Are You Using G.I. Joe?

by gary scheiner, CDE

In last issue’s Learning Curve, we explained the glycemic index (GI), and talked a little bit about how it can help you improve your diabetes management. For this month’s Thinking Like a Pancreas, we asked our resident management expert Gary Scheiner to tell us his thoughts on the GI based on his many years of experience with diabetes management.

Remember the song “War,” sung emphatically by the band “War”: War… what is it good for? Well, many people hold similar esteem for the concept of glycemic index. What good is it really? After all, some say the glycemic index is imprecise, inconsistent and limited in practicality. Or is it?

Glycemic Index (GI) refers to the rate at which carbohydrates convert into blood glucose. While virtually all carbohydrates convert into blood glucose eventually, some convert much faster than others. Pure glucose is given a GI score of 100; everything else is compared to the digestion/absorption rate of glucose. Click here to see the GI values of some common foods.

Note: for a more comprehensive list of glycemic index values, pick up a copy of: 
The Ultimate Guide to Accurate Carb Counting by Gary Scheiner, 2007, Marlowe & Company, NY, NY 
or
 New Glucose Revolution Low GI Guide to Diabetes by Jennie Brand-Miller, J, 2006, Marlowe & Company, NY, NY

What the numbers represent is the percentage of carbohydrate that turns into blood glucose in the first two hours. 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. Foods with a moderate GI (45-70) digest a bit more slowly, resulting in a less pronounced blood glucose peak approximately one to one and a half hours after they are consumed. 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 occur.

Most starchy foods have a relatively high GI; they digest easily and convert into blood glucose quickly. Exceptions include starches found in pasta and legumes. Foods that have dextrose in them (i.e. cake mixes, cookies, crackers, custards) tend to have a very high GI. 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 slower). Foods that contain fiber or large amounts of fat tend to have lower GIs than foods that do not.

what is it good for?

Why is glycemic index important? Because the effect of dietary carbohydrates is what really matters. In general, lower GI foods tend to make blood sugars easier to control. They enhance satiety and help to curb appetite. They serve as excellent fuel sources in preparation for endurance exercises, and they can also be used after very intense workouts to prevent delayed blood sugar drops.

High-GI foods tend to work best for treating hypoglycemia. Nobody wants to wait a long time for their blood sugar to come up when it is low, so consuming a food that converts into blood glucose quickly makes a world of sense. It is best to consume high-GI foods immediately prior to short bouts of exercise in order to prevent hypoglycemia. Foods such as crackers and sports drinks spend very little time in the stomach, and are less likely to cause cramping than lower-GI choices. During illness, high-GI foods are ideal because they digest very easily, requiring the stomach to do as little work as possible. For those with gastroparesis (a nerve condition causing very slow digestion), high-GI foods are recommended to prevent nausea and post-meal hypoglycemia.

For those who take insulin, knowing the glycemic index of a food helps in determining the optimal time to take mealtime insulin. Foods with a high GI (greater than 70) tend to raise blood sugar the fastest. For these types of foods, it is best to take mealtime rapid-acting insulin 15-20 minutes prior to eating. This will allow the insulin peak to coincide as closely as possible with the blood sugar peak. Taking insulin for high-GI foods just before or while eating would produce a significant after-meal blood sugar “spike”, as the insulin action would lag behind the blood sugar rise by about half an hour.

Foods with a moderate GI (approximately 45-70) as well as most “mixed” meals digest a bit slower, with a slightly less pronounced blood sugar peak approximately 60-90 minutes after eating. Taking rapid-acting insulin 15-20 minutes before eating these types of foods could produce a low blood sugar soon after eating. It is best to take mealtime insulin immediately prior to consuming foods with a moderate GI.

With low-GI (below 45) foods, the blood sugar “peak” is usually quite modest, and may take several hours to appear. Taking insulin prior to eating foods such as pasta or yogurt is likely to lead to hypoglycemia about one hour later, followed by a delayed blood sugar rise. Instead, try taking the insulin 10-15 minutes 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. One other option, available to insulin pump users, is to extend the bolus delivery over 60-90 minutes.

a matter of accuracy

How accurate is the glycemic index? Given the degree of inter-individual variability in digestion and the impact of mixing foods with different glycemic index values in the same 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: fast, moderate, and slow.

When looking at complete meals, the main source of carbohydrate should dictate the rate of blood glucose rise. For example, a meal where pasta is the primary carbohydrate should be treated as low-GI. A meal where potato is the major source of carbohydrates should be treated as high-GI. And don’t forget to take the fat content and size of the meal into account. Higher-fat meals will produce a slower blood glucose rise regardless of the nature of the carbohydrate, and larger meals usually take longer to digest than smaller meals.

So while it is true that subtle differences between GI values mean very little, major differences can certainly be used to your advantage. And we can use all the advantages we can get!

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. He and his team of Certified Diabetes Educators work with people throughout the world via phone and the internet. Gary has had type 1 diabetes for 24 years and can be reached by email or toll-free at 877-735-3648.