Alice Lichtenstein, DSc, corresponding author and director of the Cardiovascular Nutrition Laboratory at the USDA HNRCA and colleagues assessed 14 study participants' glycemic response to 50 grams of carbohydrate in the form of white bread (test food) and glucose dissolved in water (control food) on different days. This experiment was repeated three times with each individual.
"Using glucose as the control food, previous studies indicate that white bread has a glycemic index of about 70," says Lichtenstein, who is also the Gershoff professor of nutrition science and policy at the Friedman School of Nutrition Science and Policy at Tufts. "In our study the combined average was 71, virtually identical to the published value. However, quite strikingly, individual values ranged from 44 to 132. What is critical is to determine why there is such a wide range of responses among individuals.
In addition, within the same individual, test values varied by as much as 42 percent. "These results show that perhaps using glycemic index for groups is a reasonable indicator to predict chronic disease risk, but there is still considerable uncertainty when applying glycemic index to individuals," explains Lichtenstein.
Glycemic index is a scale applied to foods based on how quickly the glucose in foods is absorbed into the blood stream, relative to pure glucose. Some nutrition professionals use the glycemic index as a tool for people trying to control blood sugar, such as those with diabetes. Others use the mean glycemic index of diets to predict chronic disease risk in large groups of people. Potential confounding factors, such as the fiber or fat content of the food, are not directly factored into the calculations.
"There are many factors that can influence the glycemic index of a food," says Lichtenstein. "For example, a piece of white bread may have a high glycemic index but, if a person eats a slice of turkey and cheese with that bread, the effect of the multiple foods may result in a different glycemic index than if that person had eaten the white bread alone. Since most food is consumed as combinations during meals and snacks, there is a need to assess the significance of using glycemic index values determined on individual foods for food mixtures. Similarly, it is important to know whether the food consumed prior to a meal or snack alters subsequent glycemic response.
It is possible that we need to develop better research tools and more stringent applications for glycemic index determinations," she says. "Larger studies of diverse populations are needed to determine why inter-individual, and particularly intra-individual, glycemic index values are so variable. If we can identify the source of the variability, it will allow for more insight into the applications of the glycemic index as a tool for both researchers and in public health messages."
Lichtenstein and colleagues have received a five-year grant from the National Institute of Diabetes and Digestive and Kidney Diseases to further their understanding of the glycemic index and its utilities. The current study was supported by the U.S. Department of Agriculture's Agricultural Research Service.