Think Like a Pancreas

by Gary Scheiner

  1. the amount of carbohydrate in the meal or snack

  2. the blood sugar level at the time of the meal or snack

  3. the amount of insulin still remaining from previous boluses

  4. the amount of planned (or completed) physical activity

  For those of you who are mathematically inclined (you know who you are—your checkbooks actually balance each month), boluses are calculated as follows:

  The bolus dose = (food dose + correction dose – insulin on-board) × activity adjustment

  Later in this chapter we will also discuss the importance of bolus timing in order to keep the blood sugar from rising too high or dropping too low after eating.

  For now, let’s focus on what makes up the ultimate bolus dose of insulin.

  Part 1. Insulin to Cover Carbs

  As is the case with basal insulin, appropriate bolus insulin dosing requires individualization and fine-tuning. Each person’s needs are unique. To match insulin to food, we use something called an “insulin-to-carb (I:C) ratio.” In other words, we need to determine how many grams of carbohydrate each unit of rapid-acting insulin covers. For example, a 1-to-10 (1:10) ratio means that one unit of insulin covers 10 grams of carbohydrate, and a ratio of 1-to-20 (1:20) means that each unit covers 20 grams. If you have basic math skills (or a calculator or an electronic dosing guide—see resources section), figuring the food bolus is easy. Simply divide your carbs by your ratio. If each unit covers 10 grams and you consume 65, you will need 6.5 units of insulin (65 divided by 10 = 6.5).

  An insulin-to-carb ratio tells us how many grams of carbohydrate each unit of insulin covers.

  I:C ratios may seem a bit backward at first. A ratio of 1:10 will produce a larger insulin dose than a ratio of 1:15. A snack containing 30 grams of carb will require 3 units if you’re using a 1:10 ratio, but only 2 units if you’re using 1:15. As the second number in the ratio goes up, the amount of insulin goes down.

  The beauty of an I:C ratio is that it gives you the flexibility to eat as much or as little carbohydrate as you choose while still maintaining control of your blood sugars. However, spacing meals and snacks at least a few hours apart (three or more hours is optimal) remains important so that bolus insulin can return the blood sugar to normal before you eat and raise it again. Even if carbs are counted—and bolused for—carefully, controlling blood sugar when grazing is almost impossible. Frequent munching puts you in a perpetual state of an after-eating blood sugar rise, waiting for the bolus to kick in and bring things back down to normal.

  Incidentally, requiring different I:C ratios at different times of day is common. This is due to changes in hormone levels and physical activity, which affect insulin sensitivity. Many people find that they need their lowest I:C ratio—and thus their highest bolus—at breakfast and their highest ratio—and lowest bolus—in the middle of the day. For example, I personally need 1u:10g carb at breakfast, 1u:16g carb at lunch, and 1u:14g at dinner.

  If you already take rapid-acting insulin at your meals and your blood sugar levels are close to normal most of the time, figuring your I:C ratio is easy. Simply add up the grams of carb in your usual meals and divide by the units of rapid-acting insulin.

  For instance, Sam usually devours 45 grams of carb at breakfast and takes 5 units of rapid acting insulin. His blood sugar before breakfast and before lunch are similar, so it appears that each unit of insulin covers 9 grams of carb (45 divided by 5).

  If you have never used an I:C ratio for calculating your mealtime insulin, don’t worry: There are a number of ways to set up an initial ratio. Whichever method you choose, starting with a conservative dose is best (are you getting tired of hearing that yet?). It is better to run your blood sugar a little high than too low because frequent lows are dangerous and will make evaluating and fine-tuning your dosing formulas very difficult.

  The 500 Rule

  The 500 rule is based on an assumption that the average person consumes (via meals and snacks) and produces (via the liver) approximately 500 grams of carbohydrate daily. By dividing 500 by the average number of units of insulin you take daily (basal plus bolus, also called “total daily dose” or TDD), you should get a reasonable approximation of your I:C ratio: 500/TDD.

