Is Glycemic Index an Unreliable Indicator of Blood Sugar Response?

Glycemic index (GI) is impractical for use in food labeling or in dietary guidance at the individual level, according to a new study.

“Even under highly standardized conditions and accounting for several potential methodological and biological variables that could affect glycemic index calculations, the range of individual glycemic index values for white bread in our study was so large it would be classified as having a low value for 22 individuals (GI: 35-55), intermediate for 23 individuals (GI: 57-67), and high for 18 individuals (GI: 70-103),” said lead study author Nirupa Matthan, PhD, a scientist in the Cardiovascular Nutrition Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston.
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The investigators determined the GI value for white bread by using standardized methodology in 63 volunteers free from chronic disease and recruited to differ by sex, age (18-85 years), and body mass index (20-35 kg/m²). Volunteers randomly underwent 3 sets of food challenges involving glucose (reference) and white bread (test food), both providing 50 g of available carbohydrates. They monitored serum glucose and insulin for 5 hours following ingestion and calculated GI values by using different area under the curve (AUC) methods. Researchers measured biochemical variables by using standard assays and body composition by dual-energy X-ray absorptiometry.

The mean ± SD GI value for white bread was 62 ± 15 when calculated by using the recommended method. Among the biological factors assessed, insulin index and glycated hemoglobin values explained 15% and 16% of the variability in mean GI value for white bread, respectively.

“Rather than focusing on the glycemic index, following basic principles of healthy eating—emphasizing whole grains, legumes, fruits and vegetables, fish, and vegetable oils and limiting red meat, full-fat dairy products, and beverages high in added sugars—along with portion control and exercise are better ways to manage and control blood sugar,” Dr Matthan said.

Future research will address the variability in GI values that could be potentially introduced by differences in macronutrient (carbohydrate, protein, fat) and fiber combinations and loads, as well as by “second-meal” effects and consumption of mixed meals, she said.

—Mike Bederka

Reference:

Matthan NR, Ausman LM, Meng H, Tighiouart H, Lichtenstein AH. Estimating the reliability of glycemic index values and potential sources of methodological and biological variability [published online September 7, 2016]. Am J Clin Nutr. doi:10.3945/ajcn.116.137208.