The glycemic index (GI) is a ranking of food, based according to the rate that the particular food's carbohydrate will raise blood sugar after the food has been consumed. A low glycemic index value is 55 or less, a medium glycemic index value is 56 to 69 and a glycemic index value 70 and above is high. These values are set against and compared to glucose which has a value of 100.
Glucose is a simple carbohydrate or sugar. Simple sugars are absorbed very quickly and they give can give the body a burst of energy since the simple sugar can make it into the blood stream very quickly. Typically, the more complex carbohydrates take longer to be utilized by the body and will have a low glycemic index. However, not all complex carbohydrates have low glycemic indexes - it depends on the chemical structure of the carbohydrate, the way the body processes it.
In general terms of glycemic indexes though, the less refined a food is and the more intact the grains are, the lower the glycemic index. For example, whole oats, long grain rice, pasta and whole grain breads have lower glycemic indexes than cakes and biscuits. Foods with low glycemic indexes help you to feel fuller for longer, and so these foods may help to control weight. In theory, they should reduce fluctuations in blood glucose levels.