One of the most important concepts to understand when you are a diabetic is the concept of glycemic index (GI), the measure of how quickly carbohydrate foods affect blood glucose levels. Since the condition of diabetes impairs the body’s ability to effectively maintain proper levels of glucose in the blood it is important to make informed food choices that can reduce the speed at which carbohydrates from a meal reach the bloodstream as glucose and the total amount of glucose reaching the bloodstream. In this article we will briefly review the fundamentals of carbohydrate structure and the glycemic index of different foods. You may be in for some surprises.
As you are probably aware the three major food groups (macronutrients) include proteins, fats and carbohydrates. While fats and proteins are essential for growth and repair of tissue and membranes and their general maintenance it is the carbohydrates that provide us with most of the fuel that we burn for energy. Carbohydrates are made up of carbon, hydrogen and oxygen. These molecules are arranged in ring structures that can be linked together to form larger molecules. Carbohydrates can be as simple as single-ringed monosaccharides such as glucose, fructose (fruit sugars) and galactose (a milk sugar). They may also appear as two-ringed molecules, called disaccharides, including maltose (glucose plus glucose), sucrose (glucose plus fructose) and lactose (glucose plus galactose). Neither plants nor animals store energy in the form of these sugars as this would lead to an osmotic imbalance (the cells would take up water and burst). Therefore, sugars are converted to larger molecules for storage. In plants these are called starches, the two most common being amylopectin and amylose. Amylose is made up of glucose rings strung together in a long strand. Amylopectin is similar except it has many side branches. This difference is important because it affects their rate of enzymatic breakdown during digestion. Because of the many side chains amylopectin offers more sites for enzymes to work and is broken down more rapidly.
The Glycemic Index
How is the glycemic index (GI) actually determined? Typically the reference point against which all other foods are measured is pure glucose since that is the final product of carbohydrate digestion that enters the blood. The rate at which pure glucose enters the blood is designated a GI value of 100, all other foods being relative to this glucose standard. An important thing to keep in mind is that the GI reflects the rate or speed that different foods appear as glucose in the blood. We also need to consider the total glucose load which that food will deliver to the blood stream which is a function of the food amount and the percent carbohydrate content of the food. It is the large carbohydrate loads with high GI values that will deliver large amounts of glucose into the blood in a very short time. By regularly consuming foods geared towards a low average GI and low total carbohydrate, it should be possible to improve glycemic management in diabetes.
The GI is determined for a single type of food but in the real world the typical meal is a combination of foods composed of not just carbohydrates but also fat and protein along with fiber and micronutrients. The interaction of these foods in the digestive tract can alter the rate of digestion and absorption. The GI is still a valuable guide, however, when trying to estimate and reduce the GI of a whole meal, which should be the goal of all diabetics.
Sample calculation: For the sake of simplicity let’s say that a breakfast meal contained 100 grams of carbohydrate. This included white toast (2 slices at 13 grams each = 26 grams), hash brown potatoes (54 grams), and half a grapefruit (20 grams). First calculate the percentage each contributes to the total carbohydrate. Then multiply that by the glycemic index of each type. Then add these three numbers together. You should try doing a few of these calculations since many food combinations appear in your diet repeatedly meaning you don’t have to do calculations at every meal. You will learn to identify various food combinations and amounts as you would a spicy dish (ie. mild, medium, hot and very hot) in terms of the GI.
Points of Interest
Many people intuitively appreciate the concept of GI as it relates to various foods. What I would like to draw to your attention are some interesting specifics that could lower the GI of a meal by 50% or more.
When it comes to GI not all breads are created equal. The breads that have highly processed refined flour (white or whole wheat) are going to have a much higher GI (70) than breads made with stoneground flour (coarse) containing whole grains (GI=53). These coarse, heavy, chewy breads are digested more slowly. The problem with highly processed flours is that the high speed milling process pulverizes the starch into tiny particles increasing the surface area and accelerating the rate of digestion resulting in a higher GI, as high in fact as a Mars Almond Bar! (GI=68)
Nor is all rice created equal. Our discussion above regarding amylose and amylopectin applies especially to rice. White rice has a high GI (72) and the main starch is amylopectin. By contrast, basmati rice is mostly amylose and has a much lower GI (58). You should also try the long grain brown rice with the germ and husk as this has a favorable GI (55) and better nutritional value.
Although pasta in general already has a moderate (GI~55-65) it can be reduced by preparing it al dente (still very firm) which slows down its rate of digestion.
The GI values determined for various sugars are interesting. Table sugar, raw cane sugar and honey are all essentially sucrose (a glucose and fructose molecule linked together) and while they may vary slightly in their micronutrients they all have similar GI values (65). They are not as high as white bread either because the body is very slow to process fructose (GI=23). That also explains why many fruits have low GI values as many contain mostly fructose as a simple sugar, making them an excellent snack.
Legumes such as lentils, kidney beans, chick peas and navy beans all have GIs in the 27-38 range. Need I say more? When included in a meal they can help reduce the GI of the whole meal. In addition they are low in fat and contain high amounts of fibre that can improve your cholesterol profile.
If you are a diabetic this information may provide an additional tool to help manage blood sugars more effectively. We are all different so test your blood sugars regularly and learn which foods and combinations work for you. This doesn’t mean that you should sit down before each meal with a calculator trying to determine the GI of your meal to the nearest decimal point. The GI is simply a guide to help you understand and more accurately estimate the impact of a meal on your blood sugars. You can then make appropriate food selections.
Eric Norman is a research scientist investigating heart disease in post-menopausal women and in individuals with type II diabetes.
- Wolever, T. et al.. The Glucose Revoluation: The Authoritative Guide to the Glycemic Index. Marlowe and Company. 1999.
- Weil, A. Eating Well For Optimal Health. Alfred A. Knopf. New York. 2000.
- (You can find an extensive list of GI values for a variety of foods in The Glucose Revolution listed in the references.)