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Why should I carb count?

Accurate carb coverage in a diabetic who is on insulin is essential for success!


There are a couple of commonly used methods of dosing prandial (mealtime) insulin. Some diabetics use “set dosages” which are established by their clinician and are based on past glucose patterns. Ex. 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 promotes dosing insulin based on current glucose levels. Ex. 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.


Both methods present obvious problems. If the purpose of prandial insulin is to accurately cover the carbs in a meal, does it really make sense to dose the same amount of insulin for two completely different meals? Of course not! Any seasoned diabetic knows that a salad and a bowl of pasta have completely different BG effects. These dosing methods therefore set people up for failure.


The “sliding scale” method involves fixed doses with an added correction bolus. So, on top of the previously described “set bolus” problem, this method involves an added unit or two to cover poor BG readings. We as diabetics need to stop REACTING to our BGs and begin focusing on PREVENTING these high BGs from occurring in the first place!


So how do we be proactive? With carb counting!


Yes, it is extra work, 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!


Most of the calories from carbohydrates eaten are turned into glucose, thus they raise the BG rapidly following a meal. To prevent high post-prandial readings, carbohydrate intake needs to be balanced with exercise or insulin. Counting the carbs helps determine how much insulin is needed to cover that meal. A carb factor (ratio of inulin units to grams of carbohydrates) is then used to determine the amount of bolus insulin needed.


What determines carb factor? Usually your clinician; however, as you all know, it is incredibly important to understand how and why we do things as diabetics. It helps us to make better choices. To determine how much insulin is needed for carbohydrate intake, first determine how many carbohydrates one unit of insulin covers. The formula that I like to use is based on 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 diabetics eat this 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 under-dosing of 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 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!


Although carb counting presents some challenges, it is worth it for optimal BG control, and improved quality of life, and decreased complication risk.


Be proactive!


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