Glycemic index is determined in a research lab. Fasting people are fed 50 grams of either pure glucose or enough white bread to provide 50 grams of digestible (non-fiber) carbohydrate, and blood glucose is measured over the next 2 hours. On a different day, the same people would be provided a food in an amount to allow for 50 grams of digestible carbohydrate and again blood glucose is measured over the next 2 hours. If a food raises blood glucose to 50 percent of the rise caused by glucose then the glycemic index is 50.
Because of the difference between white bread and pure glucose, glycemic indexes determined for foods using these different standards can vary. The glycemic index scale when using pure glucose is 0 to 100 and is more common because it is a little easier for the public to use. Meanwhile, when white bread is used as the standard for determining glycemic index, several foods, such as a baked potato, rice cakes, jelly beans, and Cheerios® have a value greater than 100. When this book discusses the glycemic index of foods we will use glucose as the standard as per the values of the Human Nutrition Unit at the University of Sydney (www.glycemicindex.com).
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