Glycemic index and glycemic load
The Glycemic Index (GI) is a dietary tool that helps measure how quickly the body responds to different types of carbohydrates in foods. It is a relative ranking of how the carbohydrates in these foods impact on blood sugar levels.
The GI value is calculated by comparing the speed and extent to which a food containing 50g of carbohydrate is broken down into glucose in the blood against a 50g dose of pure glucose. For example, 50g of pure glucose is benchmarked at 100, and other foods containing 50g carbohydrates are divided into low GI foods (≤ 55), medium GI foods (56-69) and high GI foods (≥ 70), by comparison.
In essence, two foods may have the same amount of carbohydrate but very different GI values.
The GI of a food can vary due to many factors. These include how it is cooked and processed, how 'ripe' the food is, moisture content or the amount of protein or fat contained in the food.
Examples of common low, medium and high GI foods are shown in Table 1.
* Note that while one GI value is given to each food in this table, this is an average figure from a number of published studies. The GI of any particular food can vary within and between individuals. Often the degree of variation is very large - especially for foods in the medium and high category.
It is important to note that not all low GI foods are healthy. However, in general, following a low GI diet which is rich in fresh fruit, vegetables and wholegrains has been suggested to lower the risk of cardiovascular disease, type 2 diabetes and some cancers. Research continues to be conducted in this area.
The limitation of the GI system is that it compares the glycemic effect of foods containing 50g of carbohydrate, when realistically these foods are often consumed in variable amounts. To account for this concern the glycemic load was suggested.
The glycemic load builds on the glycemic index of a food by considering the serving size of the food consumed. It is a measure of both quality and quantity. To calculate glycemic load, the following equation is applied:
GL = GI x serving size (g) / 100
As with GI, the resulting numbers are classified into low (20).