How do I Calculate Glycemic Index?

Mary Beth Swayne

Calculating a glycemic index (GI) is generally considered a difficult task, which is why researchers and nutritionists have compiled a glycemic index of many common foods. Mathematical equations allow a person to calculate glycemic index of foods in a meal, provided the carbohydrates in each food and the glycemic index for each food are known. Numerous websites offer a list of general glycemic index food numbers to help calculate glycemic index; there also are websites that offer common fast-food numbers. When using these websites, it may be typical to find not only the glycemic index, but also a serving size in grams, the carbohydrates per serving, and the glycemic load (GL).

White rice has a high glycemic index.
White rice has a high glycemic index.

Although similar, the glycemic load and the glycemic index are two completely different measurements. The glycemic index is a ranking system that measures how quickly a particular carbohydrate will break down into a sugar in the human body. In contrast, the glycemic load not only takes the same measurement into account but also measures how much of the carbohydrate is in the food. For example, watermelon has a high GI but a low GL. This is due to the fact that that watermelon will quickly turn to sugar in the digestive process, but there are not many carbohydrates in the actual fruit.

Watermelon has a low glycemic load but a high glycemic index.
Watermelon has a low glycemic load but a high glycemic index.

When the GL is known, a person can calculate glycemic index by multiplying the glycemic load by 100. This number is then divided by the available carbohydrates measured in grams. Available carbohydrates are defined as the total carbohydrate content of the food minus the fiber content. The result is the glycemic index of the food.

To calculate glycemic index of a meal, add up the total carbohydrates in the entire meal, then look up the GI value for each carbohydrate. Once you have determine how many carbohydrates were consumed in each serving, take the number of carbohydrates in the serving and divide it by the total number of carbohydrates in the group. This will result in the percentage of carbohydrates contributed by the group. You can then multiply this number by the glycemic index for the group of carbohydrates. Finally, you can add all of these numbers together and the result will be the glycemic index for the meal.

Calculating the glycemic index for a particular food is nearly impossible for a person without the aid of the glycemic load. The best way to find these numbers is to look for a list created by researchers and nutritionists. These lists can help people calculate the glycemic index of a meal. Low glycemic index foods help reduce risks of heart disease and diabetes, and can help control existing diabetes.

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