What is CARB RATIO and how to calculate it?

Diabetics need carb ratio to help us calculate, how much insulin we need for a meal. It tells us how many insulin units we need to cover the amount of carbs we eat in a meal. 

For example, if our carb ratio is 1:10, this means that for 1 insulin unit we can eat 10g of carbs. And if we eat 40g of carbs in a meal, we will need 4 insulin units to cover it.

Carb ratio is an individual metric. This means that you have to calculate it in order to have a correct ratio for yourself. I'll explain below step by step, how to calculate it.

Carb ratio metric is not a static metric. It can change in one period. For example, if you increase your activity levels, you could increase insulin sensitivity and as a result your carb ratio will increase. Meaning you need less insulin for the carbs you eat.

Another variable that can impact your carb ratio are emotions. You might see that when you are emotionally more unstable, your carb ratio can decrease (you need more insulin to cover your regular meal).

Let's go through this calculation process together, step by step.

1. Step #1 is assuring we have stable night and between-meals sugars.

Why is this important? Stable night and between meal sugar means that your basal insulin is correct. It means that bolus only needs to cover blood sugar (BS) coming from a meal and doesn't have to take into account basal sugar increase or decrease.

If your basal insulin is too high (your blood sugar during the night and between meals is dropping) or too low (your blood sugar during the night and between meals is rising), you would have to add this into the carb ratio calculation and it would become too complex.

2. Step #2 is eating 30g of carbs (1 large (8–9 inches, 136 grams) banana: 31 g carbs) when it’s meal time (and there is no acting rapid or short acting insulin onboard/active (except basal)). You can choose another food for the test but it's crucial you know exactly how many grams of carbs this amount of food you will eat, has. Also important here is that you don't add other (fats, proteins) foods to this test, because they will make an impact that we don't want in this test.

Why is it important that there is no active rapid or short acting insulin on board? Let's explain this on an example.

Let's say you are trying to figure out, how much food you need to eat, when you're hungry, to not be hungry anymore, you need to be hungry to be able to measure this. You can not eat an apple 30 mins in advance and then 30 mins later measure how much food you need to eat in order to not be hungry anymore. You'll be less hungry because of the apple and this measurement (amount of food you need when you're hungry) won't be representative for cases when you're hungry.

It's the same here. If you have active rapid or short acting insulin in your bloodstream, this insulin will impact how much insulin you need to cover the meal from the carb ratio "test" you are about to perform.

So, the best time to do this "exercise" is in the morning or 4 hours after you bolused short (rapid) acting insulin for a meal or for a correction.

3. Now that we have the basics covered, the key moment is to bolus the proper amount of insulin.

If we need 3 units to bring BS back to where it was before we ate the banana, then our ratio is 1:10 (because we need 3 units to cover 30 grams of carbs: 30/3 = 10). If we need 1 unit to bring BS back to where it was before we ate the banana, then our ratio is 1:30 (because we need 1 unit to cover 30 grams of carbs: 30/1 = 30). If we need 6 units of insulin to bring BS back to where it was, before we ate the banana, then our ratio is 1:5 (because we need 6 units to cover 30 grams of carbs (30/6=5). 

If you are not sure how much insulin you need for one banana (31g of carbs), then do the bolus amount as you usually would.

• If this insulin amount brings your BS back to where it was before you ate the banana, you have the number you need for your calculation (check calculations in the previous paragraph).
• If it brings BS too low (lower than where it was before you ate the banana) or you even go into hypoglycemia, you will have to repeat the test.
• If it doesn't brings BS back to where it was before this test and BS stays higher, you will need to add more insulin to bring BS back to where it was. For example, you start with a stable glucose level GL at 90 mg/dl (5 mmol/l), you eat a banana (1 large (8–9 inches, 136 grams) with 31 g carbs) and do a bolus 2 units of rapid insulin (Fiasp for example). After 2h your GL is not at 90 mg/dl (5 mmol/l), it's at 150 mg/dl (8.4 mmol/l). This means that you need more than 2 units to get your GL back to 90 mg/dl (5 mmol/l). So maybe you will add 0.5 unit extra bolus of Fiasp and wait another 2 hours to see where it brings your glucose level. If now after another 0.5 unit your GL is very close to 90 mg/dl (5 mmol/l), you will say that you need 2.5 units of rapid insulin to cover 31g of carbs and your calculation will then be: 31/2.5=12.4. So your carb ratio is 1 : 12.4 if you want to be very precise (which is best), but sometimes people will round it to 1:12 to make things easier for the calculations in the future.

That's the whole procedure. It takes some time and some preparation, so I would recommend doing this during weekend or when you are not busy with doing 10 other tasks at the same time, so you can keep track of what is happening.

Good luck and let me know if you have any questions or comments.