The Glycemic Index: Flogging a Dead Horse?

By Thomas M.S. Wolever, MD, PHD

SUMMARY: The glycemic index (GI) is a classification of foods based on their blood glucose-raising potential. The American Diabetes Association (ADA) has questioned the clinical utility of the GI and recommends that priority should be given to the amount rather than the source of carbohydrate. Some have interpreted this to mean that all carbohydrates have a nearly equal impact on blood sugar, and some feel that the GI is now a dead issue. Nevertheless, the reasons for questioning the clinical utility of the GI are unfounded because of the following: 1) they are based on studies of single test meals, which provide insufficient evidence on which to base dietary recommendations; 2) they are based on a faulty interpretation of the studies actually cited as evidence; 3) they take no account of better designed studies showing that the GI does apply in mixed meals; and 4) they take no account of studies showing that a low-GI diet improves overall blood glucose control in persons with diabetes. The GI is a valid and potentially useful concept, but is also deceptively complex. There are a number of unresolved problems and unanswered questions, and the appropriate place for the GI in patient education is not known. However, progress cannot be made without balance and objectivity.

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TEXT: The American Diabetes Association (ADA) position on the glycemic index (GI) is that priority should be given to the amount rather than the source of carbohydrate (1). At least some individuals have interpreted this to mean that all carbohydrate foods produce the same glycemic response; for example, an advertisement for booklets on carbohydrate counting published by the ADA gives the following reason for why carbohydrate counting should be taught: "Studies have proven that . . . all carbohydrates have nearly equal impact on blood sugar." I know from anonymous comments received from manuscript reviewers that some individuals feel that the GI is no longer an issue and that continued interest is "flogging a dead horse." However, the horse is not dead, and those who suggest that it is, are misinterpreting data, misquoting the literature, and misusing statistics.

One reason for the ADA not recommending the GI is the belief that it would make life more difficult for persons with diabetes by severely limiting food choices (3). There is no evidence for this. Following any kind of therapeutic diet requires discipline. Choices have to be made resulting in the use of smaller amounts of some foods and more of others. Being on a low-GI diet does not require elimination of all high-GI foods. Indeed, there are situations where high-GI foods may be appropriate or even desirable. The primary emphasis of low-GI diet advice is using more low-GI foods. Evidence from clinical trials suggests that on a low-GI diet, the diet variety actually increases. In our studies in subjects with diabetes, the only foods avoided on the low-GI diet, which were used on the high-GI diet, were ready-to-eat breakfast cereals, instant mashed potatoes, and polished rice. By contrast, the low-GI diet contained pumpernickel bread, beans, peas, lentils, bulgur, parboiled rice, spaghetti, barley, and oat bran, all of which were absent from the high-GI diet (4).

The other reason the ADA does not endorse the GI is because of doubts about its clinical utility, which were raised by the conclusion of a small group of studies that measured glycemic responses to a single meal or, in some two meals. Early on, it was pointed out by Coulston et al. (5) that it is not valid to make dietary recommendations on the basis of the results of studies with a single test meal, an assertion with which I agree (6). Therefore, it could be argued that the ADA's doubts about the GI should be discounted because they are not based on sound evidence. However, I have been drawn into the debate about the effect of the GI on glycemic responses to mixed meals because I feel the results of many studies have been interpreted wrongly. Indeed, there is good evidence that source of carbohydrate is one of the factors that influences the glycemic response of mixed meals in subjects with diabetes and also that low-GI diets improve overall glycemic control.

 
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