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Research University Of Sidney

UNIVERSITY of SIDNEY´s Glycemic Research Service

How scientists measure the GI

A food's GI value must be measured in human subjects (we call this in vivo testing) according to an internationally standardised method.

In the standard method of GI testing, a minimum of 10 volunteers consume a 50 gram carbohydrate portion of the test food (e.g., 1 cup of rice) on one occasion and a 50 gram carbohydrate portion of the reference food on another occasion. Pure glucose dissolved in water is the usual reference food and its GI is set at 100. The test is carried out in the morning after an overnight fast. The food is eaten within 10-12 minutes, and blood glucose levels are measured at frequent intervals over the next two hours.

Each volunteer's blood glucose response to the test food is then plotted on a graph and compared with his or her response to the reference food (figure 1); that graphic response is referred to as the area under the curve - the exact percentages are calculated using a computer program.

 

Figure 1: Blood glucose response of reference and test food over 2 hours

gigraph

 

 

If the test food response area (i.e., the area under the curve) is 70 per cent of the reference food, then the GI of the test food is 70. Not everyone will give exactly the same number, of course, but the law of averages applies. If we tested them over and over again, they will all tend to congregate around the same number. Because each person is his or her own control, testing foods in volunteers with diabetes gives approximately the same GI values as testing normal subjects. In practice, the average result in the group of ten healthy individuals is the published GI value of the food.

A food's GI value cannot be predicted from its composition, carbohydrate content, or even the GI values of related foods. The only way to know a food's GI value is to test it, following the standardized methodology we've just described.

The Australian Standard: Glycemic Index of Foods has been submitted with modifications to the International Standards Organisation for possible adoption by member countries around the world including Canada, China, France, India, Japan, Malaysia, South Africa the UK and the US.

 

Recommended GI testing laboratories

Currently, few facilities around the world test GI values by following the standard international testing protocol. The Glycemic Index Foundation recommend the following:

AUSTRALIA AND NEW ZEALAND

The University of Sydney's Glycemic Index Research Service (SUGIRS)

NORTH AMERICA Glycemic Index Laboratories, Inc

UK AND EUROPE

Oxford Brookes University

Hammersmith Food Research