This is a great question and the answer can be quite confusing, so please bear with me and get ready to expand your mind!
Let’s begin by defining glycemic index and glycemic load. Glycemic index (GI) ranks carbohydrates on a scale from zero to 100 based on the degree to which they affect your blood sugar levels. A GI of 1 to 55 is considered low, 56 to 69 is medium, and 70 to 100 is high.
The lower the GI, the less impact that carb will have on raising your blood sugar. Think of GI in terms of ranking the quality of carbohydrates.
Glycemic load (GL) adds the element of quantity to the equation by showing the relationship between GI and serving size. Therefore, “Glycemic load measures the blood-sugar-raising power per serving of food.”
The formula for computing GL is to multiply the GI of a food times the amount of net carbs in a given serving size. (Net carbs equals the grams of carbohydrates in that serving size minus the grams of fiber.) Then you divide that product by 100 to yield the GL score.
A glycemic load of 10 or less is considered low, 11 to 19 is medium, and 20 or more is high.
Having said all that, it would seem that glycemic load is the value we would want to use and follow for planning our meals as a type 2 diabetic, right? However, two significant problems enter in.
First, glycemic index is a measure obtained through actual testing with humans and the rate at which certain foods actually elevate blood sugar levels. Glycemic load is merely a mathematical computation based on GI and net carbohydrates found in a specific serving size. For some reason, however, there are discrepancies in GL values listed on nutritional data information for foods.
Here’s just one example: 100g of raw watermelon has a GI of 72 (high), and net carbs of 8. Watermelon’s GL = 72 x 8 = 576/100 = 5.76 GL. But Nutrition Data lists the GL of the same serving size of watermelon at 2 GL. Now, both of these GL ratings are considered low, but there’s quite a discrepancy between the two numbers.
Let’s look at 100g of raw pear, with a GI of 38 (low), net carbs of 12. A pear’s GL = 38 x 12 = 456/100 = 4.56 GL. This time Nutrition Data lists the GL for the same serving size of pear at 3 GL. Again, there’s a significant difference between GL numbers for the pear. And according to Nutrition Data, a 100g serving of watermelon has a lower GL than a 100g serving of pear, though both are ranked low.
This brings up the second problem with GL. In Australia an extensive study was conducted regarding the relationship between GI and GL, in which 36,000 adults were followed and tested over a four-year period. The study revealed that consuming a relatively high carbohydrate diet consisting of low GI carbs led to a lower risk for developing type 2 diabetes. Conversely, those individuals who consumed high GI carbs increased their risk for developing type 2 diabetes.
The study also exposed an important fact regarding GL. Namely, it is possible for two different foods to have the same GL, but one food has a high GI and the other a low GI. This is the case with the examples above with watermelon and pears. Watermelon has a high GI and pears have a low GI, even though their GL appears to be about equal for the same serving size.
The problem is that the high GI of watermelon will generally spike blood sugar, whereas the low GI of a pear should not. But going merely by GL, one would never know that. Based on the findings from that study, it appears that glycemic index is more crucial in preventing and managing type 2 diabetes than glycemic load. Portion control is always an important factor as well.
At Barton Publishing we encourage type 2 diabetics to eat carbs with a low GI and to practice portion control. We do this through what we call the Healthy Food Plate. Here is a chart for GI values for a variety of foods as well.