Glycaemic Index (GI)
The Glycaemic Index (GI) ranks carbohydrate-rich foods based on the effect that the carbohydrate present in the food has on blood glucose levels when consumed at equal carbohydrate intakes in the food.
The problem with GI is that although it is determined from measurements of blood glucose responses to the whole foods, the responses are attributed to the carbohydrate component of the food alone. GI is calculated as the response to a portion of food containing 50 g carbohydrate as a percentage of the response to 50 g glucose, so it allows you to compare carbohydrates in foods, and foods that contain equal amounts of carbohydrates.
However, not all foods or food portions contain the same quantity of carbohydrate so it makes comparisons between foods difficult. Also the GI of food does not change with quantity, so a muesli bar has a GI of 50 whether a person consumes 50 g or 80 g, meaning that the effect of serving size is neglected.