  For example, if you take a total of 25 units of insulin in a typical day, each unit of insulin should cover approximately 20 grams of carbohydrate (500/25 = 20). If you take 60 units daily, your I:C ratio would be 1:8 (500/60 ≅ 8). (See Table 7-1.)

  Table 7-1. Determining I:C ratio from total daily insulin

  Total daily dose of Insulin (basal + bolus) Approx. I:C Ratio

  8–11 1:50

  12–14 1:40

  15–18 1:30

  19–21 1:25

  22–27 1:20

  28–35 1:15

  36–45 1:12

  46–55 1:10

  56–65 1:8

  66–80 1:7

  81–120 1:5

  >120 1:4

  The advantage to this approach is its simplicity: Only one nice round number to divide (or one easy chart to look at). The obvious weakness to this approach is that it assumes that all people are equally sensitive to insulin and eat about the same amount of food. When using this approach, those who eat very little will tend to receive too little mealtime insulin, and those who eat a great deal will tend to receive too much. This approach also assumes that the amount of insulin you are taking now is appropriate for you. If your blood sugar is frequently above or below your target range, the I:C ratio derived from this approach will likely be incorrect.

  The Weight Method

  This approach is based on the fact that insulin sensitivity diminishes as body mass increases; each unit of insulin will cover less food in a heavier person than in a lighter person. To determine a starting I:C ratio, divide 1,800 by your weight in pounds, or 850 by your weight in kilograms:

  1800/wt (lbs) 850/wt (kg)

  Table 7-2. Determining I:C ratio from weight

  Weight (lbs) Weight (kg) Approx. I:C Ratio

  <60 <27 1:30

  60–80 27–36 1:25

  81–100 37–45 1:20

  101–120 46–54 1:17

  121–140 55–64 1:14

  141–170 65–77 1:12

  171–200 78–91 1:10

  201–230 92–104 1:8

  231–270 105–123 1:7

  >270 >123 1:5

  One of the potential problems with the weight method is that it fails to consider body composition. An individual who weighs 250 pounds but is very muscular will be much more sensitive to insulin than a person of similar weight who has a great deal of body fat. Another problem is that this system fails to consider stages of growth and hormone production. A growing adolescent will require significantly more mealtime insulin than an older person who weighs the same amount. Likewise, a woman who is pregnant will require considerably more insulin than a person of similar weight who is not pregnant. (Insulin patterns during pregnancy will be discussed in detail in Chapter 8.)

  Fine-Tuning and Verifying I:C Ratios

  You should establish proper basal insulin levels before you attempt to fine-tune your boluses. Any basal insulin changes made after fine-tuning your boluses will require additional bolus adjustments.

  Fine-tuning I:C ratios is best done empirically (what my people call “trial and adjustment”). And be sure to assess the I:C ratio at each meal and snack separately because insulin sensitivity changes throughout the day.

  Detailed written records are helpful in verifying and fine-tuning I:C ratios. You should also eliminate factors other than food that might be affecting your blood sugar levels. For example, do not include data collected during or after strenuous exercise unless you do so every day at the same time. Also, don’t look at information collected during an illness or major emotional stress, at the start of a menstrual cycle, or after a low blood sugar. You should not include meals with very high fat content or unknown carb content (such as heavy restaurant meals) in your analysis.

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sp; To analyze your data, take a look at your blood sugar level before the meal and then again at least three hours later (to give the insulin a chance to finish working), without taking in calories or bolus insulin in between. Because strange things can happen on any given day, I like to consider one to two weeks of data when coming to a decision regarding the I:C ratio.

