03.How GI is measured
You cannot tell what a food’s GI is likely to be just by looking at it or knowing its nutritional value. So while there are some general rules of thumb, you’ll need to use a table of high, medium and low GI values.
The gold standard for measuring GI is to use real people: a group of volunteers are each given 50g of glucose then the effect on their blood sugar over the next two hours is measured. The GI result for glucose is given a value of 100. The same people are then given a serving of the test food, also containing 50g of carbohydrate. The effect on their blood sugar this time is compared with the first test, and the GI is calculated from the average of all the volunteers. This is a complex process and there are only a few Australian laboratories that can do this type of testing.
GI claims appear on all sorts of carbohydrate foods; some labels simply say low GI, while others give a GI test figure. There’s no food standard that specifically covers the GI labelling of foods, so you have to take the manufacturer on trust.
One way to be sure of a product’s GI is to look for the GI symbol, below. Foods with this symbol must also list the exact GI figure, near the nutrition panel, along with the words “low” or “medium”. To display this label the GI will have been reliably measured and must also meet other nutrition criteria, depending on the type of food – the food must be a healthy choice in its category.
The GI symbol is a not-for-profit program founded by the Juvenile Diabetes Research Foundation, the University of Sydney and Diabetes Australia. Companies pay a licensing fee to be part of the program and use the logo.
The University of Sydney recently check-tested a wide range of foods that have a GI claim on the label but don’t carry the GI symbol. It found eight of the 10 products it tested using the gold-standard method didn’t have the GI rating claimed.