People with type 1 diabetes who like to vary their daily intake of carbohydrates can use an insulin-to- carbohydrate ratio (ICR). This will help you get the right amount of insulin for the carbohydrates you will be eating.

The ICR means that you take one unit of basal or rapid-acting insulin for a particular amount of carbs.

Insulin-to-carbohydrate ratio in action

For example, if your ICR is 1:12 you would have to take one unit of Apidra, Fiasp, Humalog, or Novorapid for every 12 grams of carbohydrate eaten. If the meal contains 36 grams of carb and you have an ICR of 1:12, it would mean you need 3 units of rapid-acting insulin for this particular meal (36 divided by 12 = 3). A person with an ICR of 1:8 would need 4.5 units for the same amount of carbohydrates.

Your credentialled diabetes educator (CDE) or your endocrinologist can help you decide on what your ICR should be.

How is the insulin-to-carbohydrate ratio calculated?

Many diabetes healthcare professionals use what is called the 500-rule to calculate ICR. They take the number 500 and divide it by your current total daily dosage of insulin.

So first we need to establish what the Total Daily Dosage (TDD) is. It’s derived by adding all basal or long-acting and all bolus or rapid-acting insulin that is taken in a 24-hour period.

If your insulin intake varies from one day to the next it is often recommended to do this for a few days and take the average over, for example, three to four days.

To give an example, if you take Levemir, with 12 units in the morning and 14 units at night and you take 8 units of Novorapid at breakfast, lunch and dinner, your TDD will be: 12+14+8+8+8=50; and 500 divided by 50 is 10.

This means that if your ICR is 1:10 you need 1 unit of rapid-acting insulin for every 10g of carbs. This could also be written as 1.5 units per 15g serve of carbs.

To check if your ICR is correct you should check your BGL two to three hours after eating. If the BGL is 1-2 mmol/l higher than it was before the meal your ICR was spot on (and you estimated the carbs for that meal brilliantly).

If your after-meal BGL is more than 3 mmol higher than what it was before the meal, you need to consider making your carb ratio stronger (by lowering the number) or review your carb counting skills.

Rounding up or down?

Few people use an insulin pen device that can provide half units, so in most cases you will have to round off the total amount of insulin to the nearest full unit.

Should you round up or down?

This depends on your sensor glucose (SG) or blood glucose level (BGL) at the time. (If you are using continuous or flash glucose monitoring devices you should remember that for some devices a finger prick is required for insulin dosing. Talk to your CDE for more details).

Generally speaking, if your BGL is high at the time it is usually recommended that you round up. But if your BGL is on the lower side it is worthwhile rounding down (and hopefully avert a hypo). You should also consider what you will be doing during the following few hours.

If you are going to be sitting around you may want to round up, but if you are planning physical activity you would do well to round it down.

Additional tips

• Even if you use an ICR it is still important to remember to bolus before the meal. In most cases 10-15 minutes before you start eating is ideal.
• Many people have different ICRs for different meals. For example: you may have an ICR of 1:10 for breakfast, 1:8 for lunch and 1:12 for dinner.
• For the ICR to work accurately, it is important that you count the carbs correctly. The protein and fat content of meals can also affect BGLs and may need to be considered.
• You may also need to bolus for larger carb snacks.
• Taking insulin before eating and then not eating all of the planned carbohydrate may cause a hypo when the rapid-acting insulin peaks.
• Taking bolus insulin after eating will result in a high blood sugar a few hours later.

See Diabetes NSW & ACT’s events page for Carb Smart, a National Diabetes Services Scheme workshop that provides practical information about carbohydrate choices and their impact on blood glucose levels.

By Carolien Koreneff

Credentialled Diabetes Educator, Registered Nurse, FADEA