Glycemic Index Rates 50 Gram Portions of Carbs
As we have seen (How GI is measured) the glycemic index rates how rapidly a particular food raises blood-glucose. The test is performed using a food-serving that contains 50 grams of digestible carbohydrate. While providing valuable comparitive data on the glycemic effects of different foods, this test is not ideal. Why not? Because it doesn’t take into account the fact that some foods contain a lot more carbohydrate than others.
Different Percentages of Carbohydrate
Take two foods: assume both have GI values of 80, making them high GI foods. On this basis, we should eat both relatively sparingly. But suppose one of them contains 75 percent carbohydrate, whereas the other is only 5 percent carbohydrate. These foods are now quite different. The 75 percent carb food should be eaten more sparingly. The glycemic index does not allow for different carb percentages.
Glycemic Load Applies GI to Food Serving Sizes
In response to this problem of carbohydrate percentage, researchers at Harvard University invented the idea of Glycemic Load, which applies the glycemic index to normal food serving sizes. The glycemic load of a particular food rates the effect of a normal serving-size of that food on blood glucose levels.
How Glycemic Load is Calculated
The formula for calculating glycemic load is simple. Multiply the GI value of a food by the amount of carbohydrate per serving and divide the result by 100.
Example No 1
Spaghetti has a GI value of 40
A serving (1 cup) contains 52 grams of carbohydrate.
The glycemic load of spaghetti is: (40 X 52) divided by 100 = 20.8
Example No 2
An apple has a GI value of 40
A serving (medium size apple) contains 15 grams of carbohydrate.
The glycemic load of an apple is: (40 X 15) divided by 100 = 6