By Gary Scheiner MS, CDE, Integrated Diabetes Services

Gary Scheiner is owner and clinical director of Integrated Diabetes Services (www.integrateddiabetes.com; 610-642-6055), a private practice specializing in advanced training and management of children and adults on intensive insulin therapy. His team of CDEs (all of whom have Type-1 diabetes) provides consultations worldwide via phone and the internet. Gary is author of several books including “Think Like A Pancreas” and was named 2014 Diabetes Educator of the Year by the American Association of Diabetes Educators. Feel free to submit questions and comments to gary@integrateddiabetes.com

For those who take rapid-acting insulin at mealtimes and want any degree of meal planning flexibility, it is necessary to apply something known as insulin-to-carb (I:C) ratios. Now, if the mere thought of having to do math at every meal sends shivers up your spine, don’t despair. This is something that even the “mathematically challenged” can master in no time.

The Method behind the Mathematics

One of the basic assumptions we make in the use of I:C ratios is that dietary carbohydrates, which include sugars, starches and fiber, are responsible for raising blood sugar levels after meals. Fiber, however, is usually not counted since it does not break down completely and does not raise blood sugar levels. And fat and protein have minimal short-term effects, particularly when consumed as part of a carbohydrate-containing meal.

The rapid-acting insulin that we give at mealtimes is designed to offset the blood sugar rise induced by the carbohydrates. In most cases, insulin analogs such as aspart (Novolog/Novorapid), lispro (Humalog) or glulisine (Apidra) are used at meals. However, Regular insulin (Humulin R, Novolin R) may be used, but it tends to be less effective because of its slower action. Whichever insulin is used, success comes from matching the dose to the amounts of carbohydrate eaten. This is where I:C ratios come in. The I:C ratio specifies how many grams of carbohydrate are “covered” by each unit of insulin.

For example, a 1-unit-per-10-grams-of-carb (1:10) ratio means that one unit of insulin covers 10 grams of carbohydrate. A 1:20 ratio means that each unit covers 20 grams. Calculating a meal or snack dose becomes simple when you know your I:C ratio: Simply divide your carbs by your ratio. If each unit covers 10g and you have a modest 20g meal, you will need only 2 units of insulin (20 ÷ 10 = 2). If you devour 120g, you will need 12 units (120 ÷ 10 = 12). Note that using an I:C ratio of 1:10 will result in larger doses than if you use a ratio of 1:15. A 30g snack will require 3 units if using a 1:10 ratio, but only 2 units if using a 1:15 ratio. As the second number in the ratio goes up, the amount of insulin goes down. The beauty of an I:C ratio is that it gives you the flexibility to eat as much or as little carbohydrate as you choose while still maintaining good blood sugar control. It is common to require different I:C ratios at different times of day due to changes in hormone levels (which affect insulin sensitivity), physical activity (which enhances insulin sensitivity) and the amount of basal / long-acting insulin overlapping with the mealtime insulin. For most people, insulin sensitivity tends to be a bit lower in the morning than later in the day. For example, I require a 1:10 ratio at breakfast, 1:12 at lunch, and 1:15 at dinner and in the evening. But if I exercise after dinner, my I:C ratio drops to 1:25.

Initial I:C Ratios

Two methods exist for choosing an I:C starting point. Whichever method you choose, it is best to begin with a conservative approach (i.e. lower insulin doses) in order to prevent hypoglycemia. It is easier to “tighten up” than it is to try adjusting when you’re constantly recovering from low blood sugar.

The 500 Rule

This approach is based on the assumption that the average person consumes (via meals and snacks) and produces (via the liver) a total of approximately 500 grams of carbohydrate daily. By dividing 500 by the average number of units of insulin you take daily (basal insulin plus mealtime insulin), you can get a reasonable approximation of your I:C ratio.

For example, if you take a total of 25 units of insulin in a typical day, each unit of insulin should cover approximately 20 grams of carbohydrate (500 ÷ 25 = 20). If you take 60 units daily, your I:C ratio would be 1 unit per 8 grams of carb (500 ÷ 60 ≅ 8).

Avg. Units Insulin Daily (basal + bolus)Approx. I:C Ratio

8-111:50

12-141:40

15-181:30

19-211:25

22-271:20

28-351:15

36-451:12

46-551:10

56-651:8

66-801:6

81-1201:5

>1201:4

The major weakness to this approach is that it assumes that all people eat about the same amount of food and produce the same amount of glucose each day. Those who are heavy and those who eat relatively large amounts of carbohydrate will underestimate their insulin requirement with this approach; those who are lean, active or light eaters will tend to overestimate their mealtime insulin requirements.

The Weight Method

This approach is based on the general observation that insulin sensitivity decreases as body size increases. Thus, each unit of insulin will cover less carbohydrate in a heavier person than in a lighter person.

Weight (lbs)Approx. I:C Ratio

<601:30

60-801:25

81-1001:20

101-1201:18

121-1401:15

141-1701:12

171-2001:10

201-2301:8

231-2701:6

>2701:5

One of the potential problems with this system is that it fails to consider body composition. An individual who weighs 250 lbs but is very muscular will be much more sensitive to insulin than a person of similar weight who has a great deal of body fat.

Fine-Tuning and Verifying I:C Ratios

It is best to confirm your basal insulin levels before attempting to fine-tune your mealtime I:C ratios. Any basal insulin changes made after fine-tuning your mealtime doses might send you right back to the drawing board. Fine-tuning I:C ratios is best done empirically (through “trial and error”). And don’t forget to verify the I:C ratio at each meal and snack separately, as they can vary by time of day. Keep detailed written records when testing your I:C ratios. Track your blood sugar level before each meal and then again 3-4 hours later (to give the insulin a chance to work fully) with no other food, exercise or rapid insulin given between the two blood sugar readings. It is best to eliminate factors other than food that might be affecting the results of the tests.