The glycemic index (GI) of a food is essentially a measure of how quickly the carbohydrates in that food will be converted into glucose. It's expressed as a percentage of the absorption rate of glucose, which has the fastest absorption rate of any carbohydrate source. The GI of a food will therefore be between 0 and 100. You'll frequently need to know the GI of your meals if you're diabetic. This will require you to know the GI of each food in your meal.
Step 1
Obtain the total grams of carbohydrates in each food of your meal. This information must be listed on the nutritional label of each food. Assume that your meal is toast and shredded wheat with milk. The milk in this meal has 27 grams of carbohydrates, the toast has 25 g of carbohydrates and the shredded wheat has 41 g of carbohydrates.
Step 2
Compute the total carbohydrates in your meal. The milk has 27 g of carbohydrates, the toast has 25 g of carbohydrates and the shredded wheat has 41 g of carbohydrates. The total carbohydrate load in your meal is 27 + 25 + 41 = 93 g.
Step 3
Calculate the fraction of your meal's carbohydrate load contained in each food by dividing the carbohydrates in each food by the meal's carbohydrate load. The meal has a total of 93 grams of carbohydrates and the milk has 27 of those grams, so the milk contains 27/93 = 0.29 of the meal's carbohydrate load. The toast contributes 25/93 = 0.269 of the meal's carbohydrate load. The shredded wheat contributes 41/93 = 0.441 of the carbohydrate load in the meal.
Step 4
Multiply each food's fraction of the carbohydrate load of the meal by that food's GI. You can find each food's GI from a source such as glycemicindex.com. The GI of whole milk is 32, so its portion of the meal's GI is 32 x 0.29 = 9.28. The GI of toast is 70, so its portion of the meal's GI is 70 x 0.269 = 10.8. The GI of shredded wheat is 63, so its portion of the meal's GI is 63 x 0.441 = 27.8.
Step 5
Calculate the meal's total GI by adding up each food's portion of the meal's GI. The milk's portion of the meal's GI is 9.28, the toast's portion of the meal's GI is 10.8 and the shredded wheat's portion is 27.8. The meal's total GI is therefore 9.28 + 10.8 + 27.8 = 47.9.
Things You'll Need
- Glycemic index chart
