The glycemic index of a carbohydrate is measured using laboratory settings. To measure glycemic index, portions of a carbohydrate food are fed to a panel of volunteers who then give blood samples at regular intervals over 2 hours for determination of glucose levels.
Specifically, the glycemic index value of a food is assessed by giving 10 or more volunteer subjects a serving of the food containing 50 grams of digestible (available) carbohydrate. To test how the food raises blood sugar levels, a sample of blood is taken from each subject every 15 minutes during the first hour and thereafter every 30 minutes. The blood sugar level of these blood samples is measured and recorded.
The blood sugar level is plotted on a graph and the area under the curve is calculated using a computer program. The subject's response to the carbohydrate being tested is compared with the subject's sugar response to 50g of pure glucose. Since glucose standard, it is the reference food and the testing of glucose on the subject's blood sugar levels is done on a separate occasion. The average blood sugar response from 8-10 people will determine the glycemic index (GI) value of that food.