The pros and cons of using the glycemic index for carb counting and meal planning
By Tracey Neithercott
Carbs count. Eat too many and your blood glucose can spike. It’s the first lesson people with diabetes learn at diagnosis: Watch the number of carbohydrate grams you eat at each meal or snack. That’s all well and good, but what about the type of carbohydrate you choose to eat? Increasingly, researchers are asking that question. Their answer may surprise you.
The GI Revolution
Back in 1980, most people with diabetes were using carbohydrate exchange lists, and “glycemic index” was a term foreign to researchers, doctors, and patients alike. Scientists at the University of Toronto, led by David Jenkins, MD, PhD, DSc, began digging around in the body’s glucose response to different foods, and the term came into use. In a 1981 study published in The American Journal of Clinical Nutrition, the researchers listed what they called the glycemic index for 62 common foods.
The glycemic index (GI) measures the glucose response to a given number of grams of carbohydrate. Imagine a chart with a line representing glucose levels during the first two hours after you eat 50 grams of carbohydrate. The area under that line, when compared with results from a test using 50 grams of pure glucose, indicates a food’s glycemic index (graph, opposite). Foods with a high glycemic index have higher peaks and more area under the line than those with a low GI. The glycemic index, then, is a ranking of foods from zero to 100 based on blood glucose levels after eating. So 50 grams of a plain white baguette has a GI of 95 (and a taller graphed line) while 50 grams of an apple has a GI of 39 (and a shorter line).
“If you’re carb counting and if you have 20 grams of carbohydrate from an apple or a banana or rice, it’s [as if it’s] all the same,” says Thomas Wolever, BM, BCh, DM, PhD, coauthor of The New Glucose Revolution: The Authoritative Guide to the Glycemic Index—the Dietary Solution for Lifelong Health, professor in the Department of Nutritional Sciences at the University of Toronto, and coauthor of the 1981 glycemic index study. “But it’s not.”
You might imagine that the glycemic index would be useful for people with diabetes. Yet more than three decades after its discovery, the glycemic index still isn’t among the most-recommended meal-planning tools.
The Glycemic Load
Graphing Blood Glucose Response
To determine the glycemic index of foods, researchers plot glucose levels for two hours after subjects eat portions containing 50 grams of carbohydrate and compare the area under each line with that of pure glucose.
SOURCE: Adapted from The American Journal of Clinical Nutrition, January 2009
In its 2013 nutrition position statement, the American D
iabetes Association says picking low-GI foods over high-GI ones “may modestly improve glycemic control.” Yet despite the ADA’s slight encouragement, the recommendations note that while some studies showed drops in A1C (a measure of average glucose for the past two to three months) from following a low-GI diet, others found no blood glucose improvement at all.
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