Winter 2017

Volatile Index

Glycemic measurement of carbs may not be the final word on blood sugar.

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Illustration: Kevin Whipple

Big swings in your blood sugar level can lead to insulin resistance and diabetes, so eating in a way that keeps your blood sugar stable is a healthy goal. But a recent study found that the glycemic index, a long-consulted ranking of how quickly carbohydrate-containing foods raise blood sugar levels, may not be as reliable as we thought.

In repeated tests involving 63 healthy adults, researchers found that different people who ate the same amount of white bread could have different blood sugar levels, with values varying an average of 25 percent. White bread, long considered a high-glycemic food, ranked anywhere from low to high on the index. Even for the same person eating the same amount of bread at different times, the values varied by an average of 20 percent.

The study, published in the American Journal of Clinical Nutrition, suggests the glycemic index isn’t that useful.

“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, a scientist in the Cardiovascular Nutrition Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts.

“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 could be high the next time, and it may have no impact on blood sugar for me.”

The researchers, including senior author Alice H. Lichtenstein, director of the HNRCA lab and the Gershoff Professor at the Friedman School, found that gender, body-mass index, blood pressure and physical activity seemed to account for only a small amount of the variability. Bigger factors were the participants’ blood levels of insulin and glycated hemoglobin, both measures of the body’s ability to manage sugar—and evidence that glycemic index values are influenced by an individual’s metabolism.

The glycemic index was developed to help people with diabetes control their blood sugar, but the index went on to be used for food labeling and is a centerpiece for popular diets, including South Beach and the Zone.

“Based on our results, we feel strongly that glycemic index is impractical for use in food labeling or in dietary guidelines at the individual level,” Matthan said. “If your doctor told you your LDL cholesterol value could vary by 20 percent, it would be the difference between being normal or at high risk for heart disease. I don’t think many people would find that acceptable.”

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