The Glycemic Index—A Little Bit of History
Before 1981, all patients diagnosed with diabetes were given dietary exchanges to follow when planning their meals or snacks. While exchanges were formulated for all food groups, the main focus for glycemic control was on carbohydrates. At the time these guidelines were established, focusing on portions seemed appropriate since the Nutrition Facts Label was not available.
Prior to 1994, patients were also instructed to avoid simple carbohydrates like sugar, which were considered to cause a rapid rise in blood glucose levels, and not to be overly concerned about eating complex carbohydrates like starches, because these were not believed to grossly interfere with good blood glucose control. As long as the patient used the exchanges, avoided sugar and followed portion recommendations, he or she was using the diet properly to help manage blood glucose control.
But that wasn’t always what happened.
Different Carbs Cause Different Results
David Jenkins, MD, PhD, a professor of nutrition at the University of Toronto, Canada, decided to find out why.
He challenged the assumption promoted by the exchange system that all simple carbohydrates caused a rapid rise in blood glucose levels and all complex carbohydrates released glucose more slowly into the blood.
Jenkins set out to investigate exactly what happens when the body digests carbohydrates. He did this in the most logical way he could think of—by actually giving a predetermined amount of some commonly consumed carbohydrate foods to his subjects to eat and recording their resulting blood glucose levels after a designated period of time. There would be no assumptions, just sound data.
What Jenkins and his colleagues discovered was astounding. They observed that in equivalent quantities, carbohydrates in white bread (a complex carbohydrate) elevated their subjects’ blood glucose more than those in ice cream, despite its sugar (a simple carbohydrate) content. This seemed to fly in the face of the exchange system and the conventional wisdom of the time.
The Glycemic Index
As other scientists began testing more carbohydrate foods using the same methods, the results were tabulated and slowly were compiled into what we today call the glycemic index (GI).
A review of the nearly 700 foods currently listed in the glycemic index underscores the limitations of the exchange concept; the amount of carbohydrate in two similar foods may be the same, but their impact on blood glucose levels can be very different.
We now know, for example, that spaghetti and peas fall low on the index, while mashed potatoes and sticky rice are rated as high. Yet they’re all complex carbohydrates.
GI values do not define carbohydrates by their chemical structure (as in “simple” or “complex”) but rather by what those foods do to blood glucose levels in the body (quick versus slow glucose release). Jenkins (and his followers) made sure of that important distinction.
The Glycemic Index ranks carbohydrate foods on a scale of 0 to 100 based on their impact on BG levels after consumption. The higher the glycemic index, the more rapid the spike in BG levels.
|Low: 0 to 55|
|Baked beans, canned||48|
|Intermediate: 56 to 69|
|High: 70 or more|
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