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Demystifying the glycemic index

Whatever diet you are following, you've likely stumbled across the glycemic index (GI). Once only used in reference to diets for diabetes, now many popular diets have incorporated this concept into their meal plans.

What is the GI?
First, it's important to understand the term glycemic response, which is a measure of how rapidly a food affects blood sugar. Foods that have a high-glycemic response are thought to be digested and absorbed quickly, causing a rapid increase in blood sugar while those with a low-glycemic response produce a slower increase. To simplify this concept, researchers created the GI to categorize foods. Each food is given a GI or a numeric value that ranks the food based on its blood glucose response. White bread has a GI of 100 since it causes a rapid rise in blood glucose. It is used as the standard to compare other foods.

A food with a GI of less than 55 is considered to be "low" and turns to glucose more slowly. Foods with a GI greater than 70 are considered to be "high" and turn to glucose quickly. Foods with a GI value of 56-69 are considered moderate. The following list shows the GI of common foods.

High (>70)
• Instant rice
• Bagel
• Doughnut
• White bread
• Cornflakes®, Rice Krispies®, Cheerios®
• Waffle
• Baked potato
• Carrots
• Watermelon

Medium (56-69)
• Whole wheat bread
• Oatmeal
• Shredded Wheat®, Grape Nuts®
• Orange juice
• Raisins
• Pineapple

Low (<55)
• Brown rice
• Bulgur and barley
• Spaghetti
• All Bran®
• Grapes, bananas, orange, pear
• Sweet potato, green peas
• Apples and apple juice
• Low fat milk

The theory is that eating foods with a lower GI will cause a slower rise in blood sugar, making you feel fuller for a longer period of time. This is also thought to help control cravings, prevent overeating and better manage blood sugar. Eating foods with a higher GI results in a rapid increase in blood sugar, followed by the release of insulin. Blood sugar then drops quickly, often leading to hunger.

Why the controversy?
While it sounds logical, the concept of the GI has been controversial for years. The GI only reflects how quickly a food raises blood sugar levels. It does not take into account the total carbohydrate content in a typical serving of that food which is also an important part of glycemic response. As a result, the term glycemic load (GL) has emerged. The GL ranking takes the GI a step further by adjusting for the total carbohydrate factor. It is obtained by dividing the GI value by one hundred and then multiplying by the amount of total carbohydrate per typical serving of the food.

A low GL food is considered 10 or less, medium 11-19 and high would be 20 or more.

Once the total carbohydrate per typical serving size of the food is considered, it is possible that a high glycemic index food has a low glycemic load. For example, carrots have a glycemic index of 92 which is high. But, in a typical serving size of one-half cup, there is only 4.2 grams of carbohydrate. So, the calculated GL is only 3.9 which is low (GL = 92/100 X 4.2), showing that this food is unlikely to cause a disturbance in blood sugar or insulin response. The following chart will help you identify both the GI and GL of commonly eaten foods.

As the chart reveals, even foods with a high GI often have a low GL when the portion size is considered.

• Rather than select foods solely based on their GI or GL, it's most important to eat a balanced diet.
• Use common sense and evaluate foods for their overall health value.
• If you choose to plan your diet using the GI or GL as a guide, look at food values for an entire day rather than excluding a food because it has a high value. Aim to keep your GL to 80 per day.
• When you eat foods with a high GI, the smaller the serving size, the smaller the rise in your blood sugar and insulin response.
• Foods with a high GI but low GL can be included as part of a healthy diet.
• Foods with a low GI will always have a low GL.

The Yale-New Haven Nutrition Advisor is created by registered dietitians and dietetic interns who staff the Nutrition Clinic at Yale-New Haven Hospital. For information, contact the Nutrition InfoLine at (203) 688-2422.

For more information on this topic, see

• The glycemic index, Diabetes Australia-Victoria
• What is the glycemic index?, Glycemic Index Tested, The University of Sydney
• Yale-New Haven Nutrition Forum (search by keyword)
• American Diabetes Association

Other issues addressed by Yale-New Haven Nutrition Advisor:

• Nutrition 101 for students away from home
• All fats are not created equal
• The whole truth about whole grains
• Alzheimer's disease and diet
• Does fast food have to equal fat food?
• The truth about trans fats
• Understanding yogurt
• Thinking about going low-carb?
• What's to drink?
• Obesity—it's not just for grown-ups anymore
• Fruits and vegetables: Always in season
• Calcium: Not just for bones anymore!
• Mercury Rising
• A powerful B vitamin: Folate
• Vegging out
• Manic for organic
• Beta carotene
• Archive

Last revised: March 10, 2005 (jj)

Home page URL: http://www.ynhh.org

All information is intended for your general knowledge and is not a substitute for medical advice or treatment for specific medical conditions. You should seek prompt medical care for any specific health issues and consult your physician before starting a new fitness regimen.