The glycemic index (GI) is an unreliable way to rate carbohydrate-containing foods in terms of their effect on blood sugar, according to a study in the American Journal of Clinical Nutrition. The GI has been promoted as a tool for healthy eating and serves as the basis of many popular diets.
In the study, 63 people ate about three ounces of white bread after fasting overnight; the Tufts University researchers then measured their blood sugar levels and calculated the food’s GI by a standard formula. This was done six times over 12 weeks.
The resulting GIs varied by an average of 25 percent among participants for the same exact food—and by 20 percent in the same individuals. Deviations averaged 15 points (on the 100-point scale) in either direction, in effect putting white bread in all three GI categories (low, medium, high) at various times and thus “demonstrating that [the GI] is unlikely to be a good approach to guiding food choices,” the researchers concluded.
For more about the GI, see A Guide to the Glycemic Index.