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Tune In to Your Ratio(s)

by Gary Scheiner, MS, CDE

Setting your ratios
There are two commonly used methods for determining initial, or starting, insulin-to-carbohydrate ratios: the 500 rule, and the weight method. Whichever method you choose, it is best to begin with a conservative approach to prevent hypoglycemia, then fine-tune as needed.

The 500 rule. This approach is based on the assumption that the average person consumes (in meals and snacks) and produces (by the liver) approximately 500 grams of carbohydrate daily. (The liver normally secretes a small amount of glucose at all times, and it produces larger amounts when blood glucose levels fall.) By dividing 500 by the average number of units of insulin you take daily (basal plus bolus), you should get a reasonable approximation of your insulin-to-carbohydrate ratio.

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

The following list shows approximate insulin-to-carbohydrate ratios based on average total daily units of insulin (including both basal and bolus doses):

  • 8–11 units 1:50
  • 12–14 units 1:40
  • 15–18 units 1:30
  • 19–21 units 1:25
  • 22–27 units 1:20
  • 28–35 units 1:15
  • 36–45 units 1:12
  • 46–55 units 1:10
  • 56–65 units 1:8
  • 66–80 units 1:6
  • 81–120 units 1:5
  • >120 units 1:4

The obvious weakness to this approach is that it assumes all people eat about the same amount of food and produce the same amount of glucose each day. People who are heavy or who tend to eat relatively large amounts of carbohydrate will underestimate their insulin requirement with this approach; those who are lean or active or who eat relatively little will overestimate their requirements.

The weight method. This approach is based on the supposition that insulin sensitivity diminishes as body mass increases; hence each unit of insulin will cover less carbohydrate in a heavier person than in a lighter person. The following list shows approximate insulin-to-carbohydrate ratios based on weight (in pounds):

  • <60 lb 1:30
  • 60–80 lb 1:25
  • 81–100 lb 1:20
  • 101–120 lb 1:18
  • 121–140 lb 1:15
  • 141–170 lb 1:12
  • 171–200 lb 1:10
  • 201–230 lb. 1:8
  • 231–270 lb. 1:6
  • >270 lb 1:5

One of the potential problems with this system is that it fails to consider body composition. A person 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. It also fails to account for a person’s degree of insulin resistance. This presents more of a problem for people with Type 2 diabetes, who tend to be insulin-resistant, than for people with Type 1, who tend not be insulin-resistant.

Collecting data
It is best to establish your basal insulin levels before attempting to fine-tune your mealtime and snacktime bolus ratios. Any basal insulin changes made after fine-tuning your boluses will require additional bolus adjustments.

Fine-tuning bolus ratios is best done empirically, or through trial and error. You should verify your insulin-to-carbohydrate ratio for each meal and snack separately, since they can vary considerably.

Keep detailed written records when trying out different insulin-to-carbohydrate ratios. Check and note your blood glucose level before each meal and then again 3–4 hours later (to give the insulin a chance to work fully). Do not snack, exercise, or take more bolus doses between the two blood glucose readings. It is best to eliminate factors other than the food in the meal that might affect the results of your readings. For example, do not include data collected during or immediately after strenuous exercise. Don’t count data collected during an illness or major emotional stress, at the start of a menstrual cycle, or after an episode of low blood glucose (hypoglycemia). Meals with very high fat content or unknown carbohydrate content (such as restaurant meals) should not be used as part of your analysis.

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