The glycemic index, or GI, is a method of ranking foods, from 0 to 100, based on how they influence your blood sugar levels. Higher GIs indicate foods that will cause your blood sugar to rise higher and faster than foods with lower GIs. Type 1 and Type 2 diabetics can benefit from consuming low-GI diets because they help control blood sugar and insulin levels, and they can also reduce insulin resistance. The GI values of many foods have been scientifically determined and recorded, so calculating the GI of a meal is simple.
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Determine how many carbohydrates are in each portion of your meal. For example, consider a breakfast of instant oatmeal and an 8-ounce glass of orange juice. One packet of instant oatmeal contains 22 grams of carbohydrate, and 8 ounces of orange juice has 26 grams of carbohydrates.
Determine the proportion of carbohydrates each meal component adds to the meal. Divide the total grams of carbohydrates by the number of grams contributed by the individual component to find the proportion of carbs for the component. For example, there are 48 total grams of carbohydrates in the meal of orange juice and oatmeal. Divide 22 grams by 48 grams to determine that the the proportion of carbs from oatmeal is 0.46, and the proportion of carbs from the orange juice is 0.54.
Multiply the proportions for the meal components by the predetermined GI of that component. GIs of individual foods can be found in an online GI database or in the table published by "The American Journal of Clinical Nutrition." For example, the GI of oatmeal is 83. Multiply 0.46 by 83 to get 38.18. For the orange juice, multiply 0.54 by the GI of 46 to get 24.84.
Add the results of Step 3 together. In this example, add 38.18 and 24.84 to get the sum of 63.02, which is the total glycemic index of the meal.