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The glycemic index, or GI, is a method used to rank foods based on how they impact your blood glucose. It was originally created to help people with diabetes select foods to improve their glycemic control by ranking foods as low, medium, or high GI. You can’t just guess the GI of a food by looking at a food label, it has to be tested. To express the impact of a particular food, we need a reference to compare it to. 50g of pure sucrose requires little digestion so the glucose can be rapidly absorbed into the bloodstream. I’ve fasted for over 12 hours and I haven’t had any physical activity this morning, so I can drink my pure sugar drink. I’m using a continuous glucose monitor to measure my blood glucose every 5 minutes. And now I’m going to sit tight for 2 hours so we can see the impact on my blood glucose. So its the next day and now we can do an experiment to determine the glycemic index of a particular test food. I’m going to test this Dave’s Killer Bread the 21 whole grains and seeds version. We want to compare like to like. Today I’m going to eat a serving of bread that also gives me 50 grams of digestible carbohydrate. And that word digestible is actually pretty important, because of fiber, which is also a carbohydrate but we can’t actually digest it. We have to actually subtract the fiber from the total carbohydrates in the product. When I look at the nutrition facts panel, what I see is that one slice of bread has 22 grams of carbohydrates in it, but 5 of those grams are fiber. If I want to get 50 grams total then I need to eat almost three whole slices of bread. Okay, so here I have 50 grams of digestible carbohydrate of Dave’s Killer Seed bread. In this graph we see my blood glucose to the sugar drink in red and to the bread in blue. It’s pretty striking how different they are over these two hours! The bread caused a much more gradual rise in blood glucose, and the peak was substantially lower compared to the sugar drink. Now to actually calculate the glycemic index, we need to calculate the area under the curve for both the sugar drink and the bread separately. Then we divide the bread test food area by the sugar drink reference and multiply by 100 to express it as a percent. Using this method, the glycemic index for Dave’s Killer Seed bread is 34, which puts it in the low GI category. From this n of one experiment, we observed that Dave’s Killer Seed bread raises my blood glucose only 34% as much as the sugar drink over a two hour period. I was actually pretty surprised at how low the GI for this bread was. I definitely expected it to be lower than the sugar drink due to the fiber content, since fiber slows down the absorption of glucose, but I really didn’t think that it would be categorized as a low GI food because it’s still bread! And the reality is, it might not be! We can’t exactly conclude that the GI of Dave’s Killer Seed Bread is for sure 34 just from this experiment - determining the GI of a food needs to be done in a larger sample of study participants than just me, and each participant needs to do the experiment in duplicate or triplicate, meaning drinking the sugar drink on three separate days and eating the test food on three separate days. Usually that sugar drink is made of pure glucose rather that the sucrose table sugar. It also turns out that there’s a few more complications about the glycemic index and its applicability.
