The glycaemic index (GI) measures the speed with which foods are converted into glucose and therefore how quickly they enter the blood stream. The glycaemic Index uses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood glucose. Pure glucose serves as a reference point, and is given a glycaemic Index (GI) of 100. A GI of 70 or more is high, a GI of 56 to 69 inclusive is medium, and a GI of 55 or less is low.What GI does not tell you is how much of that carbohydrate is in a serving of a particular food. You need to know both things to understand a food’s effect on blood glucose levels. That is where GL (Glycaemia Load) comes in. The carbohydrate in watermelon for example, has a high GI but there isn’t a lot of it, so watermelon’s glycaemic load is relatively low.