The glycemic index (or GI) is a measure of the quality of carbohydrates found in food. The quality refers to how fast blood sugar levels are raised. It was originally brought about to help diabetics control their blood sugar but another fantastic 'side effect' was found. These diabetics were losing weight!
So now the glycemic index is a growing topic in the weight loss industry.
Each food is compared to glucose which has a glycemic index of 100. The higher the GI, the faster blood sugar rises when this food is eaten.
A high glycemic index value is >70. A medium value is 56-69. A low value is <56.
The surprising thing about this is that some foods that were traditionally thought to release energy slowly are actually digested fast, such as some breads and some rices. This means that you should now forget about the old 'simple' and 'complex' carbohydrate theories.

So How Do You Use The Glycemic Index?
You need to eat foods that are low-medium on the glycemic index scale. If you are unsure about which foods to eat, I highly recommend Professor Jennie Brand-Miller's book: The New Glucose Revolution. Jennie is the world's leading researcher on glycemic index and anything you read from her books can be taken as gospel!