Determining the GI of a food with precision is not easy. Because the GI is a reflection of the combined effect of several physiological processes, it must be determined using human subjects (in vivo testing). The protocol for in vivo GI measurements has recently been refined as a result of an international collaborative study, in which the magnitude and sources of variation of GI values obtained by experienced investigators at seven different centres were studied, using the same sources of four food samples. Centres using the prescribed methodology and capillary blood were able to achieve a highly reliable result.

Summary Of Method

To determine a food's GI rating, measured portions of the food containing 50 grams (or 25 grams in some cases) 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 glucose response curve for the two-hour period. The area under the curve (AUC) is calculated and reflects the total rise in blood glucose levels after eating the test food. The GI rating (%) of the test food is calculated by dividing the AUC for the test food by the AUC for the reference food (white bread or glucose) and multiplying by 100. 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 of that food.

Click here for further information on methodology.

The figure above shows the two hour blood sugar response
of a high-GI food (white bread, GI score = 100%)
vs a low-GI food (lentils, GI score = 40%)

What About In Vitro Testing?

In vitro (test tube) methods are also being used to estimate the GI of a food, but are in fact crude estimates for true GI values. In vitro testing does not measure the GI per se, which, by definition is a measure of glucose levels in blood. One of these in vitro tests measures 'rapidly available glucose' (RAG) (ie the glucose released within 20 minutes of the start of an incubation of food plus enzymes). Differences in food preparation, shaking rate and amount and type of enzyme can markedly affect the final result. The RAG test cannot distinguish the effect on GI of adding viscous fibre or acid to a food, and does not distinguish effects that alter gastric emptying like acidity, osmolality or concentration of sugars. It also fails to pick up subtle effects on the degree of gelatinisation with small changes to moisture and heating. These factors have all been found to affect the GI of a food with in vivo testing. For these reasons, in vitro testing is not internationally recognised as a valid means of assessing the impact of carbohydrates on blood glucose levels, and as such, these tests should not be used as an estimate of the glycemic index.

LinksContact DetailsWhat's NewFeedbackMedia ReleasesSite Map