Oh. My. Gosh. I’m writing this blog, for Mathematics and Statistics Awareness Month, and I couldn’t believe what people with type 1 diabetes (T1D) have to do, in mathematics, to reach the right blood-sugar levels. It’s insane! It’s not an overstatement to say that, to effectively manage T1D, one needs to be a mathematician.
Keep in mind, this is all hypothetical. If you have T1D, talk to your doctor/endocrinologist if you have any questions about anything in this post.
Total Daily Insulin Requirement
Let’s take a seemingly simple mathematical equation: Total daily insulin requirement (in units of insulin).
You take your weight and divide it by 4. If you weigh 160 pounds, then:
160 ÷ 4 = 40 units of insulin per day.
Easy, right? No.
Because it might be too much insulin if you are newly diagnosed and still making a lot of insulin on your own or you are sensitive to insulin; or it might be too little if you are very resistant to insulin; or, if you eat more, there’s more insulin; if you eat less, it’s less insulin. (This is a general formula, so, as always, talk to your doctor/endocrinologist about the best insulin dose for you.)
Basal Insulin Dose
Next, you need to establish the basal (also called background) insulin dose, which is generally constant from day to day. A couple of things to know about insulin:
- Approximately 40%-50% of the total daily insulin dose is when it’s NOT mealtime (basal).
- The other 50%-60% is for carbohydrate coverage (mealtime) and high blood sugar correction (bolus).
Let’s say, again, you weigh 160, and your totally daily insulin dose is 40 units (160 ÷ 4 = 40 units).
Your basal dose is 40%-50% of your total daily insulin dose. Let’s say we make it 50%, so it’s:
50% of 40 units = 20 units (of either long acting insulin or intermediate insulin).
Bolus (Carbohydrate Coverage) Insulin Dose
Okay, you’ve gotten this far. But what about mealtimes? Now you have to remember—it can be calculated using the “Rule of 500.”
You take 500, and divide it by your total daily insulin dose (40 units) to get your carbohydrate coverage ratio:
500 ÷ 40 = 12 grams of carbohydrate per 1 unit of insulin.
Easy, right? Think again.
This assumes that you have a constant response to insulin throughout the day, but insulin-to-carbohydrate ratio varies. Someone who is resistant in the morning, but sensitive at mid-day, will need to adjust the insulin-to-carbohydrate ratio at different mealtimes. The insulin-to-carbohydrate ratio might be breakfast 1:8 grams, lunch 1:15 grams, and dinner 1:12 grams.
Carbohydrate Coverage at a Meal
1 unit of insulin = 12 grams of carbohydrate.
Let’s say you’re going to have a turkey sandwich for lunch. That’s 36 grams of carbohydrate.
36 grams ÷ 12 grams = 3 units of insulin.
You’ll need 3 units of short or rapid acting insulin to cover the carbohydrate.
High Blood Sugar Correction Dose (Correction Factor)
High blood sugar correction is defined as how much 1 unit of rapid acting insulin will reduce the blood sugar so many mg/dL. This time, you have to remember the number 1800.
1800 ÷ Total Daily Insulin Dose (40 units) = 1 unit of insulin will reduce the blood-sugar level by 45 mg/dL.
Next, you have to calculate the high blood sugar correction dose.
(Actual blood sugar – target blood sugar) ÷ correction factor = high blood sugar correction dose.
Let’s say your actual blood sugar, before lunch, is 235 mg/dL, and your target is 100 mg/dL.
So 235 mg/dL – 100 mg/dL = 135 mg/dL, then we divide it by the correction factor (45), and we get 3 units of rapid acting insulin, to “correct” the blood sugar down to a target of 100 mg/dL.
Total Mealtime Dose
To get the total mealtime insulin dose, add the carbohydrate coverage dose together with the high blood sugar correction dose, so:
3 units of insulin + 3 units of insulin = 6 units of rapid acting insulin for your lunch.
And this was just lunch!