when you take your premeal doses.