Glycemic Index Update

By Marie Spano, M.S., R.D., Contributing Editor

The glycemic index (GI) measures how various carbohydrates affect blood sugar levels. It has been used to control diabetes and may hold promise as a method of controlling weight. But, recent research suggests it might be more complicated than once believed.

Glycemic index 101

GI measures a carbohydrates effect on blood glucose compared to a reference food of either glucose or white bread. Foods that are digested and absorbed rapidly, resulting in a sharp spike in blood glucose after consumption, such as potatoes and watermelon, are given a high GI (70 to 99). Foods that are digested slowly, resulting in a slow release of glucose into the bloodstream, such as vegetables, are given a low GI (55 or less). Foods with a moderate GI, including whole-wheat products and brown rice, fall within a GI range of 56 to 69 (American Journal of Clinical Nutrition, 1981; 34:362-366).

The GI has sometimes been confused with the glycemic load (GL). GL is defined as the GI of a food (divided by 100) multiplied by the grams of carbohydrate from one serving of the food. GI and GL do not go hand-in-hand; a food with a high GI will not necessarily have a high GL (American Journal of Clinical Nutrition, 2002; 76(1):5-56).

Many factors affect a foods GI. Fat, fiber and protein slow the digestion of carbohydrates and release of sugar into the bloodstream. In addition, cooking and processing methods also affect GI by altering the structure of starch and subsequently speeding up the release of sugar into the bloodstream. For example, raw carrots have a GI of 16, whereas cooked carrots have a GI of 92. And finally, ripeness, storage time, cooking method and type of carbohydrate affect the GI (American Journal of Clinical Nutrition, 2002; 76(1):5-56).

GI can be incredibly confusing and misleading for consumers. First, published GI values for the same foods vary based on testing methods and the physical and chemical characteristics of the food. In addition, a GI given for a particular food, a slice of whole-wheat bread for instance, may vary from brand to brand of whole-wheat bread based on ingredients and processing. And, GI values for commercially prepared foods may change over time based on ingredient changes. Plus, there may be differences in products, say a nutrition bar, depending on the SKU (an apple-cinnamon bar versus an almond-flavored bar, for example). Additionally, GI may vary depending on where the food is grown. Rice, for instance, varies based on botanical differences. And finally, GI values may also differ because some labs use white bread as the reference, whereas others use glucose (dextrose).

Making matters more confusing, the GI of a food does not correlate to the GI of a meal that contains that food, and many people eat mixed meals versus single foods. So, for instance, white bread consumed as part of a sandwich loaded with fiber-rich vegetables, cheese and meat will elicit a different glycemic response than plain white bread.

To see a list of GI values for some common ingredients, go to ajcn.org/content/76/1/5.full.pdf+html. A database of GI values can be found at glycemicindex.com.

GI research

According to Jennie Brand-Miller, Ph.D., professor, molecular and microbial sciences, Sydney Medical School, The University of Sydney, Australia, a low-GI diet is very helpful in a variety of contexts, including diabetes, weight loss, appetite control and more." GI has also been touted as a tool to help people reduce risk factors for diseases such as diabetes and cardiovascular disease.

The American Diabetes Association, Alexandria, VA, recognizes that GI may provide a modest additional benefit when used in conjunction with carbohydrate counting. However, monitoring carbohydrate intake through carbohydrate counting or exchanges is still the top strategy for diabetics to achieve glycemic control (Diabetes Care, 2008; 31:S61-S78). Low-GI, high-fiber diets may help lower postprandial glucose and insulin responses, improve lipid profile and possibly help decrease insulin resistance. In addition, GI may be a useful tool in diabetic populations where low-GI foods are beneficial for blood-glucose control both in the short term and long term. However, no studies to date have examined whether incorporating a low-GI, high-fiber diet can reduce the risk of diabetes or incidence of cardiovascular events in non-diabetic persons (American Journal of Clinical Nutrition, 2008; 87(1):269S-274S).

Studies examining how GI affects appetite, food intake, energy expenditure and body weight are equivocal (Obesity Reviews, 2002; 3:245-256). In addition, according to the 2010 Dietary Guidelines for Americans, glycemic index and/or glycemic load are not associated with body weight and, therefore, it is not necessary to consider these measures when selecting carbohydrate foods and beverages for weight management." While there is no evidence that GI affects weight control over the long term, there also havent been any studies examining ad libitum food intake and the selection of low versus high GI over the long term (IUBMB Life, 2011; 63:7-13).

GI seems most beneficial for diabetics when used in conjunction with carbohydrate counting. However, it isnt always easy to determine the GI of a food since the database on GI is not all-inclusive and GI is not listed on the label or packaging of most foods. In addition, the concept of GI and how cooking and processing affect GI may be too complex for some consumers to understand and effectively incorporate into their nutrition decision-making process.

Marie Spano, M.S., R.D., CSCS, is a nutrition communications expert whose work has appeared in popular press magazines, e-zines and nutrition-industry trade publications. She has been an expert guest on NBC, ABC and CBS affiliates on the East Coast. For more information, visit mariespano.com.