  For example, consider the data in Table 7-3:

  As you can see, the conclusions sometimes contradict each other (remember, this is diabetes—nothing is ever simple!). However, we can still come to a general conclusion based on the results. I would be tempted to assign an I:C ratio of 1 unit per 12 grams of carb. Here’s why:

  Table 7-3. Example log sheet

  First, I would throw out the data on June 3 due to the low reading prior to breakfast (it most likely led to a rebound high). I would also throw out the data on June 8 because it is inconsistent with every other result, and the meal was much larger than usual (perhaps it was a slow-digesting Denny’s Grand Slam breakfast). The rest of the data indicates that an I:C ratio higher than 1:12 produces a blood sugar rise; less than 1:12 produces a drop. When used, 1:12 held the blood sugar fairly steadily, as the lunch readings were within 30 mg/dl of the breakfast readings.

  Another technique for evaluating I:C ratios is to analyze the data from a continuous glucose monitor. CGM downloading offers the unique opportunity to see distinct patterns following meals. With a quick look at a single chart, we can determine whether mealtime insulin doses are too high, too low, or on target.

  For example, take a look at Figure 7-1. On most days the dinner bolus produces hypoglycemia two to three hours later. If the current I:C ratio at dinner is 1u:15g carb, trying a lower dose, such as 1u:20g carb, makes sense.

  Figure 7-1. Modal day chart from Dexcom CGM

  By contrast, the data in Figure 7-2 indicates that the blood sugar is rising and staying elevated after dinner. If the current dinner formula is 1u:15g, increasing the dose to, say, 1u:12g carb to see if this produces better results would be worthwhile.

  Figure 7-2. Glucose modal day report from Freestyle Navigator CGM

  Figure 7-3 reveals the after-meal blood sugar patterns for several days. Apparently, the blood sugar is usually above target (the shaded rectangular areas represent the postmeal target) three to four hours after the mealtime insulin was given. The statistics below the graphs indicate above-target postmeal averages. Based on this information, this person should probably increase the I:C ratio at each meal.

  Figure 7-3. Sensor overlay by meal chart from Medtronic CGM

  When analyzing your blood sugar records, the more details, the better. You might discover subtle factors that influence your blood sugar levels and require insulin dose adjustments.

  For example, one of my clients, Betty, had blood sugar readings that were above target every Sunday at lunchtime, but normal readings the rest of the week. The reason? Church. Betty is very passionate about Sunday morning services. In addition to sitting still for several hours, the adrenaline rush she gets from prayer was producing a blood sugar rise. The solution: Use her usual 1:10 formula at breakfast during the week, but increase to 1:8 on Sundays.

  Another patient, David, was experiencing very inconsistent blood sugars prior to dinner despite having the same lunch each day and using a 1:15 bolus formula every day at lunchtime. In reviewing his records, we found that most of his dinnertime lows were preceded by morning workouts; most of his dinnertime highs were on nonexercise days. The solution: Use a 1:10 formula at lunch after sedentary mornings, but decrease to 1:20 following morning exercise.

  Fine-tuning I:C ratios can be a challenging proposition, even for highly trained and experienced health professionals. Given the complexities of determining I:C ratios, having a second set of eyes look over your records is worthwhile. Don’t hesitate to ask your physician or diabetes educator to review your data just to confirm that your conclusions seem reasonable. Or give my office a call; my team would be happy to work with you on the fine-tuning process.

  (Food Dose + Correction Dose – Insulin On-Board)

  x Activity Adjustment

  Part 2. Insulin to Correct Blood Sugar

  Okay, you’ve counted your carbs and applied your I:C ratio to determine the food dose. What’s next?

  The second component of the bolus calculation is the “correction dose”: the adjustment to the standard meal bolus to fix blood sugars that are either above or below target. This adjustment improves your chances of having a blood sugar reading that is within your target range before the next meal.

  To fully understand this concept, imagine a world-famous archer (green tights and all) named Sir Gary of Kinwood. Sir Gary is trying to win the beautiful Maid Debbie’s hand by winning an archery contest. As Sir Gary focuses on his goal (the target, not Maid Debbie), what should he aim for? The bull’s-eye, of course! If he aims toward the sides or edges of the target, his chances of hitting the bull’s-eye are greatly reduced. In fact, he might miss the target completely, resulting in a chorus of laughter from the evil sheriff’s luxury suite.

