A carb-to-insulin counting ratio, or insulin-to-carbohydrate ratio (I:C), is a formula used as part of an intensive diabetes management program. An accurate I:C helps you match your mealtime insulin dose to your carbohydrate intake. It allows flexibility in the amount of carbohydrate you consume at a meal or snack while still allowing your to maintain good blood glucose control. You should work with your doctor, diabetes nurse educator or registered dietitian to estimate a starting I:C and fine-tune it over time.
About Insulin to Carbohydrate Ratios
Carbohydrates, including sugars and starches, raise blood glucose levels. Your insulin to carbohydrate ratio tells you how many grams of carbohydrate are "covered" by one unit of rapid-acting insulin. When you know this ratio, you can estimate how much meal-time insulin to take to help your body process the amount of carbohydrate you consume in a meal or snack. In general, one unit of insulin covers 12 to 15 g of carbohydrate. However, because everyone responds differently to insulin, this can range from 6 to 30 g or more of carbohydrate depending on how sensitive you are to insulin.
The 500 Rule
You can use the 500 rule, which is based on the assumption that the average person consumes a total of approximately 500 g of carbohydrate daily, to estimate a starting I:C if you have type 1 diabetes and take a rapid-acting insulin, or insulin analog. Divide 500 by your total daily insulin dose to get the grams of carbohydrate covered by one unit of insulin. For example, if your total daily insulin dose is 25 units, your I:C would be 1:20. You would take one unit of rapid-acting insulin for every 20 g of carbohydrate eaten at a meal or snack. If your total daily insulin dose is 50 units, your I:C would be 1:10. You would take one unit of rapid-acting insulin for every 10 g carbohydrate you consume. This 500 rule works best for those with type 1 diabetes who have no insulin production. With type 2 diabetes, there is usually extra internal insulin production plus resistance to insulin.
The Weight Method
The weight method is a chart that gives a starting insulin-to-carbohydrate ratio based on body weight. As body weight increases, insulin sensitivity usually decreases, so each unit of insulin will cover less carbohydrate. According to the weight method, if you weigh less than 60 lbs., your I:C is approximately 1:30, at 60 to 80 lbs., it's 1:25, at 81 to 100 lbs., it's 1:20, at 101 to 120 lbs., it's 1:18, at 121 to 140 lbs., it's 1:15, at 141 to 170 lbs., it's 1:12, at 171 to 200 lbs., it's 1:10, at 201 to 230 lbs., it's 1:8, at 231 to 270 lbs., it's 1:6, and at more than 270 lbs., it's 1:5.
Using Your Ratio
To use your I:C to match your insulin dose to the carbohydrate content of your meals, you need to know how to count carbohydrates. Milk, fruit, starches and sweets are the main carbohydrate sources. An 8-oz. serving of milk provides 12 g of carbohydrate, while servings from the fruit and starch group contain about 15 g of carbohydrate. Examples include one medium fresh fruit, 1 up of. fresh fruit, and ½ cup of canned fruit, potatoes, peas, corn, legumes or cereal. The exchange lists for meal planning include serving sizes for specific foods within each group; you should use food labels whenever they are available. Once you know the amount of carbohydrate in your meal, you can calculate your insulin dose. Divide the grams of carbohydrate in your meal by the grams of carbohydrate in your I:C. For example, if your meal contains 60 g carbohydrate and your I:C is 1:15, divide 60 by 15. You would need 4 units of insulin to cover the carbohydrate in your meal.
Fine-Tuning Your Ratio
Your starting I:C is just an estimate and you will usually need to fine-tune it by keeping a record of your carbohydrate intake and your blood glucose readings before and 3 to 4 hours after meals. Your ratio may change throughout the day depending on hormone levels, physical activity and the amount of long-acting insulin that overlaps with your meal-time insulin. To evaluate your I:C, you need to correctly calculate the amount of carbohydrate you consume at meals and you need to time your insulin dose consistently. Be aware that your I:C usually changes over time.
