Glycaemic index (GI)
A measure of the speed and extent of the rise in blood glucose (blood sugar) after eating a food containing carbohydrate.
The glycaemic index is a scale from 0 to 100 that ranks foods by their ability to raise blood glucose. Contrary to what many believe, the reference is not white bread but pure glucose, which is arbitrarily given an index of 100. Each food is measured in a laboratory by giving a fasting subject 50 g of carbohydrate from the product, then comparing the resulting blood glucose curve (over 2 hours) with that of glucose.
The blood sugar peak generally occurs 30 minutes after eating, hence the importance of timing within a meal. Three categories exist: low GI (< 55), medium GI (55-70), high GI (> 70).
A fresh angle: a food’s GI changes depending on the context of the meal. Eating a banana on its own (GI 60) does not have the same impact as eating it with butter and protein (the effective GI falls). Moreover, low-GI foods eaten in very large quantities need to be treated like high-GI ones (hence the importance of glycaemic load). One last trap: some dairy products have a low GI but a high insulin index, as insulin rises more than expected. Finally, GI only applies to foods containing carbohydrate; meat has a GI of 0, but that is not useful information.