The glycemic index, a value that aims to quantify how fast blood sugar rises after eating a given food, can vary by an average of 20% within an individual and 25% among individuals. That’s the surprising finding of a new study by scientists from the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts. The results, published in the American Journal of Clinical Nutrition, suggest the much-touted 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,” says lead study author Nirupa Matthan, PhD, a scientist in the HNRCA Cardiovascular Nutrition Laboratory. “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.”
VARYING NUMBERS: 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. Recently, however, glycemic index has also been used for broader purposes such as food labeling, and has served as the basis for several popular diets.
To study whether glycemic index values are accurate and reproducible, Matthan and her colleagues recruited 63 volunteers, who underwent six testing sessions over 12 weeks. Volunteers fasted and abstained from exercise and alcohol before each session. They then consumed either white bread – a simple carbohydrate that served as the test food – or a glucose drink, which served as a reference control, in random order. Each option contained 50 grams of available carbohydrate. Blood glucose levels were measured at multiple time points for five hours after eating, and glycemic index was calculated by standard formulas.
The average glycemic index value of white bread was 62, placing it in the “medium” category. But deviations averaged 15 points in either direction, effectively placing white bread in all three glycemic index categories. It would be considered a low glycemic index food (average values of 35 to 55) for 22 of the volunteers, medium (57 to 67) for 23 volunteers, and high (70 to 103) for 18 volunteers. Even for the same individuals, glycemic index values could differ by more than 60 points between trials.
METABOLIC RESPONSES: The variability in glycemic index values occurred despite sample sizes larger than required by standard calculations. The study cohort of 63 individuals far exceeded the 10 individuals used by typical glycemic index methodology, as did the six feeding challenges and five-hour blood glucose measuring window.
The team also tested for the influence of biological characteristics such as gender, body mass index, blood pressure and physical activity. Most factors had only a minor statistical effect on glycemic index variability. Blood insulin response as tested by insulin index and HbA1c, a measure of longer term glucose control, had the largest effect, accounting for 15% and 16% of the variability, respectively. That suggests glycemic index values are influenced by an individual’s metabolic responses to food.
IMPRACTICAL INDEX: The scientists note their findings do not suggest that a high glycemic index food may be healthy, or a low glycemic index food unhealthy. Both glycemic index and glycemic load – a value that adjusts glycemic index to serving size – reflect only foods containing carbohydrates, and no one eats food in isolation. The high variability of glycemic index and glycemic load reveals limitations in their clinical and public health applicability.
“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 adds. “If your doctor told you your LDL cholesterol value could vary by 20%, 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.”
“Reports frequently tout the benefits of choosing foods with low glycemic index and glycemic load values,” says senior study author Alice H. Lichtenstein, DSc, director of the laboratory. “Our data suggest those values may not be reliable in terms of a daily intake. A better approach to choosing foods is to consume a diet primarily composed of vegetables, fruits, whole grains, nonfat and low-fat dairy products, fish, legumes (beans) and lean meats, with preference to preparing food with liquid vegetable oils. Equally as important is to choose healthy foods and beverages you really enjoy.”
TO LEARN MORE: American Journal of Clinical Nutrition –