Glycemic Index

Your body performs best when your blood sugar is kept relatively constant.

If your blood sugar drops too low, you become lethargic and/or experience increased hunger.

And if it goes too high, your brain signals your pancreas to secrete more insulin.

Insulin brings your blood sugar back down, but primarily by converting the excess sugar to stored fat.

Also, the greater the rate of increase in your blood sugar, the more chance that your body will release an excess amount of insulin, and drive your blood sugar back down too low.

Therefore, when you eat foods that cause a large and rapid glycemic response, you may feel an initial elevation in energy and mood as your blood sugar rises, but this is followed by a cycle of increased fat storage, lethargy, and more hunger!

Although increased fat storage may sound bad enough, individuals with diabetes (diabetes mellitus, types 1 and 2) have an even worse problem.

Their bodies inability to secrete or process insulin causes their blood sugar to rise too high, leading to a host of additional medical problems.

The theory behind the Glycemic Index is simply to minimize insulin-related problems by identifying and avoiding foods that have the greatest effect on your blood sugar.

For non-diabetics, there are times when a rapid increase in blood sugar (and the corresponding increase in insulin) may be desirable.

For example, after strenuous physical activity, insulin also helps move glucose into muscle cells, where it aids tissue repair.

Because of this, some coaches and physical trainers recommend high-GI foods (such as sports drinks) immediately after exercise to speed recovery.

Also, it's not Glycemic Index alone that leads to the increase in blood sugar.

Equally important is the amount of the food that you consume.

http://nutritiondata.self.com/topics/glycemic-index

Classification	GI range	Examples
Low GI	55 or less	fructose; beans (white, black, pink, kidney, lentil, soy, almond, peanut, walnut, chickpea); small seeds (sunflower, flax, pumpkin, poppy, sesame, hemp); most whole intact grains (durum/spelt/kamut wheat, millet, oat, rye, rice, barley); most vegetables, most sweet fruits (peaches, strawberries, mangos); tagatose; mushrooms; chilis
Medium GI	56–69	white sugar or sucrose, not intact whole wheat or enriched wheat, pita bread, basmati rice, unpeeled boiled potato, grape juice, raisins, prunes, pumpernickel bread, cranberry juice, regular ice cream, banana
High GI	70 and above	glucose (dextrose, grape sugar), high fructose corn syrup, white bread (only wheat endosperm), most white rice (only rice endosperm), corn flakes, extruded breakfast cereals, maltose, maltodextrins, sweet potato, white potato, pretzels, bagels