Amy P. Campbell, MS, RD, CDE, and Richard S. Beaser, M.D.
Carbohydrate counting, a method of keeping track of the grams of carbohydrate one eats at meals and snacks, is a form of meal planning that has become increasingly popular.
It can be used by anyone with diabetes, but is particularly useful for those who have an insulin treatment program that utilizes variable doses of rapid-acting insulin. Carbohydrate counting is based on the premise that carbohydrate foods have the greatest impact on blood glucose, since almost all of it is converted to glucose, and all forms of carbohydrate have about the same effect on the blood glucose; therefore, carbohydrate foods can be easily exchanged for each other.(An interesting perspective for your patients: while one might think that a teaspoon of sugar would make blood glucose rise faster than a half-cup of potatoes, in fact the potatoes will contribute about 15 grams of carbohydrate, while a level teaspoon of sugar will only give 4 grams of carbohydrate. Therefore the potatoes will have about three times the effect on blood glucose as the table sugar. Of course, though the blood glucose responds in the same way if two foods have equal amounts of carbohydrate, not all carbohydrates are equally healthy, and this fact needs to be emphasized to patients as well.)…
The second approach to carbohydrate counting is referred to as a flexible carbohydrate plan, or advanced carbohydrate counting. This method can be used by patients who are competent in adjusting their doses of rapid acting insulin in response to the type and amount of carbohydrate desired at meals and snacks. Thus, most of these patients tend to be using programs consisting of multiple daily insulin injections or an insulin pump (see Chapter 11).
Rather than retrospectively considering the past glucose patterns to determine the current insulin dose, the flexible carbohydrate counting method focuses attention on the planned, current food intake. The more carbohydrate that one plans to consume, the more insulin is taken. In addition, further dose adjustments can be made to accommodate for the actual blood glucose level before the food is eaten. So, for example, if the patient’s pre-meal blood glucose level is above target, the patient would take a "correction" dose of insulin along with the insulin needed to cover the carbohydrate at the meal. This method works well with people who are willing to do pre-and post-prandial blood glucose monitoring and keep detailed food and blood glucose records. In addition, continuous glucose monitoring (CGM) can be instrumental in helping patients fine-tune their insulin and food intake.
Patients must have a working knowledge of food and carbohydrate content from labels, food count books, or food lists in order to estimate the grams of carbohydrate in the meals and snacks that they eat.
Necessity of Carbohydrate Counting for Patients Using an Insulin Pump
The majority of patients who are using a pump (see Chapter 11) rely on carbohydrate counting to ensure that they have programmed their pump to deliver the proper bolus dose of insulin. While the basal insulin controls blood glucose in the patient’s fasting state and typically provides 50% of one’s insulin needs, the bolus is a larger amount of insulin delivered by the pump to "cover" the food the patient eats. (A bolus can also quickly reduce a high blood glucose.) Bolus doses are matched to the carbohydrate content of each meal or snack eaten.
In order to determine what the bolus dose should be, one must first determine his insulin to carbohydrate ratio (I:Carb), which tells how much carbohydrate (thus the need for carbohydrate counting) 1 unit of insulin (rapid-acting or regular) will "cover." There are several ways to determine the I:Carb. One such method involves using what is called the 450 Rule. Here’s how this works:
- Determine the patient’s total daily dose (TDD) of insulin. This includes both rapid-acting and long-acting, or basal, insulin.
- Divide 450 by the TDD to derive the ratio. For example, if the TDD is 50 units, 450 divided by 50 equals 9. Therefore, the starting I:Carb is 1:9. This means that one unit of rapid-acting insulin "covers" 9 grams of carbohydrate.
- To use the ratio, the patient would then calculate the number of grams of carbohydrate he plans to consume at his meal. For example, if the patient will be eating 45 grams of carbohydrate, he would divide the 45 grams of carb by 9, which equals 5. This means that the patient would need to take 5 units of insulin to cover the 45 grams of carbohydrate.
Some endocrinologists advocate using the 500 Rule, rather than the 450 Rule. There is not much evidence supporting the benefits of one rule over the other, and since most I:Carb ratios need fine-tuning anyway, the initial calculation of such a ratio is merely a starting point, and the use of 450 or 500 is based upon personal preference.
The I:Carb, in general, is related to how insulin-sensitive a person is, and will vary from person to person. (The same person may have slightly different I:Carb ratios that they use for different times of the day, as well.) All patients choosing to use this advanced form of carbohydrate counting must, at least initially, be willing to check postprandial blood glucose levels in order to determine if the ratio is correct. If, for example, the 3-hour postprandial blood glucose is 40 points higher than target range (such as 100–120 mg/dl), and the patient is certain that the carbohydrate grams were accurately counted and the ratio used correctly, the ratio is probably incorrect and will need to be fine-tuned. The ratio used for each meal must be evaluated.
