The Glycaemic Index Q&As

Q:

How is the GI measured?

A:

A carbohydrate food is eaten and is broken down by the body. The carbohydrate is broken down to sugar which enters the blood stream.  Blood sugar levels are measured before the meal and at one hour intervals afterwards. The resulting measurements are plotted as a curve.  The area below the curve is compared with a standard reference, usually either glucose or white bread.  The resulting ratio is the glycaemic index of the foodstuff.  These tests are normally carried out on between 6 and 10 individuals to derive a "standard" Glycaemic Index value. However there are numerous problems associated with calculating the GI and the methodology involved is rather unsatisfactory.

Two different standards are currently in use. Some of the GI values are calculated using sugar as a standard while some are calculated using white bread. This results in two different sets of GI values for the same food.
Laboratories which calculate the GI also use different blood sampling techniques, using either blood from veins or capillaries.  The two approaches give different results.
There is considerable variation in measured glycaemic index of a food between individuals (at least 50%) therefore values based on an average are therefore not a very good representation of individual responses.
The GI of foods is significantly affected by factors such as particle size, ripeness of fruit and freshness of preparation. It is also affected by the amount of chewing.
The body's glycaemic response is also affected by the time of day at which foods are eaten and the previous meal.
Foods are normally eaten in combination, which affects the GI.  The GI of a cheese sandwich is lower than that of bread alone.
Efforts are being made to standardise GI testing at laboratories around the world, but there is still a long way to go to ensure consistency in the methods used.

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