The Glycemic Index
The glycemic index (GI) - along with its derived measure glycemic load (GL) - is a measure of the glycemic impact (resultant changes in blood sugar levels) of various foods.
These popular metrics now form the scientific basis of an ever-growing number of popular diets.
But how many dieters really understand what these two key indicators are, how they are derived, and how they should (and should NOT) be used by those seeking to control weight?
In this (2 page) section of our site we attempt to simplify the sometimes complex answers to these questions.
What Is The Glycemic Index?
The glycemic index is far different than the calorie - which, as used in diet context, is basically a measure of the energy content of foods.
Rather, the GI of a food is a measure of a food's capacity to quickly impact glucose (a sugar) levels in circulating blood.
This difference cannot be overemphasized in terms of weight loss applications; stable glucose levels do NOT mean one is losing weight, although there are other health benefits to stabilization.
Glycemic index is more precisely defined (in scientific/medical circles) as, "An indicator of the ability of different types of foods that contain carbohydrate to raise the blood glucose levels within 2 hours".
GI is expressed as a whole integer, and (theoretically at least) has a possible range of 0-100.
How Are GI Values Measured?
Glycemic index values are obtained from clinical studies conducted under controlled conditions.
These studies generally employ a set of standardized scientific protocols so that the results from different testing facilities may be used interchangeably.
Most commonly, a few (5-10) test subjects are each fed an amount of the "test food" that contains 50 g available carbohydrate.
Then, for the next 2 hours the blood glucose levels of each test subject are monitored.
After a suitable time interval, the same human subjects are each fed an amount of "reference food" (typically glucose solution) that also contains 50 g of carbohydrate. The resultant changes in blood sugar are again measured, as with the "test food".
A glycemic index value for each test food is then calculated by comparing the results of the "test food" procedure with those from the "reference food" procedure.
The most comprehensive and authoritative database on measured glycemic indices and loads was first published in 2002 (and since updated) by researchers at Sydney University's Glycemic Index Research Service, using high-quality data published in refereed scientific journals or unpublished values generated by the Sydney team itself. This data base is feely available online.
Interpreting GI Values: What Do They Tell Us?
Measuring GI values is relatively simple and straightforward (see above); interpretation of those values is not.
First, despite what you may have been led to believe, there is not uniform acceptance of what ranges of GI values should be considered "good" vs. "bad" in terms of glycemic impact. Alternate views exist.
Alternative 1: The US Government Panel
One broadly referenced set of GI value categories (low, medium, high) was established through consensus by a panel of half dozen or so select individuals convened by the US government some years ago:
Low GI = 0-55
Intermediate GI = 55-69
High GI = 70 or greater
This admittedly arbitrary GI classification scheme has since been blindly regurgitated as "the bible" on interpreting GI values by government publications and thousands of writers.
The sad part is that the very folks who continue to help promulgate this scheme have no idea why "55" should represent some sort of magic dividing line separating "low GI" (i.e., "good") foods from all others, nor apparently does anyone else.
Alternative 2: Michel Montignac
Not surprisingly, there are experts who do not agree with "Alternative 1" (above). Michel Montignac, an early pioneer of the use of the glycemic index in weight loss, feels that a "low glycemic index" threshold of 55 is too high.
According to Dr. Montignac, the GI ranking value system cited above simply does not correspond to "physiological reality", and was instead the result of the undue influence of the US food industry which did not want most of the products it marketed classified in the high GI range.
Montignac believes that a more realistic classification is instead:
Low GI = 35 or less
Intermediate GI = 35-50
High GI = greater than 50
So, which "scheme" should you use? Who knows? Take your pick. Experiment, and see which works best for you.
Second, glycemic index values tell us nothing about the nutritional value of a food.
For example a candy bar or cookie might have a low measured glycemic index (because of the high fat content), but contain high levels of saturated fats and a fair dose of ordinary table sugar, with few vitamins, minerals, fiber or other healthy nutrients.
Want to try a Snickers Diet? I hope not.
Use and Misuse of Glycemic Index
Low glycemic dieters should be ever mindful of the following:
1. Unlike the speed of light, GI values are NOT precise hard and fast numbers that one can measure repeatedly and get the same results. They are - and should be used - as approximations.
2. The glycemic index is a continuum subject to alternate interpretations - not a set of discrete values that inherently classifies foods as "good" or "bad" (in terms of glycemic impact, or otherwise).
3. The glycemic index tells us nothing about the nutritional quality or caloric "density" of a given food - only how a few test subjects responded to the carbohydrate content of that food.
4. Using glycemic index ALONE is NOT a smart way to choose your foods. Glycemic index values should only be used as only one of several guidelines in making informed decisions regarding food choices.