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Carb Counting and Insulin Math in Diabetes

As a person with diabetes who uses insulin, you develop a knack for the simple diabetes arithmetic, thanks to the necessary carb counting and insulin mathematics you perform a few times each day.  You also acquire a keen sense of balance because you are constantly finding the blood sugar equilibrium between your carbs and insulin.

The Insulin and Carb Counting Equation
A rough rule of thumb is that for every 10 grams of carbs you eat at a meal, you should inject a unit of insulin.

However, your doctor will give you guidelines, specific to you, for how much insulin you should use before every meal.  Those instructions will depend on the type or types of insulin he or she prescribes, your resistance to insulin (if you have type 2 diabetes), your body mass, your recent blood glucose test results, your level of fitness, and other factors. 

A Carb Counting Example
Let’s say you sit down with your family for dinner one evening, and you conservatively dish yourself up the following meal:

  • Baked chicken: 4 oz, 0 carb grams
  • Mashed potatoes: 4 oz, 25 carb grams
  • Caesar salad with dressing: 3 oz, 5 carb grams
  • Mixed vegetables: 3 oz, 8 carb grams
  • A medium-sized roll: 2 oz, 22 carb grams

Your carb counting estimate reveals that you are about to eat approximately 60 carb grams. So, just prior to the meal, you would inject about 6 units of insulin.

Where to Find Carb Information
Sometimes it is easy to find carb values.  Packaged foods, for instance, are required to display the number of carbs per serving on the label.  (That’s in the Nutrition Facts label that tells you about the calories, fat, etc.)

Restaurant menus are beginning to publish carb values, which is helpful to people with diabetes (as well as people without diabetes, of course).

For non-packaged food, there are plenty of resources to help you estimate the number of carbs in a serving—say, in an apple.  We list several good resources in our article on the basics of carb counting.

Carb Counting in Diabetes
Counting carbs for people with diabetes is not a precise science, but an estimate—you should keep that in mind.  Make your best estimate, and then 2 hours after you eat, check your blood glucose level (that will be your postprandial—after eating—blood glucose level).

Your blood glucose level will be the best indication of how you did balancing the carbs in your meal with the insulin you took.  At that point, if your blood glucose is a little high, you can bolus a little more insulin. If your blood glucose is a little too low, you can carb count a little snack that will get you back onto your daily diabetes plan.

You can also use your postprandial blood glucose level to help you make future insulin adjustments.  By seeing how that amount of carbs affected your blood glucose level, you can better estimate the amount of insulin you’ll need to help keep your blood glucose in your goal range.

Carb counting and all this “insulin math” may seem overwhelming, but with practice, you’ll be able to do it without thinking about it too much.  It’ll be just another part of your daily life with diabetes—one that you can easily handle as you take good care of yourself.

Updated on: October 1, 2012
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