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To read about the Tufts Research, see "Is the Glycemic Index Really Reliable?".
The take-home message from the Tufts study is that the GI value of white bread is 70. That's nothing new: The same value has been found in dozens of other studies around the world (1).
What does a GI value of 70 mean? It means that on any given day, an exchange of white bread will have approximately 70 percent of the glycemic impact of an exchange of glucose sugar.
The authors of the Tufts study erroneously concluded that the GI value of the very same bread varied from 44 to 132. That's not correct. The bread didn't vary: Its GI value remained 70. The GI is a property of the food; the person is simply the instrument of measurement. The Tufts researchers did not follow GI testing protocol, which is carefully designed to compensate for the variability inherent in using a person as a test instrument. As a result, all the Tufts study measured was the natural variation in individual responses to food at different times. Unfortunately, they mistook that for variation in the bread itself.
It's true that any one person's glycemic response varies from day to day and that glycemic response also varies from person to person. Nevertheless, the relative glycemic differences between foods are maintained. A person with diabetes can be confident that a high GI food (GI value of 70 or more) will produce a significantly higher glycemic response than a low GI food (GI value of 55 or less) 95 percent of the time. Just as the height of high tide and low tide varies from day to day and place to place, we still know that high tide will be higher than low tide on any one day at any one place. It's not the absolute level, but the difference, that's important.
Any one person's glucose tolerance may vary from day to day by as much as two-fold. (In people with diabetes, this variability is actually less than in normal subjects.) Why this occurs is not clear, but we can point to varying beta-cell responsiveness and insulin sensitivity, factors that are beyond anyone's control. The beta cells just work better on some days than on others. This variability among and within people must be managed carefully in order to detect true differences in the glycemic potential of the carbohydrates in different foods.
That's why GI testing has such a strict protocol. Ten subjects are used, each of whom is given the reference food (glucose) on three separate days. Each time, the overall fluctuation in their blood sugar is determined by testing their blood glucose eight times over a period of two hours. The findings from those three days of testing are averaged to find each person's usual response to the reference food, glucose. Next, his or her glycemic response to the test food is measured once, using the same two-hour testing protocol. Then each person's response to the test food is expressed as a percentage of their average response to the reference food. Finally, the relative responses of all ten subjects to the test food are averaged. This is the published GI value. The GI value of bread (70) means that the overall fluctuation in blood glucose after eating an exchange of white bread will be about 70 per cent of the effect of pure glucose (GI value of 100).
When it's done properly, there's nothing crude about GI testing. By taking the average of ten subjects, each of whom has undergone this painstaking process, we are simultaneously compensating for both within-subject and between-subject variability. If we were to test white bread over and over again using this protocol, we'd get the same result: a GI of 70.
The Tufts researchers did not satisfy the strict GI testing protocol. They repeatedly compared one test of the reference food (glucose) with one test of the test food (white bread). Inexplicably, they did this three times with three different groups of people. In a small sub-group of subjects who participated in all three groups (that is, who received the reference food on three occasions), the Tufts authors were able to calculate the GI according to the standard protocol. And lo and behold, they arrived at 70!
The Tufts study's within-subject variability was also heightened by the fact that it used venous sampling, not fingertip capillary sampling. Capillary testing is associated with much less variability than venous testing (2). We insist on capillary testing in the Australian Standard for GI testing (which is under review by the International Standards Organization).
The Tufts study uses the variability inherent in individual glycemic responses to criticize the GI. Were that a legitimate criticism, then that same variability could be used to denigrate carbohydrate counting as well. Can we be sure that fifteen grams of carbohydrate in white bread will always give half the glycemic response of thirty grams? No, we can't. It will also vary for the same reason: day-to-day within-subject variability. Yet carb counting is considered a cornerstone of good diabetes self-care.
Meta-analyses show that a diet based on low GI carbohydrate foods (compared to a conventional low fat diet) will reduce A1c's by an average of 0.6 of a point (3), about the same amount as a serious exercise program. Other meta-analyses show that low GI diets improve blood lipids (4) and weight control (5). Longer studies are still required, but long term prospective cohort studies indicate that diets with a low GI are likely to reduce the risk of chronic disease (6).
Professor Jennie Brand-Miller holds a Personal Chair in Human Nutrition at the University of Sydney. Her research focuses on all aspects of carbohydrates, including diet and diabetes, the glycemic index, and insulin resistance. She is Chair of the Nutrition Committee of the Australian Academy of Science, past president of the Nutrition Society of Australia, the Director of Sydney University Glycemic Index Research Service (a GI testing service for the food industry), and Chair of the Board of Directors of a non-profit company, Glycemic Index Limited, which administers a food symbol program for consumers in collaboration with Diabetes Australia and the Juvenile Diabetes Research Foundation. She has published over 200 journal articles and twenty books, including The New Glucose Revolution series, an international bestseller.
Oct 18, 2007
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