Carbohydrate-rich foods raise blood sugar levels, but the much-vaunted glycemic index doesn't really tell us which. The glycemic index (GI) is a measure of how fast the human body converts the carbohydrate in a food into glucose relative to either pure glucose itself, which is something most dieters don't eat, or white bread, which is also something dieters don't eat.
The Glycemic Index: To compute the glycemic index, scientists recruit a group of volunteers who agree to fast and then eat a 50-gram serving of the reference food, either glucose or white bread, and nothing else. Blood is drawn to see how fast blood sugar levels rise. Then they come back a second time, when they have had a chance to fast again, and eat a 50-gram serving of the test food. Once again, blood is drawn to see how fast blood sugar levels rise.
The 'Real' Glycemic Index: The problem is, nobody eats that way. Blood sugar levels, and therefore the 'real' glycemic index act very differently when foods are mixed, or served at different temperatures. And the fact that these measurements require groups of test subjects belies another important truth: your glycemic index may not be the same as the next person's.