When you eat a food, your blood sugar level rises. The food that raises blood sugar the highest is pure table sugar. Glycemic index is a ratio of how high a particular food raises blood sugar in comparison to how high table sugar raises blood sugar levels. Foods whose carbohydrates break down slowly release glucose into the bloodstream slowly, so blood sugar levels do not rise high and therefore these foods have low glycemic index scores. Those that break down quickly cause a high rise in blood sugar and have a high glycemic index.
All foods are made up of carbohydrates, fats and protein. Carbohydrates are made up of sugars in singles, doubles and combinations of sugars up to thousands (starch) and millions (fiber). Only single sugars can be absorbed into your bloodstream. You cannot absorb even a double sugar.
A high rise in blood sugar causes sugar to stick to the outer membranes of cells and damage the cell to increase risk for diabetes, heart attacks, strokes, cancers, and premature death. The higher the rise in blood sugar, the greater the cell damage.
All foods can be classified by how high they raise your blood sugar after you eat them. This is called Glycemic Index, that measures the rate that carbohydrate-containing foods raise a person's fasting level of blood sugar and need for insulin. Sugary drinks, all foods made from flour (breads and pastas), and other processed carbohydrates cause the highest rises in blood sugar and therefore the highest glycemic indexes. The lowest rises in blood sugar include foods that are not processed. Most beans, whole grains and non-starchy vegetables have low glycemic index.
However, if you try to use Glycemic Index tables to guide your choice of foods, you will see things that should bother an intelligent person. A carrot has almost the same glycemic index as sugar. That is ridiculous. You know that a carrot is a far wiser choice for dieters or diabetics than table sugar.
To deal with this conflict, researchers developed a new measure to rank foods called Glycemic Load (GL). To calculate glycemic load, the grams of carbohydrate in a serving of food are multiplied by that food's glycemic index/100.
Carrots and potatoes both have a high glycemic index, but using the GL index, carrots dropped from high GI of 131 to a GL of 10. Potatoes fall from a GI of 121 to a GL of 45. Air-popped popcorn, with a glycemic index of 79, has a GL of 4.
Foods that are mostly water or air will not cause a steep rise in your blood sugar even if their glycemic index is high. That's why Glycemic Load is more useful. However, all of these tools should be used for research and not for your daily selection of foods. Use your own common sense and eat plenty of fruits, vegetables, whole grains, beans and other seeds. Limit refined foods, particularly sugared drinks, foods made with any type of added sugar, foods made from any type of flour, or milled grains such as white rice.
Recommended reading: Eat, Drink, and Be Healthy: The Harvard Medical School Guide to Healthy Eating (Simon & Schuster, June 2001), by Harvard School of Public Health professor and researcher Walter Willett, M.D.