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glycemic index

A measure devised around 1981 to indicate how fast an ingested carbohydrate becomes glucose in the human bloodstream.¹ British spelling, glycaemic index. Symbol, GI. Persons with diabetes use the glycemic index in regulating their diet. Generally speaking, the more processed a food, the higher its glycemic index, although there are many exceptions.

Tests on human volunteers are used to establish the glycemic index for a foodstuff.^1,2 Typically, 10 healthy people who have fasted overnight eat a quantity of the food containing 50 grams of carbohydrate. Their blood sugar levels are then measured every 15 to 30 minutes for 2 hours. Graphing the average of these readings against time yields a rising and falling curve. The area under the curve can be calculated. This area is divided by the area under the curve from a measurement made in the same way for a reference substance, usually either white bread or pure glucose. The resulting number is multiplied by 100 to give the glycemic index. The glycemic index for the reference substance (white bread or glucose) is therefore 100. Indexes based on glucose can be converted to the white bread scale by multiplying them by 1.43.

A related measure, the glycemic load³, describes a serving rather than a foodstuff. It is the GI of the foodstuff times the carbohydrate content of the serving expressed in grams, divided by 100.

Researchers have suggested that consumption of high glycemic index foods is related to the development of, not just type 2 diabetes, but obesity, gall bladder disease, coronary heart disease, and other conditions. Such claims are as yet controversial. Some even question the role of GI in continuing care for diabetics, often on the grounds that tracking intake this way is too complicated. Unlike, say, calories, it is often not possible to assign the same GI to all the forms in which a food appears. For example, the smaller the particles which compose a food stuff, the faster they are digested. Bread made from coarse, stone-ground flour will have a lower glycemic index than bread made from the same wheat berries but very finely ground. Pureed vegetables will generally have a higher glycemic index than the same vegetable in the unpureed form.

1. David J. A. Jenkins, Thomas M. S. Wolever, Rodney H. Taylor, Helen Barker, Hashmein Fielden, Janet M. Baldwin, Allen C. Bowling, Hillary C. Newman, Alexandra L. Jenkins and David V. Goff.
Glycemic index of foods: a physiological basis for carbohydrate exchange.
American Journal of Clinical Nutrition, vol. 34, pages 362-366 (March 1981)
The full text is available free at www.ajcn.org.

2. D. S. Ludwig.
The glycemic index: physiological mechanisms relating to obesity, diabetes, and cardiovascular disease.
JAMA, vol. 287, no. 18, pages 2414-2423 (2002).

3. Walter C. Willett.
Eat, Drink, and Be Healthy: The Harvard Medical School Guide to Healthy Eating.
New York: Simon and Schuster, 2001.

resources

Anne Raben.
Glycemic index and metabolic risks: how strong is the evidence?
American Journal of Clinical Nutrition, vol. 100, no. 1, pages 1-3 (July 2014). An editorial.
doi: 10.3945/ajcn.114.090415
The full text is available free at www.ajcn.org.

Many Web sites offer listings of the glycemic indices of various foodstuffs. Among the best are:

www.mendosa.com/gilists.htm

www.glycemicindex.com

http://care.diabetesjournals.org/content/31/12/2281/suppl/DC1 (download pdf's)

The two latest versions of the "international table" of glycemic indices:

Kaye Foster-Powell, Susanna H. A. Holt, and Janette C. Brand-Miller.
International table of glycemic index and glycemic load values: 2002.
The American Journal of Clinical Nutrition, vol. 76, pages 5-56 (2002).

Fiona S. Atkinson, Kaye Foster-Powell and Jennie C. Brand-Miller.
International Tables of Glycemic Index and Glycemic Load Values: 2008.
Diabetes Care, vol. 31, no. 12, pages 2281-2283 (Dec. 2008).
doi.org/10.2337/dc08-1239

Several prominent institutions provide good web pages on the GI, including the American Diabetes Assn.:

www.diabetes.org/food-and-fitness/food/planning-meals/glycemic-index-and-diabetes.html

and the Linus Pauling Institute at Oregon State University:

(http://lpi.oregonstate.edu/infocenter/foods/grains/gigl.html).