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How Useful Is The Glycemic Index?

New research shows varied individual responses to GI values may be confusing for patients

Just how valid is the glycemic index as a guide to suitable foods for people with diabetes? The glycemic index (GI) of a food indicates the speed with which blood sugar can be expected to rise after a person eats it. Each food gets a score out of 100 on the index, for example, 40 for baked beans. Glycemic load is a measure that applies the GI to a portion of food. The glycemic load for a 150-gram serving of baked beans would be 6.

GI is used to help people with diabetes to control their blood glucose. Some food labels carry GI measurements. More recently, a number of popular diets have been based on GI. There are lists available for people to check the GI and glycemic load of different foods. However, its usefulness is controversial, because individual responses to a particular food can vary.

Scientists from the Jean Mayer United States Department of Agriculture (USDA) Human Nutrition Research Center on Aging (HRNCA) at Tufts University have questioned whether GI is a valid measure after randomized, controlled, repeated tests on 63 healthy adults suggested otherwise.

In a study, the volunteers participated in six testing sessions over 12 weeks. They fasted and abstained from exercise and alcohol before each session. During the session, they ate either white bread or a glucose drink, in random order. The bread was the test food, and the drink was a reference control.

Fast facts about GI values

  • Plain white baguette scores 95 on the glycemic index.
  • White wheat flour bread scores 75.
  • An average apple scores 36.
  • Hummus, a chick-pea dip, scores 6.

Each item contained 50 grams of available carbohydrate. Blood glucose levels were measured several times over the next 5 hours, and GI was calculated using standard methods. The findings put the average GI value of white bread at 62, making it a “medium” GI food, but scores varied by 15 points in either direction.

Individual responses varied by up to 60 points between tests. In 22 participants, the blood sugar response was low, in 23, it was medium, and in 18, it was high. This range effectively put white bread in all three GI categories. These readings reflect long-term glucose control.

In this study, these factors showed the highest range in measures, around 15 percent to 16 percent difference. This suggests that an individual’s metabolic response to food impacts their GI values. As a result, the scientists have questioned how useful the GI is for predicting a food’s impact on blood glucose levels.

Lead author Nirupa Matthan points out that a person consuming the same amount of the same food three times should have a similar blood glucose response every time, but this did not happen.  As a result, she suggests that GI values are “unlikely to be useful in guiding food choices.”

So, if a food that is low glycemic index for you one time you eat it and could be high the next time, and it may have no impact on blood sugar, then how useful could it possibly be?

The glycemic index variability values can be due to the differences in macronutrient (carbohydrate, protein, fat), including fiber combinations, which prevents you from getting an accurate number for the glycemic index.

The team used a higher number of participants, more tests and a longer measuring window than most GI tests do. They also looked at how the biological factors such as sex, body-mass index (BMI), blood pressure, and physical activity impact GI variability. In most cases, the impact was minor.

The authors point out that the results do not mean that food with a high GI is necessarily healthy, nor that one with a low GI is unhealthy, but it does imply that the usefulness of these measures for managing clinical and public health may be limited.

Practice Pearls:

  • The glycemic index value of a food can vary by as much as 20% within an individual and 25% among healthy adults.
  • The findings suggest glycemic index has limited value in predicting how foods impact blood sugar levels.
  • Individual blood sugar responses after consuming a fixed amount of white bread could range across all three glycemic index categories (low, medium or high), which makes the glycemic index is impractical for use in food labeling or for dietary guidance at the individual level.

Reference:

Matthan NR, et al. Am J Clin Nutr. 2016;doi:10.3945/ajcn.116.137208. Sept. 12, 2016