The glycemic index (GI) of a food indicates the rate at which the carbohydrates in that food are broken down into glucose. This value ranges from 0 to 100 with higher values indicating a faster conversion to glucose. The GI is important to diabetics because they must actively manage their blood sugar level. You can determine the GI for an entire meal when you know the GI and amount of carbohydrates in each food you consume.
Step 1
Determine the number of grams (g) of carbohydrates in each food. For example, assume your meal is 1.5 ounces of shredded wheat, 15.5 fluid ounces of milk and a banana. The shredded wheat has 34 g of carbohydrates, the milk has 25 g and the banana has 23 g.
Step 2
Add the carbohydrates in each food to obtain the total carbohydrates in the meal. The 34 g in the shredded wheat, the 25 g in the milk and the 23 g in the banana provide 82 g of carbohydrates in the meal.
Step 3
Calculate the fraction of carbohydrates that come from each food. The entire meal has 82 g of carbohydrates, of which 34 g are from the shredded wheat. So, the shredded wheat provides 34 / 82 = 0.41 of the total carbohydrates in the meal. Similarly, the milk provides 25 / 82 = 0.3 of the total carbohydrates. The banana has 23 / 84 = 0.29 of the total.
Step 4
Compute the fraction of the meal's GI that comes from each food. This requires you to multiply the fraction of carbohydrates provided by each food with the GI for that food. Find the GI for each food in the database at the website Glycemicindex.com. For example, shredded wheat has a carbohydrate fraction of .41 and a GI of 63, so the shredded wheat provides a GI fraction of 0.41 x 63 = 25.8. Milk has a carbohydrate fraction of 0.3 and a GI of 32, so the milk provides a GI fraction of 0.3 x 32 = 9.6. The banana has a carbohydrate fraction of 0.29 and a GI of 58, so the banana provides a GI fraction of 0.29 x 58 = 16.8.
Step 5
Derive the total GI for the meal by adding the GI fractions for each food. The GI fraction for the shredded wheat is 25.8, the GI fraction for the milk is 9.6 and the banana is 16.8. The GI for the entire meal, therefore, is 25.8 + 9.6 + 16.8 = 52.2.
Tips and Warnings
- Your Personal Nutrition Guide defines low GI foods as those with a GI of 50 or less. High GI foods are those with a GI of 70 or more. An increase in the portion size of a particular food in a meal will make the GI of the meal closer to the GI of that particular food. For example, shredded wheat has a GI of 63, so increasing the portion size of the shredded wheat will cause the GI of the meal to be closer to 63.
Things You'll Need
- Nutritional labels
- Calculator
- Glycemic index chart
