Insulin Sensitivity Factor (ISF) and Insulin-to-Carbohydrate Ratios (ICR) for Advanced Carb Counting

Insulin is the hormone necessary to move glucose from the bloodstream into the cells. Insulin keeps blood glucose within a normal range. With diabetes, insulin is either not available or is not being used properly by the body. Insulin treatment through injections or an insulin pump is always necessary for people with type 1 diabetes because the body no longer produces insulin. With type 2 diabetes, the body does not use insulin properly. Insulin injections may be part of the treatment plan with type 2 diabetes also. 

The main goal of diabetes treatment is the regulation of blood glucose to achieve individualized blood glucose targets. There is no one nutritional approach to managing diabetes, nor is there one insulin or medication prescription that is appropriate for everyone with diabetes. 

Evidence-based guidelines from the Academy of Nutrition and Dietetics recommends education on carbohydrate counting using insulin-to-carbohydrate ratios (ICRs) for persons taking multiple daily insulin injections or insulin pump therapy. Insulin-to-carbohydrate ratios estimate how many grams of carbohydrates are covered with 1 unit of short-acting or rapid-acting insulin. For normal-weight adults, a typical starting point for ICR is 1:10, meaning 1 unit of rapid-acting insulin is estimated to cover 10 grams of carbohydrate.  

Carbohydrate intake records, insulin doses, and blood glucose record keeping before and two hours after eating a meal are all necessary to fine-tune your ICR. Your ICR may be different for breakfast than for lunch. It may also change at different stages of life. No two people have the same ICR.  

Another tool that can help manage blood glucose for individuals taking multiple daily doses of insulin or using an insulin pump is an insulin sensitivity factor (ISF). The insulin sensitivity factor is also called a correction factor.  This is used to calculate how much insulin you need to take to bring your glucose level down to a pre-determined target range. A sensitivity factor is either subtracted from or added to the pre-meal insulin dose.  As with the ICR, the ISF is different for different people. Some people may need 1 unit of insulin to lower blood glucose by 50 mg/dl and someone else may need 1 unit of insulin to lower blood glucose by 25 mg/dl.  

How sensitive you are to insulin and your body size affects your ISF. An ISF of 1:40 is typical for an average weight adult with type 1 diabetes.  One unit of rapid-acting insulin is estimated to drop blood glucose by 40 mg/dl.  A person with type 2 diabetes is generally less sensitive to insulin than someone with type 1 diabetes.  Physical activity, illness and hormones can all affect your sensitivity to insulin. If you use an insulin pump or take multiple daily doses of insulin, work with your health-care team to determine your ICR and ISF for better blood glucose control.   

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