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Carb ratios

Updated: Mar 19

I know I get into a lot of complicated diabetes stuff on this blog, but today I want to take it back to basics...meal blousing and carbohydrate ratios. Why are they so important?

There are a couple commonly used methods of dosing prandial (mealtime) insulin. Many people use “set dosages” which are established by their clinician and are based on past glucose patterns. For example; someone might give 8 units for breakfast, 10 units for lunch, and 6 units for dinner every day independent of glucose levels, type of carb intake, or amount of carb intake.


The “sliding scale” method uses insulin dosing based on current glucose levels. For example, if the person’s BG is between 100-150mg/dl they will give 5 units for a meal; if it is between 150-200mg/dl, 6 units; and between 200-250mg/dl, 7 units etc. It’s basically the “set dosages” method with an added unit or two to cover poor BG readings.


Both methods present obvious problems. First, they are based on reaction instead of proactivity. Instead of using a “sliding scale” to correct high BG’s, we should be focusing on preventing these high BGs in the first place. Second, any seasoned PWD knows that a salad and a bowl of pasta have completely different BG effects. Dosing the same amount of insulin for both meals is crazy, lazy, and quite frankly dangerous. I truly believe that these dosing methods set people up for failure.


So how do we be proactive? With carb counting!


Yes, it is extra work and a totally different mindset, but it is important! Counting carbohydrates allows us to quantify how much or how little a meal will raise our BGs. Most people on a basal-bolus insulin regimen use ~50% basal insulin and ~50% bolus insulin to provide the most stability. Therefore, prandial doses account for 50% of daily BG control! It is important for them to be accurate!


Carbohydrate intake needs to be balanced with exercise or insulin. Counting the amount of carbs you eat will help determine how much insulin is needed to cover that meal. You can then use your carb factor (ratio of insulin units to grams of carbohydrates) to determine the amount of bolus insulin needed.


To determine your carb factor, you first need to determine how many carbohydrates one unit of insulin will cover for you. This should always be done with the help of your clinician. The formula that I like to use is based on your total daily dose (TDD). It is very simple, but it only works if you have an accurate TDD (if you are having frequent hyper- or hypoglycemia your TDD is probably inaccurate).


The formula is called “The 500 Rule” and involves dividing 500 by your TDD. A person using a small TDD will therefore have a large carb factor (500/20u TDD=carb factor of 25), and a person with a large TDD will have a small carb factor (ex: 500/100u TDD= carb factor of 5). Both people have very different insulin needs and should therefore be treated differently. For instance, If both PWD eat a muffin with 50 grams of carbs, the person with the carb factor of 25 will dose 2 units whereas the person with the carb factor of 5 will dose 10u. With “’the 500 rule”, carb factors can be specified to each person to prevent over or underdosing prandial insulin.


Keep in mind, the carb factor established by “the 500 rule” is a rough guestimate and will need to be tested and titrated. Before testing your carb factor, make sure to do basal testing in order to rule out your basal insulin as a possible cause of highs and lows.


How to test carb factor:


Start the test when your BGs are at goal and you have not had any food for 3.5-4 hrs.


Divide the total grams of carbs by your carb factor to get your carb bolus amount.


Take your carb bolus (wait 15-20 min) and then eat.


Check your BG hourly. You will know you have an accurate carb factor if your BG is within 40mg/dl of your original BG 2-hr post prandial and within 30mg/dl of your original BG 4-hrs post-prandial.


If your carb factor passes this test, great! If your BG ends up higher than these targets, strengthen your carb factor by 1 (if tested carbF is 10, weaken to 9). If your BG ends up lower than these targets, weaken your carb factor by 1 (if tested carbF is 10, strengthen to 11). Then retest!


Remember, although carb counting presents some challenges, it is worth it when you think about the importance of optimal BG control regarding quality of life and decreased complication risk. Remember it is important to be proactive about your control!




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