What’s the glycaemic index?

The glycaemic index (GI) is a way of comparing the impact that different foods have on blood glucose levels.

Jennie Brand-Miller and her colleagues at Sydney University have tested hundreds of foods on groups of volunteers. Each volunteer is given a portion of the test food that provides 50g of available (digestible) carbohydrate.

Blood glucose levels are measured over the next two hours. The impact of the test food on blood glucose is compared with that of 50g of pure glucose.

Glucose itself has a GI of 100. A food producing half the blood glucose response of pure glucose has a GI of 50.

Foods can be divided into three bands:

  • Low GI (less than 55)
  • Medium GI (55-70)
  • High GI (over 70)

To reduce surges in blood sugar, you try to eat more low-GI foods and less high-GI foods. On this basis, watermelon, which has a GI of 72, looks like something to avoid.

But remember that the tests are carried out on a portion of the food that contains 50g of digestible carbohydrate. Well, you have to eat about 1kg of melon to get that much carbohydrate. There’s not a lot of carbohydrate in one slice of watermelon, so even though it’s rapidly digested, a normal portion of watermelon has a very modest impact on blood glucose.

By contrast, some foods with low GI numbers (such as pasta) can cause a big rise in blood glucose because they are densely packed with carbohydrate.

So GI can be very misleading and it’s more helpful to be guided by GL.

 

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