New study casts doubt on reliability of glycemic index as an indicator of blood sugar response

USA — The glycemic Index, a value that aims to quantify how fast blood sugar rises after eating it, may not be a reliable indicator blood sugar response, according to a new study by scientists at the Jean Mayer USDA Human Nutrition Research Center on Aging. 

The scientists conducted controlled, repeated tests involving 63 healthy adults and found out that blood sugar responses after consuming a fixed amount of white bread could range across all three glycemic index categories (low, medium, or high). 

The team found that the average glycemic index value of white bread for the study population was 62, placing it in the category of a “medium” glycemic index food. 

However, deviations averaged 15 points in either direction, effectively placing white bread in all three glycemic index categories.  

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It would be considered a low glycemic index food (average values of 35 to 55) for 22 of the volunteers, intermediate glycemic index (57 to 67) for 23 volunteers, and high glycemic index (70 to 103) for 18 volunteers.  

What’s even casts more doubt on GI’s effectiveness is that even within the same individual, glycemic index values could differ by more than 60 points between trials. 

Part of this variability could be attributed to insulin index and an individual’s baseline HbA1c levels (amount of sugar attached to haemoglobin). 

Therefore, an individual’s metabolic responses to food has a greater bearing on glycemic index values of the food they eat and cannot reliably be used as a standard measure of the GI of foods. 

Developed as a way to help diabetic individuals control their blood sugar, glycemic index is intended to represent the inherent effect a food has on blood sugar levels.  

However, glycemic index is becoming used for broader purposes such as food labeling and has served as the basis for several popular diets. 

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The study, published in the American Journal of Clinical Nutrition on Sept. 7, suggests glycemic index has limited utility as a tool to predict how a food affects blood sugar levels. 

“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., 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.” 

In the concluding remarks, the authors note their findings do not suggest that a high glycemic index food may be healthy, or that a low glycemic index food is unhealthy.  

What the study reveals however is that the high variability of glycemic index and glycemic load indicate limitations in their clinical and public health applicability, and suggest that glycemic index estimates and subsequent associations with chronic disease risk be reconsidered.

Influenced by insights from the study, Senior study author Alice H. Lichtenstein now advises that a better approach to choosing foods is to consume to choose healthy foods and beverages you really enjoy. 

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