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Since its discovery in 1921, insulins have improved dramatically and saved countless lives. Modern day, synthetic, rapid acting insulins such as Humalog, Novolog, Apidra, Fiasp, Admelog, etc.. are designed to control high blood sugars and cover carbohydrates. These insulins have, in large part, helped many people with diabetes live normal lives without major complications. Rapid acting insulins on average will be in a person’s body for about 3-6 hours. Once an injection is taken, Active Insulin Time (AIT) and Insulin on Board (IOB) become quite relevant.
AIT refers to how long insulin will be working in the body once it has been taken. IOB refers to how much insulin is actually active in the bloodstream after it has been injected. Your doctor or endocrinologist will often give you a specific time window of when not to give another dose of rapid acting insulin. For example, he or she might tell you, “your active insulin time is 3 hours, so don’t take another shot of insulin until at least 3 hours is past.” The idea of waiting is to avoid “insulin stacking.” Think of insulin stacking like walking up the stairs. Each stair goes up and plateaus on the top of each step but then goes back up with the start of the next stair. This is what insulin stacking is, a shot of rapid acting insulin is given initially and begins to plateau/leave the body when another shot is given, resulting in another increase of the insulin levels. Insulin stacking makes low blood sugar more likely and makes it difficult to know how much insulin is active in the body. Additionally, it can be challenging to figure out if basal or bolus doses need to be changed. Knowing your AIT and IOB can help you control your blood sugars.
As mentioned above AIT is about 3-6 hours for most people who are using rapid acting insulin. Let’s meet somewhere in the middle and call it 4 hours for this next example of calculating IOB for someone on multiple daily injections of rapid acting insulin. If we assume that after each hour the same amount of insulin has left the body, we can break down each hour into 25% based on 4 hours of AIT (with longer AITs, IOB can be calculated with the corresponding percentages of each hour). With each hour passed, 25% less of the initial dose will be active in the bloodstream. If someone gave 3 units of rapid acting insulin 3 hours ago and gave 1 unit of rapid acting insulin 4 hours ago, the IOB would be 1.75 units of rapid acting insulin in the bloodstream at the current time.
Hour 1 - 100%
Hour 2 - 75%
Hour 3 - 50%
Hour 4 - 25%
Hour 5 - 0%
Example: (50% x 3) + (25% x 1) = 1.75 units of IOB.
To avoid insulin stacking, if another dose of insulin needs to be given for a high sugar level or carbohydrates, 1.75 units should be subtracted from the dose. For example, if 5 units is the dose that is calculated to give, the person would subtract 1.75 units from 5 units to get 3.25 units. If this person is using shots, the dose is 3 units. An insulin pump will track this for you and make recommendations on insulin doses by looking at the IOB in addition to the pre-programmed settings.
If calculating your IOB seems daunting, there are apps that can help you with this. The following apps have features that can help you with insulin dosing:
mySugr
Diabetes:M
Glucose Buddy
Blue Loop
One Drop
Diabetes Connect
PredictBGL
RapidCalc
Insulia
Accu-Chek
BlueStar
As you can see AIT and IOB are important concepts to understand and can directly impact blood sugar management. Don’t guess any longer on the insulin dose you need, talk with your doctor today (or as soon as possible) about incorporating IOB calculations into your diabetes management. If you enjoyed this post, check out some of my other blog articles or take a look at my social media platforms on YouTube, Facebook, and Instagram.
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