The Glycemic Index: What You Need To Know
The glycemic index (GI) was developed as an indicator of the comparative ability of different food types to raise our blood sugar (glucose) levels shortly after consumption.
Why is this information useful? Because certain kinds of foods (types of carbohydrates) are capable of causing large and rapid "spikes" in circulating glucose levels. If uncontrolled, this "roller coaster" effect can have adverse health impacts.
Knowing which foods are likely to have such an effect helps us control our diet so as to maintain blood sugar levels within that fairly narrow range at which our bodies function properly. And that is where the utility of the glycemic index lies; that is why it was developed.
How Is Glycemic Index Measured?
Glycemic index values are obtained from clinical studies of human test subjects, conducted under controlled conditions.
The subjects are fed a measured amount of the "test food", and blood glucose levels are carefully monitored for the next 2 hours. Some time later, the same subjects are fed an amount of "reference food" (typically glucose solution) containing the same amount of carbohydrate as the "test food".
The glycemic index value for each test food is then calculated by comparing the averaged results of the "test food" and "reference food" trials (the area under the "test food" curve is divided by that under the "reference food" curve (see Figure 1).
GI is expressed as a whole integer ranging from 0-100. The most comprehensive and authoritative database on measured glycemic indices is published and maintained by researchers at Sydney University; it is freely available online.
GI Values: What Do (and Don't) They Tell Us?
To productively use the glycemic index as a guide to making food choices, it is absolutely ESSENTIAL to understand that published GI values of foods are simply rough approximations ("indicators") of the relative glycemic impact of different foods on the "average" human.
They are not - and should never be used as - precise measures of the glycemic impact that different food choices will have on YOU. It is decidedly unproductive (and unscientific) to include - or exclude - particular foods from your diet solely on the basis of a few points difference in publiched glycemic index values.
Also, despite what you may have been led to believe, there is by no means uniform acceptance among "experts" of what ranges of GI values should qualify carbohydrates as "good" vs. "bad" in terms of glycemic impact. Alternate views exist.
Alternative 1: The US Government Panel Scale
The most 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. The panel contained representation by the commercial food industry. The admittedly arbitrary GI classification scheme emerging from this panel has since been blindly regurgitated as "the bible" on GI values, both in government publications and the efforts of thousands of writers.
Low GI = 0-55
Intermediate GI = 55-69
High GI = 70 or greater
So, while foods with a GI below 55 are now widely touted as "low glycemic", the sad truth is that none the very folks who continue to help promulgate this scheme can give you a research-based explanation of why "GI=55" should represent some sort of magic dividing line between "good" and "bad" carbs.
Alternative 2: The Montignac Scale
Not surprisingly, there are experts who do not agree with "Alternative 1" (above). Michel Montignac, an early European 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.
Instead, Montignac believes that a more realistic, science-based GI scale is as follows:
Low GI = 35 or less
Intermediate GI = 35-50
High GI = greater than 50
So, which "scheme" should you use? That's up to you of course. Opinions differ, and no two people have exactly the same metabolisms, genetic makeup, activity patterns and such. That said, I have found that (at least for myself) the Montignac scale seems far more realistic and delivers more productive results in terms of weight control and general health.
Glycemic Index values and Good Nutrition
Finally, keep in mind that the GI vlaue of a food tells us NOTHING about nutritional quality or caloric "density".
For example a candy bar or cookie might have a low glycemic index (because of very high fat content), but contain lots of calories along with high levels of saturated fats and a large dose of table sugar, with few healthy nutrients.
Clearly then, using glycemic index ALONE is NOT a smart way to choose your foods.
Summary: Use and Misuse of Glycemic Index
The key "take away" points from this discussion are:
- GI values are approximations - NOT precise measures - of glycemic impact. Your reaction to a particular food (in terms of glycemic impact) can be expected to vary with such factors as age, activity levels, time of day, and other foods consumed.
- Broad classes of foods cannot readily be assigned a single GI value - the glycemic index of potatoes, bananas, etc. will vary according to variety, stage of ripeness, manner of preparation and cooking, and other such information.
- Glycemic index values alone should never be used as the basis for choosing your diet - other information (i.e., nutrition) is also essential to making healthy food choices.
- The published glycemic index values of foods are ONLY indicators of the impact of that food when eaten ALONE; the glycemic impact of foods taken in combination (meals) is generally unknown.
- The glycemic index alone tells us NOTHING about the comparative impacts on blood sugar of "normal" portions of different foods. To derive that key piece of meal planning information an alternate "indicator" - called "glycemic load" - was developed (see next page).
Updated April 15, 2014