Glycemic Index: What is it?

The glycemic index is about foods high in carbohydrates. Foods high in fat or protein don't cause your blood sugar level to rise much.

A lot of people still think that it is plain table sugar that people with diabetes need to avoid. The experts used to say that, but the glycemic index shows that even complex carbohydrates, like baked potatoes, can be even worse.

When you make use of the glycemic index to prepare healthy meals, it helps to keep your blood sugar levels under control. This is especially important for people with diabetes, although athletes and people who are overweight also stand to benefit from knowing about this relatively new concept in good nutrition.

Recent studies of large numbers of people with diabetes show that those who keep their blood sugar under tight control best avoid the complications that this disease can lead to. Most experts agree that what works best for people with diabetes—and probably the rest of us as well—is regular exercise, little saturated or trans fat (partially hydrogenated oils), and a high-fiber diet.

The recommendations to exercise and eat more fiber and less saturated and trans fats is excellent advice—as far as it goes. The real problem is carbohydrates. The official consensus remains that a high-carbohydrate diet is best for people with diabetes. However, some of the experts, led by endocrinologists like Dr. Richard K. Bernstein, recommend a low-carbohydrate diet, because carbohydrates can raise blood sugar to dangerous levels.

But not all carbohydrates act the same. Some are quickly broken down in the intestine, causing the blood sugar level to rise rapidly. These carbohydrates have a high glycemic index.

Please note, however, that a GI value tells you only how rapidly a particular carbohydrate turns into sugar. It doesn't tell you how much of that carbohydrate is in a serving of a particular food. Four extensions of the glycemic index concept noted in the bibliography below address this limitation.

• One extension is called the glycemic load, which is the glycemic index of a food times its carbohydrate content in grams. Harvard School of Public Health professor and researcher Walter Willett, M.D., and his associates developed this concept as long ago as 1997, when they published journal articles on the subject. But it is only in their Harvard Women's Health Watch article and Dr. Willett's new book (see bibliography below) that they have published many of the GL numbers. For example, these resources have nice but very short lists of a few foods for which they have calculated the glycemic load (note in particular the high GI and low GL of watermelon). Now, however, Jennie Brand-Miller and her associates at the University of Sydney have calculated the GL of all 750 foods for which GI numbers have been calculated. This table was published originally in the July 2002 issue of the American Journal of Clinical Nutrition, and I publish an extract of that table on a Web page on this site as authorized by Professor Brand-Miller. She also published the table in the new version of her book, The New Glucose Revolution, (Marlowe, January 2003).

• Des Buchhorn similarly developed the adjusted carbohydrate exchange, which refines carbohydrate exchange lists by multiplying the glycemic index of a food by its carbohydrate content to indicate the likely effect of a food on blood glucose.

• J.A. Monro, a scientist at the New Zealand Institute for Crop & Food Research, developed the concept of relative glycemic potency, which compares equal weights of foods.

• Derek Paice, an engineer who has type 2 diabetes, similarly developed the concept of the substance glycemic index based on a fixed weight of foods.

Before the development of the glycemic index beginning in 1981, scientists assumed that our bodies absorbed and digested simple sugars quickly, producing rapid increases in our blood sugar level. This was the basis of the advice to avoid sugar, a proscription recently relaxed by the American Diabetes Association and others.

Now we know that simple sugars don't make your blood sugar rise any more rapidly than some complex carbohydrates do. Of course, simple sugars are simply empty calories, and still should be minimized for that reason.

Many of the glycemic index results have been surprises. For example, baked potatoes have a glycemic index considerably higher than that of table sugar.

This material is intended for informational purposes only and is not a substitute for the medical advice of your doctor or any other health care professional. Always consult with your physician if you are in any way concerned about your health.

 © 2003 SLPM Self-care