For my contribution this month, I wanted to share an important lesson I learned about twenty years ago from Peggy Wong at the UCSF Diabetes Teaching Center. It concerns how long insulin lasts after you push down that plunger and create a “depot” of insulin under your skin.
We know a shot of insulin does not make a perfect sphere when injected, but it does create a pool of insulin in the subcutaneous tissue that approximates a ball or sphere. After the insulin is injected, it starts to be absorbed by the tissue it actually contacts. As you will see from the formulas and examples below, the larger the shot, the more time it takes to be absorbed by your body.
In the chart below, we have calculated the volume and surface area for seven different-sized injections. You might remember from a past math class that the radius of a sphere is the distance from the center to the surface of the sphere. The chart shows that as the radius (r) grows, the amount of surface area in relation to the volume gets smaller and smaller.
For example, let’s say you test your blood sugar and it is 220 mg/dl. You decide that you need 4 units of insulin to bring it back to 100 mg/dl.
Let’s look at the chart to see the difference between taking one shot of 4 units versus taking four shots of 1 unit. Both provide equal amounts of insulin.
With the 4-unit shot, there is only approximately 0.6 mm of surface area available for the insulin to “escape” into the surrounding tissues. With four shots of 1 unit each, there is actually 0.96 mm of surface area – about fifty percent more!
So which one do you think is going to hit the blood faster? Yes, the one with the most surface area. In fact, I have had healthcare pros recommend splitting up a dose like this to get a high blood sugar down. And one of the syringe makers once explored the possibility of making a “sprinkler” syringe, in which the insulin would come out of holes up and down the shaft of the needle, creating many small depots and, therefore, faster absorption.
Volume = 4/3 πr3
Surface = 4πr2
|units of insulin*||surface area|
in mm squared
|*1 unit of insulin is .01 ml|
Looking at the chart, you can see that as the shot gets larger, there is less and less surface area as compared to the volume of the shot. This forces the insulin to wait longer to be absorbed, because the insulin molecules in the middle of the injected ball won’t come into contact with the tissue until the insulin molecules surrounding them get absorbed first.
Another problem with large shots is variability in absorption. The larger the injected ball of insulin, the longer it will be there, and, therefore, the more unpredictable it becomes. Absorption is affected by body movement, which can hasten absorption, and by changing temperatures, both within the body and in the environment outside the body. Hot tubs, for instance, speed up absorption. If you get into a hot tub after taking a large shot, more insulin is mobilized by the heat than would be after a small shot.
I feel that all insulin users should try to understand as much as they can about this powerful, wonderful, lifesaving drug. So it is crucial to understand the mechanics of how insulin is absorbed. I hope that this little math lesson helps.