Dr Alan Barclay
How scientists measure a food’s GI value
GI testing has a very strict protocol in order to detect true differences in the glycemic potential of the carbohydrates in different foods. Here’s how we test the GI of a food following the protocol set out by the International Standards Organization:
Step 1. Ten volunteers consume a fifty-gram carbohydrate portion of the reference food on three separate days. Pure glucose dissolved in water is the usual reference food, and its GI is set at 100. The test is carried out in the morning after an overnight fast. The solution is consumed within ten to twelve minutes, and blood glucose levels are measured eight times over the next two hours. The findings from those three days of testing are averaged to find each person’s usual response to the reference food.
Step 2. Next, we measure the individuals glycemic response to a fifty-gram carbohydrate portion of the test food (e.g., approximately one cup of cooked rice) once, using exactly the same two-hour testing protocol.
Step 3. Then we calculate each person’s response to the test food as a percentage of his or her average response to the reference food. We do this by plotting his or her blood glucose response to the test food on a graph and comparing this with the response to the reference food; the response can be summarized as the area under the curve—the exact value of which is calculated using a computer program.
Step 4. Finally, we average the responses of all ten volunteers to the test food; this is the GI value which we publish. If the average test food response area (i.e., the area under the curve) is only 40 percent of the reference food, then the GI of the test food is 40. Not everyone will give exactly the same number, of course, but the law of averages applies. If we tested them over and over again, people would all tend to congregate around the same number.
Because each person is his or her own control, testing foods in volunteers with diabetes or prediabetes gives approximately the same GI values as testing people who don’t have diabetes.
In practice, the average result in the group of ten healthy people is the published GI value of the food. At least 240 blood glucose assays (the technical term for the test measuring the blood glucose) will have been made to generate that number. In some labs, up to 640 assays are
made. So there is nothing crude about GI testing.
For example, the GI value of bread (70) means that the overall fluctuation in blood glucose after eating a serve of white bread will be about 70 percent of the effect of an equivalent amount of carbohydrate from pure glucose (GI value of 100).
If you want to know more about GI testing or find an accredited laboratory to test your food product, drop me an email: firstname.lastname@example.org
For more information about the GI Symbol Program
Dr Alan W Barclay, PhD
Chief Scientific Officer
Glycemic Index Foundation (Ltd)
Phone: +61 (0)2 9785 1037
Mob: +61 (0)416 111 046
Fax: +61 (0)2 9785 1037