Measuring the GI
To determine a food’s GI value, measured portions of the food containing 50 grams of available carbohydrate (or 25 grams of available carbohydrate for foods that contain lower amounts of carbohydrate) are fed to 10 healthy people after an overnight fast. Finger-prick blood samples are taken at 15-30 minute intervals over the next two hours. These blood samples are used to construct a blood sugar response curve for the two hour period. The incremental area under the curve (iAUC) is calculated to reflect the total rise in blood glucose levels after eating the test food. The GI value is calculated by dividing the iAUC for the test food by the iAUC for the reference food (same amount of glucose) and multiplying by 100 (see Figure 1). The use of a standard food is essential for reducing the confounding influence of differences in the physical characteristics of the subjects. The average of the GI ratings from all ten subjects is published as the GI for that food.
The GI of foods has important implications for the food industry. Some foods on the Australian market already show their GI rating on the nutrition information panel. Terms such as complex carbohydrates and sugars, which commonly appear on food labels, are now recognised as having little nutritional or physiological significance. The WHO/FAO recommend that these terms be removed and replaced with the total carbohydrate content of the food and its GI value. However, the GI rating of a food must be tested physiologically and only a few centres around the world currently provide a legitimate testing service. The Human Nutrition Unit at the University of Sydney has been at the forefront of glycemic index research for over two decades and has tested hundreds of foods as an integral part of its program. Jennie Brand Miller is the senior author of International Tables of Glycemic Index published by the American Journal of Clinical Nutrition in 1995 and 2002 and by Diabetes Care in 2008.