The "Glycemic Index" concept was originally developed a team of scientists lead by Dr. David Jenkins at the University of Toronto in 1981; it is now gaining widespread acceptance as part of a dietary strategy.

It was written about back in 1989 in Michel Montignac's book, "Dine Out and Lose Weight", and has since spawned several variants ("Sugar Busters", "The G.I. Factor", "The New Glucose Revolution", "The G-Index Diet" ... the list goes on).

The Glycemic Index of a food is the measure of the rise in the level of glucose that occurs in your bloodstream, after that food is ingested.

When you eat foods with a high glycemic index, your blood sugar levels shoot through the roof.

This causes your pancreas to secrete insulin, which is your body's way of getting your blood sugar level back to normal.

However, the secretion of this insulin:

Furthermore, as foods with a low glycemic index ("G.I.") are absorbed more slowly, the calories from the food you eat are more likely to be burned throughout the day as energy, rather than stored as fat.

In fact, studies have shown that even when calorie intake is the same, you can lose more weight eating low G.I. foods rather than high G.I. foods.

The Glycemic Index of a food is derived by comparing the rate of digestion to that that food, with the rate of digestion of pure glucose. Glucose* is assigned a Glycemic Index of 100, and the tested food is charted against this standard.

* glucose if the most widely accepted reference food, however some other systems use white bread instead.

Foods with a high Glycemic Index (70 and above) are those that break down quickly and cause a spike in blood sugar levels.

Foods with a low Glycemic Index (55 and below) break down more slowly and steadily, resulting a more sustained supply of energy.

Following is two other explaniations of Glycemic index:

1. The extent to which a carbohydrate increases blood glucose, and therefore insulin, in the bloodstream can be quantified by a number called the Glycemic Index. A higher glycemic index translates to a faster and more dramatic response. A lower glycemic index indicates a more gradual response. For example, the ingestion of glucose (glycemic index=100) quickly invokes the dramatic chain of reactions described above. On the other hand, the ingestion of fructose (glycemic index=23) results in a more prolonged response because its glycemic index is much lower. During exercise, high glycemic index carbohydrates help maintain blood sugar levels, while low glycemic index carbohydrates do not because the response is not dramatic enough. Prior to exercise, high glycemic index carbohydrates have the potential to induce a poorly timed state of hypoglycemia, while low glycemic index carbohydrates do not. Hence the importance of timing.

2. Quick biochemistry lesson: The glycemic index is a system whereby all carbohydrate-containing foods are ranked by number. The higher the number, the faster the food empties from the stomach into the small intestine, where it's broken down to blood sugar, or glucose. And thus, the greater the surge of sugar from the intestine into the bloodstream. Some scientists believe this "surge" can lead to a number of health problems:

2a.Overweight: When blood sugar spikes dramatically in the bloodstream, there's a matching spike in the hormone insulin to remove that sugar from the blood. That means glucose empties from the bloodstream relatively quickly, which, the theory goes, causes hunger to return more quickly and more calories to be eaten over the course of the day than if glucose left the bloodstream more gradually.

2b. Diabetes: Insulin is made in the pancreas, and the thinking is that if the pancreas is overworked to keep secreting large insulin loads, it eventually will give out, or at least lose efficiency in its insulin delivery. And the bloodstream, in turn, will chronically have more sugar in it than it should—the hallmark of diabetes.

2c. Heart disease: Studies have indicated that secreting a lot of insulin is associated with higher levels of blood fats called triglycerides and lower levels of "good" HDL-cholesterol, which helps keep arteries unclogged.

2d. Consistent with that line of research, findings from Harvard's Nurses' Health Study have found that among some 75,000 women, those who followed diets with the lowest glycemic load (the glycemic index of a food multiplied by the number of carbohydrate grams it contains) were less likely to develop heart disease over a 10-year period than their high-glycemic-load counterparts

Source for Glycemic index chart taken from http://www.southbeachdietguide.com/gi-charts.html

A complete table containing Glycemic Index values for thousands of foods can be viewed at the American Journal of Clinical Nutrition website.

For you convenience, a table of common foods appears below: