Insulin: How Does It Work?
How does insulin actually get from the injection site to the bloodstream? When an insulin peaks in 4 to 6 hours, does that mean it takes that long for the insulin to get into the blood, or does it get to the blood sooner and just get activated in 4 to 6 hours? What makes one insulin peak in 1 hour and another in 4 to 6 hours? Christine Carr, Avon Lake, Ohio
Roger Austin, ms, rph, CDE, Responds: When you inject insulin under the skin, you are actually injecting molecules of insulin, which break apart once injected.
The basic insulin molecule is called a "hexamer," a six-part crystal, which first breaks into groups of two-part molecules called "dimers," which then further break apart into single molecules called "monomers." It is the monomer that is then absorbed into the capillaries and distributed throughout the bloodstream to actually begin lowering blood glucose levels.
The rate at which the insulin molecule breaks down is determined by the arrangement of amino acids in the molecule. The amino acids are assembled in two interwoven chains of a precise sequence. The newer insulins such as lispro, aspart, glulisine, detemir, and glargine each have unique amino acid sequences that in turn control this rate at which the insulin molecule breaks apart. The first three, which are usually used at mealtime, break apart and are absorbed more quickly than regular insulin. Detemir and glargine, on the other hand, have been altered so that they are absorbed into the bloodstream in a slow and steady manner. (In the past, we had to add different substances, such as protamine and zinc, to prolong the speed at which the insulin breaks apart in the body.)
When we talk about an insulin having its "peak effect," or "peaking," that refers to the time after injection when the dose has its maximum effect. For example, rapid-acting insulins such as lispro, aspart, or glulisine start working within 10 to 15 minutes after injection, but have their "peak," or maximum effect, usually about 1 or 2 hours after injection. After that, the action tends to diminish over time, so that the duration of effect from injection to clearance from the body is usually 4 to 5 hours.