The Glycemic Index (GI) employs a rating system from 0 to 100 in order to compare the blood glucose or glycemic response to various foods, primarily complex carbohydrates. The glycemic response is the change in blood glucose after eating certain foods measured from blood samples. Glucose is a byproduct of starch digestion (other by products are galactose and fructose).
The GI is determined by comparing blood sample results of volunteers given a test food (50 g of carbohydrate) with blood sample results after ingesting a control food, usually pure glucose (50 g of carbohydrate). Foods then can be compared based on a percentage of the control food. For example, cornflakes have a GI of 81 or 81 percent of the blood glucose response to the same amount of carbohydrate in pure glucose (100 percent).
Glycemic Load (GL)
Another important factor that affects blood glucose levels and insulin responses is the glycemic load. The glycemic load is a measure of the amount of carbohydrate consumed that affects blood glucose levels and insulin responses. The GL is obtained by multiplying the glycemic index (GI) by the amount of carbohydrate provided from the food in grams and then dividing this amount by 100.
Glycemic Load = [Glycemic Index (GI) x food (grams)] / 100
Eating 14 g (one slice) of white bread, which has a GI of 73, then provides a Glycemic Load of 10.
Glycemic Load= [73 x 14] / 100
=  / 100
Why are the GI and GL so important?
There is scientific evidence showing that foods high in glycemic load are associated with developing type 2 diabetes (diabetes mellitus). Foods found to be especially associated with type 2 diabetes are potatoes, white rice, white bread, and carbonated beverages.
Eating foods with high glycemic loads have been associated with obesity. For example, when you eat cornflakes (GL of 42) your body secretes insulin, which then rapidly drops your blood glucose level(within several hours). This drop in blood glucose than signals a hunger response so that the body can once again raise your blood glucose levels in your body. If you then eat foods high in GL again then the cycle repeats itself.
What to do?
Your best bet is to eat foods that have low glycemic loads such as fruits (with low GL), nuts/seeds, whole grains, and low starch vegetables. Stay away from sugar, candy, soda, cookies, and cakes that have little or no nutrition. These “treats” only feed food addictions and result in becoming overweight.