If you think you've mastered everything there is to know about carb counting, it's time for a little revelation. Not all carbs are created equal. Another factor to consider is the influence of the Glycemic Index.
Glycemic Index (GI) refers to the speed with which carbohydrates convert into blood glucose. While virtually all carbohydrates (except for fiber and a portion of sugar alcohols) convert into blood glucose eventually, some forms convert much faster than others. Technically, GI tells us the percentage of carbohydrates that have entered the bloodstream as glucose within the first two hours. For example:
|Cream of Wheat||70|
|Macaroni & Cheese||64|
|Ice Cream, vanilla||62|
|Yogurt, low fat||33|
|Fruits & Juices|
|Red Kidney Beans||19|
Foods with a high GI (greater than 70) tend to digest and convert to blood glucose the fastest, with a significant blood glucose "peak" occurring in 30-45 minutes. Examples include bread, potatoes, cereal and instant rice. Foods with a moderate GI (approximately 45-70) digest a bit more slowly, resulting in a less pronounced blood glucose peak approximately 60 minutes after eating. Examples include ice cream, orange juice, cake and pizza. Foods with a low GI (below 45) tend to make a gradual appearance in the bloodstream. The blood glucose peak is usually quite modest, and may take several hours to appear. Examples include pasta, chocolate, milk, yogurt and beans.
Most starchy foods have a relatively high GI; they digest easily and convert into blood glucose quickly. Exceptions include the types of starches (called "straight chain" starches) found in pasta and legumes. Because these starches pack together very tightly, digestive enzymes take a while to break them apart - thus causing a slow, progressive blood glucose rise.
Foods that contain dextrose tend to have a very high GI, and are optimal for treating hypoglycemia. Fructose (fruit sugar) and lactose (milk sugar) are slower to convert into blood glucose. Table sugar (sucrose) has a moderate GI because it contains a combination of glucose (which is very fast) and fructose (which is somewhat slower). Foods that contain fiber or large amounts of fat tend to have lower GIs than foods that do not.
Why is glycemic index important?
|Glycemic Index Fact Box|
Because the effect of dietary carbohydrates is what really matters. High-GI foods tend to make blood sugars spike quite high right after eating. This rapid rise and fall can be detrimental to one's physical and mental performance on a daily basis, and may increase the risk for long-term complications. The slower a carbohydrate digests, the less immediate and dramatic its impact will be. Low-glycemic index foods tend to make blood sugars rise more gradually. They enhance satiety and help to curb appetite. They help to lower triglyceride levels and prevent "reactive" hypoglycemia, and they serve as excellent fuel sources in preparation for endurance exercise.
For those who take rapid-acting insulin at mealtimes, knowing the glycemic index also helps to determine the optimal timing of the insulin.
If the main source of carbohydrate in a meal has a high GI value (greater than 70), it is best to take mealtime rapid-acting insulin 20-30 minutes prior to eating. This will allow the insulin peak to coincide as closely as possible with the post-meal blood glucose rise. Taking insulin for high-GI foods just before or during a meal would produce a significant after-meal "spike", as the insulin would lag behind the blood glucose rise by about half an hour. Of course, if your blood glucose is low or dropping quickly prior to a meal, it is best not to delay eating after taking your insulin.
For meals with a moderate GI (approximately 45-70) taking rapid-acting insulin 20-30 minutes before eating could produce a blood glucose drop soon after the meal. It is best to take the insulin 10-15 minutes prior to foods with a moderate GI.
For foods with a low GI (below 45) taking insulin prior to eating is likely to lead to hypoglycemia about an hour later, followed by a delayed blood glucose rise once the food finally kicks in. Instead, try taking the insulin soon after eating. A second option is to split the rapid-acting insulin into two parts: half given with the meal, the other half about an hour later. A third option is to take Regular insulin with the meal, rather than a rapid-acting analog. For those who use an insulin pump, the bolus delivery can be extended over 60-90 minutes.
A Matter of Accuracy
How accurate is the glycemic index? Given the variability in digestion and the impact of mixing foods with different glycemic index values into one meal, there is no way to tell exactly when a specific food or meal will cause the blood glucose level to peak. What glycemic index is good for is categorizing foods according to their relative impact: slow, medium and fast.
There really is not much difference between a food with a GI of 50 and one with a GI of 60. But there should be a significant difference between an 80 and a 35. Try adjusting the timing of your insulin to better match the expected rate of digestion, and see what happens - particularly with your after-meal blood glucose levels. Diabetes management, by its very nature, is all about "trial and adjustment," so if you don’t get it right the first time, don't give up!
Brand-Miller, J, et al, The Glucose Revolution. 1998, Marlowe & Company, NY, NY.
Brand-Miller, J. and Foster-Powell, K. New Glucose Revolution Low GI Guide to Diabetes. 2006, Marlowe & Company, NY, NY.
Geil, Patti. From Jelly Beans to Kidney Beans: What Diabetes Educators Should Know About the Glycemic Index. The Diabetes Educator. 2001; 27:4. 505-508.
Editor's note: Gary Scheiner MS, CDE is Owner and Clinical Director of Integrated Diabetes Services, a private consulting practice located near Philadelphia for people with diabetes who utilize intensive insulin therapy. He is the author of several books, including Think Like A Pancreas: A Practical Guide to Managing Diabetes With Insulin and The Ultimate Guide to Accurate Carb Counting. He and his team of Certified Diabetes Educators work with people throughout the world, providing advanced self-management training and blood sugar fine-tuning via phone and the internet. Gary has had Type-1 diabetes for 25 years and has worn & trained on every make & model of continuous glucose monitor. He can be reached at firstname.lastname@example.org, or toll-free at 877-735-3648.
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