Rapid-acting insulin, taken before meals and snacks, is the preferred choice for bolus insulin, but short-acting regular insulin will do if rapid-acting insulin is not an option. High-fat meals are absorbed more slowly and may best be covered by short-acting regular insulin, which has a longer duration of action than rapid-acting insulin.
Rapid-acting insulin takes around 10–20 minutes to kick in, so you’ll want to take it about 10-20 minutes before a meal or snack. Ultra-rapid-acting insulin will start acting sooner. One good rule of thumb is that if your pre-meal glucose is >100 mg/dL or >5.5 mmol/L, take insulin 10 min before eating; if it is >200 mg/dL or >11.1 mmol/L , take insulin 20 min before eating; and if it is >300 mg/dL or >16.6 mmol/L, take insulin 30 min before eating.
The pre-meal insulin bolus amount is based on a carbohydrate dose plus a correction dose. Working with a dietitian and your diabetes provider is the best way to figure out the doses. Bolus doses need to be individualized to ensure that blood glucose doesn’t go too high or too low.
Some people need different amounts of insulin for the same amount of carbs at different times of the day. For example, teenagers and adults typically need more insulin for the carbs at breakfast than later in the day, whereas school-aged children often need less insulin for lunch compared with the rest of the day.
The main factors that determine bolus doses are:
- Carbohydrate count (insulin-to-carb ratio).
- Current blood glucose level (correction dose).
- Insulin remaining from previous boluses (“active insulin”).
- Physical activity.
- The rate of change (a consideration for those who use CGM).
The amount of insulin you take when you eat depends on your insulin-to-carb ratio (I:C), or how many grams of carbohydrates each unit of insulin will cover. A good starting carb ratio for a relatively lean adult is 1:15 (1 unit of insulin for every 15 grams of carbs eaten).
Each person has unique insulin needs, so the I:C is personal. The insulin-to-carb ratio is typically between 1:5 and 1:20 for adolescents and adults and between 1:10 and 1:40 for children. However, these vary by person and time of day and should be worked out with your healthcare team.
This value estimates how much your blood glucose drops for every unit of bolus insulin you take. Sensitivity factors vary by person and time of day.
Often we start with a number of 50 mg/dL or 2.8 mmol/L in adolescents and adults, which means 1 unit drops the blood glucose by 50 mg/dL or 2.8 mmol/L. In children, the starting dose is determined by body weight and where the child is in puberty, anywhere between 50 and >200 mg/dL or 2.8 and >11.1 mmol/L.
Rapid-acting insulin typically stays in the body for 4-6 hours, so there’s likely to be some insulin left in your system if you eat meals or snacks less than 6 hours apart. This can result in “stacking,” in which people give too much insulin too soon after the last dose, causing a low blood glucose reaction to occur. Insulin pumps calculate active insulin automatically and subtract it from the bolus dose.
If you’re planning to go for a long run, mow the lawn, or engage in some other form of activity after a bolus, factor that into the dose. Activity acts like another insulin, lowering blood glucose in its own way. These adjustments in insulin doses help protect against blood glucose lows. You may need to factor in physical activity that occurred before your bolus too, as the effects of exercise on blood glucose can last for up to 24 hours.