The glycemic index is based off values obtained by monitoring a person’s blood sugar after consuming food. Each food listed on the index is rated. Another way to think of it is how complex a carbohydrate is or how long it takes to break down in the body. The slower it breaks down the more useful it is. Foods with low GI cause the body to digest slowly, leaving the body feeling full longer and allowing you to eat less calories.
Of course if you are looking for instant energy your best shot is for something with a high GI such as sucrose (sugar).
Here is an example;
1 cup of Brown Rice = GI 55
1 cup of White Rice = GI 64
If you compare the two rice choices the brown rice has a lower GI. The brown rice is a better choice and will digest slower giving the body time to use the energy instead of storing it as fat.
If you would like a list of common foods on the Glycemic Index just ask or visit glycemicindex.com and enter any food you eat.