We get a lot of questions about food and diabetes, especially as it pertains to counting carbohydrates and dosing insulin for them.

So in this weekend’s edition of our Ask D’Mine advice column, our own Wil Dubois takes a look at this topic wearing both his hat as a former clinical educator as well as a person living with T1D himself for many years.


Important Things to Know About Carbohydrate Counting

In the beginning there was the carbohydrate exchange. And everyone agreed it was good. Newly diagnosed type 1’s were taught to recognize a serving of food that contained 15 carbs and were told to inject one unit of insulin for every exchange they consumed to “cover” the meal.

It worked, and some folks still use this system, but it’s not without its problems. Chief among them is the fact that not every type 1 needs the same amount of insulin for 15 carbs of food, and running a close second is the fact that 15 carbs isn’t a real-world serving of any food or drink on the planet.

So somewhere in the murky evolution of diabetes care a more sophisticated system was developed. The system is called carb counting, and it’s a form of mathematics designed to optimize, personalize, and perfect every drop of insulin that you take, while keeping your blood sugar in check.

And like any form of math there are different levels of complexity.

Let’s start with basic addition. Job one in carb counting is to simply add up the carbs in your meal. Well, it sounds simple, but there are a number of trap doors to deal with, and the first is how to determine the number of carbs that you’re supposed to add up. The primary tool at our disposal is the FDA Nutrition Facts label that pretty much every food in a box, can, or bag is required to have. In fact, it’s the Facts label—which was required by law starting in 1993— that most likely gave birth to carb counting. And as an interesting side note, this label continues to evolve, with changes in how added sugars are reported rolling out next year.

So if you eat from a box, can, or bag, adding up your carbs is relatively simple, right? Well… Maybe. Here’s the thing: The label gives the carbs for a pre-determined serving of any given food, but the container the food is in might have one, two, three, or even ten servings. Even containers that logically seem to be a single serving, like a can of soda or a lunch-sized bag of potato chips, actually have two servings per container. Or you may not agree on the FDA’s notion of what constitutes a serving of a given food. So you need to use some higher math to work out how many servings you’re actually going to eat. This isn’t rocket science math, but you do need to be aware of it.

If you don’t eat from a box, can, or bag (and good for you!), adding up your carbs takes a little more work. But there are any number of books, apps, and web data bases that list carb counts for these free-range foods. Calorie King, who used to print that handy little carb book we all carried in our back pockets before smart phones, has both an excellent app and an excellent website. One of the things I like about the app is that I can change the serving size from the standard serving to what I’m actually going to eat, and the app will re-crunch the math for me.

Then beware of occult carbs. The common sugar substitute family known as sugar alcohols—common in many “sugar free” candies—have a reduced glycemic impact, so you should use only half of their carb count in your math problems.

So much for addition. Now let’s talk about subtraction. Because here’s the thing, the carb count on the label includes fiber, which actually has no impact on blood sugar—which is what we’re really interested in. So to determine impact carbs, we need to subtract the fiber from the carbs, either from each element of the meal individually, or for the whole kit and caboodle at once. The math doesn’t care which way you do it. This step, often skipped as an annoyance by many PWDs is actually more important than you’d think. Some foods have enough fiber that it can really make a difference in the ultimate bolus. Fiber is conveniently listed right below carbs on the Nutrition Facts label.

Now it’s time for some multiplication. Well division, actually, but division is just multiplication spelled backwards. At least in a mathematical sense. Once you have your carb count, you need to divide it by your insulin-to-carb ratio, or IC ratio, to get the amount of insulin that you need to take to cover the meal. 

How do you know your IC ratio? The simple answer is that you ask your endo or your diabetes educator. The more complex answer is that if you add up your total daily dose of insulin (basal and fast-acting) and divide it into 500 you’ll get your IC ratio. Where’d the number 500 come from? It’s the theoretical daily average intake of food combined with the typical daily liver output of glucose. Assuming that you’re taking the right amount of insulin in the first place, this little equation will get you close to your IC ratio. Or you can use your weight to ball park it.

Now comes algebra, geometry, trigonometry and calculus

Don’t panic. I’m just kidding. But beyond the basic add up the carbs, subtract the fiber, and figure the bolus, there are a number of optional enhancements that can take carb counting to the next level.

The first of these is adjusting for out-of-target blood sugar, a.k.a. taking a correction. How often is your blood sugar perfect when you sit down to eat? Yeah. Don’t feel too bad. I think my blood sugar has only been perfect before a meal once, and that was before I had diabetes. So here’s the thing, as you’re going to go through the trouble of taking insulin for the food anyway, why not take the opportunity to fix your blood sugar at the same time?

If you’re above target, it’s a simple matter to use your correction factor to add some more insulin to the meal bolus so you can cover both the food and correct the blood sugar. If you’re below target, you can use subtraction to reduce the meal bolus to avoid the risk of having it send you yet lower. Don’t know your correction factor? Ask your doc, your educator, or divide that total daily dose into 1,800 this time. And, no, I don’t have a clue where the number 1,800 came from. 

But wait, there’s more. Blood sugar isn’t a static thing and many of us now have continuous glucose monitors (CGM). You can add glucose trend data to your carb counting for the ultimate in proactive prophylactic blood glucose corrections. This can be done using set percentage adjustments based on the trend arrows, or by making unit adjustments.

Ready for graduate school mathematics? Insulin is like the last guest at a good party; it hangs around for a while. But like that last guest, its energy winds down toward the end. Still, it’s ready to tie one more on if more people suddenly show up. Yes, I’m talking about the risk of insulin stacking, which happens any time you take more than one bolus within a four-hour period. The best way to track this hanger-on insulin, called insulin on board, or IOB, is with technology.

Dealing the onset, peak, and duration of insulin action in your head is impossible for most people. It used to be that you needed a pump to track IOB, but now handy apps like RapidCalc can help tremendously. If fact, properly set up, RapidCalc can do a lot of the math we’ve talked about today. It’s the best $7.99 you’ll ever spend on your diabetes.

So I think you can see the problem here. You can spend so much time on the math that your food is cold and inedible by the time you figure out how much insulin to take, and then there’s no point in taking any at all. This is a problem we didn’t have when we used the exchange method.

Still, the beauty of carb counting is that you can choose how complex to make it, and you don’t have to use the same level of complexity every day or at every meal. 

And we can all agree that’s a good thing, too. 


This is not a medical advice column. We are PWDs freely and openly sharing the wisdom of our collected experiences — our been-there-done-that knowledge from the trenches. Bottom Line: You still need the guidance and care of a licensed medical professional.