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Glycemic Index: The Big Picture


I have promised for several months to write more about the infamous Glycemic Index. To prepare for this discussion, I went back and followed the GI family tree to share with you.

Before the 1980's, physicians and dieticians who were working with diabetic patients recommended a certain number of "carbohydrate" exchanges. These would include such things as bread, rice, potato, cereal, etc. Little consideration was given to ANY DIFFERENCE that the starches might have in the body.

In 1981 David Jenkins published an article in the American Journal of Clinical Nutrition describing different blood sugar effects of certain selected starches. He gave 34 men and woman 50 gram servings of different starches and measured the effect on their blood sugar. He called the results the Glycemic Index.

The foods with the lowest GI were legumes (lentil/29 and soy/15). The highest GI went to parsnips at 97 and potatoes weighed in at 70. The foods were eaten alone rather than as part of a complete meal. Scientists continued to explore the usefulness of the GI.

In 1993, David Trout published a study which discussed the complexity of measuring the Glycemic Index. He found that a number of variables have an effect on the scores. Things like the source of the starch (legume, grain or tuber), the species of the starch, the way it was cultivated, the cooking methods, and the degree of ripeness all contributed variation. In addition, the reduction of the starch particle size before, during and after cooking all affected the GI.

The majority of the studies (approximately 2/3) were being done with diabetics, and no studies were being done to compare the effect of the same foods, cooked in the same way, on diabetic compared to normal individuals. No discussion of gender differences was done.

In 1995, Kaye Foster-Powell published the International Tables of Glycemic Index in which she reviewed and compiled the 73 studies which had been done on the subject. Her compilation was partially funded by the Australian Sugar industry. She found great variations for the same foods and in some cases could offer no explanation. For example, porridge (i.e. oatmeal) varied from as low as 42 to a high of 75. Significantly, she noted that as particle size decreases, the GI increases.

She identifies the concern that some foods have been rated as "good" or "bad" on the basis of their GI. As you know, several recent authors have suggested that potatoes are "bad" because they have a high GI. However, the GI was developed for use in diabetic diets and was never intended to be used in isolation.

The total amount of the carbohydrate eaten, the amount and type of fat, and the fiber content all affect absorption rate and the impact on the blood sugar. Where does this leave us? I think the guideline of eating brown things rather than white things is useful.

A bagel scores 72 in the GI and lentils score 29. That one is pretty obvious. And remember that we are wanting to elicit an insulin response in the evening while we are sleeping. We want our blood sugar to rise so our bodies will produce insulin to help the transport of tryptophan into our brains to make serotonin.

If you choose a carbohydrate which is too slow, (i.e. with a very low GI), you will not get the desired effect. Putting butter on your potato seems to slow it to an ideal intervention. And, lastly, but quite significantly, potatoes scored the very highest in a test measuring the satisfaction index of certain starches.

I still love Mr. Spud. And, if he doesn't work for you, choose an alternative that helps you feel better.

* Glycemic Index of foods: a physiological basis for carbohydrate exchange. Jenkins, D., et al. Am J Clin Nut, 34: MARCH 1981.pp. 362-366
* Prediction of glycemic index for starchy foods. Trout, D, Behall, KM and Osilesi, O., Am J Clin Nut, 1993;58.873-8.
* International tables of glycemic index. Foster-Powell, K and Miller, Am J Clin Nut, 1995:62:871S-93S.



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