Clinical meaning
Understanding insulin pharmacokinetics — onset, peak, and duration of each formulation — is essential for NP prescribing to replicate physiological insulin secretion and prevent both hyperglycemia and hypoglycemia.
Physiological insulin secretion has two components: basal secretion (continuous low-level insulin release suppressing hepatic glucose output between meals and overnight, approximately 50% of daily insulin production) and bolus/prandial secretion (rapid insulin release in response to meals, peaking within 30-60 minutes of eating, approximately 50% of daily production). Insulin therapy attempts to replicate this physiology using long-acting basal insulins and rapid-acting mealtime insulins.
Rapid-acting insulin analogs (lispro, aspart, glulisine) have been engineered through amino acid substitutions that prevent hexamer formation, allowing monomeric absorption from the injection site. Onset 10-15 minutes, peak 1-2 hours, duration 3-5 hours. Inject within 15 minutes of eating. Ultra-rapid formulations (Fiasp — aspart with niacinamide for faster absorption, Lyumjev — lispro with treprostinil and citrate) have onset within 2-5 minutes. These are used for mealtime (bolus) coverage and correction doses.
Short-acting (regular) insulin forms hexamers in solution that must dissociate to monomers before absorption. Onset 30 minutes, peak 2-4 hours, duration 6-8 hours. Must be injected 30 minutes before meals. Used in insulin drips (IV — only insulin type given IV), sliding scales, and mixed insulin regimens. Also used in insulin pumps historically but largely replaced by rapid analogs.
Intermediate-acting (NPH/isophane) insulin is complexed with protamine and zinc, creating a crystalline suspension that delays absorption. Onset 1-2 hours, peak 4-12 hours (variable and unpredictable), duration 12-18 hours. Must be gently rolled (NOT shaken) before use to resuspend crystals. The unpredictable peak increases hypoglycemia risk. Appears cloudy. Can be mixed with regular insulin in the same syringe (draw clear — regular — first, then cloudy — NPH).