While most people are now familiar with GI and the benefits of eating more low GI foods, the difference between glycemic index (GI) and glycemic load (GL) is something many people get confused about.
The GI is essentially a way of ‘ranking’ carbohydrate foods according to the speed at which they cause our blood glucose levels to rise and fall. High GI foods are those which are more quickly digested and absorbed while low GI foods break down more slowly, gradually releasing glucose into the bloodstream.
However the overall effect that a food has on blood glucose levels is dependent on both the nature (GI) of the carbohydrate it contains and the amount you eat (i.e. the grams of carbs). Glycemic load takes both of these factors into account and is calculated by multiplying the GI of the food by the amount of carbohydrate per serve and then dividing by 100. For example an apple has a GI of 40 and contains 15 grams of carbohydrate so has a GL of 6 (40 x 15)/100.
While foods with a high carbohydrate content and those with higher GI values will generally have the highest GL, this also means that small amounts of a high GI food may have only modest effects on blood glucose levels while large amounts of a low GI food can still raise blood glucose and insulin levels significantly.
What this means in practice, is that there is no need to totally avoid foods that have a high GI but are low in carbohydrate and nutrient-dense – a good example of this would be watermelon. On the other hand, just because a food is low GI, it doesn’t mean you can eat as much as you like, particularly if you are watching your weight or your blood glucose or insulin levels. If you are choosing lower GI foods and being mindful of portion sizes, then you are already taking GL into account.