Study questions reliability of glycemic index

by Jeff Gelski
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Glycemic index foods
Determining the glycemic index value of food items is unlikely to be a good approach to guiding food choices.

BOSTON — Determining the glycemic index value of food items is unlikely to be a good approach to guiding food choices, according to the conclusion of a study appearing on-line Sept. 7 in The American Journal of Clinical Nutrition.

Scientists from Tufts University in Boston conducted randomized, controlled, repeated tests involving 63 healthy adults. They found the glycemic index of a given food may vary by an average of 20% within an individual and 25% among individuals. White bread and a glucose drink were used in the study.

“Glycemic index values appear to be an unreliable indicator even under highly standardized conditions and are unlikely to be useful in guiding food choices,” said Nirupa Matthan, Ph.D., lead study author and a scientist in the Cardiovascular Nutrition Laboratory at the Jean Mayer U.S.D.A. Human Nutrition Research Center on Aging at Tufts University. “If someone eats the same amount of the same food three times, their blood glucose response should be similar each time, but that was not observed in our study. A food that is low glycemic index for you one time you eat it could be high the next time, and it may have no impact on blood sugar for me.”

The glycemic index (G.I.) measures how a carbohydrate-containing food raises blood glucose, according to the American Diabetes Association, Alexandria, Va. Because the type of carbohydrate may affect blood glucose, using the G.I. may be helpful in “fine-tuning” blood glucose management, according to the association.

“These criticisms of G.I. because of between-individual and within-individual variation in glucose response are nothing new, but they don’t discount the value of the G.I. as a characteristic of a food,” said Jennie Brand-Miller, a professor of human nutrition at the University of Sydney in Sydney, Australia, and director of the Glycemic Index Foundation, a not-for-profit health promotion charity. “It’s still useful to distinguish between high G.I. and low G.I. foods.

“The best everyday analogy is high tide and low tide. The height of the tides varies from bay to bay (place to place) and from day to day, but on any given day, it’s still useful to distinguish between high tide and low tide. Similarly, on any given day, a low G.I. meal will give us a lower blood glucose profile than a high G.I. meal.”

Glycemic index foods
The glycemic index measures how a carbohydrate-containing food raises blood glucose.

Grants from the National Institute of Diabetes and Digestive and Kidney Diseases, the National Center for Advancing Translational Sciences, and the U.S. Department of Agriculture supported the Tufts University study.

The 63 adults in the study underwent six tasting sessions over 12 weeks. They fasted and abstained from exercise and alcohol before each session. In random order they then consumed either white bread, which served as a test food, or a glucose drink, which served as a reference control. The bread and the drink each contained 50 grams of available carbohydrates. Blood glucose levels were measured at multiple time points for 5 hours after eating. Standard formulas calculated the glycemic index.

The average glycemic index value for white bread was 62. Deviations averaged 15 points in either direction, however. Within the same person, G.I. values could differ by more than 60 points between trials.

Alice Lichtenstein, TUFT
Alice H. Lichtenstein, senior scientist and director of the Cardiovascular Nutrition Laboratory

“Reports frequently tout the benefits of choosing foods with low glycemic index and glycemic load values,” said senior study author Alice H. Lichtenstein, senior scientist and director of the Cardiovascular Nutrition Laboratory. “Our data suggest those values may not be reliable in terms of a daily intake. A better approach to choosing foods is to consume a diet primarily composed of vegetables, fruits, whole grains, non-fat and low-fat dairy products, fish, legumes (beans), lean meats with preference to preparing food with liquid vegetable oils, and equally as important, to choose healthy foods and beverages you really enjoy.”

The researchers said the study’s findings do not suggest a food with a high glycemic index may be healthy or that a food with a low glycemic index may be unhealthy. They said glycemic index estimates and subsequent associations with chronic disease risk need to be reconsidered.

“Based on our results, we feel strongly that glycemic index is impractical for use in food labeling or in dietary guidelines at the individual level,” Dr. Matthan said. “If your doctor told you your L.D.L. cholesterol value could vary by 20%, it would be the difference between being normal or at high risk for heart disease. I don’t think many people would find that acceptable.” 

David J.A. Jenkins, M.D., Ph.D., C. Sc., led a University of Toronto team that invented the glycemic index in 1981. Today he is a professor in the Department of Nutritional Sciences at the University of Toronto.

“I think that Alice (Lichtenstein) and her group have done a good job in identifying biological variation to acute test meals,” Dr. Jenkins said of the Tufts University study. “However, this is well-known, and I'm sure was no surprise to Alice and her group.

“Differences in response within and between individuals occur to both diet and drugs, and I think we all have to be aware of this fact. Some blood chemicals are more tightly controlled than others, and fasting levels tend to have less variability than post-meal excursions, like the glucose rise.”

He said research from Thomas M.S. Wolever, D.M., Ph.D., a professor in the Department of Nutritional Sciences at the University of Toronto, has shown beans and barley always will have a lower G.I. than bread even if included in a mixed meal.

“For me, the real issue is how low does the G.I. of a food have to be to make it useful to add to therapeutic diets for diabetes complications and cardiovascular risk reduction?” Dr. Jenkins said. “Can you make a low G.I., higher vegetable protein bread we can give to patients?”

Dr. Jenkins said he and his colleagues are planning studies to address such questions.

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