Once the I:Carb ratio is established, the patient’s bolus dose can be calculated by taking into account the following factors:
- the amount of carbohydrate the patient plans to eat
- the I:Carb ratio
- the patient’s blood glucose at the time the bolus is being calculated
- how much 1 unit of insulin will reduce the patient’s blood glucose when it is above her target range (the patient’s sensitivity factor)
- the length of time since the last bolus
One other important component of advanced carbohydrate counting involves the use of the sensitivity, or correction, factor. The Insulin:Carb ratio covers carbohydrate consumed at a meal. But if the pre-meal blood glucose level is above target, additional insulin is required to lower the glucose level back to target range. This is where use of the sensitivity factor comes in. The sensitivity factor is defined as the amount of blood glucose (mg/dl) that is lowered by 1 unit of rapid-acting insulin. Joslin Diabetes Center uses the 1500 Rule to calculate the sensitivity factor, although other institutions may prefer to use the 1800 Rule. Again, the point is that these formulas are merely starting points, and, just as with the I:Carb, the sensitivity factor needs to be evaluated and fine-tuned, if necessary.
The steps for calculating the sensitivity factor are as follows:
- Determine the total daily dose (TDD) of insulin, which includes both rapid-acting insulin and long-acting insulin.
- Divide 1500 by the TDD to determine the sensitivity factor. For example, if the TDD is 50, divide 1500 by 50 and the result is 30. This means that a unit of rapid-acting insulin will lower the blood glucose level by approximately 30 points.
Most people have a sensitivity factor between 30 and 50, although patients who are extremely insulin resistant may have a sensitivity factor of 10, for example, and those who are very insulin sensitive may have a sensitivity factor of 80.
Here is an example as to how the sensitivity factor is used:
- The patient’s pre-meal blood glucose level is 250, and his target blood glucose is 100. Therefore, the blood glucose is 150 points higher than target.
- Divide 150 by the sensitivity factor of 30, which equals 5.
- The patient would need to add 5 units of rapid-acting insulin to his mealtime dose (calculated using his I:Carb ratio) to bring his blood glucose level back to target range postprandially.
The sensitivity factor is most commonly used when pre-meal blood glucose levels are high, as in the example above. It may also be used to correct a high blood glucose level apart from meals. Patients using pump therapy may need to correct apart from meals, for example. However, patients should be instructed not to randomly correct for high blood glucose levels, and should receive appropriate instruction from their healthcare team before doing so, in order to limit episodes of potentially severe hypoglycemia.
Most likely, patients interested in pump therapy will have been referred to specialists in diabetes care and management, and this specialist or, more likely, a diabetes team, will have helped the patient determine his or her I:Carb ratio, sensitivity factor, and the appropriate basal and bolus dose, and will have taught him or her how and when adjustments are necessary. As a member of the patient’s overall healthcare team, however, it is important for the primary care provider to understand these basics of intensive therapy (see Chapter 10).
Evaluation of the Success of a Nutrition Plan
Whatever meal planning approach is selected, it is important that it be an integrated part of an overall diabetes treatment program initiated to reach specific goals of therapy. These goals must be measured, and if they are not being reached, adjustments in the treatment are needed.
For a person with type 2 diabetes whose diabetes is being controlled by meal planning and physical activity alone, specific metabolic goals have been defined by Joslin and by the ADA: achieve an A1C level that is below 7%. If this goal is not met within a defined time period — 6–8 weeks — then more aggressive treatment is warranted.
The meal plan may need to be adjusted and/or the patient may need to start pharmacologic therapy. Antidiabetes medication therapies are usually the next step. Similarly, if medical nutrition therapy and medications fail to achieve targeted glycemic goals, progression to insulin must be considered. Monitoring progress towards reaching goals and helping evaluate the overall effectiveness of the meal plan can be done using a number of parameters:
- Glucose control: Are the glucose monitoring (SMBG) results approaching target levels? Is there improvement in the fasting, premeal and postprandial levels? Is hypoglycemia or hyperglycemia a problem? Is the A1C level below 7%?
- Lipid Levels: Are the lipid levels (LDL, HDL, triglycerides) improving?
- Weight: Is the patient losing or gaining weight (if that is the desired goal)?
- Patient Satisfaction: Is the patient satisfied with the approach chosen? Is he or she meeting overall nutritional needs? What barriers is he or she having? Can the patient sustain the diabetes treatment plan long-term?
Next Joslin Excerpt: Food Labels
Copyright © 2010 by Joslin Diabetes Center. All rights reserved. Reprinted with permission. Neither this book nor any part thereof may be reproduced or distributed in any form or by any means without permission in writing from Joslin. Note: Joslin does not endorse products or services.
You can purchase the updated 2nd Edition of JOSLIN’S DIABETES DESKBOOK at:
Please Note: Reasonable measures have been taken to ensure the accuracy of the information presented herein. However, drug information may change at any time and without notice and all readers are cautioned to consult the manufacturer’s packaging inserts before prescribing medication. Joslin Diabetes Center cannot ensure the safety or efficacy of any product described in this book.
Professionals must use their own professional medical judgment, training and experience and should not rely solely on the information provided in this book to make recommendations to patients with regard to nutrition or exercise or to prescribe medications.
This book is not intended to encourage the treatment of illness, disease or any other medical problem by the layperson. Any application of the recommendations set forth in the following pages is at the reader’s discretion and sole risk. Laypersons are strongly advised to consult a physician or other healthcare professional before altering or undertaking any exercise or nutritional program or before taking any medication referred to in this book.