Nav: Home

High variability suggests glycemic index is unreliable indicator of blood sugar response

September 07, 2016

BOSTON (September 7, 2016)--The glycemic index of a given food, a value that aims to quantify how fast blood sugar rises after eating it, 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 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."

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.

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 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.

An unreliable guide

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. 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 . Even within the same individual, glycemic index values could differ by more than 60 points between trials.

"Reports frequently tout the benefits of choosing foods with low glycemic index and glycemic load values. 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), lean meats with preference to preparing food with liquid vegetable oils, and equally as important, to choose healthy foods and beverages you really enjoy," said senior study author Alice H. Lichtenstein, D.Sc., senior scientist and director of the Cardiovascular Nutrition Laboratory at the USDA HNRCA. Lichtenstein is also the Gershoff Professor at the Friedman School of Nutrition Science and Policy at Tufts.

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: sex, body-mass index, blood pressure, physical activity, and several others. Most factors had only a minor statistical effect on glycemic index variability. Blood insulin response as measured by insulin index and HbA1c, a measure of longer term glucose control, had the largest effect, accounting for 15 and 16 percent of the variability, respectively.

The authors note their findings do not suggest that a high glycemic index food may be healthy, or that a low glycemic index food unhealthy. Both glycemic index and glycemic load--a value that adjusts glycemic index to serving size--reflect only food containing carbohydrates, and no one eats food in isolation. The high variability of glycemic index and glycemic load reveal limitations in their clinical and public health applicability, and glycemic index estimates and subsequent associations with chronic disease risk needs to be reconsidered, say the authors.

"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."
-end-
Additional authors on this study are Lynne M. Ausman, D.Sc., professor at the Friedman School, Huicui Meng, Ph.D., postdoctoral associate in the Cardiovascular Nutrition Lab at the USDA HNRCA, and Hocine Tighiouart, M.S., statistical associate at the Tufts Clinical and Translational Science Institute and clinical assistant professor of medicine at Tufts University School of Medicine.

This study was supported by awards from the National Institute of Diabetes and Digestive and Kidney Diseases (DK073321) and the National Center for Advancing Translational Sciences (TR001064), both of the National Institutes of Health, and the U.S. Department of Agriculture's Agricultural Research Service.

Matthan, N.R., Ausman, L.M, Meng, H., Tighiouart, H., and Lichtenstein, A.H., "Estimating the reliability of glycemic index values and potential sources of methodological and biological variability." Am J Clin Nutr 2016;104:1-10. doi: 10.3945/ajcn.116.137208.

About the Jean Mayer USDA Human Nutrition Research Center on Aging and the Friedman School of Nutrition Science and Policy

For three decades, the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University has studied the relationship between good nutrition and good health in aging populations. Tufts research scientists work with federal agencies to establish the USDA Dietary Guidelines for Americans, the Dietary Reference Intakes, and other significant public policies. The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy at Tufts University is the only independent school of nutrition in the United States. The school's eight degree programs - which focus on questions relating to nutrition and chronic diseases, molecular nutrition, agriculture and sustainability, food security, humanitarian assistance, public health nutrition, and food policy and economics - are renowned for the application of scientific research to national and international policy.

Tufts University, Health Sciences Campus

Related Blood Sugar Articles:

High blood sugar following surgery common, increases risk of complications
High blood sugar, also known as hyperglycemia, occurs when the body does not produce enough insulin to turn blood glucose into energy.
Researchers use modified insulin and red blood cells to regulate blood sugar
Researchers have developed a new technique that uses modified insulin and red blood cells to create a glucose-responsive 'smart' insulin delivery system.
Surprising link between blood sugar and brain cancer found
New research further illuminates the surprising relationship between blood sugar and brain tumors and could begin to shed light on how certain cancers develop.
Diabetes app forecasts blood sugar levels
Glucoracle is a new app for people with type 2 diabetes that uses a personalized algorithm to predict the impact of particular foods on blood sugar levels.
Electroacupuncture may improve regulation of blood sugar in overweight and obese women
For women who are overweight or obese and are unable to exercise, new research appearing online in The FASEB Journal suggests combining acupuncture with an electrical current may help.
KU Leuven researchers unravel how stevia controls blood sugar levels
What makes stevia taste so extremely sweet? And how does the sweetener keep our blood sugar level under control?
Artificial pancreas improves blood sugar control in young kids
An artificial pancreas, which delivers insulin in an automated way to individuals with type 1 diabetes, appears to be safe and effective for use in children ages 5 to 8 years, a new study finds.
Size matters when it comes to keeping blood sugar levels in check
Keeping blood sugar levels within a safe range is key to managing both type 1 and type 2 diabetes.
A better blood sugar test for diabetes
A new method for estimating blood sugar levels can cut diagnostic errors by more than 50 percent compared to the current widely used blood test, according to a study of more than 200 diabetic patients.
Scientists devise a more accurate way to gauge blood sugar averages in diabetes
Marrying advanced math with standard blood-sugar tests, Harvard Medical School scientists have devised a more accurate way to measure three-month average blood sugar levels in people with diabetes.

Related Blood Sugar Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Anthropomorphic
Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#532 A Class Conversation
This week we take a look at the sociology of class. What factors create and impact class? How do we try and study it? How does class play out differently in different countries like the US and the UK? How does it impact the political system? We talk with Daniel Laurison, Assistant Professor of Sociology at Swarthmore College and coauthor of the book "The Class Ceiling: Why it Pays to be Privileged", about class and its impacts on people and our systems.