When calculating glycemic index (GI) values, glucose is arbitrarily given the highest GI value: 100. To assign a GI value to another type of carb, a complex process is used to compare the blood sugar response elicited by the test carb to the blood sugar response provoked by glucose.

Then the test carb is assigned a GI value somewhere below 100, based upon its overall effect on blood glucose in comparison to pure glucose.

Previous studies have assigned white bread an average GI value of about 70, indicating that white bread has approximately seventy percent of the glycemic impact of pure glucose. But now a study of individual blood glucose responses to white bread has raised questions about how reliable that number is.

In the study, performed by researchers from Tufts University, twenty-three healthy adults were seen for up to three sets of testing, each set consisting of two visits. During one visit, they were given fifty grams of carbs in the form of white bread, and their blood sugar response to the bread over a two-hour period was measured.

On the second visit, their blood sugar response to fifty grams of glucose was measured. By using a calculation to compare their response to each food, the GI value of the bread was generated.

By averaging the GI values derived from the participants who completed all three sets of tests, this study produced a GI value of about 70 for the bread, very close to the previously published value. However, when the researchers looked at each individual's response to the white bread on different days, they found GI values for the bread that ranged from 44 to 132.

The between-individual variation in blood glucose responses was less, about 17 percent, but the within-individual variation was about 43 percent. The researchers believe that their findings cast some doubt on the utility of the glycemic index.

Sources: Medpage Today, Diabetes Care, September 2007