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Harvard Commentaries
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Food for Thought Food for Thought
 

Understanding the Glycemic Index


January 07, 2013


By Brittany Sinensky, Dietetic Intern
Brigham and Women's Hospital

Over three decades ago, Dr. David J. Jenkins introduced the glycemic index (GI). It's a measure of how quickly blood sugar rises when you eat particular foods. Since then, the GI has become an effective treatment tool for people with diabetes.

More recently, health care professionals are using the GI and the newer glycemic load (GL) to help prevent or treat other chronic diseases. These include cardiovascular disease and cancer.

In the world of nutrition information overload today, many people are wondering if the GI is the new gold standard for helping people make good dietary choices — even healthy people.

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Understanding the Numbers

The GI measures exactly how quickly or slowly carbohydrates (sugars and starch) are digested. Foods are assigned a number by measuring their effect on blood sugar. This is done in a laboratory.

The GL also is a number. It takes into account how much carbohydrate gets into the bloodstream and how quickly it gets there. The GL is estimated by multiplying the food's GI value by the number of grams of available carbohydrate in a specific serving size of a food and dividing by 100.

The GI and GL values help people manage their glycemic response (rise or fall of blood sugar) after eating certain foods. Foods with a high glycemic index or load will cause a greater glycemic response than foods with a low GI or GL.

Some foods, such as watermelon, have a high GI (72), but a low GL (4). That's because a realistic serving size lowers the effect that watermelon has on raising blood sugar.

Here are the GI and GL for common foods:

GI value

Classification

Foods

Less than 55
Low
Apples, chickpeas, low-fat skim milk
55-69
Medium
Peas, sourdough rye bread, low-fat vanilla ice cream
70-100
High
Waffles, Gatorade, corn flakes

GL value

Classification

Foods

Less than or equal to 10
Low
Hummus, cashews, mango
11-20
Medium
Sweet potato, corn chips, bananas
More than 20
High
Raisins, instant white rice, cranberry juice cocktail (from USA)

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What the Research Shows

How a food affects blood sugar is not just important for people with diabetes. According to research, it's also important for people with other chronic diseases, such as cardiovascular and cancer. A meta-analysis in 2008 found that diets with a high GI or GL independently increased the risk of:

  • Type 2 diabetes
  • Heart disease
  • Gallbladder disease
  • Breast cancer

The study provides strong evidence that diets with a high GI, high GL or both, increase the risk of chronic lifestyle-related diseases.

The GI can also help people who are overweight or obese lose weight. Studies show that people lost more weight on a low GI diet than on high GI or popular calorie-restricted diets. People also maintain their weight losses better when eating a diet moderately high in protein and low in GI.

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Tips for Using the Glycemic Index and Glycemic Load

For many, a diet based on GI and/or GL may seem difficult to follow. That's because:

  • These values are not clearly displayed on the nutrition facts labels.
  • You can't calculate them on your own, or even estimate them based on the nutrient composition of a single food or meal.

However, despite these drawbacks, there is overwhelming evidence that everyone could benefit from following a low-GI and/or low-GL diet. In fact, it may be easier to stick to a diet using the GI and/or GL as a guideline than trying to stick to a stringent diet in which you can only eat certain foods. Also, there don't seem to be any safety concerns in adopting this diet.

Here are some tips to use when using the GI or GL to help you make food choices:

  • Use this list of GI and GL values (which starts on page 5) to choose foods. Stick it on your fridge.
  • Use either the GI or GL values. Both are useful, but switching back and forth between the two lists may make your head spin with numbers.
  • Do not avoid foods that are known to be a good source of nutrients just because they have a high GI or GL (i.e. watermelon). They will still offer lots of health benefits, such as vitamins, minerals and fiber.

Barclay, A.W., Petocz, P., et al. "Glycemic index, glycemic load, and chronic disease risk – A meta-analysis of observational studies." The American Journal of Clinical Nutrition. 2008; 87: 627-637.

Larsen, T.M., Dalskov, S-M., et al. "Diets with High or Low protein content and glycemic index for weight-loss maintenance." The New England Journal of Medicine. 2010; 363: 2102-2113.

Brand-Miller, J., McMillan-Price, J., et al. "Dietary glycemic index: Health implications." Journal of the American College of Nutrition. 2009; 28: 446S-449S.

Thomas, D.E., Elliot E.J., & Baur, L. (2007). "Low glycaemic index or low glycaemic load diets for overweight and obesity." Cochrane Database of Systematic Reviews, Issue 3. Art. No.: CD005105.

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Brittany Sinensky is a dietetic intern at Brigham and Women's Hospital in Boston, MA. She holds a Bachelor of Science in Marketing from Providence College and a Bachelor of Science in Nutrition from Long Island University.

 

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