What is the Glycemic Index (GI)?
Glycemic Index (GI) is a comparative system that ranks carbohydrate-based foods to indicate how fast and to what extent the food will raise the blood glucose level.
After consuming a carbohydrate-based food, the blood glucose rises and gradually falls to baseline levels usually over a period of 2-3 hours. This is the glycemic response.
When a food is tested for its GI, the glycemic response to the food is compared to the glyycemic response to a reference (glucose).
The amount of available (digestible) carbohydrate in the reference and the test food must be the same. Therefore GI reflects the quality or type of carbohydrate in a food rather than the quantity.
Foods containing carbohydrates, which are slowly digested and absorbed and therefore raise the blood glucose gradually and to a lesser extent, have a low GI. Foods containing carbohydrates, which are more quickly digested and absorbed and therefore have a greater effect on blood glucose levels, have a high GI.
GI is divided into three categories:
- Low (GI <=55)
- Medium (GI = 56-69)
- High (GI >=70)
Use of the GI
Knowledge of the GI value, allows for manipulation of the diet to obtain desired benefits. For example, inclusion of low GI foods in the diet has been shown to:
- Reduce risk of diabetes
- Improve blood glucose management in diabetes
- Reduce risk of cardiovascular disease
- Improve cholesterol
- Improve insulin sensitivity
- Reduce hunger and food intake
- Promote weight loss
- Reduce risk of some cancers
In addition, successful sport nutrition is related to the timely incorporation of either high or low GI foods before, during or after the event.
In vivo (in humans) GI testing
GI testing should be conducted according to an internationally established methodology, and within Australia, in accordance with the Australian Standard, Glycemic Index of Foods (AS 4694-2007).
In summary:
- Ten or more healthy subjects are required to determine the GI of a food.
- A portion of the test food is consumed which contains 50 grams of available carbohydrate and its effect on blood glucose levels is measured over the next two hour period.
- On three other occasions, a 50 gram portion of glucose (reference food) is consumed and the effect on blood glucose levels is measured over the subsequent two hour period.
- The GI is determined by dividing the area under the curve (AUC) of the test food by the average AUC of the three reference food tests in each participant.
- The final GI value is the average GI value of 10 or more participants.
In vitro (laboratory) GI testing
Currently, in vitro GI estimate testing cannot replicate all aspects of the complex human digestive process. Thus a true GI figure for use in advertising and product labelling must involve an in vivo test of the food product.
However, GI estimate testing if achieved by a methodology well validated against in vivo GI testing, can provide a useful tool during product development and refinement for the purpose of comparing various prototypes at a lower cost as a prelude to in vivo GI testing the final product.
Further information
Baker IDI is no longer providing a GI testing service. If you require in vivo GI testing we recommend you contact Dr Fiona Atkinson at Sydney University Glycemic Index Research Service (SUGiRS) on (02) 9351 6018 or email: sugirs@mmb.usyd.edu.au