How Glycemic Index Values are Determined


Glycemic Index values are determined from the calculation of the influence a given food item has in increasing glucose (blood sugar) levels during human consumption. And when we say ‘food’, we are talking more specifically about carbohydrate content in foods. To have a standard system that we can use to compare foods, this obviously has to be carried out using some form of scientific method.

The Human Nutrition Unit at Sydney University is one of the main facilities that determines Glycemic Index values, and has been at the forefront of this type of research for over a decade. Surprisingly (at least to me!) there are actually only a handful of research groups around the globe that provide reputable testing and acceptable results. To date there are a little over 400 foods that have Glycemic Index values.

Glycemic Index values (aka glycemic index scores, glycemic index rankings, glycemic index rating, glycemic index numbers, glycemic index list) are determined by monitoring healthy volunteers (typically 10 or more), for the two hour period subsequent to them consuming an amount of the food being tested (the test food) that contains exactly 50 grams of available carbohydrates (meaning it can be processed by the body). During this period the blood sugar (glucose) levels are measured.

For each participant, the amount under their 120 minute glucose response (this is known as the test food glucose AUC*) for this food is then recorded. At a later date, the same group of individuals eat the same quantity in glucose sugar, and again their glucose response is measured during the same time frame (known as the reference food glucose AUC).

For each person tested, Glycemic Index values for the test foods are subsequently determined by dividing the reference food glucose AUC value by the test food glucose AUC.

e.g. ‘test food glucose AUC’ / ‘reference food glucose AUC’ = Glycemic Index value

From here all the test Glycemic Index values are totaled and divided by the number test subjects to give an average Glycemic Index value. This is how we come by the final Glycemic Index values we see in GI charts today.

(*AUC = Area Under the Curve)

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