  Remember, blood sugar control is far from an exact science; we’re happy just to hit the target somewhere. To do so, we must always aim for the center. If your target range is 60 mg/dl (3.3 mmol/l) to 140 mg/dl (7.8 mmol/l), aim for 100 (5.5). If your range is 70 (3.8) to 170 (9.4), aim for 120 (6.7). If your acceptable range is 80 (4.4) to 200 (11.1), aim for 140 (7.8). By aiming for the center, you increase your chances of landing within your target range.

  Your target blood sugar should be a single number near the midpoint of your acceptable target range.

  Here’s how correction boluses work. If you designate your target blood sugar as 120 mg/dl (6.7 mmol/l) and your premeal reading happens to be 175 (9.7), you will need to add extra correction insulin to your meal dose. Does this guarantee that you will be on target at your next reading? No, but it sure increases the odds that your blood sugar will come down somewhat and land within your target range. If you don’t add any correction insulin, it’s like aiming to be 175 (9.7) again next time. In archery terms, it’s like aiming for the outer edge of the target instead of the bull’s-eye.

  Sometimes correction boluses involve taking insulin away from your usual meal dose. If your blood sugar is 80 (4.4) and your target is 110 (6.1), you will need to deduct some insulin from your meal dose. This increases your chances of raising the blood sugar somewhat and being within the target range by the next meal. If you don’t reduce your meal dose when your blood sugar is below your target, you increase your chances of missing the target completely and experiencing a low blood sugar before the next meal. Incidentally, if your blood sugar is below target and you don’t plan to eat, there is nothing to deduct the correction dose from. A small snack should take you up toward your target blood sugar.

  To figure out the correction dose of insulin, three pieces of information are needed:

  1. Your current blood sugar level.

  2. Your target blood sugar.

  3. Your sensitivity factor (sometimes called a “correction factor”). This has nothing to do with how good a listener you are or your willingness to miss a football game for the sake of going shopping with your partner. Rather, your sensitivity factor is how much each unit of insulin is expected to lower your blood sugar.

  Each person’s sensitivity to insulin is unique. In general, the heavier you are and the more insulin you take, the less sensitive you will be to each unit. Certain conditions (growth, pregnancy, premenses, illness, stress, surgery, use of steroid medications) also reduce insulin sensitivity, albeit on a temporary basis.

  Determining your sensitivity factor is similar to determining your I:C ratio. We start with an estimate based on a mathematical formula and then fine-tune based on actual blood sugar results.

  The formula that I find works best for figuring the daytime correction factor is called the 1700 (94) rule. Take your total daily insulin, including basal and bolus insulin, and divide into 1700 (94 if measuring
blood sugar in mmol/l).

  When choosing an initial sensitivity factor, I recommend leaning toward the higher end of the range. For instance, if you average 80 units a day, you might choose 25 (1.4) as your initial sensitivity factor. This reduces the risk of overcorrecting your highs and winding up with low blood sugar. Below are some examples.

  Stan takes an average of 28 units of insulin daily. Applying the 1700 rule, we get:

  1700 / 28 = 61 (94 / 28 = 3.4)

  This means that every unit of rapid-acting insulin should lower his blood sugar approximately 61 mg/dl, or 3.4 mmol/l. To simplify the calculation and start off conservatively, we’ll round up to 65 (3.5).

  Because Stan’s target blood sugar is 110 (6.1), he should add 1 full unit for every 65 (3.5) points over 110 (6.1) . . . and subtract 1 unit for every 65 (3.5) points below 110 (6.1). Expressed as a formula, his correction dose is:

  In mg/dl: (current blood sugar – 110) / 65

  In mmol/l: (current blood sugar – 6.1) / 3.5

  Table 7-4. Estimating the sensitivity factor based on total daily insulin

  Average total daily insulin (units) (all basal + all boluses) Approx. Sensitivity factor (mg/dl) how much 1 unit lowers blood sugar Approx. Sensitivity factor (mmol/l) how much 1 unit lowers blood sugar