How Insulin Affects Blood Sugar Levels
The Insulin Sensitivity Factor: Your Key Number
One unit of insulin typically lowers blood glucose by approximately 30-50 mg/dL (1.7-2.8 mmol/L) in adults with diabetes, though this varies significantly based on individual insulin sensitivity. 1
The precise amount that 1 unit of insulin lowers blood sugar is calculated using the insulin sensitivity factor (ISF), also called the correction factor, which is individualized for each patient 1:
- Formula: ISF = 1500 ÷ Total Daily Dose (TDD) of insulin 1
- For example, if your total daily insulin dose is 50 units: 1500 ÷ 50 = 30, meaning 1 unit of insulin will lower blood glucose by approximately 30 mg/dL 1
- Alternative formula: ISF = 1700 ÷ TDD (some clinicians use this variation) 2
Factors That Determine Individual Response
Your insulin sensitivity varies throughout the day and depends on multiple factors:
- Body weight and insulin resistance: Patients with higher insulin resistance require more insulin to achieve the same glucose reduction 1, 2
- Time of day: Morning hours often require more insulin per gram of carbohydrate due to counter-regulatory hormones like cortisol and growth hormone (the "dawn phenomenon") 1
- Physical activity level: Exercise increases insulin sensitivity, meaning less insulin is needed to lower glucose 1
- Acute illness or stress: These conditions increase insulin requirements by 40-60% or more 2
Practical Application for Mixtard (Premixed Insulin)
For Mixtard specifically, the glucose-lowering effect is more complex because it contains both short-acting and intermediate-acting insulin components:
- The short-acting component begins working within 30 minutes and peaks at 2-4 hours 3
- The intermediate-acting component provides coverage for 12-16 hours 3
- Critical warning: Mixing different insulin types in the same syringe can impair the quick action of the short-acting component, resulting in less predictable glucose lowering 3
Calculating Your Correction Dose
When blood glucose is above target, use this algorithm:
- Determine your current blood glucose and target glucose 1
- Calculate the difference (e.g., 250 mg/dL - 100 mg/dL = 150 mg/dL above target) 1
- Divide by your ISF: 150 ÷ 30 = 5 units of correction insulin needed 1
Common Pitfalls to Avoid
- Never "stack" correction doses: The pump memory system prevents giving too many corrections too close together, as insulin from the previous dose may still be active 1
- Don't use the same calculation for basal insulin adjustments: Basal insulin is titrated based on fasting glucose patterns over several days, not single readings 1, 2
- Avoid treating hypoglycemia with protein-rich foods: Use 15 grams of pure glucose or fast-acting carbohydrates instead 2
- Don't assume the same ISF applies 24 hours a day: Many patients require different correction factors for different times of day 1
When to Recalculate Your ISF
Your insulin sensitivity factor should be recalculated periodically (every few weeks to months), not daily: 2