The amount of carbohydrates depends on your appetite, the composition of the meal, the amount of energy you need and personal preferences. The bolus insulin dose varies in a similar fashion. If you use a glucose sensor, you can increase or decrease the mealtime dose based on the direction and rate of change of the trend arrow.
Food rich in protein or fat or both, such as pizza, a large steak, a substantial portion of fatty fish, or potatoes containing fat, can also influence the need for insulin and its absorption rate. When consuming these, it may be necessary to administer slightly more rapid-acting insulin than what would be given for carbohydrates alone. Especially with foods high in fat, it may also be necessary to split the rapid-acting insulin dose in two, injecting half before the meal and the remaining half about 1-2 hours after the meal, as fat slows the absorption of carbohydrates.
To determine the dosage for mealtime insulin, an insulin-to-carbohydrate ratio (ICR) is established for different times of the day. The following indicative 500-rule can be used to evaluate the mealtime insulin dose.
Insulin-Carbohydrate Ratio (ICR)
How much 1 unit of rapid-acting insulin covers carbohydrates:
Divide the number 500 by the total daily insulin (basal insulin + mealtime insulins) unit amount.
Examples:
For a person whose insulin requirement is a total of 50 units per day:
500 / 50 = 10
1 unit of rapid-acting insulin covers about 10 g of carbohydrates
in other words, 10 g of carbohydrate requires 1 unit of rapid-acting insulin.
For a person whose insulin requirement is a total of 30 units per day:
500/30 = 16.7
1 unit of rapid-acting insulin covers about 16 g of carbohydrates
in other words, 10 g of carbohydrate requires about 0.6 units of rapid-acting insulin.
For most normal-weight adults, the insulin-to-carbohydrate ratio is 0.8-1.2 units / 10 g of carbohydrate. For those who are insulin-sensitive, the ratio can be 0.4-0.5 units / 10 g of carbohydrate, and for those who are insulin-resistant, 2-3 units / 10 g of carbohydrate. The mealtime insulin dose and timing of the injection have been correctly estimated if the blood sugar is no more than 2-3 mmol/l higher about two hours after the meal than before the meal.
When starting multiple daily injections as an outpatient treatment, one might initially administer 0.5 units / 10 g of carbohydrate and then refine the appropriate ratio based on glucose sensor results or blood sugar meal pair measurements. Due to morning insulin resistance, a larger ratio is often needed for breakfast. A smaller ratio might be needed for supper, especially if the day has included physical activity.