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.
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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