The glycemic index of a given food, a value that aims to quantify how fast blood sugar rises after eating that food, can vary by an average of 20 percent within an individual and 25 percent among individuals, report scientists from the Jean Mayer USDA Human Nutrition Research Center on Aging (USDA HRNCA) at Tufts University. In randomized, controlled, repeated tests involving 63 healthy adults, researchers found that individual blood sugar responses after consuming a fixed amount of white bread could range across all three glycemic index categories (low, medium, or high). Part of this variability could be attributed to insulin index and baseline HbA1c levels, which reflect long-term glucose control--evidence that glycemic index values are influenced by an individual's metabolic responses to food. The new study, published online in the American Journal of Clinical Nutrition on September 7, 2016 suggests that glycemic index has limited utility as a tool to predict how a food affects blood sugar levels. The article is titled “Estimating the Reliability of Glycemic Index Values and Potential Sources of Methodological and Biological Variability.” "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 lead study author Nirupa Matthan, Ph.D., a scientist in the Cardiovascular Nutrition Laboratory at the USDA HNRCA. "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.
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